Failure detection circuit, dynamic brake device, and motor drive device

Abstract

This failure detection circuit comprises: a brake control unit for controlling a dynamic brake circuit which brakes a motor; a rotating speed detection unit for detecting the rotating speed of the motor; a coasting distance estimation unit for calculating, on the basis of parameters regarding the motor and the rotating speed of the motor detected by the rotating speed detection unit at the time when the dynamic brake circuit starts braking, an estimation value of a coasting distance from where the dynamic brake circuit starts braking to where the motor stops; a coasting distance actual measurement value acquisition unit for acquiring an actual measurement value of the coasting distance from where the dynamic brake circuit starts braking to where the motor stops; a comparison unit for comparing, with a prescribed threshold value, a value obtained by calculation based on the estimation value of the coasting distance and the actual measurement value of the coasting distance; and a determination unit for determining, on the basis of a comparison result from the comparison unit, whether or not there is a failure in the dynamic brake circuit.

Description

Fault detection circuit, dynamic braking device and motor drive device

[0001] The present disclosure relates to a fault detection circuit, a dynamic braking device, and a motor drive device.

[0002] Dynamic braking circuits that apply dynamic braking by shorting the motor's input terminals are widely used in motor drive devices that power machines such as machine tools and robots. A dynamic braking circuit typically consists of a dynamic braking circuit switch (hereinafter simply referred to as the "switch") installed across the motor's input terminals and a dynamic braking resistor (DB resistor) connected in series with the switch. To apply dynamic braking, the supply of drive power to the motor is cut off, and the switch is closed to short-circuit the motor's input terminals (between the motor winding phases). Even when the motor is electrically disconnected from the power source, a field magnetic flux exists, and the motor, rotating by inertia, functions as a generator. The generated current flows through the closed switch into the dynamic braking resistor, generating a deceleration torque in the motor.

[0003] One type of dynamic braking circuit failure is an open switch failure. Switch open failures include a failure caused by the switch itself, in which the switch does not close but remains open despite a braking command, and a failure caused by the drive control system not being able to properly generate a braking command to close the switch. If the dynamic braking circuit fails, it will not be able to brake the motor.

[0004] Japanese Patent Application Laid-Open No. 2000-253687 Japanese Patent Application Laid-Open No. 2021-016224 Special Publication No. 2018-536376

[0005] There is a demand for a technique that can accurately detect a fault in a dynamic braking circuit with a simple structure.

[0006] According to one aspect of the present disclosure, a fault detection circuit comprises: a brake control unit that controls a dynamic braking circuit that brakes the motor; a rotational speed detection unit that detects the rotational speed of the motor; a coasting distance estimation unit that calculates a coasting distance estimate, which is the coasting distance of the motor that is estimated to be required from the start of braking by the dynamic braking circuit to the stop of the motor, based on the rotational speed of the motor at the start of braking by the dynamic braking circuit detected by the rotational speed detection unit and motor-related parameters; a coasting distance measurement value acquisition unit that acquires, from the rotational speed detection unit, the coasting distance measured, which is the coasting distance of the motor that is actually measured from the start of braking by the dynamic braking circuit to the stop of the motor; a comparison unit that compares a value calculated based on the estimated coasting distance and the measured coasting distance with a predetermined threshold; and a determination unit that determines whether or not a fault has occurred in the dynamic braking circuit based on the result of the comparison by the comparison unit.

[0007] FIG. 1 is a diagram showing a fault detection circuit, a dynamic braking device, and a motor drive device according to an embodiment of the present disclosure. FIG. 2 is a diagram showing a fault detection circuit, a dynamic braking device, and a motor drive device according to a modification of an embodiment of the present disclosure. FIG. 3 is a cross-sectional view showing the structure of a friction brake device provided in a motor drive device according to a modification of an embodiment of the present disclosure, illustrating a state in which braking is applied to the motor. FIG. 4 is a cross-sectional view showing the structure of a friction brake device provided in a motor drive device according to a modification of an embodiment of the present disclosure, illustrating a state in which braking of the motor is released. FIG. 5 is a diagram illustrating an example of a relationship between a coasting distance actual measurement value and a coasting distance estimated value calculated according to Equation 43 in a motor drive device according to a modification of an embodiment of the present disclosure. FIG. 6 is a flowchart showing an operation flow of fault detection processing according to an embodiment and a modification thereof. FIG. 7 is a flowchart showing an operation flow when sequential calculation of a coasting distance estimated value is performed according to Equation 43 in fault detection processing according to a modification of an embodiment of the present disclosure.

[0008] Hereinafter, embodiments of a fault detection circuit, a dynamic braking device, and a motor drive device will be described with reference to the drawings. In the following description, components having the same or similar functions are designated by the same reference numerals. Duplicate descriptions of those components may be omitted. The scale of the drawings has been appropriately changed to facilitate understanding.

[0009] In the following description, a converter that converts AC power supplied from an AC power source into DC power and outputs it is also referred to as a "rectifier device," "rectifier circuit," "rectifier," or "forward converter." An inverter that converts DC power into AC power and outputs it is also referred to as an "inverter." A "DC link" refers to a circuit portion that electrically connects the DC output side of a converter and the DC input side of an inverter, and is also referred to as a "DC link unit," "DC link," "DC link unit," "DC bus," or "DC intermediate circuit." The "on" state of a switch means that the switch is closed, forming an electric path through the switch. The "off" state of a switch means that the switch is opened, interrupting the electric path through the switch. Furthermore, the "rotational speed" of a motor refers to the "rotational angular velocity of the rotor or rotating shaft" of the motor. The "coasting distance" is also referred to as the "braking distance." Dynamic braking is also referred to as "DB."

[0010] <Overall Configuration of One Embodiment of the Present Disclosure> FIG. 1 is a diagram illustrating a fault detection circuit, a dynamic braking device, and a motor drive device according to one embodiment of the present disclosure.

[0011] In one embodiment of the present disclosure and the modified examples described below, a case is shown in which a three-phase AC motor 3 is driven by a motor drive device 1 connected to an AC power source 2. The number of phases of the AC power source 2 is not particularly limited in each embodiment and each modified example, and may be, for example, three-phase or single-phase. Examples of the AC power source 2 include a three-phase 400V AC power source, a three-phase 200V AC power source, a three-phase 600V AC power source, and a single-phase 100V AC power source. Here, as an example, each AC power source 2 is three-phase. Furthermore, the number of motors 3 is not particularly limited in each embodiment, and may be multiple. When multiple motors 3 are provided, a dynamic braking device 100 is provided for each motor 3. Here, as an example, one motor 3 is provided. Machines in which the motor 3 is provided include, for example, machine tools and robots.

[0012] A motor drive device 1 according to one embodiment of the present disclosure includes a converter 101, an inverter 102, a motor control unit 103, a smoothing capacitor 104, and a dynamic braking device 100. Although not shown here, a power line that supplies power to drive the motor control unit 103 and the dynamic braking device 100 is provided on a system separate from a power line that supplies power from the AC power supply 2 to the converter 101.

[0013] The converter 101 converts AC power supplied from the AC power source 2 into DC power and outputs it to a DC link. In the example shown in FIG. 1 , since the AC power source 2 is a three-phase AC power source, the converter 101 is configured as a three-phase bridge circuit. If the AC power source 2 is a single-phase AC power source, the converter 101 is configured as a single-phase bridge circuit. Examples of the converter 101 include a diode rectifier, a PWM switching control rectifier, and a 120-degree conduction rectifier. For example, if the converter 101 is configured as a diode rectifier, the converter 101 is configured as a three-phase bridge circuit of diodes. If the converter 101 is configured as a PWM switching control rectifier or a 120-degree conduction rectifier, the converter 101 is configured as a three-phase bridge circuit of semiconductor switching elements and diodes connected in anti-parallel to the semiconductor switching elements. Examples of the semiconductor switching elements include FETs, IGBTs, thyristors, GTOs, and transistors, but other semiconductor switching elements may also be used. An AC reactor, an AC line filter, an electromagnetic contactor, and the like may be provided on the AC input side of the converter 101, but these are not shown here.

[0014] A smoothing capacitor 104 is electrically connected to the DC link between the converter 101 and the inverter 102. The smoothing capacitor 104 is sometimes referred to as a "DC link capacitor" or a "direct-current link capacitor." The smoothing capacitor 104 has the function of suppressing oscillations in the DC output of the converter 101 and the function of storing DC power used by the inverter 102 to generate AC power. Examples of the smoothing capacitor 104 include an electrolytic capacitor and a film capacitor. A pre-charging circuit for pre-charging the smoothing capacitor 104 may be provided, but is not shown here.

[0015] The motor control unit 103 executes control to drive the motor 3. The motor control unit 103 generates a drive command to control the rotational speed, position, or torque of the rotor of the motor 3 based on the rotational speed of the motor 3 (rotational speed feedback), the current flowing through the windings of the motor 3 (current feedback), a rotational speed command, a torque command, a position command, an operation program for the motor 3, and the like.

[0016] The inverter 102 is connected to the converter 101 via a DC link. The inverter 102 converts DC power supplied from the converter 101 into AC power for driving the motor and outputs the converted AC power. The inverter 102 is composed of a three-phase bridge circuit of semiconductor switching elements and diodes connected in antiparallel to the semiconductor switching elements. Examples of the semiconductor switching elements include FETs, IGBTs, thyristors, GTOs, and transistors, but other semiconductor switching elements may also be used. In response to a drive command from the motor control unit 103, the inverter 102 converts DC power in the DC link into AC power for driving the motor and outputs the converted AC power to the motor 3. As a result, the motor 3 is driven based on the AC power output from the inverter 102. In response to a drive command from the motor control unit 103, the inverter 102 converts AC power regenerated during deceleration of the motor 3 into DC power and outputs the converted DC power to the DC link.

[0017] The dynamic braking device 100 includes a fault detection circuit 10 and a dynamic braking circuit 40.

[0018] The dynamic braking circuit 40, under the control of the brake control unit 11, shorts the input terminals of the motor 3 via a dynamic braking resistor 41, thereby generating a deceleration torque in the motor 3 and braking the motor 3.

[0019] The dynamic braking circuit 40 includes a dynamic braking resistor 41 and a switch 42 connected in series with the dynamic braking resistor 41. As shown in the figure, a series circuit consisting of the dynamic braking resistor 41 and the switch 42 is provided between the input terminals of the motor 3 (between the phases of the windings of the motor 3). The switch 42 is formed by a relay, a semiconductor switching element, or the like, and its on / off (open / closed) state is controlled by a brake command (on / off command) generated by the brake control unit 11. When braking the motor 3 using the dynamic braking circuit 40, the inverter 102 cuts off the supply of drive power to the motor 3, and the brake control unit 11 outputs an on command as a brake command, turning on the switch 42 in the dynamic braking circuit 40 and shorting the input terminals of the motor 3 via the dynamic braking resistor 41. In this case, even though the motor 3 is electrically disconnected from the power supply, a field magnetic flux is present, and the motor 3, rotating by inertia, functions as a generator, generating a dynamic braking current. The dynamic braking current flows into the dynamic braking resistor 41 via the turned-on switch 42, where it is converted into Joule heat and consumed. As a result, a deceleration torque is generated in the motor 3. This deceleration torque brakes the motor 3, and the motor 3 coasts to a certain extent before finally coming to a stop.

[0020] The fault detection circuit 10 detects a fault in the dynamic braking circuit 40.

[0021] The fault detection circuit 10 includes a brake control unit 11 , a rotational speed detection unit 12 , a coasting distance estimation unit 13 , a coasting distance actual measurement value acquisition unit 14 , a comparison unit 15 , a determination unit 16 , and a memory unit 17 .

[0022] The brake control unit 11 controls the dynamic braking circuit 40, which brakes the motor 3. When braking the motor 3, the brake control unit 11 outputs an ON command as a brake command to the switch 42 of the dynamic braking circuit 40. Upon receiving the ON command, the dynamic braking circuit 40 turns on (closes) the switch 42 to short-circuit the input terminals of the motor 3 via the dynamic braking resistor 41. On the other hand, when not braking the motor 3, the brake control unit 11 outputs an OFF command as a brake command to the switch 42 of the dynamic braking circuit 40. While the OFF command is being received, the dynamic braking circuit 40 keeps the switch 42 OFF (open).

[0023] The rotational speed detection unit 12 detects the rotational speed of the motor 3 via an encoder (not shown) attached to the motor 3. Alternatively, the rotational speed detection unit 12 may indirectly detect the rotational speed of the motor 3 via an encoder (not shown) attached to a rotating body driven by the motor 3. The rotational speed is the number of rotations per unit time. Information related to the rotational speed detected by the rotational speed detection unit 12 is sent to the coasting distance estimation unit 13, the coasting distance actual measurement value acquisition unit 14, the brake control unit 11, etc.

[0024] The coasting distance estimator 13 calculates a coasting distance estimate based on the rotational speed of the motor 3 detected by the rotational speed detector 12 when the dynamic braking circuit 40 starts braking and parameters related to the motor 3. The coasting distance estimate is the coasting distance of the motor 3 estimated to be required from the start of braking by the dynamic braking circuit 40 until the motor comes to a stop. When the dynamic braking circuit 40 applies braking to the motor 3 rotating at a certain rotational speed, the motor 3 coasts for a certain distance before coming to a complete stop. The coasting distance of the motor 3 occurring from the start of braking by the dynamic braking circuit 40 until the motor 3 comes to a complete stop can be theoretically estimated according to a predetermined calculation formula using the rotational speed of the motor 3 at the start of braking and various parameters. In other words, the coasting distance estimate is a theoretical value that is not affected by any malfunctions in the dynamic braking circuit 40. Details of the coasting distance estimate process performed by the coasting distance estimator 13 will be described later.

[0025] The parameters used in the coasting distance estimation process by the coasting distance estimator 13 include the back electromotive force constant of the motor 3, the motor resistance value, the number of motor poles, and the motor inductance, but other parameters may also be included. Also, some parameters may be omitted, and the coasting distance estimate may be calculated using approximate values ​​or approximate expressions as appropriate.

[0026] The memory unit 17 stores parameters used in the coasting distance estimation unit 13's estimation process for the coasting distance estimate value, such as the back electromotive force constant of the motor 3, the motor resistance value, the number of motor poles, the motor inductance, and other values.

[0027] The coasting distance actual value acquisition unit 14 acquires, from the rotational speed detection unit 12, the coasting distance actual value, which is the coasting distance of the motor 3 measured from when the dynamic braking circuit 40 starts braking until the motor 3 stops. The rotational speed detection unit 12 sends the rotational speed of the motor 3, and the coasting distance actual value acquisition unit 14 acquires (calculates) the coasting distance actual value of the motor 3 using the rotational speed of the motor 3.

[0028] The comparison unit 15 compares the deviation, which is a value calculated based on the coasting distance estimate value and the coasting distance actual measurement value, with a predetermined threshold. The deviation indicates the degree of deviation between the coasting distance estimate value and the coasting distance actual measurement value. The deviation can be obtained, for example, by dividing the absolute value of the difference between the coasting distance estimate value and the coasting distance actual measurement value by the coasting distance estimate value. The greater the deviation between the coasting distance estimate value and the coasting distance actual measurement value, the larger the deviation value becomes.

[0029] The determination unit 16 determines whether or not the dynamic braking circuit 40 has failed based on the results of the comparison by the comparison unit 15. If the result of the comparison by the comparison unit 15 indicates that the deviation exceeds the threshold, the determination unit 16 determines that the dynamic braking circuit 40 has failed. If the result of the comparison by the comparison unit 15 indicates that the deviation does not exceed the threshold, the determination unit 16 determines that the dynamic braking circuit 40 has not failed (is normal).

[0030] The estimated coasting distance is a theoretical value calculated according to a predetermined formula using the rotational speed of the motor 3 at the start of braking and various parameters, and is a theoretical value that is not affected by a fault in the dynamic braking circuit 40. On the other hand, the measured coasting distance is obtained from the actually measured rotational speed of the motor 3, and therefore varies depending on whether or not there is a fault in the dynamic braking circuit 40. If there is no fault in the dynamic braking circuit 40, the measured coasting distance and the estimated coasting distance will be nearly identical, and the degree of deviation will be small. If there is an open circuit fault in the dynamic braking circuit 40, the dynamic braking circuit 40 will not be able to apply the brakes to the motor 3 appropriately, and the measured coasting distance will be larger than the estimated coasting distance, i.e., the degree of deviation will also be large. Therefore, in one embodiment of the present disclosure, the comparison unit 15 compares the deviation with the threshold value, and the determination unit 16 determines that the dynamic braking circuit 40 has a malfunction if the deviation exceeds the threshold value, and determines that the dynamic braking circuit 40 has no malfunction (is normal) if the deviation does not exceed the threshold value.

[0031] If the rotational speed of the motor 3 is too low when the dynamic braking circuit 40 begins to apply braking, the amount of heat consumed by the dynamic braking resistor 41 will be small, and dynamic braking will not be achieved. When dynamic braking is not achieved, the coasting distance measurement acquired by the coasting distance measurement acquisition unit 14 will contain a large error. Therefore, to enable the fault detection circuit 10 to properly perform its fault detection process, the rotational speed of the motor 3 used in the estimation process by the coasting distance estimator 13 and the acquisition process by the coasting distance measurement acquisition unit 14 when the dynamic braking circuit 40 begins to apply braking must be equal to or greater than a predetermined rotational speed. The "predetermined rotational speed" here refers to the lower limit of the rotational speed of the motor 3 at which dynamic braking by the dynamic braking circuit 40 is properly achieved. Generally, the resistance value of the dynamic braking resistor 41 in the dynamic braking circuit 40 is set to a value that enables the dynamic braking circuit 40 to properly brake the motor 3 during normal operation of the motor drive device 1. Therefore, if the fault detection process of the fault detection circuit 10 is started at least when the motor 3 is rotating at the rotation speed during normal operation of the motor drive device 1, there is little possibility that the fault detection accuracy will deteriorate.

[0032] The threshold values ​​used in the comparison process by the comparison unit 15 and the determination process by the determination unit 16 may be set taking into account the operating method of the motor drive device 1 and the dynamic braking device 100, the application environment, the required control accuracy, economy, safety, and so forth. For example, the threshold values ​​may be set based on the relationship between the coasting distance measured when the motor drive device 1 is experimentally operated and an open fault is intentionally induced in the dynamic braking circuit 40, and the coasting distance measured when the dynamic braking circuit 40 is normal. For example, if the deviation is expressed as a percentage, the threshold value may also be expressed as a percentage. The threshold value expressed as a percentage may be set to, for example, several percent to several tens of percent. Note that the values ​​given here are merely examples, and other values ​​may also be used. Note that the threshold value may be stored in a rewritable storage unit (not shown) and rewritable by an external device, which allows the threshold value to be changed to an appropriate value as needed even after it has been set.

[0033] The determination result by the determination unit 16 may be displayed on, for example, a display device (not shown). Examples of the display device include a standalone display device, a display device attached to the motor drive device 1, a display device attached to a higher-level control device (for example, a robot controller or a numerical control device), and a display device attached to a personal computer or a mobile terminal.

[0034] Furthermore, if the determination result by the determination unit 16 indicates that the dynamic braking circuit has failed, the determination unit 16 may output an alarm signal, for example. The alarm signal is sent to an external host control device (not shown) via a communication interface within the motor drive device 1. Upon receiving the alarm signal, the host control device may control a display device (not shown) to display a message indicating that the dynamic braking circuit has failed, or may control an audio device (not shown) to emit an alarm sound. Furthermore, upon receiving the alarm signal, the host control device may control the motor 3 to stop rotation by a method other than braking using the dynamic braking circuit 40, or may control an emergency stop of a machine (such as a robot or a machine tool) in which the motor drive device 1 is installed. The host control device includes a processor and memory. The processor may be, for example, an IC, an LSI, a CPU, an MPU, or a DSP.

[0035] The fault detection process by the fault detection circuit 10 may be performed, for example, during a test run of the motor drive device 1, during maintenance, and / or during start-up before the start of normal operation. Furthermore, even during normal operation of the motor drive device 1, if there is a timing at which braking by the dynamic braking circuit 40 does not adversely affect the normal operation, the fault detection process by the fault detection circuit 10 may be performed at that timing.

[0036] <Process for Estimating Coasting Distance Estimated Value in One Embodiment of the Present Disclosure> As a calculation formula used to calculate a coasting distance estimated value in one embodiment of the present disclosure shown in FIG. 1, for example, a formula derived as follows can be used.

[0037] When the switch 42 is turned on to apply braking to the motor 3 by the dynamic braking circuit 40, the value of the energy consumption contribution resistance R [Ω] that contributes to the energy consumption for braking the motor 3 is calculated by multiplying the value of the motor winding resistance by R m [Ω], the value of the cable resistance is R c [Ω], the value of the dynamic brake resistor 41 is R DB When [Ω], it is expressed as in Equation 1.

[0038]

[0039] The back electromotive force constant of the motor 3 is K v [Vsec / rad], rotational angular velocity is ω [rad / sec], and the number of motor poles is p. The stator voltage of the first phase of the three phases of the motor 3 is e I [V] is expressed as in Equation 2.

[0040]

[0041] The current of the first phase of the three phases of the motor 3 is i I When [A] is used, the circuit equation of Equation 3 holds.

[0042]

[0043] If D is a differential operator, Equation 3 can be expressed as Equation 4.

[0044]

[0045] Define Equation 5 as a cofunction with C1 as a constant.

[0046]

[0047] However, since Equation 6 holds, i I is a periodic function and C1=0.

[0048]

[0049] A particular solution expressed by Equation 7 is obtained with A and B as constants.

[0050]

[0051] By substituting the above particular solution into Equation 3 and comparing the resulting equation with Equation 2, Equations 8 and 9 are obtained.

[0052]

[0053]

[0054] Here, A and B are expressed as in Equation 10 and Equation 11.

[0055]

[0056]

[0057] By rearranging the above equations, the first phase current i I [A] can be expressed as in Equation 12.

[0058]

[0059] where θ α is expressed as in Equation 13.

[0060]

[0061] Second phase stator voltage e II and current i II and the third phase stator voltage e III and current i III and the first phase stator voltage e I and current i I Since the phases are shifted by 2 / 3π and −2 / 3π, respectively, the stator power P [W] can be expressed as in Equation 14.

[0062]

[0063] Here, the torque constant K t [N·m / Ap] is expressed as in Equation 15.

[0064]

[0065] cosθ α can be expressed as in Equation 16.

[0066]

[0067] By substituting Equations 15 and 16 into Equation 14, the power P [W] of the stator of the motor 3 is obtained as shown in Equation 17.

[0068]

[0069] Since the energy applied to the stator of the motor 3 is equal to the energy of the rotor (torque x angle = power x time), Equation 18 holds true. In Equation 18, the torque of the rotor of the motor 3 is T [N], and the rotation angle of the rotor is θ [rad].

[0070]

[0071] Substituting equation 17 into equation 18 and rearranging it yields equation 19. The rotational angular velocity of the rotor is assumed to be ω [rad / sec].

[0072]

[0073] The equation of motion of the rotor of the motor 3 is expressed by Equation 20. In Equation 20, the rotor moment of inertia is expressed as J [kgm 2 ]

[0074]

[0075] Substituting the torque T [N] expressed by equation 19 into equation 20 and solving it yields equation 21. In equation 21, C2 is a constant.

[0076]

[0077] In Equation 21, if ω=ω0 at t=0, the constant C is obtained as shown in Equation 22. Let ω0 [rad / sec] be the rotational angular velocity of the motor 3 when braking by the dynamic braking circuit 40 begins.

[0078]

[0079] Equation 21 is rearranged using equation 22 to obtain equation 23. t shown in equation 23 is the time required from the start of braking of the motor 3 until the rotational speed of the motor 3 reaches ω [rad / sec].

[0080]

[0081] Therefore, the coasting distance estimate s [rad] is expressed as in Equation 24.

[0082]

[0083] In Equation 24, the rotational angular velocity ω [rad / sec] of the motor 3 at the start of braking by the dynamic braking circuit 40 is expressed as shown in Equation 25.

[0084]

[0085] The coasting distance estimator 13 can calculate the coasting distance estimate s [rad] in accordance with Equation 24 and Equation 25, based on the rotational speed ω [rad / sec] of the motor 3 at the start of braking by the dynamic braking circuit 40, which is detected by the rotational speed detector 12, and parameters related to the motor 3.

[0086] 2 is a diagram showing a fault detection circuit, a dynamic braking device, and a motor drive device according to a modification of an embodiment of the present disclosure. This modification adds a non-excitation-activated friction brake device 105 to the motor drive device 1 described with reference to FIG.

[0087] A motor drive device 1 according to an embodiment of the present disclosure includes a converter 101, an inverter 102, a motor control unit 103, a smoothing capacitor 104, a friction braking device 105, and a dynamic braking device 100. Although not shown here, power lines that supply power to drive the motor control unit 103, the dynamic braking device 100, and the friction braking device 105 are provided in a system separate from the power line that supplies power from the AC power supply 2 to the converter 101.

[0088] The converter 101, the inverter 102, the motor control unit 103, and the smoothing capacitor 104 are as described with reference to FIG.

[0089] The dynamic braking device 100 includes a fault detection circuit 10 and a dynamic braking circuit 40. The dynamic braking circuit 40 has been described with reference to FIG. 1.

[0090] The fault detection circuit 10 detects a fault in the dynamic braking circuit 40. The fault detection circuit 10 includes a brake control unit 11, a rotational speed detection unit 12, a coasting distance estimation unit 13, a coasting distance actual measurement value acquisition unit 14, a comparison unit 15, a determination unit 16, and a storage unit 17.

[0091] The rotation speed detection unit 12 and the coasting distance actual measurement value acquisition unit 14 are as described with reference to FIG.

[0092] The coasting distance estimator 13 calculates a coasting distance estimate based on the rotational speed of the motor 3 at the start of braking by the dynamic braking circuit 40 detected by the rotational speed detector 12, parameters related to the motor 3, and the braking torque and shaft friction torque of the friction brake device 105. Details of the process of estimating the coasting distance estimate by the coasting distance estimator 13 will be described later.

[0093] The memory unit 17 stores parameters used in the estimation process of the coasting distance estimation unit 13 to estimate the coasting distance estimate value, such as the back electromotive force constant of the motor 3, the motor resistance value, the number of motor poles, and the motor inductance, as well as the braking torque and axial friction torque of the friction brake device 105.

[0094] The comparison unit 15 compares the degree of deviation, which is a value calculated based on the coasting distance estimated value and the coasting distance actual measurement value, with a predetermined threshold value.

[0095] The determination unit 16 determines whether or not the dynamic braking circuit 40 has failed based on the results of the comparison by the comparison unit 15. If the result of the comparison by the comparison unit 15 indicates that the deviation exceeds the threshold, the determination unit 16 determines that the dynamic braking circuit 40 has failed. If the result of the comparison by the comparison unit 15 indicates that the deviation does not exceed the threshold, the determination unit 16 determines that the dynamic braking circuit 40 has not failed (is normal).

[0096] The threshold values ​​used in the comparison process by the comparison unit 15 and the determination process by the determination unit 16 are as described with reference to Fig. 1. Furthermore, the process performed after the determination result is obtained by the determination unit 16 is as described with reference to Fig. 1.

[0097] The brake control unit 11 controls the dynamic braking circuit 40 that brakes the motor 3 and the friction brake device 105 .

[0098] The control of the dynamic braking circuit 40 by the brake control unit 11 is as described with reference to FIG. 1.

[0099] When the motor 3 is braked by the friction brake device 105, the brake control unit 11 controls so that no current flows through the brake coil of the friction brake device 105. When the motor 3 is not braked by the friction brake device 105, the brake control unit 11 controls so that a current flows through the brake coil of the friction brake device 105.

[0100] 3 and 4 are cross-sectional views showing the structure of a friction brake device provided in a motor drive device according to a modified embodiment of the present disclosure, illustrating a state in which the motor is braked.

[0101] As shown in FIGS. 3 and 4 , in the friction brake device 105, a friction plate 21 is disposed between an armature 22 and an end plate 23. A hub 32 is spline-connected to the friction plate 21. The hub 32 and the motor shaft 31 are integrated, for example, by shrink fitting, so that the friction plate 21 rotates in conjunction with the rotation of the motor shaft 31. The end plate 23 and a spacer 27 are connected with bolts 28, and the armature 22 is connected to the spacer 27 so as to be movable toward and away from the friction plate 21. A spring 24 and a brake coil 25 are provided within a core 26. As shown in FIG. 3 , in a non-excited state where no current flows through the brake coil 25, the armature 22 is pressed firmly against the friction plate 21 by the elastic force of the spring 24, and the friction plate 21 is sandwiched between the armature 22 and the end plate 23 and cannot rotate. As a result, the motor shaft 31 connected to the friction plate 21 also cannot rotate, and the motor is braked (braked state). 4, in an excited state where a brake current flows through the brake coil 25, an electromagnetic force is generated in the core 26 that overcomes the elastic force of the spring 24 that was pressing the armature 22 against the friction plate 21, thereby attracting the armature 22 to the core 26 and releasing the friction plate 21 from contact with the armature 22 and the end plate 23. As a result, the friction plate 21 and therefore the motor shaft 31 can rotate freely, and the motor is no longer braked (brake-released state).

[0102] <Estimation process of coasting distance estimated value in a modified example of an embodiment of the present disclosure> In the modified example of an embodiment of the present disclosure shown in Figures 2 to 4, the calculation formula used to calculate the coasting distance estimated value may be, for example, a formula derived as follows.

[0103] In this modification, when braking the motor 3, the dynamic braking circuit 40 operates to generate a deceleration torque, and the friction brake device 105 operates to generate a braking torque and a shaft friction torque.

[0104] In the embodiment of the present disclosure described with reference to FIG. 1, the equation of motion of the rotor of the motor 3 is expressed by Equation 20, but in this modification, the equation of motion of the rotor of the motor 3 is expressed by Equation 26. In Equation 26, the torque of the rotor of the motor 3 is T [N], and the braking torque and shaft friction torque due to the operation of the friction brake device 105 are T f Let [N].

[0105]

[0106] When the torque T [N] expressed by the equation 19 is substituted into the equation 26, the equation 27 is obtained.

[0107]

[0108] Here, X shown in Equation 28 and Y shown in Equation 29 are introduced.

[0109]

[0110]

[0111] By transforming Equation 27 using Equation 28 and Equation 29, Equation 30 is obtained.

[0112]

[0113] Transforming equation 30 yields equation 31.

[0114]

[0115] 4Y≦X 2 When (i.e., pLT f ≦K t K v ), Equation 31 can be expressed as Equation 32. In Equation 31, C3 is a constant.

[0116]

[0117] 4Y>X 2 When (i.e., pLT f >K t K v , Equation 31 can be expressed as Equation 33. In Equation 32, C4 is a constant.

[0118]

[0119] The right side of Equation 31 is replaced by Equation 34.

[0120]

[0121] 4Y≦X 2 When (i.e., pLT f ≦K t K v ), Equation 32 can be expressed as Equation 35.

[0122]

[0123] 4Y>X 2 When (i.e., pLT f >K t K v ), Equation 33 can be expressed as Equation 36.

[0124]

[0125] 4Y≦X 2 When (i.e., pLT f ≦K t K v (When t = 0 and ω = ω0 in Equation 35, a constant C is obtained, and by further rearranging Equation 32, Equation 37 is obtained. The value t in Equation 37 is the time required from the start of braking of the motor 3 until the rotational speed of the motor 3 reaches ω [rad / sec].

[0126]

[0127] 4Y>X 2 When (i.e., pLT f >K t K v (When t = 0 and ω = ω0 in Equation 36, a constant C is obtained, and by further rearranging Equation 33, Equation 38 is obtained. The value t in Equation 38 is the time required from the start of braking of the motor 3 until the rotational speed of the motor 3 reaches ω [rad / sec].

[0128]

[0129] 4Y≦X 2 When (i.e., pLT f ≦K t K vWhen (a) and (b), the coasting distance estimate s [rad] is expressed as in Equation 39.

[0130]

[0131] Here, α, β, δ, and γ are defined as in Equation 40.

[0132]

[0133] Substituting Equation 40 into Equation 39, Equation 41 is obtained.

[0134]

[0135] 4Y≦X 2 When (i.e., pLT f ≦K t K v (when the rotational speed ω is greater than the rotational speed ω [rad / sec] of the motor 3 at the start of braking by the dynamic braking circuit 40, as detected by the rotational speed detection unit 12, parameters related to the motor 3, and the braking torque and shaft friction torque of the friction brake device 105, are used to calculate the coasting distance estimate s [rad] in accordance with Equation 41 and Equation 25.

[0136] Alternatively, Δt corresponding to the amount of change Δω in the rotational speed (rotational angular speed) ω [rad] obtained by the rotational speed detection unit 12 may be calculated, and the estimated coasting distance s may be calculated by taking the sum of the products of Δω and Δt. 2 The magnitude relationship with f and K. t K v Regardless of the magnitude relationship with the coasting distance s, the coasting distance estimate s can be calculated as follows.

[0137] For example, if the rotational speed ω [rad / sec] of the motor 3 detected by the rotational speed detection unit 12 at the start of braking by the dynamic braking circuit 40 is set to n [rad / sec] and Δω is set to 1, the recurrence formula shown in Equation 42 is obtained.

[0138]

[0139] From equation 42, the coasting distance estimate s can be obtained as shown in equation 43.

[0140]

[0141] The coasting distance estimation unit 13 can calculate the coasting distance estimate s [rad] according to Equation 43, based on the rotational speed ω [rad / sec] of the motor 3 at the start of braking by the dynamic braking circuit 40 detected by the rotational speed detection unit 12, parameters related to the motor 3, and the braking torque and shaft friction torque of the friction brake device 105.

[0142] FIG. 5 is a diagram illustrating an example of the relationship between the coasting distance measurement value and the coasting distance estimate value calculated according to Equation 43 in a motor drive device according to a modified embodiment of the present disclosure. As described above, the calculation process for the coasting distance estimate value according to Equation 43 uses discretized data related to the rotational angular velocity. Therefore, the coasting distance estimate value can be calculated sequentially even before the motor 3 has completely stopped due to braking. In the example shown in FIG. 5, braking of the motor 3 begins at time 0, and the motor 3 has completely stopped at time t2. Because the coasting distance estimate value can be calculated sequentially according to Equation 43, the fault detection circuit 10 can determine whether or not there is a fault at time t1, before the motor 3 has completely stopped.

[0143] <Fault detection processing operation according to an embodiment of the present disclosure and its modified examples> Fig. 6 is a flowchart showing the fault detection processing operation according to an embodiment of the present disclosure and its modified examples. The flowchart shown in Fig. 6 is applicable to both the embodiment described with reference to Fig. 1 and the modified examples described with reference to Figs. 2 to 5.

[0144] When the motor 3 is rotating at or above a predetermined rotational speed, in step S101, braking of the motor 3 is initiated. Braking of the motor 3 is performed by the dynamic braking circuit 40 in the embodiment described with reference to Figure 1, and by the dynamic braking circuit 40 and the friction brake device 105 in the modified example described with reference to Figures 2 to 5.

[0145] In step S102, the rotational speed detection unit 12 detects the rotational speed of the motor 3 when the dynamic braking circuit 40 starts braking.

[0146] In step S103, the failure detection circuit 10 determines whether the motor 3 has completely stopped.

[0147] In step S104, the coasting distance estimating unit 13 calculates an estimated coasting distance value. The processing of step S104 and the processing of step S103 may be executed in reverse order.

[0148] In step S105, the coasting distance actual value acquisition unit 14 acquires, from the rotational speed detection unit 12, the coasting distance actual value, which is the coasting distance of the motor 3 measured from when the dynamic braking circuit 40 starts braking until the motor 3 stops. The processing of step S104 may be executed in a reverse order to the processing of step S105.

[0149] In step S106, the comparison unit 15 calculates a deviation degree indicating the degree of deviation between the coasting distance estimated value and the coasting distance actual measurement value.

[0150] In step S107, the comparison unit 15 compares the deviation degree with a threshold value.

[0151] If it is determined in step S107 that the deviation exceeds the threshold value, the determining unit 16 determines in step S108 that the dynamic braking circuit 40 has a malfunction.

[0152] If it is determined in step S107 that the deviation does not exceed the threshold value, the determining unit 16 determines in step S109 that the dynamic braking circuit 40 is not faulty (is normal).

[0153] It should be noted that the coasting distance estimated value can be calculated sequentially in the calculation process of the coasting distance estimated value according to Equation 43. In this case, the processes of steps S104 and S105 in FIG. 6 may be interchanged.

[0154] Alternatively, when sequential calculation of the coasting distance estimated value is performed according to Equation 43, processing may be performed according to the flowchart shown in Fig. 7. Fig. 7 is a flowchart showing an operation flow when sequential calculation of the coasting distance estimated value is performed according to Equation 43 in the fault detection processing according to a modified example of the embodiment of the present disclosure.

[0155] When the motor 3 is rotating at a predetermined rotation speed or higher, in step S201, braking of the motor 3 is started. The braking of the motor 3 is performed by the dynamic braking circuit 40 and the friction brake device 105.

[0156] In step S202, the rotational speed detection unit 12 detects the rotational speed of the motor 3 when the dynamic braking circuit 40 and the friction brake device 105 start braking.

[0157] In step S203, the failure detection circuit 10 determines whether the motor 3 has completely stopped.

[0158] If it is not determined in step S203 that the motor 3 has completely stopped, the coasting distance estimating unit 13 calculates an estimated coasting distance value in step S204.

[0159] In step S205, the coasting distance actual measurement value acquisition unit 14 acquires from the rotational speed detection unit 12 the coasting distance actual measurement value, which is the coasting distance of the motor 3 measured from the time the dynamic braking circuit 40 (and the friction brake device 105) starts braking until the motor 3 stops.

[0160] In step S206, the comparison unit 15 calculates a deviation degree indicating the degree of deviation between the coasting distance estimated value and the coasting distance actual measurement value.

[0161] In step S207, the comparison unit 15 compares the deviation degree with a threshold value.

[0162] If it is determined in step S207 that the deviation exceeds the threshold value, the determining unit 16 determines in step S208 that the dynamic braking circuit 40 has failed, and then ends the process.

[0163] If it is determined in step S207 that the deviation does not exceed the threshold value, the determining unit 16 determines in step S209 that the dynamic braking circuit 40 is not faulty (is normal), and the process returns to step S203.

[0164] The processing from steps S203 to S209 is repeatedly executed until it is determined in step S203 that the motor 3 has stopped, or until it is determined in step S208 that the dynamic braking circuit 40 has failed.

[0165] <Processor and Memory> The motor drive device 1 includes at least one processor, which is an arithmetic processing device. Examples of the arithmetic processing device include an IC, an LSI, a CPU, an MPU, and a DSP. The arithmetic processing device includes a brake control unit 11, a rotational speed detection unit 12, a coasting distance estimation unit 13, a coasting distance measurement value acquisition unit 14, a comparison unit 15, a determination unit 16, a motor control unit 103, and other processing units. Each of these units included in the arithmetic processing device is a functional module implemented by, for example, a program executed on the processor. For example, if the brake control unit 11, the rotational speed detection unit 12, the coasting distance estimation unit 13, the coasting distance measurement value acquisition unit 14, the comparison unit 15, the determination unit 16, the motor control unit 103, and other processing units are implemented in the form of a program, the functions of each unit can be realized by operating the arithmetic processing device in accordance with the program. The programs for executing the processes in the brake control unit 11, the rotational speed detection unit 12, the coasting distance estimation unit 13, the coasting distance actual value acquisition unit 14, the comparison unit 15, the determination unit 16, the motor control unit 103, and other processing units may be provided in the form of being recorded on a computer-readable recording medium such as a semiconductor memory, a magnetic recording medium, or an optical recording medium. Alternatively, the brake control unit 11, the rotational speed detection unit 12, the coasting distance estimation unit 13, the coasting distance actual value acquisition unit 14, the comparison unit 15, the determination unit 16, the motor control unit 103, and other processing units may be realized as semiconductor integrated circuits in which programs for realizing the functions of the respective units are written.

[0166] The motor drive device 1 also includes at least one memory serving as a storage device. The memory includes the brake control unit 11, the rotational speed detection unit 12, the coasting distance estimation unit 13, the coasting distance measurement value acquisition unit 14, the comparison unit 15, the determination unit 16, the storage unit 17, the motor control unit 103, and various storage units within the other processing units. Examples of the memory include electrically erasable and recordable nonvolatile memory such as EEPROM (registered trademark), or high-speed read / write random access memory such as DRAM or SRAM. The storage unit may also have a configuration such as an HDD (hard disk drive) or an SSD (solid state drive). The memory stores programs for operating the brake control unit 11, the rotational speed detection unit 12, the coasting distance estimation unit 13, the coasting distance measurement value acquisition unit 14, the comparison unit 15, the determination unit 16, the motor control unit 103, and the other processing units. The memory also stores the rotational speed detected by the rotational speed detection unit 12, data related to the coasting distance estimated value calculated by the coasting distance estimation unit 13, data related to the coasting distance actual value acquired by the coasting distance actual value acquisition unit 14, the comparison result by the comparison unit 15, the determination result by the determination unit 16, etc. The memory also stores parameters used in the coasting distance estimation unit 13's estimation process for the coasting distance estimated value. The memory stores various programs and data related to the fault detection circuit 10. The memory stores various programs and data related to the dynamic braking device 100. The memory stores various programs and data related to the friction braking device 105. The memory stores various programs and data related to the motor drive device 1.

[0167] Advantages of an embodiment and its modifications of the present disclosure According to an embodiment and its modifications of the present disclosure, a fault in a dynamic braking circuit can be accurately detected with a simple structure. One type of fault in a dynamic braking circuit is an open-circuit fault of a switch. According to an embodiment and its modifications of the present disclosure, an open-circuit fault of a switch in a dynamic braking circuit can be accurately detected with a simple structure. An operator can quickly and reliably determine the state of the switch in the dynamic braking circuit based on the fault determination result. Therefore, if the operator determines based on the fault determination result that a switch has an open-circuit fault, the operator can take action such as replacing or repairing the switch with the open-circuit fault or the drive control system that is unable to properly generate a braking command to close the switch. Furthermore, each time a fault determination result is obtained, the results can be stored in memory and compiled into a database, which can be used to predict failures and for preventive maintenance.

[0168] A conventional method detects open faults by configuring the switches in a dynamic braking circuit as 1a1b contact relays and monitoring their contact signals. However, this conventional method requires a circuit to monitor the contact signals of the 1a1b contact relay, which increases the implementation circuitry and costs. In contrast, one embodiment and its modified examples of the present disclosure do not require additional implementation circuitry, making it possible to avoid increases in the size and cost of the fault detection circuit. Furthermore, because fault detection is performed by arithmetic processing, the structure of the fault detection circuit can be simplified.

[0169]

[0003] Also, a conventional method for detecting an open fault involves turning a switch in a dynamic braking circuit on and off to apply a DC link voltage to a dynamic braking resistor for a short period of time and monitoring the current that flows during this period. However, with this conventional method, when fault detection processing is performed, even if the switch is normally functioning, an abnormal current that differs from that during normal dynamic braking operation flows through the switch, which is likely to cause degradation and failure of the switch that was originally normal. According to one embodiment and its variants of the present disclosure, when fault detection processing is performed, the same current flows through the switch as during normal dynamic braking operation, preventing degradation and failure of the switch that was originally normal.

[0170] Although the present disclosure has been described in detail above, the present disclosure is not limited to the individual embodiments and individual variations described above. Various additions, substitutions, modifications, partial deletions, etc. are possible for these embodiments and variations within the scope of the gist of the present disclosure, or within the scope of the gist of the present disclosure derived from the content of the claims and their equivalents. These embodiments and variations can also be implemented in combination. For example, in the above-described embodiments and variations, the order of each operation and the order of each process are shown as examples and are not limited to these. The same applies when numerical values ​​or mathematical formulas are used in the description of the above-described embodiments and variations.

[0171] <Supplementary Notes> The following supplementary notes are further disclosed regarding the above-described embodiment and modifications.

[0172] (Supplementary Note 1) A fault detection circuit comprising: a brake control unit that controls a dynamic braking circuit that brakes a motor; a rotational speed detection unit that detects the rotational speed of the motor; a coasting distance estimation unit that calculates a coasting distance estimate, which is the coasting distance of the motor that is estimated to be required from the start of braking by the dynamic braking circuit to the stop of the motor, based on the rotational speed of the motor at the start of braking by the dynamic braking circuit detected by the rotational speed detection unit and motor-related parameters; a coasting distance measurement value acquisition unit that acquires, from the rotational speed detection unit, the coasting distance measured, which is the coasting distance of the motor that is actually measured from the start of braking by the dynamic braking circuit to the stop of the motor; a comparison unit that compares a value calculated based on the coasting distance estimate and the coasting distance measurement with a predetermined threshold value; and a determination unit that determines whether or not a fault has occurred in the dynamic braking circuit based on the result of the comparison by the comparison unit. (Supplementary Note 2) The fault detection circuit according to Supplementary Note 1, wherein the comparison unit compares a deviation indicating the degree of deviation between the estimated coasting distance value and the measured coasting distance value with a threshold, and the determination unit determines that the dynamic braking circuit has a fault if the comparison by the comparison unit results in the deviation being greater than the threshold. (Supplementary Note 3) The fault detection circuit according to Supplementary Note 2, wherein the deviation is a value obtained by dividing the absolute value of the difference between the estimated coasting distance value and the measured coasting distance value by the estimated coasting distance value. (Supplementary Note 4) The fault detection circuit according to Supplementary Note 1, wherein the rotational speed of the motor at the start of braking by the dynamic braking circuit, which is used for estimating the estimated coasting distance value by the coasting distance estimation unit and for obtaining the measured coasting distance value by the measured coasting distance value acquisition unit, is a value equal to or greater than a predetermined rotational speed. (Supplementary Note 5) The fault detection circuit according to Supplementary Note 1, wherein the parameters include at least one of a back electromotive force constant of the motor, a motor resistance value, the number of motor poles, and a motor inductance. (Supplementary Note 6) A dynamic braking device comprising: the fault detection circuit according to any one of Supplementary Notes 1 to 5; and a dynamic braking circuit that, under control of a brake control unit, shorts input terminals of the motor via a dynamic braking resistor to generate a deceleration torque in the motor, thereby braking the motor.(Supplementary Note 7) A motor drive device comprising: an inverter that converts supplied DC power into AC power for driving the motor and outputs the AC power; a motor control unit that executes control to drive the motor; and the dynamic braking device described in Supplementary Note 6. (Supplementary Note 8) The motor drive device described in Supplementary Note 7, further comprising a friction brake device that brakes the motor by pressing an armature against a friction plate to which the motor shaft is coupled, and releases the brake of the motor by separating the armature from the friction plate, wherein the coasting distance estimator calculates a coasting distance estimate based on the rotational speed of the motor at the start of braking of the dynamic braking circuit detected by the rotational speed detector, the parameters, and the braking torque and shaft friction torque of the friction brake device.

[0173] REFERENCE SIGNS LIST 1 Motor drive device 2 AC power supply 3 Motor 10 Fault detection circuit 11 Brake control unit 12 Rotational speed detection unit 13 Coasting distance estimation unit 14 Coasting distance actual measurement value acquisition unit 15 Comparison unit 16 Determination unit 21 Friction plate 22 Armature 23 End plate 24 Spring 25 Brake coil 26 Core 27 Spacer 28 Bolt 31 Shaft 32 Hub 40 Dynamic braking circuit 41 Dynamic braking resistor 42 Switch 100 Dynamic braking device 101 Converter 102 Inverter 103 Motor control unit 104 Smoothing capacitor 105 Friction braking device

Claims

1. A fault detection circuit comprising: a brake control unit that controls a dynamic braking circuit that brakes a motor; a rotational speed detection unit that detects the rotational speed of the motor; a coasting distance estimation unit that calculates a coasting distance estimate, which is the coasting distance of the motor that is estimated to be required from the start of braking by the dynamic braking circuit to the stop of the motor, based on the rotational speed of the motor at the start of braking by the dynamic braking circuit detected by the rotational speed detection unit and parameters related to the motor; a coasting distance measurement value acquisition unit that acquires from the rotational speed detection unit a coasting distance measurement value, which is the coasting distance of the motor that is actually measured from the start of braking by the dynamic braking circuit to the stop of the motor; a comparison unit that compares a value calculated based on the coasting distance estimate and the coasting distance measurement value with a predetermined threshold value; and a determination unit that determines whether or not the dynamic braking circuit has a fault, based on the result of the comparison by the comparison unit.

2. The fault detection circuit according to claim 1, wherein the comparison unit compares a degree of deviation indicating the degree of deviation between the estimated coasting distance value and the measured coasting distance value with the threshold value, and the determination unit determines that the dynamic braking circuit has a fault if the comparison by the comparison unit indicates that the degree of deviation exceeds the threshold value.

3. A fault detection circuit according to claim 2, wherein the deviation is a value obtained by dividing the absolute value of the difference between the coasting distance estimate value and the coasting distance actual measurement value by the coasting distance estimate value.

4. A fault detection circuit as described in claim 1, wherein the rotational speed of the motor at the start of braking by the dynamic braking circuit, which is used for estimating the coasting distance estimated value by the coasting distance estimation unit and for acquiring the coasting distance actual value by the coasting distance measured value acquisition unit, is equal to or greater than a predetermined rotational speed.

5. The fault detection circuit of claim 1, wherein the parameters include at least one of the back-emf constant of the motor, a value of motor resistance, a number of motor poles, and a motor inductance.

6. A dynamic braking device comprising: a fault detection circuit according to any one of claims 1 to 5; and a dynamic braking circuit that, under control of the brake control unit, shorts the input terminals of the motor via a dynamic braking resistor, thereby generating a deceleration torque in the motor and braking the motor.

7. A motor drive device comprising: an inverter that converts supplied DC power into AC power for driving a motor and outputs the AC power; a motor control unit that executes control to drive the motor; and the dynamic braking device according to claim 6.

8. A motor drive device according to claim 7, further comprising a friction brake device that brakes the motor by pressing an armature against a friction plate to which the motor shaft is connected, and releases the brake of the motor by separating the armature from the friction plate, wherein the coasting distance estimation unit calculates the coasting distance estimate based on the rotational speed of the motor detected by the rotational speed detection unit at the start of braking of the dynamic braking circuit, the parameters, and the braking torque and shaft friction torque of the friction brake device.

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