Methods and circuits for detecting over-temperature of motor windings

Abstract

This application relates to a method and circuit for detecting over-temperature in a motor winding. A hardware-based detection system (10) specifically includes signal generation circuitry (22, 38, 54, 58, 62) for generating a signal functionally related to the current, a reference current, and a duration in the motor winding. The system may also include a comparator circuit (66) for comparing the generated signal with a reference signal, and thereby for detecting an over-temperature state in the motor winding. If desired, a compensation circuit (410) can be used to generate a variable reference signal as a function of ambient temperature. A method for operating the detection system (10) is also disclosed. If desired, the detection system (10) can be fully implemented in hardware using a simple analog circuit architecture.

Description

[0001] This application claims priority to Indian Patent Application No. 201841032510 (filed on August 30, 2018). The entire disclosure of Indian Patent Application No. 201841032510 is hereby incorporated by reference in its entirety. TECHNICAL FIELD

[0002] The present application relates to a method and circuit for detecting over-temperature in a motor winding. BACKGROUND

[0003] Electric motor drives are used in many applications, including but not limited to household appliances, industrial machines, telecommunications devices, medical devices, building automation devices, and automotive devices. When designing a motor drive for certain applications, it can be desirable to ensure that the device can operate safely and reliably. Safety and reliability have different aspects, including human / operator safety and safe operation of the device itself. In particular, it can be important, or even necessary, to protect an operator, the device, or the area surrounding the device from an over-temperature condition or a fire. Thus, it can be desirable to monitor an electric motor drive so that it does not develop an over-temperature condition in one or more of its windings or in a stator or rotor that includes a shaft or motor body.

[0004] Indeed, one of the potential hazards associated with an electric motor drive is a fire or hot surface due to an over-temperature in one or more motor windings. For some devices, it can be desirable to continuously monitor the temperature of the motor windings to ensure that there is no over-temperature or related fire, especially in the event of an overload or other abnormal condition or fault. Many safety standards mention maximum allowable temperatures for different classes of motor windings. It can be desirable for a designer to ensure that the winding temperature will remain within such limits. SUMMARY

[0005] The present disclosure overcomes the drawbacks of the prior art to a large extent. The present disclosure relates to a hardware-based detection system comprising a signal generation circuit for generating a signal functionally related to a current in a motor winding, a reference current, and a duration of time; and a comparator circuit coupled to the signal generation circuit for comparing the generated signal to a reference signal, and thereby for detecting an over-temperature condition in the motor winding.

[0006] The present disclosure also relates to a detection system comprising a signal generation circuit for generating a signal functionally related to a current in a motor winding, a reference current, and a duration of time; a comparator circuit for comparing the generated signal to a variable reference signal, and thereby for detecting an over-temperature condition in the motor winding; and a compensation circuit for generating the variable reference signal as a function of the reference current and an ambient temperature.

[0007] The present disclosure also relates to a method of operating a hardware-based detection system. The method comprises causing a signal generation circuit to generate a signal functionally related to a current in a motor winding, a reference current, and a time duration; and causing a comparator circuit to compare the signal to a reference signal and thereby detect an over-temperature condition in the motor winding. BRIEF DESCRIPTION OF DRAWINGS

[0008] Figure 1 is a plot of the I 2 t curve (or thermal limit curve) for a typical multi-phase motor;

[0009] Figure 2 is a block diagram of a detection system coupled to a drive system, wherein the detection system is configured to generate a signal indicative of an over-temperature condition in a winding in a multi-phase motor;

[0010] Figure 3 is a waveform plot of the detection system in Figure 2

[0011] Figure 4 is a circuit diagram of a portion of the detection system in Figure 2

[0012] Figure 5 is a block diagram of a system for compensating for environmental conditions, which can be used in conjunction with the detection system in Figure 2 and 4 DETAILED DESCRIPTION

[0013] Referring now to the drawings in which like elements are represented by like reference numbers and other characters of identification throughout the various figures, a plot of the I Figure 1 t curve 110 for a typical motor winding is shown in 2 The instantaneous energy dissipated in a motor winding is proportional to the square of the winding current I P and the temperature in the winding depends on the time duration t of the energy dissipation. A motor can be designed to work continuously at a nominal or rated current I N Any load current I N above the nominal current I P will cause excessive losses that cannot be removed by the cooling means of the motor. As a result, when the load current I P is greater than the nominal current I N the motor temperature rises and eventually exceeds the rated temperature.

[0014] When the load current I P is only slightly greater than the nominal current I N the time t required for the motor temperature to exceed the rated temperature can be relatively long, but eventually the rated temperature will be exceeded. When the load current I P ​​​significantly greater than the nominal current I N When, as shown in the case of Figure 1 , the motor temperature exceeds the rated temperature, the time t required for this can be relatively short. When the load current I P is less than the nominal current I N , the motor temperature should not exceed the rated temperature, regardless of the duration t.

[0015] In general, when the load current I P is equal to the nominal current I N , the motor winding dissipates an energy E NOM as follows:

[0016] E NOM = I N,RMS 2 x R x t,

[0017] where I N,RMS is the root mean square (RMS) value of the nominal current I N , R is the resistance of the winding carrying the current, and t is the duration of the energy dissipation.

[0018] When the load current I P is greater than the nominal current I N ( as shown in the case of Figure 1 ), the motor winding generates an excess energy E P as follows:

[0019] E P = (I P,RMS 2 - I N,RMS 2 ) x R x t,

[0020] where I P,RMS is the root mean square value of the load current I P .

[0021] Since R can be considered constant (ignoring variations in R with temperature), the latter equation can be simplified to a proportional relationship as follows:

[0022] E P a (I P,RMS 2 - I N,RMS 2 ) x t.

[0023] Figure 2This is a block diagram illustrating a hardware-based detection system 10 coupled to a motor drive system 12. The motor drive system 12 can, for example, be configured to operate a household appliance (not shown). The detection system 10 is configured to detect over-temperature (OT) conditions in the motor drive system 12. In the illustrated configuration, the motor drive system 12 has a three-phase motor 14 coupled to a three-phase inverter 16, and corresponding plurality of motor windings electrically connected to lines 15, 17, 19, and corresponding current sensing elements 18, 20 electrically connected in series with the winding connection lines 15, 17. The motor drive system 12 can be a multiphase system, wherein the multiphase motor is driven by a multiphase inverter. The motor drive system 12 illustrated here uses sensing elements 18, 20 to sense the current through two of the three motor windings of the three-phase motor 14. Examples of current sensing elements can be current-sensing resistors or Hall effect sensors or any other suitable sensing element, but are not limited thereto.

[0024] The illustrated detection system 10 has in-line sensors 22, 24 for sensing (monitoring) corresponding currents in the windings connected to lines 15, 17 using sensing elements 18, 20, and for outputting current signals on corresponding conductors 26, 28. Sensors 22, 24 are electrically coupled to opposite ends of sensing elements 18, 20 via appropriate electrical connections 30, 32, 34, 36. In the illustrated configuration, the number of in-line sensors 22, 24 (2) is one less than the number of phases of motor 14 (3). However, this disclosure should not be limited to the systems and apparatus shown in the figures and described in this detailed description. Typically, for an N-phase motor, no more than N-1 current sensors may be required, especially when the drive system has adequate grounding leakage insulation or protection. Typically, for an N-phase motor, N current sensors may be required, especially when the drive system has no grounding leakage insulation or protection or when each motor phase winding is independent and there is no electrical connection between the phase windings.

[0025] In addition, such as Figure 2 As shown, the detection system 10 includes RMS calculator circuits 38 and 40, which receive current signals on lines 26 and 28 and output corresponding root mean square (RMS) signals on wires 42 and 44. The RMS calculator circuits 38 and 40 calculate the corresponding RMS value (IS) of the sensed winding current. P,RMS The RMS signal corresponds to the corresponding load current I sensed by the respective sensor elements 18, 20 and sensors 22, 24 in the windings connected to lines 15, 17. P,RMS .

[0026] Following the RMS calculator circuits 38 and 40, first multiplier and subtractor circuits 54 and 56 are provided, which are used to generate a first difference signal (whose value corresponds to I) based on the corresponding RMS value.P,RMS 2 -I N,RMS 2 ), and second multiplier and subtractor circuits 58 and 60, which are used to generate a second difference signal based on the corresponding RMS value (the value of which corresponds to I). N,RMS 2 -I P,RMS 2 ).

[0027] In the example shown, I N,RMS This is the rated nominal motor current of the motor drive system 12. That is, I... N,RMS This is the motor current, at which the temperature will not reach the over-temperature (OT) threshold for an infinitely long time. When the drawdown of the motor drive system 12 is less than the nominal current I... N,RMS At that time, the motor drive system 12 will be cooled to a temperature less than the corresponding I. N,RMS The rated current and the rated temperature. Therefore, by calculating the second difference (I) N,RMS 2 -I P,RMS 2 The cooling curve is generated by integrating the first difference (I) with time in appropriate integrator circuits 62 and 64. P,RMS 2 -I N,RMS 2 Subtract the second difference from the first. As discussed in more detail below, the multiplier and subtractor circuits 54, 56, 58, and 60 may include appropriate operational amplifiers.

[0028] In the illustrated configuration, integrator circuits 62 and 64 receive the difference signal and generate a time-integrated signal. Comparator circuits 66 and 68 are provided for comparing the integrated signal with a reference signal on line 70. The reference signal on line 70 may correspond to a predetermined fixed I. 2 The reference value t, or as discussed in more detail below, the reference signal on line 70 can correspond to the variable I that varies as a function of ambient temperature. 2 The tRef_comp value. Therefore, the detection system 10 has first signal generation circuits 22, 38, 54, 58, 62 for generating a first signal on line 80. The first signal (on line 80) is functionally related to the current (I) in the first motor winding. P,RMS (It is associated with line 15), reference current (I) N,RMS The detection system 10 also has a first comparator circuit 66 coupled to first signal generation circuits 22, 38, 54, 58, 62, for comparing the first signal with a reference signal I. 2The tRef_comp is compared, and thus used to detect an over-temperature condition in the first motor winding. Furthermore, the detection system 10 has second signal generation circuits 24, 40, 56, 60, and 64 for generating a second signal on line 82. The second signal (on line 82) is functionally related to the current (I) in the second motor winding. P,RMS (associated with line 17), reference current (I) N,RMS The detection system 10 also has a second comparator circuit 68, which is coupled to second signal generation circuits 24, 40, 56, 60, 64, for comparing the second signal with a reference signal I. 2 The tRef_comp is compared, and thus used to detect over-temperature conditions in the second motor winding.

[0029] like Figure 2 As shown, by way of example, the difference signal can be output from multiplier and subtractor circuits 54, 56, 58, 60 and input to the corresponding integrator circuits 62, 64 on wires 72, 74, 76, 78, while the time integration signal can be output from the corresponding integrator circuits 62, 64 and input to the corresponding comparator circuits 66, 68 on other wires 80, 82.

[0030] During operation, whenever the value of the corresponding integral signal exceeds I... 2 t reference signal (or variable I) 2 When the value of tRef_comp is reached, one (or both) of the comparator circuits 66 and 68 issues an OT fault signal on the corresponding wires 84 and 86. The outputs from the comparator circuits 66 and 68 are combined at node 90 such that an OT status warning signal is generated on wire 92 whenever at least one of the comparator circuits 66 and 68 issues an OT fault signal. If necessary, wire 92 can be operatively connected to a suitable operator interface (not shown) to warn the operator of the presence of an OT status. Alternatively, wire 92 can be operatively connected to a suitable electrical or mechanical controller (not shown) for automatically taking corrective action, for example, by disconnecting the motor drive system 12 from the power supply (not shown).

[0031] like Figure 3 As shown, when the load current I of one of the windings P,RMS Continuously compared to the nominal current I N,RMS When x is large, the integral value represented by the output signals from the corresponding integrator circuits 62 and 64 [(I P,RMS 2 -I N,RMS 2 [×t] increases with time. As discussed in more detail below, when the motor windings connected to lines 15 and 17 are already at the load current I... P,RMSThe work was greater than t OT For a certain period of time, the output signal generated by the corresponding integrator circuits 62 and 64 exceeds I. 2 The reference signal y is used, which causes an OT status warning signal to be generated on the merging conductor 92.

[0032] When the actual motor current exceeds the nominal current, the corresponding motor windings will heat up. When the load current I... P,RMS Greater than the nominal current I N,RMS However, when the quantity is only relatively small, the time t taken by the detection system 10 to generate an OT status warning signal is... OT Relatively long. When the load current I P,RMS Greater than the nominal current I N,RMS When the quantity is relatively large, the time t taken by the detection system 10 to generate an OT status warning signal is... OT Correspondingly shorter. When the load current I P,RMS It is always less than the rated current I in all windings connected to lines 15 and 17. N,RMS hour( Figure 3 If the case shown is not shown, then the detection system 10 shown will not generate an OT status warning signal.

[0033] According to one aspect of this disclosure, the nominal current I can be obtained from information provided on the manufacturer's nameplate (not shown). N,RMS The nameplate can be attached to the motor drive system 12 at an appropriate value. For example, if the nameplate indicates that the motor drive system 12 is configured to carry a 2-A winding current (specifying an ambient temperature of up to 25°C), where the motor windings will heat to a rated temperature (e.g., 100°C) after an indefinite period of time (steady-state temperature), then I N,RMS = 2A. This means that the motor windings connected to lines 15 and 17 can each continuously carry 2A at an ambient temperature of 25°C. However, this disclosure should not be limited to the configurations and values ​​described herein, which are intended to characterize non-limiting examples of this disclosure.

[0034] Figure 4 A circuit-level implementation of a portion of the detection system 10 is shown, wherein a voltage equal to the current sensed in the first winding connected to line 15 is in I P The signal I is available and is supplied to the non-inverting input 100 of the first operational amplifier 102 via a suitable resistor 104. P Is Figure 2The equivalent signal at the output of the first online sensor 22 at signal output line 26. Motor windings typically carry alternating (bipolar) current. When alternating current is sensed, the output of sensor 22 is typically bipolar (with both positive and negative polarities). To simplify the sensor circuitry and other circuitry, a unipolar output is typically used, where the operational amplifier can be operated using a unipolar power supply VCC to the reference signal ground. To achieve this, the outputs of sensors 22 and 24 are level-shifted by a voltage VCC / 2 (not shown here), and the level-shifted signal is... Figure 2 Lines 26 and 28 are available. Signal I P ( Figure 4 ) is in Figure 2 The level-shifted (by VCC / 2) voltage signal available at line 26 (or line 28) is equivalent to the voltage signal connected to... Figure 2 The current flowing in the motor windings of lines 15 and 17. When the winding current is zero, the corresponding I... P The signal voltage is VCC / 2. When the winding current is positive (current flows into the motor winding), the corresponding I... P When the signal voltage is greater than VCC / 2 and the winding current is negative (current flows out of the motor winding), the corresponding I P The signal voltage is less than VCC / 2. Operational amplifier 102 is the first RMS calculator 38 ( Figure 2 The first operational amplifier 102 outputs through wire 101 ( Figure 4 Available on, the output is transmitted via a suitable diode 106, provided to the inverting input 108 of operational amplifier 102 via a suitable resistor 111, coupled to ground via resistor 111 and a suitable capacitor 112, provided to the non-inverting input 114 of a second operational amplifier 116 via resistor 111 and another suitable resistor 118, and coupled to ground via resistors 111, 118 and another suitable resistor 120.

[0035] therefore, Figure 4 The circuit shown includes a peak detector, which includes, but is not limited to, a first operational amplifier 102, a diode 106, and associated resistors 111 and capacitors 112. In operation, the peak detector tracks a first current sensor 22. Figure 2 The peak value of the output of ). At the same time, the second operational amplifier 116 approximates the first difference (I) by calculating the peak value of the output. P,RMS 2 -I N,RMS 2 ) can be used to calculate the motor winding heat. For example, a range within an appropriate range (which could be from I) can be used. P To I NThe approximation is achieved using an operational amplifier gain of 1.2V / V. However, as stated above, this disclosure should not be limited to the configurations and values ​​described herein. The output of the heating amplifier circuit on wire 128 is connected to the non-inverting input 130 of the integrator / third operational amplifier 140.

[0036] INOM, which is equivalent to the nominal current I N,RMS The voltage, level-shifted (by VCC / 2), is applied to the inverting input 124 of the second operational amplifier 116 via a suitable resistor 126 on wire 122. The output of the second operational amplifier 116 on wire 128 is applied to the non-inverting input 130 of the third operational amplifier 140 via a suitable resistor 142, and to the inverting input 124 of the second operational amplifier 116 via another suitable resistor 144. In operation, Figure 2 The first RMS calculator circuit 38 ( Figure 2 ) and difference (I P,RMS 2 -I N,RMS 2 Calculator 54 includes a first operational amplifier 102. Figure 4 The first operational amplifier 140 (used as a peak detector) and the second operational amplifier 116 (used as a gain stage) are also present. The non-inverting input 130 of the third operational amplifier 140 is electrically coupled to ground via a suitable capacitor 145.

[0037] The current signal transmitted through diode 106 is also provided to the inverting input 146 of the fourth operational amplifier 148 via resistor 111 and another suitable resistor 150. INOM, which is equivalent to the nominal current I... N,RMS The voltage, level-shifted (by VCC / 2), is applied to the non-inverting input 152 of the fourth operational amplifier 148 via a suitable resistor 154 on wire 122, and is electrically coupled to ground via resistor 154 and another suitable resistor 156. The output of the fourth operational amplifier 148 on wire 158 is applied to the inverting input 160 of the third operational amplifier 140 via suitable resistors 162 and 164, and to the inverting input 146 of the fourth operational amplifier 148 via a suitable resistor 166.

[0038] During operation, when the motor current is less than the nominal current, the corresponding motor windings will be cooled to a temperature lower than the current corresponding to the rated current I. N,RMS The rated temperature. The fourth operational amplifier 148 approximates the second difference (I) by calculating the temperature. P,RMS 2 -I N,RMS 2 Cooling can be calculated using, for example, within an appropriate range (which could be from I). N To IP The approximation is achieved using an operational amplifier gain of 0.48V / V. The output of the cooled amplifier circuit on line 158 is connected to the inverting input 160 of the integrator / third operational amplifier 140.

[0039] If necessary, it can be based on nominal I. 2 The value of t is used to tune the time constant of the integrator's RC circuit. For this purpose, capacitor 145 and resistor 142, as well as capacitor 170 and resistor 164, form the RC time constant. The capacitances C of capacitors 145 and 170 can be equal, and the resistances R of resistors 142 and 164 can be equal, in which case the integrator gain is equal to 1 / RC. Simultaneously, resistor 172 receives the FB signal on line 314 to detect when the motor current is below the rated current I for an extended period before abnormal operation occurs. N,RMS Simulated motor preheating, where the abnormal motor current is greater than the nominal current I. N,RMS .

[0040] The output of the third operational amplifier 140 is coupled to the base of an NPN bipolar junction transistor (BJT) 168. The collector of the BJT transistor 168 is connected to the inverting input 160 of the third operational amplifier 140 via a capacitor 170. The capacitor 170 is connected in parallel with resistor networks 172 and 164. The emitter of the transistor 168 is electrically coupled to ground via a suitable resistor 174.

[0041] The collector of BJT transistor 168 is also connected to the inverting input 190 of the fifth operational amplifier (i.e., the first comparator) 66 via a suitable metal-oxide-semiconductor field-effect transistor (MOSFET) 194 and wire 80. MOSFET 194 is connected to the power supply VCC via a suitable resistor 196 and a suitable capacitor 198. The non-inverting input 200 of the fifth operational amplifier / first comparator 66 receives a variable reference signal I on wire 70. 2 t Ref_comp (or fixed reference signal I) 2(t Reference). When the signal applied to the inverting input 190 is greater than the reference signal, the comparator circuit 66 generates an OT fault signal on line 84. The OT fault signal is transmitted through a suitable resistor 202 and a wire 84, which is grounded through a suitable capacitor 204. If necessary, a BJT 168 (at the output of the third operational amplifier 140) and a MOSFET 194 (associated with the fifth operational amplifier 66) can be used to ensure that the integrating capacitor 170 does not discharge in the event of a power failure, but instead retains its charge in the detection system 10, thereby ensuring a capacitive charge retention effect similar to that of a motor retaining heat until it dissipates. The values ​​of the RC circuit consisting of components 170, 172, and 164 are chosen such that the RC time constant of the circuit is equal to the cooling time constant of the motor.

[0042] In operation, the gains of the second and third operational amplifiers 116 and 140 are adjusted together to obtain a linear approximation of the square circuit near the winding current IP. Therefore, the first, second, and third operational amplifiers 102, 116, and 140 work together to achieve the desired RMS and square function of the first difference signal. The second operational amplifier 116 is part of the first difference calculation circuit 54 (…). Figure 2 The components, the fourth operational amplifier 148 ( Figure 4 ) is the component of the corresponding second difference calculation circuit 58.

[0043] The preceding description has assumed that the motor drive system 12 uses a current of I. P When the motor drive system starts up, its initial temperature is equal to the ambient temperature. However, if the motor drive system 12 operates for an extended period at a current slightly lower than its rated current, and then a sudden abnormality causes the motor current to increase beyond the rated current, the initial temperature at which overheating begins is not the ambient temperature, but rather close to the over-temperature (OT) threshold, and therefore the time to reach the OT threshold is shorter. On the other hand, if the motor drive system 12 operates for an extended period at a current much lower than its rated current, the initial temperature is close to the ambient temperature, and the time required to reach the OT threshold is longer.

[0044] To resolve the above situation, Figure 4 The circuit shown can also have sixth and seventh operational amplifiers 300, 302, which are coupled to appropriate resistors 304, 306, 308, 310, 312 for use in load current I P Less than the nominal current I N The initial state is set in the integrator circuit 140. Specifically, when the load current I... P Below the nominal current I NAt this time, a signal FB is generated on line 314 (electrically connected to resistors 172, 162, 164) to set the initial state in integrator circuit 140. During operation, the VIPEAK signal is applied to the non-inverting input of the sixth operational amplifier 300 via resistor 304 on line 316. The non-inverting input of the sixth operational amplifier 300 is grounded via resistor 310. Simultaneously, the VCC / 2 signal is applied to the inverting input of the sixth operational amplifier 300 via resistor 306, which has a feedback resistor 308.

[0045] The output of the sixth operational amplifier 300 is applied to the non-inverting input of the seventh operational amplifier 302, and the VINT signal is applied to the inverting input of the seventh operational amplifier 302 on line 80. The VINT signal is the same as the signal applied to the inverting input of operational amplifier 66. The output of the seventh operational amplifier 302 is applied to resistor 312 and MOSFET 316 to generate the FB signal on wire 314. Operational amplifier 300 is a differential amplifier in which the VIPEAK minus VCC / 2 signal is generated. Refer to the above

[0028] paragraph (which begins with the phrase " Figure 4 The circuit-level implementation of a portion of the detection system 10 is shown…”), the VCC / 2 voltage signal corresponds to zero current and therefore the output of operational amplifier 300 can be considered as the equivalent of the winding current I with appropriate gain. P Or I P,RMS The peak voltage. Operational amplifier 302 is configured as a comparator. When the non-inverting input of operational amplifier 302 is greater than the VINT signal at the inverting input of operational amplifier 302, the output of operational amplifier 302 goes high (equal to VCC) and MOSFET switch 316 turns on, making the FB voltage approximately equal to zero volts. When I P,RMS Less than I N,RMS At that time, the output of the second amplifier 116 at line 128 is zero volts, and the VINT signal at line 80 remains constant when the FB signal is pulled to ground by turning on MOSFET 316.

[0046] If necessary, at least the second, third, fourth, and fifth operational amplifiers 116, 140, 148, and 66 can be formed on or within a single chip. If necessary, at least the second, third, fourth, fifth, sixth, and seventh operational amplifiers 116, 140, 148, 66, 300, and 302 can be formed on or within a single chip.

[0047] Figure 4 The system shown has multiple stages for implementing an RMS calculator, a square circuit, an integrator, and a comparator, as well as for responding to a connection to line 15 by generating an OT fault signal on line 84. Figure 2The OT state in the first winding of the circuit. Another circuit (not shown, but essentially the same) Figure 4 The system shown is the same and can be coupled to the second winding connected to line 17. Figure 2 ) and output node 90, for responding to the OT state in the second winding connected to line 17 by generating an OT fault signal on line 86.

[0048] Figure 5 It is used to generate a variable (compensated) reference signal I on conductor 70. 2 Block diagram of the compensation circuit 69 for tRef_comp. The value of the compensated reference signal varies as a function of the ambient temperature, while the fixed I... 2 The value of the reference signal t can be a constant for the motor drive system 12 in a given working environment. Figure 5 The compensation circuit 69 shown generates a variable reference signal as a function of ambient temperature.

[0049] like Figure 5 As shown, one or more analog temperature sensors 402 can be provided for generating a temperature signal representing the ambient temperature on line 404, and a calibration circuit 406 for generating a calibration signal corresponding to the temperature signal on line 408. A compensation circuit 410 applies a scaling signal to a fixed I... 2 The reference signal is used to generate a compensated reference signal on line 70. In the illustrated configuration, the compensation circuit 410 adds a scaling signal to I. 2 t reference signal or from I 2 The scaling signal is subtracted from the reference signal.

[0050] Typically, when the ambient temperature sensed by one or more ambient temperature sensors 402 increases, the value of the compensation reference signal on line 70 decreases, and vice versa. As a result, when the ambient temperature increases, the value of the compensation reference signal on line 80 ( Figure 2 The load current-based signal on the circuit (which causes an OT warning state to be generated) decreases, and vice versa. The term "ambient temperature" refers to the temperature near the motor drive system 12, in a location unaffected by the heat generated by the motor windings electrically connected to the lines 15, 17. The sensor 402 can be placed if desired, without any complex assembly requirements. Relatively simple, low-cost sensors are suitable for this purpose.

[0051] The apparatus and method described herein for hardware implementation of detecting over-temperature conditions in motor windings have many advantages. The circuit structure can be based on well-known and readily available amplifiers and comparators, and therefore can be easily implemented. They relate to the indirect determination of winding over-temperature, and therefore do not require a temperature sensor configured for direct measurement of winding temperature.

[0052] The detection system 10 can have a simple analog chip structure and does not require a separate printed circuit board (PCB). System 10 can advantageously be implemented on the same PCB supporting the motor drive system 12 without any complex requirements and can occupy less space because sensors 22, 24 can be configured in-line. As a result of these features, the system shown can be manufactured at a reduced cost while still maintaining the desired accuracy compared to known systems. Moreover, improved integration can be achieved because the solution architecture can utilize relatively common operational amplifiers.

[0053] Another advantage of the systems 10 described herein is that they are not adversely affected by aging. In contrast, known devices with direct temperature sensors are prone to aging problems, such as sensor misalignment, and are susceptible to improper or impractical maintenance.

[0054] The above description is an example. This disclosure is intended to cover changes, modifications, and variations to the subject matter described herein that are included within the scope of this application (including the appended claims). As used herein, the term "comprising" means including but not limited to. The term "based on" means at least partially based on. Furthermore, wherever an element “a,” “an,” “first,” or “another,” or its equivalent, is enumerated in this disclosure or the claims, it should be interpreted as including one or more such elements, neither requiring nor excluding two or more such elements.

Claims

1. A hardware-based detection system, comprising: a first signal generation circuit for generating a first signal functionally related to a current in the first motor winding, a reference current, and a time duration; and a first comparator circuit coupled to the first signal generation circuit for comparing the first signal to a reference signal, and thereby for detecting an over-temperature condition in the first motor winding based on the comparison; wherein the first signal generation circuit is further configured to generate a cooling curve, the first signal generation circuit comprising: a first multiplier and subtractor circuit configured to generate a first difference signal, a value of the first difference signal corresponding to I P,RMS greater than I N,RMS for a first duration P,RMS 2 -I N,RMS 2 where I N,RMS 2 corresponds to a root mean square value of a nominal current and I P,RMS 2 corresponds to a root mean square value of a current in the first motor winding, and wherein an over-temperature condition is reached after an indefinitely long time at the nominal current; a second multiplier and subtractor circuit configured to generate a second difference signal, a value of the second difference signal corresponding to I P,RMS less than I N,RMS for a second duration N,RMS 2 -I P,RMS 2 ; and an integrator circuit configured to receive the first difference signal and the second difference signal, and to generate the cooling curve by subtracting the second difference signal from the first difference signal on a time-integrated basis.

2. The detection system of claim 1, further comprising a first sensor for generating a current signal representative of the current in the first motor winding.

3. The detection system of claim 2, further comprising an RMS calculator circuit coupled to the sensor for generating an RMS signal based on the current signal.

4. The detection system of claim 3, further comprising an operational amplifier coupled to the RMS calculator circuit and the first comparator circuit for generating the first signal.

5. The detection system of claim 1, further comprising a second signal generation circuit for generating a second signal functionally related to a current in a second motor winding, the reference current, and the duration of time; and a second comparator circuit coupled to the second signal generation circuit for comparing the second signal to the reference signal, and thereby for detecting an over-temperature condition in the second motor winding.

6. The detection system of claim 5, wherein the second signal generation circuit comprises an integrator circuit, and wherein the second signal is functionally related to a square of the current in the second motor winding minus a square of the reference current and is integrated over the duration of time.

7. The detection system of claim 1, wherein the reference signal comprises a variable reference signal, and wherein the detection system comprises a compensation circuit for generating the variable reference signal as a function of an ambient temperature.

8. A system, comprising: a first sensor for sensing a current in a first motor winding; and the hardware-based detection system of claim 1.

9. The system of claim 8, wherein the sensor is an in-line current sensor for sensing the current in the motor winding.

10. The system of claim 8, further comprising an operational amplifier coupled to the comparator circuit for generating the signal functionally related to the current in the motor winding, the reference current, and the duration of time.

11. The system of claim 8, further comprising: a second sensor for sensing current in a second motor winding; a second signal generation circuit for generating a second signal functionally related to the current sensed by the second sensor, the reference current, and the duration; and a second comparator circuit coupled to the second signal generation circuit for comparing the second signal to the reference signal and thereby for detecting an over-temperature condition in the second motor winding.

12. A method of operating a hardware-based detection system, comprising: causing the signal generation circuit to generate a signal functionally related to the current in the motor winding, the reference current, and the time duration; and causing a comparator circuit coupled to the signal generation circuit to compare the signal to a reference signal, thereby detecting an over-temperature condition in the motor winding based on the comparison; a first difference signal is generated, a value of the first difference signal corresponding to I P,RMS greater than I N,RMS for a first duration of time P,RMS 2 - I N,RMS 2 where I N,RMS 2 corresponds to a root mean square value of a nominal current and I P,RMS 2 corresponds to a root mean square value of a current in the first motor winding, and wherein the over-temperature condition is reached after an indefinitely long time at the nominal current; generating a second difference signal, a value of the second difference signal corresponding to I P,RMS less than I N,RMS for a second duration N,RMS 2 -I P,RMS 2 ; and generating a cooling curve on a time-integrated basis by subtracting the second difference signal from the first difference signal.

13. The method of claim 12, further comprising detecting the current in the motor winding.

14. The method of claim 12, further comprising causing a compensation circuit to vary the reference signal as a function of ambient temperature.

15. The method of claim 14, further comprising using a transistor to maintain charge in the detection system during a loss of power.

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