Starting method of BLDC motor applied to reciprocating compressor

Abstract

The present invention relates to a method for starting a BLDC motor (10) with a trapezoidal shaped induction voltage under high load conditions using a modified two-level discrete fixed-time or fixed-frequency hysteresis current controller, the BLDC motor (10) being driven by an inverter bridge (30) with only one current sensor (20) placed on the busbar and controlled by a processing unit associated with an analog-to-digital converter, the novelty of which consists essentially in the fact that the intensity of the current (Im) of the BLDC motor is controlled by a two-level discrete fixed-time or fixed-frequency hysteresis current controller.

Description

[Technical Field]

[0001] The present invention relates to reciprocating compressors that include a brushless three-phase direct current (BLDC) motor with trapezoidally induced voltage.

[0002] More specifically, the present invention relates to a starting method for a BLDC motor without mechanical position sensors, typical of refrigeration systems that have blocking valves to improve efficiency under conditions of high load and uneven pressure.

[0003] The method according to the present invention is carried out by driving a BLDC motor through a three-phase inverter bridge with only one current sensor located on the busbar, controlled by a processing unit associated with an analog-to-digital (A / D) converter. [Background technology]

[0004] The motor commonly used to drive variable displacement reciprocating compressors is the Brushless Direct Current motor - BLDC, due to its high power density and high efficiency, in addition to its simplicity of drive and reduced number of parts required to perform such a drive, which makes the cost of the refrigeration system cheaper.

[0005] Figure 1 shows a system used to drive a state-of-the-art variable displacement reciprocating compressor. As shown in Figure 1, the system is powered by a DC power source V cc and a current sensor 20 in the bus bar, which measures the bus bar current i busbarThe BLDC motor 10 includes a sensor for measuring the current i, an analog-to-digital converter, and a current controller. The current controller is associated with six switches S1-S6, which are responsible for the strength of the current applied to the BLDC motor 10. Furthermore, the switches S1-S6 are associated in parallel with six freewheeling diodes D1-D6. The switches S1 and S2 and the freewheeling diodes D1 and D2 are associated with the A phase of the BLDC motor 10, and are responsible for the current i a The switches S3 and S4 and the freewheeling diodes D3 and D4 are associated with the B phase of the BLDC motor 10, and the current i b The switches S5 and S6 and the freewheeling diodes D5 and D6 are associated with the C phase of the BLDC motor 10, and the current i c Furthermore, six operating switches S1-S6 and six freewheeling diodes D1-D6 constitute an inverter bridge (30).

[0006] In addition, in the upper half of FIG. 2, the induced voltage waveform e on the BLDC motor 10 is shown. a , e b , e c and the desired current i a , i b , i c , and the bottom half shows the waveforms of the command signs applied to switches S1-S6 in a typical drive of BLDC motor 10.

[0007] The basic principle of operating a three-phase BLDC motor is to drive only two phases of the motor during each step of the motor's drive, while the third phase remains open. The start and end of each phase's drive must coincide with the flat region of the induced voltage waveform to provide continuous torque to the BLDC motor. Therefore, the drive of a three-phase BLDC motor can be divided into six steps, each of which has a 60-degree electrical angle, within a 360-degree electrical cycle. This is called a six-step drive, as shown in Figure 2. Furthermore, an important feature of six-step drive is that when moving from one electrical position to the next, the open phase is driven, and one of the two activated phases is opened while the other remains activated.

[0008] Six-step drive of a three-phase BLDC motor is generally achieved by a three-phase inverter bridge, which is responsible for the electrical switching of the BLDC motor switches and the magnitude of the voltage applied to the motor, according to the basic principles of drive described above. Driving the motor phases is achieved by driving only two switches with electrical positions, one on the upper branch of the inverter bridge and the other on the lower branch of the inverter bridge, each operating at a 120° electrical angle. Additionally, the magnitude of the power applied to the BLDC motor can be controlled by pulse-width modulation (PWM) of one or both switches operating at each electrical position.

[0009] Additionally, because only two phases of a BLDC motor are activated in each drive step, it is common in low-cost applications for motor current monitoring to be performed indirectly by only one current sensor placed on the busbar. However, such a configuration imposes a series of limitations that need to be addressed, as will be explained further below.

[0010] A simpler way to control a BLDC motor is to control the voltage applied to the motor based on the error between the desired rotational speed and the actual rotational speed. A simpler starting of a BLDC motor consists of applying an open-mesh voltage ramp, with voltage increases and time decreases at each electrical position, up to the lowest speed that the voltage controller allows for closed-mesh drive.

[0011] However, the problem with such control types is that they have a slow response time and do not allow for fast control of the BLDC motor torque as required in some applications, such as starting a reciprocating compressor with non-uniform pressures, a typical requirement for refrigeration systems with shut-off valves.

[0012] A solution known in the prior art to provide a faster solution consists of controlling the torque applied to a BLDC motor by means of a current controller that controls the current circulating in the motor. This type of controller includes several control meshes: an outer mesh for controlling the rotational speed and an inner mesh for controlling the torque via current control.

[0013] Hysteresis current controllers are very popular due to their simplicity of application, their ability to provide fast dynamic response to transients, and their inherent ability to limit the maximum current. Another feature of hysteresis current controllers is their robustness; such type of controller is independent of the motor parameters.

[0014] Furthermore, conventional hysteretic current controllers control the actual current to lie within a narrow operating range close to the desired current, and the current control is turned off whenever the actual current exceeds the upper limit of such operating range, and turned on again when the actual current exceeds the lower limit of the operating range.

[0015] A conventional hysteresis current controller for a BLDC motor uses three current sensors, one for each phase of the motor. The phase currents of the BLDC motor are compared to reference currents using three independent comparators.

[0016] An alternative approach would be to use only one current sensor on the busbar, since ideally only two motor phases are active at each activation step. However, using only one current sensor on the busbar poses some limitations, primarily with regard to the switching method used.

[0017] A switching method commonly used with hysteresis current controllers is the bipolar switching method shown in Figure 3a. In bipolar switching, pulse width modulation is applied to two switches that actuate at specific electrical positions. The bipolar switching method is simpler and uses +V in the phases corresponding to the two switches that are actuated. CC and -V CC However, compared to unipolar switching, current swells, switching frequency, and switching losses are higher.

[0018] In the unipolar switching scheme, pulse width modulation is applied to only one of the two phases that operates at a particular electrical position, while the other phase remains continuously turned on, as shown in Figure 3b. In this way, in the unipolar switching scheme, pulse width modulation is applied to only the switches of one of the two operating phases, which reduces switching losses and reduces the voltage applied to the phases between 0 and +V. CC or 0 and -V CC Since the voltage amplitude applied to the motor is reduced, the switching frequency is also reduced.

[0019] Unipolar switching schemes can also be divided into several switching patterns. Differences between unipolar patterns can consist of limitations related to the switch's drive circuit, the possibility of current feedback through the busbar of an interrupted phase when changing electrical position, and input when conducting an open phase through a freewheeling diode. (Y.-s. Lai and Y.-k. Lin, "Assessment of Pulse-Width Modulation Techniques for Brushless DC Motor Drives," Conference Record of the 2006 IEEE Industry Applications Conference Forty-First IAS Annual Meeting, 2006, doi: 10.1109 / IAS.2006.256754.)

[0020] Among such unipolar switching patterns, two patterns are widely known and used: PWM_ON and ON_PWM patterns.

[0021] In the PWM_ON pattern exemplarily shown in Figure 4a, pulse width modulation is applied during the first 60° of electrical movement, and then the switches are kept switched on (ON) for the last 60° of electrical movement. The PWM_ON pattern prevents the freewheeling current of the interrupted phase (blocking phase) from returning through the busbar even while one of the switches is open.

[0022] However, in the ON_PWM pattern exemplarily shown in Figure 4b, the sequence is reversed and the switch remains switched on (ON) for the first 60° of electrical movement, after which pulse width modulation is applied for the last 60° of electrical movement. The ON_PWM pattern forces the freewheeling current of the interrupted phase back through the busbar while the switch is open.

[0023] Despite the advantages highlighted with respect to unipolar switching schemes, there are very important limitations when associating such unipolar schemes with a hysteretic current controller with only one current sensor on the busbar. This limitation arises because the motor current is no longer circulated by the busbar while only one switch is open, making it impossible to correctly compare the motor current with the current limits of a conventional hysteretic controller. Therefore, recovery of the switch drive is expected, and the resulting current can increase from cycle to cycle, as shown in Figure 5.

[0024] The prior art shows several alternatives to conventional hysteretic current controllers that can overcome some of the limitations presented so far, such as the limitation of using unipolar switching schemes in hysteretic current controllers with only one current sensor in the busbar.

[0025] In this regard, a paper by Jahns published in January 1991 (T.M. Jahns, R.C. Becerra, and M. Ehsani, "Integrated current regulation for a brushless ECM drive," IEEE Transactions on Power Electronics, vol. 6, no. 1, pp. 118-126, Jan. 1991, doi: 10.1109 / 63.65010.) proposes a method for regulating the current in a BLDC motor using current sensors integrated into the switches, instead of using only current sensors in the busbars.

[0026] Based on this topology, Jahn proposes motor current control by applying an analog fixed-time hysteresis current controller, using only the current sensor associated with the lower switch, and opening only one of such switches. Rather than waiting for the current to drop to the lower protection current limit, as in conventional hysteresis current control techniques, a fixed-time hysteresis current controller was chosen because the integrated current sensor is in series with the switch and cannot measure the current circulating through the diode when the switch is open.

[0027] Furthermore, Jahns proposes using a current sensor integrated with the upper switch as rear overcurrent protection, but the current protection limit is fixed and adjusted beyond the maximum current limit used to regulate the motor current. Therefore, such current protection would only act under abnormal operating conditions to ensure the integrity of the motor and system electronics.

[0028] The drawback of the method proposed by Jahns is the need to integrate current sensors in all switches and the need for a current controller for each switch, which increases the cost and reduces the reliability of the solution.

[0029] A paper by Becerra published in 1991 (RC Becerra, TM Jahns, and M. Ehsani, "Four-quadrant sensorless brushless ECM drive," in [Proceedings] APEC 91: Sixth Annual Applied Power Electronics Conference and Exhibition, 1991, doi: 10.1109 / APEC.1991.146165) proposed a drive for a BLDC motor capable of four-quadrant operation without a rotor position sensor. The control and drive strategy proposed by Becerra is very similar to that proposed by Jahns, with some minor differences.

[0030] As an alternative to the analog fixed-time hysteresis current controller proposed by Jahns, Becerra proposed an analog fixed-frequency hysteresis current controller. The limitation to Becerra's approach is the same as that present in Jahns, namely, the loss of the ability to measure the current when the switch is open. The advantage of applying fixed-frequency modulation would be that it would simplify the EMC filter (electromagnetic compatibility) associated with the inverter input circuit.

[0031] Becerra also proposed a different pulse width modulation pattern for current control than Jahns, the ON_PWM mentioned above, which forces the return of the freewheeling current of the phase interrupted by the busbar while one of the switches is open. This allows the freewheeling current of the open phase to disappear more quickly, while maintaining indirect position sensing by monitoring the potential induced in this open phase.

[0032] Despite the improvements introduced by Becerra, a drawback is the need for current sensors integrated with every switch, and for a current controller for each switch, which increases the cost and reduces the reliability of the solution.

[0033] A paper by Wei, published in 2015 (Wei, Y.; Xu, Y.; Zou, J. & Li, Y. Current, "Limit Strategy for BLDC Motor Drive With Minimized DC-Link Capacitor," IEEE Transactions on Industry Applications, Institute of Electrical and Electronics Engineers (IEEE), 2015, 51, 3907-3913), proposes a current limiting strategy using only one current sensor in the busbar to drive a BLDC motor with a low-capacitance capacitor in the busbar. The purpose of the proposed strategy is to prevent excessive voltage rise in the busbar caused by the motor's freewheeling current returning to the capacitor. To achieve this, Wei uses a fixed-time hysteresis current controller associated with PWM_ON type pulse width modulation.

[0034] However, Wei did not at all concern himself with the limitations of his proposed solution during starting or failed starting. In this case, low motor speeds or even a reversal of direction can cause current control to become ineffective, and the current will eventually increase beyond the desired limit, despite the use of a discrete, but fixed-time hysteresis current controller as proposed by Wei. This limitation and the resulting overcurrent in these conditions can be explained as follows:

[0035] 6 shows a state-of-the-art starting method using a one-level, discrete, fixed-time hysteresis current controller. Here, when applying a current to the BLDC motor 10 to generate maximum torque during starting, the voltage applied to the BLDC motor 10 varies with each sampling period T s The BLDC motor current I measured at measurement instant X after m is the first maximum current limit I max1When the first protection time T off1 is then re-established by closing switch S1.

[0036] The magnitude of the current applied to the BLDC motor 10 must be as large as possible at start-up, i.e., the maximum applied current, in order to generate maximum torque in the BLDC motor 10. However, the applied current must be less than the maximum current I allowed for healthy operation of the BLDC motor 10. desmag , i.e., it must be less than the current that will not damage the magnets of the BLDC motor 10.

[0037] Figure 6 also shows a prior art starting method using a one-level discrete fixed-frequency hysteresis current controller, where the voltage applied to the motor is increased every sampling period T s The current I of the BLDC motor measured at a later measurement instant X m is the first maximum current limit I max1 When the pulse width modulation period T PWM At , it is re-established by closing switch S1.

[0038] The first protection time T off1 After the first maximum current limit I max1 The recovery of the switch drive by exceeding m is not circulated by the busbar current sensor 20 and interferes with its measurement. pwm However, start-up recovery can be performed.

[0039] As can be seen in FIG. 7a, which represents an exemplary reference position for analyzing the readings of the BLDC motor current 10, when the BLDC motor 10 is driven, the busbar current i busbaris equal to the current circulating in the phase of the BLDC motor 10. However, as can be seen in Figure 7b, at a later moment in the same electrical position, the busbar current i busbar is null during such a step, the BLDC motor current I is measured by the current sensor 20 in the busbar. m It is impossible to measure.

[0040] As already mentioned above, the advantages of opening only one switch include reduced instantaneous torque fluctuations of the BLDC motor 10, reduced switching frequency, and reduced commutation losses.

[0041] However, at very low speeds, such as during initial starting or failed starting, the first protection time T off1 After or in the next pulse width modulation period T PWM However, simply closing the switch after the first switch may not be sufficient to prevent overcurrent in the BLDC motor 10.

[0042] In the case of a separate current controller, this overcurrent is measured by the minimum time the switch remains on, which is equal to the current sampling time T S Therefore, the predicted drive recovery, which is related to the minimum time the switch remains on due to the discrete control characteristic, can cause a gradual increase in the current limit, as shown in Figure 5.

[0043] FIG. 8 shows an equivalent model of the BLDC motor 10 when driving phases A and B of the prior art, where “L” is the equivalent inductance of phases A and B, “R” is the equivalent resistance of phases A and B, and “E” is the equivalent induced voltage between phases A and B.

[0044] Furthermore, FIG. 6 illustrates driving a prior art BLDC motor 10 with a one-level discrete fixed-time or fixed-frequency hysteresis current controller, where the first step is to turn on switches S1 and S4 for a first time period T on The second step consists of closing the first protection period T off1 and the pulse width modulation period T pwm includes the first and second steps.

[0045] The currents in the first and second steps can be modeled as follows: TIFF0007785077000001.tif24152

[0046] where i(t) is the current in the first step, I1 is the initial current in the first step, i2(t) is the current in the second step, I2 is the initial current in the second step, and τ is the electrical time constant. TIFF0007785077000002.tif12143

[0047] In practice, the pulse width modulation period T pwm is much smaller than the electrical time constant τ, the behavior of the currents i1(t) and i2(t) can be approximated as a first-order linear equation, with the slope set by its derivative at the first instant. TIFF0007785077000003.tif29158

[0048] where α1 is the slope of the current in the first step, and α2 is the slope of the current in the second step.

[0049] Therefore, the current variation in both the first and second steps can be approximated as: TIFF0007785077000004.tif25158

[0050] where Δi1 is the current variation in the first step and Δi2 is the current variation in the second step.

[0051] Thus, at very low speeds, such as during the initial start-up or when a start-up failure occurs, the induced voltage E of the BLDC motor 10 is very low or practically null, and the busbar voltage V cc Considering that is much higher than the voltage drop across resistor R due to the current I1 circulating through the BLDC motor 10, time T on is the protection time T off1 Even if it is shorter, the current fluctuation increment ΔI1 in the first step can be higher than the current fluctuation decrement ΔI2 in the second step.

[0052] Other types of current controllers have been developed to control the current. Among these other topologies, predictive controllers and deadbeat controllers can be highlighted. However, these types of controllers depend on the motor parameters and their rotational speed. Speed ​​dependency is a concern, especially during compressor start-up, due to the initial acceleration required. Speed ​​also oscillates depending on the load profile, which is a concern for reciprocating compressors.

[0053] Furthermore, there are several other solutions that have been developed to improve the performance of such classical solutions.

[0054] For example, U.S. Patent Publication US5019756, entitled "PROCESS AND ELECTRONIC CIRCUIT FOR CONTROLLING A BRUSHLESS DIRECT CURRENT MOTOR," published May 28, 1991, describes a control system for a BLDC motor during the starting phase. The publication describes that starting occurs in three steps. 1) The first two switch combinations are applied to inject current into the two phases of the motor, forcing the rotor into a known electrical position for a certain amount of time. 2) Apply two open mesh steps and then wait a period of time while checking whether the induced voltage on the motor coil is detectable. 3) When an induced potential is detected by the analog position sensor, a self-control step is entered.

[0055] Publication US5857349, published on January 12, 1999, for an invention entitled "REFRIGERATING APPARATUS, AND REFRIGERATOR CONTROL AND BRUSHLESS MOTOR STARTER USED IN SAME," describes a set of three commutation sequences (A, B, and C) to be applied during motor starting. Each pattern is a sequence of nine open-mesh commutations with different timings, and the timings vary depending on the level of the existing load. Pattern A is reserved for light loads, and pattern C is reserved for heavy loads. Starting always begins with pattern A, and changes to the next pattern in the event of a fault.

[0056] The document JP200328073, published on August 26, 2004, entitled "Exposure Method and Exposure Apparatus," proposes a start-up scheme using rotation speed and current control. However, the proposal in JP200328073 does not clearly define the type of current controller, and does not consider all the limitations presented in this document.

[0057] Patent document US2018 / 152120, published on May 31, 2018, and titled "ELECTRIC WORKING MACHINE AND METHOD FOR CONTROLLING ELECTRIC WORKING MACHINE," discloses an electric work machine including a motor, a first switching element, a second switching element, a rectifier element, and a controller. The controller is configured to perform stop control by maintaining completion of a first current path via the first switching element for a predetermined time and interrupting the second current path via the second switching element in response to a stop condition being satisfied for stopping the supply of power from a power source to the motor while the motor is rotating.

[0058] It is important to emphasize that the torque generated by a BLDC motor is proportional to the current applied to the motor, as well as the magnetic force generated by its permanent magnets. Therefore, if maximum torque is desired during motor startup, the current applied to the motor should be increased to its maximum, taking into account that this current is applied in synchronization with the motor's induced voltage.

[0059] A problem with the prior art is the fact that the starting methods are based on adjusting the voltage applied to the motor based on the speed of the motor, and in addition, the prior art starting methods tend to have poor dynamic response when subjected to high loads because they are unable to generate the motor's maximum torque.

[0060] Another problem in the prior art occurs in low-cost systems with only one current sensor in the busbar when applying maximum intensity current to a BLDC motor and, without proper control of the level of such current, causing the BLDC motor to generate maximum torque, which can be damaging to the BLDC motor, especially if a starting failure occurs. Summary of the Invention

[0061] SUMMARY OF THE INVENTION It is an object of the present invention to provide a starting method which avoids the drawbacks of the prior art.

[0062] Such an object is achieved by a method for starting a BLDC motor with trapezoidal induced voltage under high load conditions using a two-level discrete fixed-time or fixed-frequency hysteresis current controller, the BLDC motor being driven by an inverter bridge with only one current sensor located on the busbars and controlled by a processing unit associated with an analog-to-digital converter, the method comprising the steps of: - driving the BLDC motor with a six-step drive technique that drives only two phases at a specific electrical position; applying a maximum magnitude current to the BLDC motor by a closed current control mesh to generate a maximum torque in the BLDC motor; - applying a unipolar switching pattern of on_PWM type, in which each switch is driven during a 120° electrical period in the following way: each switch is kept continuously on during the first 60° electrical period and is modulated with pulse width modulation during the last 60° electrical period; - controlling the magnitude of the BLDC motor current by a two-level discrete fixed-time or fixed-frequency hysteresis current controller; The first level of protection is - turning off the first switch if, during a first protection time, the current of the BLDC motor measured at a measurement instant exceeds a first maximum current limit; and at the end of the first protection time, turning off the second switch while keeping the first switch turned off for a second protection time; The second level of protection is and turning off the first switch and the second switch for a third protection time if the current of the BLDC motor measured at the measurement instant exceeds a second maximum current limit.

[0063] Furthermore, the method according to the present invention comprises the first maximum current limit being lower than the second maximum current limit, the second maximum current limit being lower than the maximum allowed current value.

[0064] Furthermore, the method according to the present invention comprises setting the second maximum current limit by subtracting the allowable maximum current by the second maximum error, and setting the first maximum current limit by subtracting the second maximum current limit by the first maximum error.

[0065] Furthermore, the method according to the invention comprises the difference between the value of the second maximum current limit and the value of the maximum allowable current being greater than the value of the second maximum error increment.

[0066] The method according to the invention also comprises the step of: the difference between the value of the second maximum current limit and the value of the maximum allowable current is greater than the value of the second maximum error increment.

[0067] Furthermore, the method according to the present invention comprises a first protection time that ensures that the reduction in the BLDC motor current is greater than a first maximum error in the event of a successful start.

[0068] Furthermore, the method according to the invention comprises that the second guard time must be less than or equal to the sampling period.

[0069] The method according to the invention also consists in phasing out the second protection time when the BLDC motor reaches one-quarter of its nominal rotation speed or performs a full mechanical revolution.

[0070] In addition, the method according to the invention has the advantage that the gradual disappearance of the second protection time must take place in approximately 1 second.

[0071] Furthermore, the method according to the invention comprises a third protection time that ensures that the reduction in current of the BLDC motor is greater than the second maximum error increment even in the event of a starting failure.

[0072] Furthermore, in the method according to the present invention, the third protection time may be replaced with the following condition: When the current of the motor measured at the measurement instant is lower than the first maximum current limit, the two switches are switched on again.

[0073] In the method according to the present invention, the third protection time may also be replaced with the following condition: - When the current of the motor exceeds a second maximum current limit, starting is aborted.

[0074] Furthermore, the method according to the invention allows the first protection time to be adjusted depending on the speed of the motor.

[0075] Furthermore, the method of the present invention replaces the step of turning off the first switch for a first protection time when the measured BLDC motor current at the measurement instant exceeds the first maximum current limit with the following: the first switch is turned off when the measured BLDC motor current at the measurement instant exceeds a first maximum current limit, and is turned on again only at the beginning of the next correction period of the pulse width modulation.

[0076] Furthermore, the method according to the invention consists in the first switch that is turned off being the switch associated with the phase that continues to be driven after the change in electrical position.

[0077] Furthermore, the method according to the invention comprises the use of opening both switches at the end of the second protection time in order to be able to carry out a monitoring of the maximum current circulating through the BLDC motor.

[0078] Furthermore, the method according to the invention ensures more effective protection against possible overcurrents.

[0079] Another advantage of the method according to the invention is that it provides the protection required mainly in the initial moments of the drive or when a starting failure occurs, when the induced voltage is so low that the action of the control applied to the first maximum current limit may not be sufficient for a possible overcurrent.

[0080] Furthermore, another advantage of the present invention is that by opening the two switches at the end of the first maximum current limit protection, the application of the busbar voltage during the sampling period is compensated by applying a reverse busbar voltage also during the sampling period, thereby preventing the current from escaping the control of the current controller.

[0081] Another advantage of the method according to the invention consists in ensuring that when opening the two switches during the second protection period, the busbar on which the current sensor is located allows the larger current circulated by the motor to flow instantaneously, thus ensuring monitoring and control of the total current and allowing a more effective protection of the motor. [Brief explanation of the drawings]

[0082] Objects and advantages of the present invention will become apparent from the detailed description of the embodiments that follow and the non-limiting drawings that appear at the end of this document.

[0083] [Figure 1]FIG. 1 shows a prior art electronic circuit and a BLDC motor associated with the electronic circuit.

[0084] [Figure 2] FIG. 2 shows the induced voltage waveform, desired current, and command code of a prior art six-step driving method for a BLDC motor.

[0085] [Figure 3] Figure 3 shows the difference between (A) bipolar and (B) unipolar switching types for driving a prior art BLDC motor.

[0086] [Figure 4a] FIG. 4a shows a PWM_on pattern applied to a prior art switch.

[0087] [Figure 4b] FIG. 4b shows the on_PWM pattern applied to a prior art switch.

[0088] [Figure 5] FIG. 5 illustrates the effect of sampling on a discrete hysteretic controller associated with prior art unipolar switching.

[0089] [Figure 6] FIG. 6 is a diagram showing the command code of a prior art fixed-time hysteresis current controller.

[0090] [Figure 7a] FIG. 7a is a diagram showing the relationship between the phase currents and the busbar currents of a motor when only two phases are conducting in the prior art. [Figure 7b] FIG. 7b is a diagram showing the relationship between the phase currents and the busbar currents of the motor when only two phases are conducting in the prior art. [Figure 7c] FIG. 7c is a diagram showing the relationship between the phase currents and the busbar currents of the motor when only two phases are conducting in the prior art.

[0091] [Figure 8] FIG. 8 is a diagram showing a model equivalent to a BLDC motor when driven by the A and B phases according to the prior art.

[0092] [Figure 9] FIG. 9 shows the command code of a modified hysteretic current controller according to the present invention.

[0093] [Figure 10] FIG. 10 shows the results of applying current protection with a two-level modified hysteresis current controller according to the present invention.

[0094] [Figure 11a] FIG. 11a shows the relationship between the motor phase currents and the busbar currents when three phases are conducting in accordance with the present invention. [Figure 11b] FIG. 11b shows the relationship between the motor phase currents and the busbar currents when three phases are conducting in accordance with the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0095] The present invention describes a method for starting a BLDC motor 10 using a two-level, discrete, modified fixed-time or fixed-frequency hysteresis current controller using only current sensors 20 in the busbars.

[0096] As shown in FIG. 1, current control is achieved by a closed mesh operatively associating current sensors 20 in the busbars, A / D converters, current controllers, and the BLDC motor 10, such that the voltage applied to the BLDC motor 10 is the difference between the desired applied current and the actual applied current.

[0097] As shown in FIG. 9, the starting method using the modified two-level fixed time and discrete hysteresis current controller, which is the object of the present invention, includes a second protection time T during which two switches, e.g., S1 and S4, are open. off2The third step is to set the BLDC motor current I as shown in Figure 7c. m is forced to flow in the reverse direction by the bus bars, applying a negative voltage to the BLDC motor 10 and forcing such current to decrease.

[0098] Optionally, still according to FIG. 9, the start-up method uses a fixed frequency and includes a pulse width modulation correction period (period) T MPWM Set the pulse width modulation correction period T MPWM During this time, both switches, e.g., S1 and S4, are turned on during their respective correction periods T MPWM At the end of time T off2 Furthermore, one of the switches, for example, switch S1, is opened only when the current is equal to or exceeds the first maximum current limit I max1 It may be opened when it exceeds the next correction period T MPWM It will only be turned on again at the start of

[0099] The difference between a fixed-time hysteresis current controller and a fixed-frequency hysteresis current controller is that the fixed-time controller uses T off1 is fixed, and T MPWM is variable, and at fixed frequency TMPWM is fixed, and T off1 In both cases, the time T on is variable and depends on the operating conditions, and the time T off2 is fixed. TIFF0007785077000005.tif41154

[0100] The current equation in the third step (equation i3(t)), the approximation of this current by a straight line with a slope defined by the derivative of the current i3(t) at the initial moment (t=0) (equation α3), and the current change equation in the third step (equation Δi3) are defined as follows: TIFF0007785077000006.tif45158

[0101] In this way, even when the rotation speed is very low, such as during start-up or when start-up fails, the induced voltage E of the BLDC motor 10 is very low or substantially zero, and the busbar voltage V cc Considering that the voltage drop across the resistor R is much higher than the voltage drop across the resistor R, the increment of the current fluctuation in the first step ΔI1 due to the current I1 circulating in the BLDC motor 10 is off2 is the first time period T on Therefore, if the current is already equal to the first maximum current limit I max1 Considering the worst case scenario when the first time period T on is the sampling period T S If the second protection time T off2 is the sampling period T S Therefore, even under these conditions, it is possible to ensure that the current variation Δi resulting from the three steps Δi1, Δi2, and Δi3 is less than zero, preventing overcurrent as shown in the following equation:

[0102] In the above expansion, the natural solution of the RL circuit is I1R / L T on , I2R / L·T off1 , and I3R / L·T off2 The part is I1, I 2、 and I3, in other words, the larger current I2 is used as the reference, and the time portion t On , T off1 , and T off2 and the period of the pulse width modulation T pwm The total is calculated as: TIFF0007785077000007.tif12158

[0103] According to FIG. 9, the first maximum error ΔI in current limit due to sign sampling implemented by a one-level modified discrete fixed-time or fixed-frequency hysteresis current controller max1 is calculated according to the following formula for the sampling period Ts A first time period T equal to on , and the nulled induced voltage En can be obtained taking into account the equalization of the first step.

[0104] Therefore, the first maximum current limit I max1 is the maximum allowable current I desmag The first maximum error ΔI max1 must be lower than the value obtained by subtracting TIFF0007785077000008.tif43158

[0105] However, this first level of protection does not prevent the current from continuing to increase even after the first switch with the first level of protection is opened in the event of a start failure due to a reversal of the rotation direction, which is typical of a reciprocating compressor. In this case, the induced voltage E of the BLDC motor 10 is made negative, here E rev During switching, the BLDC motor current I m This can increase rather than decrease the reversed induced voltage E of the BLDC motor 10, as shown by the following equation: rev is higher than the voltage drop across the resistance R of the BLDC motor 10.

[0106] In the above expansion, the part related to the natural solution of the circuit RL and the part related to the back electromotive force E rev The same simplification is performed for the part related to time t On , T off1 , and T off2 The part is grouped and the period of the pulse width modulation is T pwm The total is calculated as:

[0107] Therefore, the present invention provides a second maximum current limit I max2 The present invention also proposes a second protection level that defines a limit for the measured current of the BLDC motor I at the measurement instant X. m When the current reaches the third protection time T, the switches S1 and S4 are open, as illustrated in FIG. off3Furthermore, according to Figure 10, the motor current I m is forced in the reverse direction by the bus bars, applying a negative voltage to the BLDC motor 10 and more effectively forcing down such current.

[0108] Thus, as can be seen in FIG. 10, the starting method using a two-level modified discrete fixed-time or fixed-frequency hysteresis current controller according to the present invention operates as follows: First, a maximum current limit I max1 At the first protection level associated with the first protection time T off1 To ensure effective current protection, switches S1 and S4 are open for a first protection time T off1 The second protection time T off2 Then, the second maximum current limit I max2 where switches S1 and S4 are connected for a third protection time T off3 and the BLDC motor current i m is forced to flow by the busbars and reduced more quickly, ensuring the reliability of the BLDC motor 10. TIFF0007785077000009.tif11158

[0109] Second maximum current limit I max2 The value of is the second maximum error ΔI that can occur due to the increase in current caused by the reversal of the direction of rotation. max2 It is worth emphasizing that the second maximum error ΔI max2 is the back electromotive force E of the BLDC motor 10. rev and can be set by the following formula:

[0110] Thus, the second maximum current limit I max2 is the maximum allowable current I desmag The second maximum error ΔI max2 must be set taking into consideration the following:

[0111] Additionally, a second maximum current limit Imax2 Adding the first current limit I max1 The second maximum current limit I max2 The first maximum error ΔI max1 The first maximum current limit I max1 needs to be redefined.

[0112] Also, the first maximum current limit I max1 The value of is the motor current I under normal operating conditions. m must be set to control the second maximum current limit I max2 However, it must be ensured that the second maximum current limit I max2 The value of is set to the maximum allowable current I so that it only acts in case of a failed start. desmag The system must be designed to protect against TIFF0007785077000010.tif13158

[0113] The duration of the third protection time is the second maximum error ΔI max2 can be set as a function of

[0114] For fixed-time hysteresis controllers, the third protection time T off3 The period of pulse width modulation T mpwm This can be set by the lower correction period of the lower on-time T on is the sampling period T s Therefore, the first protection time T off1 , the second protection time T off2 , and sampling time T s is equal to

[0115] Alternatively, in the case of a fixed frequency hysteresis controller, the third protection time T off3 The period of correction T mpwv It can be easily set by

[0116] Finally, the third protection time T off3 is the motor current I mis the second current limit I max2 If exceeded, such current strength is indicative of reverse rotation of the BLDC motor 10 and is evidence of a failed start, so that starting can be aborted.

[0117] Another challenge associated with using only one current sensor 20 on the busbar is measuring higher currents when all three phases of the BLDC motor 10 are conducting, for example, during transient conditions immediately after an electrical position change, or during a fault in electrical position sensing, such as occurs during a start-up failure due to a reversal of the direction of rotation of the BLDC motor 10.

[0118] Figure 11a shows switches S1 and S4 turned on with current flowing in phase A and out phase B, resulting in unwanted current flowing in phase C. This unwanted current in phase C is temporary and may be caused by a change in electrical position or by a poor starting condition due to a reversal of direction of rotation. TIFF0007785077000011.tif8144

[0119] Referring again to FIG. 11a, the current i flowing through the busbar current sensor 20 bus is the current i in phase A A However, the current i in phase C is c still exists, and the current i in phase B b is characterized by the sum of the currents in the other two phases.

[0120] where i>0 and i c >0, so i b >0.

[0121] In this sense, in order to monitor the maximum current circulating in the BLDC motor 10, the present invention provides a second protection time T off2We propose to use two switches that are open at the end of the switching period (cycle) for i. In this case, the maximum current circulating in such a phase while the switches are open flows through the busbar current sensor 20. In this case, as shown in Figure 11b, i b =i a +i c This becomes:

[0122] Finally, the second protection time T off2 may be disabled as soon as a successful start is identified, which may be identified when the BLDC motor 10 successfully rotates fully or when the speed of the BLDC motor 10 reaches a minimum speed considered safe, such as 1 / 4 of the motor's nominal speed.

[0123] Second protection time T off2 The deactivation of the second protection time can be done in stages, gradually reducing its activation time with each mechanical revolution until it is completely deactivated. The gradual deactivation of the second protection time can be done, for example, over a period of about 1 second.

[0124] In addition to the embodiments described above, the same inventive concept is intended to apply to other alternatives or possibilities of using the present invention.

[0125] While the present invention has been described with reference to certain preferred embodiments, example For example, similar problems are found when three current sensors are used in series with the lower switches instead of only one current sensor in the inverter busbar, which can be solved with the present invention.

Claims

1. A method for starting a BLDC motor (10) with trapezoidal induced voltage under high load conditions using a two-level discrete fixed-time or fixed-frequency hysteresis current controller, the BLDC motor (10) being driven by an inverter bridge (30) having only one current sensor (20) located on a busbar and controlled by a processing unit associated with an analog-to-digital converter, comprising the steps of: - driving said BLDC motor (10) with a six-step driving technique that drives only two phases at a specific electrical position; - applying a current of maximum intensity to the BLDC motor (10) by a closed mesh of current control in order to generate a maximum torque in the BLDC motor (10); applying a unipolar switching pattern of on_PWM type, in which each switch is driven during an electrical period of 120° in the following way: during the first electrical period of 60°, each switch is kept continuously on and during the last electrical period of 60°, it is modulated by pulse width modulation, and further comprising: The two-level discrete fixed-time or fixed-frequency hysteresis current controller controls the current (I ｍ and controlling the intensity of the The first level of protection is: - first protection period (T ｏｆｆ１ ) during the measurement, the current (I ｍ ) First Maximum Current Limit (I ｍａｘ１ turning off first switches belonging to the portions of the inverter bridge associated with the two phases driven at a particular electrical position when - the first protection period (T ｏｆｆ１ At the end of the second protection period (T ｏｆｆ２ keeping the first switch off and turning off second switches belonging to portions of the inverter bridge associated with the two phases driven at specific electrical positions during The second level of protection is The current (I) of the BLDC motor measured at the measurement instant (X) ｍ ) is the second maximum current limit (I ｍａｘ２ ) exceeds the third protection period (T ｏｆｆ３ turning off the first switch and the second switch during The first maximum current limit (I ｍａｘ１ ) is the second maximum current limit (I ｍａｘ２ ) lower than The method, wherein the second protection level is activated after activation of the first protection level.

2. The second maximum current limit (I ｍａｘ２ ) is the maximum allowable current (I ｄｅｓｍａｇ 2. The method of claim 1, wherein the temperature is lower than 100°C.

3. The second maximum current limit (I ｍａｘ２ ) is the maximum allowable current (I ｄｅｓｍａｇ ) to the second maximum error (ΔI ｍａｘ２ ) and the first maximum current limit (I ｍａｘ１ ) is the second maximum current limit (I ｍａｘ２ ) to the first maximum error (ΔI ｍａｘ１ 3. The method according to claim 1, wherein the value is set by subtracting the value of the reference signal from the reference signal.

4. The first maximum current limit (I ｍａｘ１ ) and the second maximum current limit (I ｍａｘ２ ) is the first maximum error (ΔI ｍａｘ１ 2. The method of claim 1, wherein the value of .lamda. is greater than the value of .lamda..

5. The second maximum current limit (I ｍａｘ２ ) and the allowable maximum current (I ｄｅｓｍａｇ ) is the second maximum error (Δ Ｉｍａｘ２ 3. The method of claim 2, wherein the value of .lamda. is greater than the value of .lamda..

6. The first protection period (T ｏｆｆ１ ) is the current (I ｍ ) is the first maximum error (ΔI ｍａｘ１ 2. The method of claim 1, wherein the method further comprises:

7. The second protection period (T ｏｆｆ２ ) is the sampling period (T ｓ 2. The method of claim 1, wherein the number of vertices must be less than or equal to 1.

8. The second protection period (T) begins when the BLDC motor (10) reaches 1 / 4 of its nominal rotation speed or performs a full mechanical rotation. ｏｆｆ２ 8. The method of claim 7, wherein the .alpha.-to- ...

9. The second protection period (T ｏｆｆ２ 9. The method of claim 8, wherein the gradual disappearance of the signal must occur in approximately 1 second.

10. The third protection period (T ｏｆｆ３ ) is the current (I ｍ ) is the second maximum error (ΔI ｍａｘ２ 2. The method of claim 1, wherein the method further comprises ensuring that the value of .times. ...

11. The third protection period (Toff3) satisfies the following conditions: - the current (I) of the motor measured at the measurement instant (X) ｍ ) is the first maximum current limit (I ｍａｘ１ ), the two switches are turned on again.

2. The method of claim 1, wherein:

12. The third protection period (T ｏｆｆ３ )teeth, - the current of the motor (I ｍ ) is the second maximum current limit (I ｍａｘ２ ) is exceeded, the start is aborted.

2. The method of claim 1, wherein:

13. The first protection period (T ｏｆｆ１ 13. The method according to claim 1, wherein the rotational speed of the motor is adjustable.

14. The current (I) of the BLDC motor measured at the measurement instant (X) ｍ ) is the first maximum current limit (I ｍａｘ１ ) is exceeded, the first protection period (T ｏｆｆ１ turning off the first switch during - the current (I) of the BLDC motor measured at the measurement instant (X) ｍ ) is the first maximum current limit (I ｍａｘ１ ) is exceeded, the first switch is turned off, and the next correction period (T ＭＰＷＭ ) and switch it on again only at the beginning of 2. The method of claim 1, wherein:

15. 2. The method of claim 1, wherein the first switch that is turned off is the switch associated with the phase that continues to be driven after the change in electrical position.

16. The second protection time T ｏｆｆ２ 16. A method according to any one of claims 1 to 15, characterized in that it utilizes the opening of the two switches at the end of

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