JP5997724B2 - Method for controlling a brushless permanent magnet motor - Google Patents

Description

  The present invention relates to a method for controlling a brushless permanent magnet motor.

  There are ongoing efforts to improve the efficiency of brushless permanent magnet motors.

  There is a growing need to improve the efficiency of brushless permanent magnet motors.

  The present invention provides a method for controlling a brushless permanent magnet motor, the method commutating when the motor winding is delayed with respect to the zero crossing of the back electromotive force in the winding, and each electrical half. Dividing the cycle into conduction periods followed by a primary freewheel period; dividing the conduction period into a first excitation period, a secondary freewheel period and a second excitation period; and Energizing the windings and freewheeling the windings during each freewheel period.

  For permanent magnet motors, the torque to current ratio is maximized when the phase current waveform matches the back electromotive force waveform. Therefore, improvement in motor efficiency is achieved by forming the phase current waveform to better match the back electromotive force waveform. During excitation, the phase current can rise faster than the back electromotive force around the zero crossing in the back electromotive force. As a result, if the winding is commutated prior to or synchronously with the zero crossing, the phase current will quickly lead to the back electromotive force. By delaying commutation until after each zero crossing of the back electromotive force, the increase in phase current can be more closely followed by the increase in back electromotive force. As a result, the efficiency of the motor can be improved.

  Although commutation is delayed, the phase current can nevertheless rise at a faster rate than the back electromotive force. As a result, the phase current can ultimately lead to a counter electromotive force. The secondary freewheel period plays a role in preventing the phase current from rising in an instant. As a result, the phase current can be more closely followed by the back electromotive force increase during the conduction period, thereby further improving efficiency.

  The secondary freewheel period can occur when the back electromotive force in the winding increases, and the primary free wheel period can occur when the back electromotive force mainly decreases. The primary freewheel period utilizes the inductance of the winding so that torque continues to be generated by the phase current without additional power being drawn from the power source. As the back electromotive force decreases, less torque is generated for a given phase current. Thus, by freewheeling the windings during periods when the back electromotive force is reduced, the efficiency of the motor can be improved without adversely affecting the torque.

  The length of the secondary freewheel period may be shorter than each of the primary freewheel period, the first excitation period, and the second excitation period. As a result, the secondary freewheel period plays a role of preventing the phase current from rising instantly without adversely affecting the electric power of the motor.

  The method includes exciting the winding with a supply voltage and changing the length of the conduction period in response to changes in supply voltage and / or motor speed. As a result, better control over the power of the motor can be achieved.

  As the supply voltage decreases, less current and less power is driven into the motor for the same conduction period. Similarly, as the motor speed increases, the magnitude of the back electromotive force induced in the winding increases. Less current and less power is driven into the motor over the same conduction period. Thus, to compensate for this, the method may include increasing the conduction period in response to a decrease in supply voltage and / or an increase in motor speed.

  A change in the magnitude of the supply voltage used to excite the winding affects the rate at which the phase current rises. Changes in motor speed affect the length of each electrical half cycle and the rate at which the back electromotive force increases. Furthermore, changes in motor speed affect the magnitude of the back electromotive force and the rate at which the phase current increases. Thus, the method may include delaying commutation by a delay period and changing the delay period in response to changes in supply voltage and / or motor speed. This has the advantage that the efficiency of the motor can be improved when the motor operates over various supply voltages and / or motor speeds. Further, the amount of current and power driven by the windings is affected by changes in supply voltage and / or motor speed. By changing the delay period in response to changes in supply voltage and / or motor speed, better control over motor input or output power can be achieved.

  The method may include increasing the delay period in response to an increase in supply voltage and / or a decrease in motor speed. As the magnitude of the supply voltage increases, the rate at which the phase current rises increases. As the motor speed decreases, the speed at which the back electromotive force increases decreases. In addition, the magnitude of the back electromotive force is reduced, thus increasing the phase current at a faster rate. By increasing the delay period in response to an increase in supply voltage and / or a decrease in motor speed, the phase current waveform better matches the back electromotive force waveform in response to a change in supply voltage and / or motor speed. Can be done. As a result, the efficiency of the motor can be improved when operating over various supply voltages and / or speeds.

  The method may include controlling the motor in the manner described in any of the preceding paragraphs when operating at a speed greater than 50 krpm. At this relatively high speed, the length of each electrical half cycle is relatively short and the magnitude of the back electromotive force is relatively large. Both of these factors indicate that precommutation is required to drive sufficient current and power to the phase winding to maintain such speed. In fact, it appears that prior commutation is necessary to accelerate to such speeds. Nevertheless, Applicants have confirmed that once these speeds are reached, commutation can be delayed to improve motor efficiency.

  The method may include controlling the motor in the manner described in any of the preceding paragraphs when operating over a speed range extending to at least 5 krpm, more preferably at least 10 krpm. As a result, improved motor efficiency can be achieved over a relatively wide speed range.

  The present invention also provides a control circuit configured to perform the method described in any of the preceding paragraphs, and a motor assembly comprising the control circuit and a brushless permanent magnet motor.

  The control circuit may include an inverter coupled to the motor windings, the gate driver module and the controller. Next, the gate driver module controls the switch of the inverter according to the control signal received from the controller. The controller generates a control signal that commutates the winding when delayed with respect to the zero crossing of the back electromotive force in the winding. The controller is also responsible for dividing each electrical half-cycle into a conduction period and a primary freewheel period, and dividing the conduction period into a first excitation period, a secondary freewheel period, and a second excitation period. The controller then energizes the windings during each excitation period and generates a control signal that causes the windings to freewheel during each freewheel period.

  In order that the present invention may be more readily understood, embodiments thereof will now be described by way of example with reference to the accompanying drawings, in which:

1 is a block diagram of a motor assembly according to the present invention. It is a circuit diagram of a motor assembly. It is a figure which shows the detail of the permitted state of an inverter according to the control signal issued by the controller of a motor assembly. It is a figure which shows the various waveforms of the motor assembly at the time of operate | moving in acceleration mode. It is a figure which shows the various waveforms of the motor assembly at the time of operate | moving in high power mode. It is a figure which shows the various waveforms of the motor assembly at the time of operate | moving in a low power mode.

  The motor assembly 1 shown in FIGS. 1 and 2 is supplied with power by a DC power source 2 and includes a brushless motor 3 and a control circuit 4.

  The motor 3 includes a 4-pole permanent magnet rotor 5 that rotates in relation to the 4-pole stator 6. The conductors wound around the stator 6 are coupled together to form a single phase winding 7.

  The control circuit 4 includes a filter 8, an inverter 9, a gate driver module 10, a current sensor 11, a voltage sensor 12, a position sensor 13, and a controller 14.

  The filter 8 includes a link capacitor C1 that flattens a relatively high-frequency ripple resulting from the switching of the inverter 9.

  Inverter 9 includes four power switches Q1-Q4 for all bridges that couple phase winding 7 to the voltage rail. Each of switches Q1-Q4 includes a freewheel diode.

  The gate driver module 10 drives opening and closing of the switches Q1 to Q4 according to the control signal received from the controller 14.

  The current sensor 11 includes a shunt resistor R1 disposed between the inverter and the zero voltage rail. The voltage across the current sensor 11 provides a measurement of the current in the phase winding 7 when connected to the power supply 2. The voltage applied to the current sensor 11 is output to the controller 14 as a signal I_PHASE.

  The voltage sensor 12 includes voltage dividers R2 and R3 disposed between the DC voltage rail and the zero voltage rail. The voltage sensor outputs a signal V_DC to the controller 14, which represents a reduced measurement of the supply voltage provided by the power supply 2.

  The position sensor 13 includes a Hall effect sensor disposed in the slot opening of the stator 6. The sensor 13 outputs HALL which is a logically high or low digital signal according to the direction of the magnetic flux passing through the sensor 13. Therefore, the HALL signal provides a measurement of the angular position of the rotor 5.

  The controller 14 includes a microcontroller having a processor, a storage device, and a plurality of peripheral devices (for example, an AD converter, a comparator, a timer, etc.). The storage device stores instructions executed by the processor, control parameters used by the processor during operation of the motor assembly 1, and a look-up table. The controller 14 is involved in controlling the operation of the motor 3 and generates four control signals S1 to S4 for controlling each of the four power switches Q1 to Q4. The control signal is output to the gate driver module 10, and the gate driver module drives opening and closing of the switches Q1 to Q4 accordingly.

  FIG. 3 is a diagram summarizing the permitted states of the switches Q1 to Q4 in accordance with the control signals S1 to S4 output by the controller 14. Hereinafter, the terms “set” and “clear” are used to indicate that the signal has been logically high and low, respectively. As can be seen from FIG. 3, the controller 14 sets S1 and S4 to excite the phase winding 7 from left to right, and clears S2 and S3. Conversely, the controller 14 sets S2 and S3 to clear the phase winding 7 from right to left and clears S1 and S4. The controller 14 clears S1 and S3 to set the phase winding 7 to freewheel, and sets S2 and S4. The freewheeling action allows the current in the phase winding 7 to be recirculated around the low side loop of the inverter 9. In the present embodiment, the power switches Q1 to Q4 can conduct in both directions. Thus, the controller 14 closes both the low side switches Q2 and Q4 during freewheeling so that current flows through the switches Q2 and Q4 rather than the low efficiency diodes. It is conceivable that the inverter 9 may include a power switch that is conductive only in one direction. In this case, the controller 14 clears S1, S2 and S3 and sets S4 to freewheel the phase winding 7 from left to right. Thereafter, the controller 14 clears S1, S3 and S4 and sets S2 in order to freewheel the phase winding 7 from right to left. Thereafter, the low-side loop current of inverter 9 flows down through the closed low-side switch (eg, Q4) and flows up through the diode of the open low-side switch (eg, Q2).

  The controller 14 operates in one of three modes: an acceleration mode, a low power mode, and a high power mode. Both the low power mode and the high power mode are steady state modes. The controller 14 receives and periodically monitors the power mode signal POWER_MODE to determine which steady state mode should be used. If the power mode signal is logically low, the controller 14 selects the low power mode, and if the power mode signal is logically high, the controller 14 selects the high power mode. When operating in the low power mode, the controller 14 drives the motor 3 over an operating speed range of 60 to 70 krpm. When operating in the high power mode, the controller 14 drives the motor 3 over an operating speed range of 90 to 100 krpm. The acceleration mode is then used to accelerate the motor 3 from rest to the lower limit of each operating speed range.

  In all three modes, the controller 14 commutates the phase winding 7 in response to the edge of the HALL signal. Each HALL edge corresponds to a change in the polarity of the rotor 5 and thus a change in the polarity of the back electromotive force induced in the phase winding 7. More specifically, each HALL edge corresponds to a zero crossing of the back electromotive force. The commutation involves reversing the direction of the current through the phase winding 7. As a result, when current flows through the phase winding 7 from left to right, commutation involves exciting the winding from right to left.

In the following discussion, the speed of the motor 3 is often referred to. The speed of the motor 3 is determined from the interval between successive edges of the HALL signal, hereinafter referred to as the HALL period.
Acceleration mode At a speed slower than 20 krpm, the controller 14 commutates the phase winding 7 in synchronization with each HALL edge. At a speed of 20 krpm or higher, the controller 14 commutates the phase winding 7 prior to each HALL edge. In order to commutate the phase winding 7 prior to a particular HALL edge, the controller 14 operates in response to the preceding HALL edge. In response to the preceding HALL edge, the controller 14 subtracts the preceding period T_ADV from the HALL period T_HALL to obtain the commutation period T_COM.

T_COM = T_HALL-T_ADV
The controller 14 commutates the phase winding 7 at time T_COM after the preceding HALL edge. As a result, the controller 14 commutates the phase winding 7 prior to the subsequent HALL edge by the preceding period T_ADV.

  Regardless of whether the commutation is synchronous or preceding, the controller 14 continuously excites and freewheels the phase winding 7 over each electrical half-cycle when operating in the acceleration mode. More specifically, the controller 14 excites the phase winding 7, monitors the current signal I_PHASE, and freewheels the phase winding 7 when the current in the phase winding 7 exceeds a predetermined limit. The freewheel then continues for a predetermined freewheel period, during which time the current in the phase winding 7 drops to a level below the current limit. When the freewheel period ends, the controller 14 excites the phase winding 7 again. This process of exciting and freewheeling the phase winding 7 continues over the entire length of the electrical half-cycle. Therefore, the controller 14 switches from excitation to freewheel multiple times during each electrical half cycle.

  FIG. 4 is a diagram illustrating waveforms of a HALL signal, a back electromotive force, a phase current, a phase voltage, and a control signal over a few HALL periods when operating in the acceleration mode. In FIG. 4, the phase winding 7 is commutated in synchronism with the HALL edge.

  At a relatively slow speed, the magnitude of the back electromotive force induced in the phase winding 7 is relatively small. Therefore, the current in the phase winding 7 rises relatively quickly during excitation and falls relatively slowly during freewheeling. In addition, the length of each HALL period and the length of each electrical half cycle are relatively long. As a result, the frequency at which the controller 14 switches from excitation to freewheel is relatively high. However, as the rotational speed increases, the magnitude of the back electromotive force increases, so the current increases at a slower speed during excitation and decreases at a faster speed during freewheeling. Note that the length of each electrical half-cycle decreases. As a result, the frequency of switching decreases.

The controller 14 continues to operate in the acceleration mode until the speed of the rotor 5 reaches the lower limit of the operating speed range of the selected power mode. Thus, for example, when the high power mode is selected, the controller 14 continues to operate in the acceleration mode until the speed of the rotor 5 reaches 90 krpm.
The high power mode controller 14 commutates the phase winding prior to each HALL edge. Pre-commutation is achieved in the same manner as described above for the acceleration mode.

  When operating in the high power mode, the controller 14 divides each electrical half-cycle into conduction periods followed by a freewheel period. The controller 14 then excites the phase winding 7 during the conduction period and freewheels the phase winding 7 during the freewheel period. The phase current does not exceed the current limit during excitation. As a result, the controller 14 switches from excitation to freewheeling only once during each electrical half cycle.

  The controller 14 excites the phase winding 7 during the conduction period T_CD. When the conduction period ends, the controller 14 freewheels the phase winding 7. At that time, the freewheel continues indefinitely until the controller 14 commutates the phase winding 7. Therefore, the controller 14 controls the operation of the motor 3 using the two parameters of the preceding period T_ADV and the conduction period T_CD.

  FIG. 5 is a diagram illustrating waveforms of a HALL signal, a back electromotive force, a phase current, a phase voltage, and a control signal over a few HALL periods when operating in the high power mode.

  The magnitude of the supply voltage used to excite the phase winding 7 can vary. For example, the power source 2 may include a battery that discharges during use. Alternatively, the power source 2 may include an AC power source, a rectifier, and a smoothing capacitor that provides a relatively smooth voltage, but the RMS voltage of the AC power source may vary. The change in the magnitude of the supply voltage affects the amount of current driven by the phase winding 7 during the conduction period. As a result, the electric power of the motor 3 is susceptible to changes in the supply voltage. In addition to the supply voltage, the power of the motor 3 is susceptible to changes in the speed of the rotor 5. Just as the speed of the rotor 5 changes (for example, in response to a change in load), the magnitude of the back electromotive force also changes. As a result, the amount of current driven into the phase winding 7 during the conduction period can vary. Therefore, the controller 14 changes the preceding period and the conduction period in accordance with the change in the magnitude of the supply voltage. Further, the controller 14 changes the preceding period according to the change in the speed of the rotor 5.

  The controller 14 stores a voltage lookup table comprising a preceding period T_ADV and a conduction period T_CD for each of a plurality of various supply voltages. The controller 14 also stores a speed lookup table comprising speed compensation values for each of a plurality of different rotor speeds and various supply voltages. The look-up table stores values that achieve a particular input or output power at each voltage and speed point. In the present embodiment, the lookup table stores values that achieve constant output power in the motor 3 over various supply voltages and operating speed ranges in high power mode.

  The V_DC signal output by the voltage sensor 12 provides a measurement of the supply voltage, while the length of the HALL period provides a measurement of the rotor speed. The controller 14 indexes the voltage reference table using the supply voltage and selects the phase period and the conduction period. The controller 14 then indexes the speed lookup table using the rotor speed and supply voltage to select a speed compensation value. The controller 14 then adds the selected speed compensation value to the selected phase period to obtain a speed compensation phase period. The commutation period T_COM is then obtained by subtracting the speed compensation phase period from the HALL period T_HALL.

  The speed reference table stores a speed compensation value that depends not only on the speed of the rotor 5 but also on the magnitude of the supply voltage. The reason is that as the supply voltage decreases, the net effect of the specific speed compensation value on the output power of the motor 3 becomes smaller. By storing a speed compensation value that depends on both the rotor speed and the supply voltage, better control over the output power of the motor 3 can be achieved in response to changes in the rotor speed.

Note that two lookup tables are used to determine the preceding period. The first lookup table (ie, voltage lookup table) is indexed using the supply voltage. The second lookup table (ie, the speed lookup table) is indexed using both the rotor speed and the supply voltage. Since the second lookup table is indexed using both rotor speed and supply voltage, it may be questioned that it requires two lookup tables. However, the advantage of using two lookup tables is that different voltage resolutions can be used. The output power of the motor 3 is relatively susceptible to the magnitude of the supplied power. In contrast, the effect that the speed compensation value has on the output power is only slightly affected by the supply voltage. Thus, by using two lookup tables, a finer voltage resolution can be used for the voltage lookup table and a coarser voltage resolution can be used for the velocity lookup table. As a result, relatively good control over the output power of the motor 3 can be achieved through the use of a smaller lookup table, which reduces the memory requirements of the controller 14.
The low power mode controller 14 commutates the phase winding 7 when delayed with respect to the HALL edge. Delayed commutation is achieved in a manner similar to the preceding commutation. In response to the HALL edge, the controller 14 adds a delay period T_RET to the HALL period T_HALL in order to obtain the commutation period T_COM.

T_COM = T_HALL + T_RET
The controller 14 then commutates the phase winding 7 at time T_COM after the HALL edge. As a result, the controller 14 commutates the phase winding 7 at time T_RET after the subsequent HALL edge.

  When operating in the low power mode, the controller 14 divides each half of the electrical period into conduction periods followed by a primary freewheel period. Thereafter, the controller 14 divides the conduction period into a first excitation period, a subsequent secondary freewheel period, and a subsequent second excitation period. The controller 14 then energizes the phase winding 7 during each of the two excitation periods and freewheels the phase winding 7 during each of the two freewheel periods. As in the high power mode, the phase current does not exceed the current limit during excitation. Therefore, the controller 14 switches from excitation to freewheel twice during each electrical half cycle.

  FIG. 6 is a diagram illustrating the waveforms of the HALL signal, the back electromotive force, the phase current, the phase voltage, and the control signal over a few HALL periods when operating in the low power mode.

  As in the high power mode, the controller 14 changes the delay period and the conduction period according to the change in the magnitude of the supply voltage, and the controller 14 changes the delay period according to the change in the speed of the rotor 5. . Thus, the controller 14 stores a further voltage look-up table comprising various delay periods T_RET and various excitation periods T_EXC for various supply voltages. The controller 14 also stores additional speed look-up tables with speed compensation values for various rotor speeds and various supply voltages. Therefore, the reference table used in the low power mode is different from the reference table used in the high power mode only in that the table stores a delay period, not a preceding period, and an excitation period, not a conduction period. Like the high power mode, the lookup table used in the low power mode stores values that achieve a constant output power for the motor 3 over the same range as the supply voltage and over the operating speed range of the low power mode.

  During operation, the controller 14 indexes the voltage lookup table using the supply voltage and selects the delay period and the excitation period. The selected excitation period is then used to define both the first excitation period and the second excitation period, i.e., during the conduction period, the controller 14 causes the phase winding during the selected excitation period. 7 is excited, the phase winding 7 is freewheeled during the secondary freewheel period, and the phase winding 7 is excited again during the selected excitation period. As a result, a secondary freewheel period occurs in the middle of the conduction period.

  Compared to the high power mode, the excitation of the phase winding 7 differs in two important respects. First, the controller 14 delays commutation. Second, the controller 14 introduces a secondary freewheel period into the conduction period. The reason and advantage of these two differences will be described below.

  When operating in high power mode, a prior commutation is required to achieve the required output power. As the speed of the rotor 5 increases, the HALL period decreases, so the constant (L / R) becomes increasingly important when related to phase inductance. Furthermore, the back electromotive force induced in the phase winding 7 increases, which affects the rate at which the phase current rises. Therefore, it becomes increasingly difficult to drive current and power to the phase winding 7. By commutating the phase winding 7 prior to each HALL edge and thus prior to the zero crossing of the back electromotive force, the supply voltage is instantaneously boosted by the back electromotive force. As a result, the direction of the current through the phase winding 7 reverses more quickly. In addition, the phase current is made to introduce a back electromotive force, which helps compensate for the slower rate of current rise. The subsequent phase current generates a short-term negative torque, but the phase current is usually more than compensated by subsequent gain at positive torque.

  When operating in the low power mode, the length of the HALL period increases, and therefore the back electromotive force increases at a slower rate. In addition, the magnitude of the back electromotive force is reduced, so that the current in the phase winding 7 rises at a faster rate for a given supply voltage. Thus, the back electromotive force increases at a slower rate, but the phase current increases at a faster rate. Thus, it is not necessary to commutate the phase winding 7 prior to the HALL edge to achieve the desired output power. Furthermore, for reasons explained below, the efficiency of the motor assembly 3 is improved by delaying commutation.

  During excitation, the torque to current ratio is maximized when the phase current waveform matches the back electromotive force waveform. Therefore, improvement in the efficiency of the motor 3 is achieved by forming the phase current waveform to better match the back electromotive force waveform, ie, by reducing the harmonic component of the phase current waveform with respect to the back electromotive force waveform. Is done. As described in the preceding paragraph, when operating in the low power mode, the back electromotive force increases at a slower rate while the phase current increases at a faster rate. In fact, when operating in the low power mode, the phase current rises faster than the back electromotive force when the magnitude of the back electromotive force is relatively low, ie, approximately around the zero crossing of the back electromotive force. As a result, if the phase winding 7 is commutated prior to or in synchronization with the HALL edge, the phase current will lead back electromotive force more quickly. In the high power mode, it was necessary for the phase current to first induce a back electromotive force to compensate for the shorter HALL period and the slower rise in phase current. However, in the low power mode, it is not necessary for the transmission current to induce a back electromotive force to achieve the required output power. By delaying commutation until after each HALL edge, the phase current more closely follows the increase in back electromotive force. As a result, the efficiency of the motor assembly 1 is improved.

  The secondary freewheel period serves to further improve the efficiency of the motor 3. As a result of the delayed commutation, the phase current more closely matches the phase current of the counter electromotive force. Nevertheless, the phase current continues to rise at a faster rate than the back electromotive force rate. As a result, the phase current eventually exceeds the back electromotive force. By introducing a relatively small secondary freewheel period into the conduction period, the increase in phase current is momentarily prevented, so that the increase in phase current closely follows the increase in counter electromotive force. As a result, the harmonic component of the phase current waveform with respect to the back electromotive force waveform is further reduced, and thus the efficiency of the motor 3 is further increased.

  The acceleration mode is used to accelerate the motor 3 from the stationary state to the lower limit of each operation speed range. As a result, the controller 14 operates in an acceleration mode between 0 and 90 krpm when the high power mode is selected, and between 0 and 60 krpm when the low power mode is selected. Regardless of which power mode is selected, the controller 14 commutates the phase winding 7 in synchronism with the HALL edge between 0 and 20 krpm. The controller 14 then commutates the phase winding 7 prior to the HALL edge when the motor 3 accelerates from 20 to 90 krpm (high power mode) or from 20 to 60 krpm (low power mode). When operating in the acceleration mode, the controller 14 uses a preceding period in which the acceleration of the rotor 5 is fixed and maintained. Regardless of whether the high power mode or the low power mode is selected, the controller 14 indexes the voltage lookup table used in the high power mode using the supply voltage to select the preceding period. The selected preceding period is then used by the controller 14 during the acceleration mode. It is contemplated that the efficiency of the motor 3 can be improved by using a preceding period that varies with the rotor speed. But then this requires an additional lookup table. Furthermore, the acceleration mode typically does not last long and the controller 14 operates primarily in a low power mode or a high power mode. As a result, the efficiency improvement that can be made by changing the preceding period in the acceleration mode is unlikely to contribute significantly to the overall efficiency of the motor 3.

  When switching from the acceleration mode to the low power mode, the controller 14 switches from the preceding commutation to the delayed commutation. When operating in the low power mode, the controller 14 can drive sufficient current and power to the phase winding 7 during each electrical half-cycle and at the same time uses delayed commutation to improve the efficiency of the motor assembly 1. . In contrast, during commutation, prior commutation is necessary to ensure that sufficient current and power is driven into the phase winding 7 during each electrical half-cycle. If the controller 14 delays or synchronizes commutation during acceleration, the rotor 5 will not be able to accelerate to the required speed. Thus, while delayed commutation can be used to maintain the rotor speed over the speed range of 60-70 krpm, prior commutation is required to ensure that the rotor accelerates to 60 krpm.

  Although it is known to delay commutation when operating at relatively slow speeds, it is not known at all to delay commutation when operating at relatively high speeds, i.e., speeds exceeding 50 krpm. At these relatively high speeds, each relatively short length of HALL period and the magnitude of the back electromotive force should indicate that a prior commutation is required. In fact, precommutation is necessary to accelerate the motor to such speed. However, the Applicant has confirmed that once these speeds are reached, commutation can be delayed to improve the efficiency of the motor 3.

  In the embodiment described above, the controller 14 uses two steady state modes, a high power mode and a low power mode. In the high power mode, the controller 14 commutates the phase winding 7 prior to the zero crossing of the back electromotive force. In the low power mode, the controller 14 commutates the phase winding 7 with a delay to the zero crossing of the counter electromotive force. The preceding period and the delay period can each be regarded as a phase period T_PHASE, and the commutation period T_COM is defined as follows.

T_COM = T_HALL-T_PHASE
If the phase period is positive, commutation occurs before the HALL edge (ie, leading commutation), and if the phase period is negative, commutation occurs after the HALL edge (ie, delayed commutation). Using the same scheme to commutate the phase winding 7 in both high and low power modes simplifies control. However, it is conceivable that various methods can be used to commutate the phase winding 7. For example, when operating in the low power mode, the controller 14 may simply commutate the phase winding 7 at time T_RET after each HALL edge.

  In the above-described embodiment, the controller 14 changes only the phase period (that is, the preceding period in the high power mode and the delay period in the low power mode) according to the change in the rotor speed. Compared to the conduction period, the input power of the motor 3 is typically more affected by changes in the phase period. Therefore, better control over the output power of the motor 3 can be achieved by changing the phase period. Nevertheless, despite these advantages, the controller 14 can instead change only the conduction period in response to changes in rotor speed. Alternatively, the controller 14 can change both the phase period and the conduction period in response to changes in the rotor speed. This may be necessary, for example, when the output power of the motor 3 cannot be adequately controlled by changing only the phase period. Alternatively, an improvement in the efficiency of the motor 3 can be achieved by changing both the phase period and the conduction period in response to changes in the rotor speed. However, the disadvantage of changing both the phase period and the conduction period is that an additional look-up table is required, thus making further demands on the memory of the controller 14.

  In the above-described embodiment, the controller 14 changes the phase period and the conduction period according to the change in the supply voltage. This then has the advantage that the efficiency of the motor 3 can be better optimized at each voltage point. Nevertheless, it may be possible to achieve desirable control over the output power of the motor 3 by changing only one of the phase period and the conduction period. Since the output power of the motor 3 is more affected by changes in the phase period, better control over the output power of the motor 3 can be achieved by changing the phase period.

  Therefore, it may be said that the controller 14 changes the phase period and / or the conduction period in response to changes in the supply voltage and the rotor speed. Although the two periods can be changed in response to changes in supply voltage and rotor speed, it is contemplated that the controller 14 can change the period in response to only one of supply voltage and rotor speed. . For example, the voltage provided by the power supply 2 can be relatively stable. In that case, the controller 14 may change the phase period and / or conduction period in response to changes in only the rotor speed. Alternatively, the motor 3 may need to operate at a constant speed or over a relatively small range of speeds within the low and high power modes. In this case, the controller 14 can change the phase period and / or the conduction period in response to a change in only the supply voltage. Thus, in a more general sense, the controller 14 may be said to change the phase period and / or conduction period in response to changes in supply voltage and / or rotor speed. Further, without storing the voltage reference table or speed reference table, it may be said that the controller 14 stores a power reference table with various control values for various supply voltages and / or rotor speeds. Each control value then achieves a specific output power at each voltage and / or speed point. Controller 14 then indexes the power lookup table using the supply voltage and / or rotor speed and selects a control value from the power lookup table. The control value is then used to define the phase period or conduction period.

  In the above-described embodiment, the controller 14 stores a reference table including a conduction period for use in the high power mode and an excitation period for use in the low power mode. However, the same level of control can be achieved by different means. For example, without storing a reference table for the conduction period and the excitation period, the controller 14 can store a reference table for the primary freewheel period, which is likewise the magnitude and / or rotation of the supply voltage. The speed of the child 5 is used to index. At that time, the conduction period is obtained by subtracting the primary freewheel period from the HALL period, and each excitation period is obtained by subtracting the primary freewheel period and the secondary freewheel period from the HALL period by 2 It is obtained by dividing.

T_CD = T_HALL-T_FW_1
T_EXC = (T_HALL-T_FW_1-T_FW_2) / 2
Here, T_CD is a conduction period, T_EXC is each of the first and second excitation periods, T_HALL is a HALL period, T_FW_1 is a primary freewheel period, and T_FW_2 is a secondary freewheel period. is there.

  In the embodiment described above, the secondary freewheel period occurs in the middle of the conduction period. This is accomplished by ensuring that the same excitation period is used to define the length of the first excitation period and the second excitation period. There are at least two advantages to ensuring that the secondary freewheel period occurs in the middle of the conduction period. First, the harmonic components of the phase current are better balanced over the two excitation periods. As a result, if the lengths of the two excitation periods are different, the overall harmonic component of the phase current over the conduction period tends to decrease. Secondly, the lookup table need only store one excitation period for each voltage point. As a result, less memory is required for the lookup table. Despite the advantages described above, it may be desirable to change the position of the secondary freewheel period in response to changes in supply voltage and / or rotor speed. This can be accomplished by using a lookup table that stores the first and second excitation periods for various voltages and / or speeds.

  The controller 14 uses a secondary freewheel period with a fixed length. This has the advantage of reducing the memory requirements of the controller 14. However, alternatively, the controller 14 may use a secondary freewheel period that varies in response to changes in supply voltage and / or rotor speed. In particular, the controller 14 may use a secondary freewheel period that increases as the supply voltage increases or the rotor speed decreases. As the supply voltage increases, the current in the phase winding 7 rises at a faster rate during excitation, provided that the rotor speed, and thus the magnitude of the back electromotive force, does not change. As a result, the harmonic component of the phase current waveform related to the back electromotive force waveform tends to increase. By increasing the length of the secondary freewheel period in response to an increase in supply voltage, the increase in phase current is prevented for a long period, and therefore the harmonic components of the phase current waveform can be reduced. As the rotor speed decreases, the length of the HALL period increases, so the back electromotive force increases at a slower speed. Furthermore, if the supply voltage does not change, the magnitude of the back electromotive force decreases, and thus the current in the phase winding 7 increases at a faster rate. As a result, as the rotor speed decreases, the back electromotive force increases at a slower speed, but the phase current increases at a faster speed. Therefore, the harmonic component of the phase current waveform related to the back electromotive force waveform tends to increase. By increasing the secondary freewheel period in response to the decrease in rotor speed, the increase in phase current is prevented for a long period, and therefore the harmonic components of the phase current waveform can be reduced. Thus, increasing the secondary freewheel period in response to increasing supply voltage and / or decreasing rotor speed may further improve efficiency.

  The length of the secondary freewheel period is relatively short and is intended only to prevent the phase current from rising in an instant. Therefore, the secondary freewheel period is shorter than both the primary freewheel period and each excitation period. The actual length of the secondary freewheel period depends on certain characteristics of the motor assembly 1, such as the inductance of the phase winding 7, the magnitude of the supply voltage, the magnitude of the back electromotive force, and the like. Regardless of length, the secondary freewheel period occurs during the period of back electromotive force rising in the phase winding 7. This is in contrast to the primary freewheel period that occurs predominantly but not entirely during the period of decreasing back EMF. During the primary freewheel period, the inductance of the phase winding 7 is utilized so that torque is continuously generated by the phase current without additional power being drawn from the power source 2. As the back electromotive force decreases, less torque is generated for a given phase current. Therefore, by freewheeling the phase winding 7 during the period when the back electromotive force is lowered, the efficiency of the motor assembly 1 can be improved without adversely affecting the torque.

DESCRIPTION OF SYMBOLS 1 Motor assembly 2 DC power supply 3 Brushless motor 4 Control circuit 5 Rotor 6 Stator 7 Phase winding 8 Filter 9 Inverter 10 Gate driver module 11 Current sensor 12 Voltage sensor 13 Position sensor 14 Controller

Claims (11)

A method for controlling a brushless permanent magnet motor, comprising:
Commutating the motor winding when delayed relative to the zero crossing of the back electromotive force in the winding;
Dividing each electrical half-cycle into conduction periods followed by a primary freewheel period;
Dividing the conduction period into a first excitation period, a secondary freewheel period, and a second excitation period;
Exciting the motor windings during each excitation period;
Freewheeling the winding during each freewheel period ,
The length of the secondary freewheel period is shorter than each of the primary freewheel period, the first excitation period, and the second excitation period.   The method of claim 1, wherein the secondary freewheel period occurs when back electromotive force in the winding increases, and the primary free wheel period occurs mainly when back electromotive force decreases. 3. The method according to claim 1, comprising: exciting the winding with a supply voltage; and changing the length of the conduction period according to a change in the supply voltage or the speed of the motor. The method described. 4. The method of claim 3 , comprising increasing the length of the conduction period in response to a decrease in the supply voltage or an increase in speed of the motor. A step of commutating the winding when delayed by a delay period with respect to a zero crossing of the counter electromotive force in the winding; a step of exciting the winding with a supply voltage; and a length of the delay period. and a step of changing in accordance with the speed change of the voltage or the motor, the method according to any one of claims 1 to 4. 6. The method of claim 5 , comprising increasing the delay period in response to an increase in the supply voltage or a decrease in the speed of the motor. Commutating when the winding is delayed with respect to the zero crossing of the back electromotive force, dividing each electrical half-cycle into the conduction period and the primary freewheel period, and operating at a speed faster than 50 krpm to time, the said conduction period first exciting period, the a step of dividing the secondary freewheel period and the second exciting period, the method according to any one of claims 1 to 6. Commutation when the winding is delayed with respect to the zero crossing of the back electromotive force, dividing each electrical half-cycle into the conduction period and the primary freewheel period, and a speed range extending to at least 5 krpm when operating over, the said conduction period first exciting period, the and a secondary freewheel period and the step of dividing said second exciting period, according to an item any of claims 1 to 7 the method of. 9. The method of claim 8 , comprising driving the motor with constant power over the speed range. A control circuit for a brushless permanent magnet motor, adapted to perform the method according to any one of claims 1 to 9, the control circuit. A motor assembly comprising a brushless permanent magnet motor and a control circuit according to claim 10 .

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