JP4147383B2 - DC brushless motor parallel drive circuit - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a parallel drive circuit for driving a plurality of DC brushless motors connected in parallel with each other in order to operate a plurality of fans, pumps, and the like at the same speed, and is particularly applicable to applications requiring low noise. Thus, it is related to a suitable parallel drive circuit.
[0002]
[Prior art]
The present applicant has previously filed this kind of parallel drive circuit as Japanese Patent Application No. 2001-22412 (hereinafter referred to as the prior application).
First, the invention of the prior application will be described below.
[0003]
FIG. 7 shows the overall configuration of the drive circuit according to the invention of the prior application, in which two DC brushless motors MA and MB are driven by one drive circuit. In FIG. 7, E is a DC power source, and a three-phase bridge circuit composed of semiconductor switching elements T1 to T6 is connected to the DC power source E. The DC brassless motors MA, MB having the same configuration are connected in parallel to the phase output terminals U, V, W of the three-phase bridge circuit, and the hall elements HU, HV, HW provided in the respective phases are rotor positions. The detection circuits 21 and 22 are connected.
[0004]
The rotor position detection signals of the motors MA and MB output from the detection circuits 21 and 22 are input to the signal selection circuit 40, and the motor MA and MB position detection signals to be used can be switched and selected. It has become. The signal selection circuit 40 outputs a control signal for the switching signal generation circuit 30 to turn on / off the switching elements T1 to T6 according to the selected position detection signal of the motor.
In FIG. 7, 11 is a stator, 12 is a rotor, and CU, CV, and CW are phase coils of the stator 11.
[0005]
As shown in FIG. 8, the signal selection circuit 40 includes XOR (exclusive OR) gates IC1 and IC2 to which the position detection signal of the motor MA is input, and NAND gates IC3 to IC7 connected to the output side of the XOR gate IC2. NAND gates IC8 to IC10 to which the position detection signal of the motor MB is input, resistors R1 to R10 such as pull-up resistors, capacitors C1, diodes D1 to D7 on the output side of the NAND gates IC5 to IC10, and outputs Side transistors TR1 to TR3.
The signal selection circuit 40 receives one or two phases of the motors MA and MB with respect to the switching signal generation circuit 30 when a rotor position detection signal for each phase of the motors MA and MB as shown in FIG. The transistors TR1 to TR3 operate so as to output control signals for driving the switching elements.
[0006]
The operation of the circuit of FIG. 7 will be described below with reference to FIGS.
Assuming that the motors MA and MB are operating at the same speed in synchronization, the respective rotor position detection signals are output in synchronization as shown in FIG. In FIG. 9, a signal related to the motor MA is indicated by A, and a signal related to the motor MB is indicated by B.
Here, the rotor position detection signals of the motors MA and MB shown in FIG. 9 are substantially equal to the output signals of the Hall elements HU, HV and HW in FIG.
[0007]
In order to operate both motors MA and MB in synchronization with the rotor position detection signal, the rotor rotation angles (space angles) in FIG. 9 are 0 °, 60 °, 120 °, 180 °, 240 °, and 300 °. It is necessary to change the switching signal output from the switching signal generation circuit 30 as the position detection signal changes with timing.
[0008]
On the other hand, the rotation angle in FIG. 9 is between 0 ° and 60 °, between 60 ° and 120 °, between 120 ° and 180 °, between 180 ° and 240 °, between 240 ° and 300 °, and 300 °. Between 0 ° and 0 °, the position detection signals of the motors MA and MB remain unchanged and remain constant (for example, between 0 ° and 60 °, the U phase is used as the position detection signal of the motors MA and MB). In addition, a state in which a W-phase signal is detected continues, and a state in which only a U-phase signal is detected as a position detection signal for the motors MA and MB continues between 60 ° and 120 °.
[0009]
Therefore, there is no adverse effect even if the position detection signal of the motor MA and the position detection signal of the motor MB are switched while the rotor position detection signal remains unchanged and remains constant as described above.
For example, a switching signal for driving the motor MA using the position detection signal of the motor MA (the motors MA and MB are connected in parallel and may be a switching signal for driving the motor MB) is output. In this case, even if the switching signal for driving the motor MB by switching to the position detection signal of the other motor MB (which may also be a switching signal for driving the motor MA) is output. If it is performed during a period when there is no change in the position detection signal, there is no fear that the applied voltage of the motor changes suddenly at the moment of switching. Further, if the drive is switched at a cycle shorter than the cycle of the rotor position detection signal, the operation is less likely to become unstable.
[0010]
For this reason, in this prior art, the angle is between 0 ° and 60 °, between 60 ° and 120 °, between 120 ° and 180 °, between 180 ° and 240 °, between 240 ° and 300 °, At the time of 30 °, 90 °, 150 °, 210 °, 270 °, 330 ° between 300 ° and 0 ° (360 °), the signal selection circuit 40 causes the rotor position detection signals of the motors MA and MB to be transmitted to the motor. The switching signals for driving the motors MA and MB are output based on the selected rotor position detection signal.
[0011]
That is, as shown in FIG. 9, for example, the rotor position detection signal of the motor MA is selected between 330 ° and 30 °, and the switching signal generating circuit 30 based on this signal causes the U-phase coil CU and W-phase coil CW to be selected. A switching signal is output so that current is supplied to the power supply (periods are different). Further, the rotor position detection signal of the motor MB is selected between 30 ° and 90 °, and based on this signal, the switching signal generation circuit 30 energizes the U-phase coil CU and the W-phase coil CW (periods are respectively Create a switching signal as follows.
Thereafter, similarly, the rotor position detection signal of the motor MA is selected between 90 ° and 150 °, and based on this signal, the switching signal generating circuit 30 switches the switching signal to energize the U-phase coil CU and V-phase coil CV. The rotor position detection signal of the motor MB is selected between 150 ° and 210 °, and based on this signal, the switching signal generating circuit 30 switches the switching signal so that the U-phase coil CU and the V-phase coil CV are energized. Create
[0012]
In FIG. 9, for convenience of explanation, the position detection signals of the motors MA and MB are switched at angles of 30 °, 90 °, 150 °, 210 °, 270 °, and 330 °, but the switching angle is limited to these values. Not between 0 ° and 60 °, between 60 ° and 120 °, between 120 ° and 180 °, between 180 ° and 240 °, between 240 ° and 300 °, as described above. A similar effect can be obtained if the position detection signals of the motors MA and MB are switched at an angle between 0 ° and 0 ° (360 °) with no change.
[0013]
When the operation of the signal selection circuit 40 shown in FIG. 8 is confirmed, any of the rotor position detection signals (U phase, V phase, W phase) of the motors MA, MB in FIG. Is also expressed by logic (1, 0, 1), and these are input as position detection signals of the motors MA and MB in FIG. 8, the output side transistors TR1, TR2, TR3 (U phase) are output by the logic circuit in FIG. , V phase, W phase) output signal logic is (1, 0, 1), and position detection signals (U phase, V phase, W phase) of motor MA, MB between 60 ° and 90 ° ) Is logic (1, 0, 0), the logic of the output signals of the output side transistors TR1, TR2, TR3 (U phase, V phase, W phase) is (1, 0, 0). Coincides with the change of the output (control signal) of the signal selection circuit in the period of 30 ° to 90 ° Understood.
[0014]
Through the operation as described above, the two motors MA and MB can be stably driven in parallel by a single drive circuit in synchronization with the rotor position detection signal.
[0015]
[Problems to be solved by the invention]
However, in the above prior art, when the motor is stopped, the two motors MA and MB are not always stopped synchronously (at the same rotor position).
If the operation is started from a state in which the two motors are not synchronized and the speed is rapidly increased, a large circulating current may flow irregularly and a large noise may be generated from the motors until the motors are synchronized.
Further, in some cases, there has been a problem in that both motors are not synchronized indefinitely and continue to generate large noises.
[0016]
Accordingly, the present invention is intended to provide a parallel drive circuit for a DC brushless motor in which noise caused by operation in an asynchronous state is reduced.
[0017]
[Means for Solving the Problems]
In order to solve the above problems, the invention described in claim 1 is directed to switching signal generating means for driving a plurality of DC brushless motors connected in parallel to each other at the same speed by a driving circuit having a plurality of semiconductor switching elements. Is a parallel drive circuit of a DC brushless motor that creates a switching signal of the switching element using a rotor position detection signal of each motor,
While the rotor position detection signal of each motor does not change, the rotor position detection signal used for generating the switching signal is switched between the motors, and the switching signal is generated using the rotor position detection signal of the motor after switching. In addition, in a parallel drive circuit including signal selection means for outputting a control signal to the switching signal generation means,
Means for passing a direct current through the stator coil of each motor for a certain period after startup;
Speed control means for accelerating each motor to a predetermined speed after the predetermined period has elapsed.
[0018]
According to the second aspect of the present invention, in order to drive a plurality of DC brushless motors connected in parallel to each other at the same speed by a drive circuit having a plurality of semiconductor switching elements, the switching signal generating means includes a rotor position of each motor. A DC brushless motor parallel drive circuit that creates a switching signal of the switching element using a detection signal,
While the rotor position detection signal of each motor does not change, the rotor position detection signal used for generating the switching signal is switched between the motors, and the switching signal is generated using the rotor position detection signal of the motor after switching. In addition, in a parallel drive circuit including signal selection means for outputting a control signal to the switching signal generation means,
Means for passing a direct current through the stator coil of each motor for a certain period after startup;
Speed control means for operating each motor at a low speed after the predetermined period of time has elapsed and then accelerating to a predetermined speed.
[0019]
According to a third aspect of the present invention, in order to drive a plurality of DC brushless motors connected in parallel to each other at the same speed by a drive circuit having a plurality of semiconductor switching elements, the switching signal generating means includes a rotor position of each motor. A DC brushless motor parallel drive circuit that creates a switching signal of the switching element using a detection signal,
While the rotor position detection signal of each motor does not change, the rotor position detection signal used for generating the switching signal is switched between the motors, and the switching signal is generated using the rotor position detection signal of the motor after switching. In addition, in a parallel drive circuit including signal selection means for outputting a control signal to the switching signal generation means,
Means for passing a direct current through the stator coil of each motor for a certain period after startup;
Position signal error detection means for detecting an error in the rotor position detection signal of each motor;
Speed control means for operating each motor at a low speed after the predetermined period has elapsed, and accelerating each motor to a predetermined speed after an error detected by the position signal error detection means during the low speed operation falls below a specified value, It is provided.
[0020]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
First, FIG. 1 shows the overall configuration of the drive circuit according to the first embodiment of the present invention, in which two DC brushless motors MA and MB are driven by one drive circuit. The same components as those in FIG. 7 are denoted by the same reference numerals. The drive circuit of FIG. 1 corresponds to the invention described in claims 1 and 2.
[0021]
In FIG. 1, a DC power source E, a three-phase bridge circuit comprising semiconductor switching elements T1 to T6, DC brassless motors MA and MB, Hall elements HU, HV and HW, rotor position detection circuits 21 and 22, a signal selection circuit 40, and The connection configuration of the switching signal generation circuit 30 is the same as that in FIG.
Reference numeral 50 denotes a speed setting circuit for setting the rotation speeds of the motors MA and MB in accordance with the operation command. The set speed by the circuit 50 is sent to the speed control circuit 60, and the speed control circuit 60 sets the motors MA and MB in accordance with the set speeds. The switching elements T <b> 1 to T <b> 6 are controlled to be turned on and off via the switching signal generating circuit 30.
[0022]
Reference numeral 70 denotes a direct current setting circuit 70 for setting a direct current to be supplied to the stator coils CU, CV, and CW of the motors MA and MB in accordance with the operation command, and the output is applied to the speed control circuit 60.
That is, the set speed output from the speed setting circuit 50 and the DC current set value output from the DC current setting circuit 70 are input to the speed control circuit 60. The speed control circuit 60 switches the switching signal according to the input. A control signal is sent to the generation circuit 30 to control the rotational speed and current of the motors MA and MB.
During the operation of the DC current setting circuit 70, the output of the speed setting circuit 50 is held at zero by the control signal output from the setting circuit 70.
[0023]
Here, when the motors MA and MB are stopped while being out of synchronization, if a direct current is passed through the stator coils of the motors MA and MB, the permanent magnet of the rotor 12 is attracted to a predetermined position with respect to the stator coils. . Therefore, the positional relationship between the stator 11 and the rotor 12 of each motor MA, MB is the same, and a synchronized state can be realized.
The invention according to claim 1 pays attention to the above points, and for a certain period of time after starting, a direct current is passed through the stator coils of the motors MA and MB to synchronize both the motors MA and MB, and then a predetermined speed is reached. Accelerated until.
[0024]
FIG. 2 shows an embodiment of the invention of claim 1 and shows outputs of the speed setting circuit 50 and the DC current setting circuit 70 in FIG.
In FIG. 2, when an operation command is input at time t1 (at the time of starting), the direct current setting circuit 70 outputs a direct current setting value to the speed control circuit 60 for a certain period of time from time t1 to time t2. A direct current is passed through the stator coils of the motors MA and MB via the switching signal generation circuit 30. As a result, as described above, the permanent magnet of each rotor 12 is attracted to a predetermined position with respect to each stator coil, and the positional relationship between the stator 11 and the rotor 12 of each motor MA, MB becomes equal and synchronized. . During this time, the output of the speed setting circuit 50 is held at zero.
[0025]
Next, at time t2, the output of the DC current setting circuit 70 is made zero, and thereafter, the output of the speed setting circuit 50 is gradually increased to accelerate to the set speed V2.
Thereby, since it can accelerate in the state which synchronized both motor MA and MB, generation | occurrence | production of a noise can be prevented beforehand.
[0026]
However, when the motor's bearing lubricant is stopped at a low temperature, the viscosity of the lubricant is high, so if you immediately accelerate from the stopped state, you may not be able to accelerate well while maintaining synchronization. is there. In such a case, if the motor is operated at a low speed for a while, the viscosity of the lubricating oil decreases, so that it is possible to establish synchronization at the time of the low speed operation and to accelerate thereafter.
Therefore, in the invention described in claim 2, after a direct current is passed through the stator coils of the motors MA and MB for a certain period of time after starting, the motor is further operated at a low speed for a certain period of time that is the same as or different from the above, and then until a predetermined speed is reached. Accelerated.
[0027]
FIG. 3 shows an embodiment of the invention of claim 2 and shows outputs of the speed setting circuit 50 and the DC current setting circuit 70 in FIG.
In FIG. 3, when an operation command is input at time t4, which is the start time, the direct current setting circuit 70 outputs the direct current set value to the speed control circuit 60 for a certain period of time from time t4 to time t5, A direct current is passed through the stator coils of the motors MA and MB via the switching signal generation circuit 30. Thereby, as described above, the permanent magnet of each rotor 12 is attracted to a predetermined position with respect to each stator coil, and the positional relationship between the stator 11 and the rotor 12 of each motor MA, MB becomes equal and synchronized. Become. During this time, the output of the speed setting circuit 50 is held at zero.
[0028]
Next, at time t5, the output of the DC current setting circuit 70 is made zero, and at the same time, the output of the speed setting circuit 50 is set to V1 (the output of the speed setting circuit 50 is a voltage value, but the voltage value corresponding to the set speed V1 is also convenient. V1. V2 is the same) and is maintained until time t6. During this period t5 to t6, the motors MA and MB are operated at a low speed V1, so that even if the synchronous operation is difficult due to the high viscosity of the lubricating oil of the bearing, the difficulty is eliminated and the synchronization is sufficiently established. It becomes.
Then, after time t6, the speed setting circuit 50 gradually increases the set speed and accelerates to V2.
[0029]
As a result, the motors MA and MB can be accelerated while being reliably synchronized, and noise can be prevented from occurring.
As described above, the fixed time from time t4 to t5 and the fixed time from time t5 to t6 may be the same or different.
[0030]
Next, the invention described in claim 3 is configured such that the synchronization confirmation of the two motors MA and MB is performed more strictly.
That is, in the state where the motors MA and MB are rotating synchronously, the error of the rotor position detection signal of each motor MA and MB is almost eliminated. Accordingly, in the invention of claim 3, after starting, a direct current is passed through the stator coil for a certain period of time, and thereafter an error in the rotor position detection signal of each motor MA, MB is detected during low speed operation, and the error is below a specified value. When it came to it, it was judged that it reached the same state without fail, and acceleration was started.
[0031]
FIG. 4 is a circuit diagram showing a second embodiment of the present invention, and corresponds to the invention described in claim 3. The same components as those in FIG. 1 are denoted by the same reference numerals, and different portions will be mainly described below.
[0032]
In the second embodiment, the rotor position detection signals of the motors MA and MB are input to the position signal error detection circuit 80, and the result of determining whether the position detection signal error is greater than or equal to a specified value is output. Is added to the speed setting circuit 50.
Other configurations are the same as those in FIG.
[0033]
FIG. 5 shows the relationship among the outputs of the position signal error detection circuit 80, the speed setting circuit 50, and the direct current setting circuit 70.
In FIG. 5, when an operation command is input at time t7, which is a start time, the DC current setting circuit 70 outputs a DC current setting value to the speed control circuit 60 for a certain period of time from time t7 to time t8. A direct current is passed through the stator coils of the motors MA and MB via the switching signal generation circuit 30. Thereby, as described above, the positional relationship between the stator 11 and the rotor 12 of each of the motors MA and MB becomes equal and synchronized. During this time, the output of the speed setting circuit 50 is held at zero.
[0034]
Next, at time t8, the output of the DC current setting circuit 70 is made zero, and at the same time, the output of the speed setting circuit 50 is maintained at V1 for a while. After this time t8, since the motors MA and MB are operated at a low speed V1, the difficulty of synchronization due to the high viscosity of the lubricating oil in the bearing before starting is eliminated, and the motor MA and MB are sufficiently synchronized. Thereafter, when the error of the rotor position detection signal becomes equal to or less than the specified value at time t9, the output of the position signal error detection circuit 80 changes, and the speed setting circuit 50 determines that the motors MA and MB are synchronized and gradually increases the set speed. Increase the speed and finally accelerate to the set speed V2.
As a result, the acceleration can be started after the motors MA and MB have surely reached the synchronous state, so that there is no possibility of noise due to the rapid acceleration in the asynchronous state.
[0035]
FIG. 6 is a timing chart showing the operation in a state in which the motors MA and MB are slightly out of synchronization in this embodiment.
Errors in the rotor position detection signals of both motors MA and MB occur for each of the U phase, V phase, and W phase. The position signal error detection circuit 80 can use all of these three-phase errors. However, in order to avoid an increase in cost due to the complexity of the circuit configuration, if one-phase or two-phase errors are used. good.
The error of the rotor position detection signal can be easily detected from the exclusive OR of the rotor position detection signals of the motors MA and MB.
[0036]
As shown in FIG. 6, even when the motors MA and MB are slightly out of synchronization, the rotor position detection signal remains unchanged and remains constant, for example, the angle is between 15 ° and 60 °. 75 ° to 120 °, 135 ° to 180 °, 195 ° to 240 °, 255 ° to 300 °, 315 ° to 0 ° (360 °) 30 °, 90 ° , 150 °, 210 °, 270 °, and 330 °, the signal selection circuit 40 alternately selects the position detection signals of the motors MA and MB between the motors, and selects the motor based on the selected position detection signals. A switching signal for driving MA and MB is output.
[0037]
That is, the position detection signal of the motor MA is selected between 330 ° and 30 °, and the switching signal generation circuit 30 is energized to the U-phase coil CU and the W-phase coil CW based on this signal (periods are different). Outputs a switching signal. Further, the position detection signal of the motor MB is selected between 30 ° and 90 °, and the switching signal generating circuit 30 is energized to the U-phase coil CU and the W-phase coil CW based on this signal (periods are different). Create a switching signal.
Thereafter, similarly, a position detection signal of the motor MA is selected between 90 ° and 150 °, and based on this signal, the switching signal generating circuit 30 sends a switching signal so as to energize the U-phase coil CU and V-phase coil CV. The position detection signal of the motor MB is selected between 150 ° and 210 °. Based on this signal, the switching signal generation circuit 30 generates a switching signal so that the U-phase coil CU and the V-phase coil CV are energized. To do.
[0038]
When both the motors MA and MB are synchronized, the rotor position detection signal is as shown in FIG. 9, and the error of the rotor position detection signal is eliminated. In this embodiment, after the motors MA and MB are synchronized, the position detection signals of the motors MA and MB are alternately displayed between the motors by the signal selection circuit 40 at, for example, 30 °, 90 °, 150 °,. The switching signal generation circuit 30 may output the switching signal based on the selected position detection signal.
[0039]
A sine wave PWM control method is well known in which the switching element is PWM (pulse width modulation) controlled so that the applied voltages to the motors MA and MB are equivalently sine waves. If this method is applied to the switching signal generation circuit 30 and the switching element is driven by the PWM pulse, the motors MA and MB can be operated more stably.
In the above embodiment, the case where two DC brushless motors are operated in parallel has been described. However, the present invention can also be applied to a case where three or more motors are operated in parallel.
[0040]
【The invention's effect】
As described above, according to the present invention, a plurality of DC brushless motors can be stably driven in parallel with low noise by a single drive circuit, and a plurality of fans, pumps, etc. that require low noise. Can be provided at low cost.
[Brief description of the drawings]
FIG. 1 is a circuit diagram showing a first embodiment of the present invention.
FIG. 2 is an operation explanatory diagram of a speed setting circuit and a direct current setting circuit in FIG. 1;
FIG. 3 is an operation explanatory diagram of a speed setting circuit and a direct current setting circuit in FIG. 1;
FIG. 4 is a circuit diagram showing a second embodiment of the present invention.
5 is an operation explanatory diagram of a speed setting circuit, a position signal error detection circuit, and a direct current setting circuit in FIG. 4;
FIG. 6 is a timing chart showing an operation when there is an error in the rotor position detection signal of each motor.
FIG. 7 is a circuit diagram showing a conventional technique.
8 is a circuit diagram showing a configuration of a signal selection circuit in FIG. 7;
9 is a timing chart showing the operation of FIG.
[Explanation of symbols]
E DC power sources T1 to T6 Switching elements U, V, W Output terminals MA, MB DC brushless motors CU, CV, CW Coils HU, HV, HW Hall element 11 Stator 12 Rotor 21, 22 Rotor position detection circuit 30 Switching signal generation circuit 40 signal selection circuit 50 speed setting circuit 60 speed control circuit 70 DC current setting circuit 80 position signal error detection circuit

Claims (3)

In order to drive a plurality of DC brushless motors connected in parallel to each other at the same speed by a drive circuit having a plurality of semiconductor switching elements, a switching signal generating means uses the rotor position detection signal of each motor to A parallel drive circuit of a DC brushless motor that creates a switching signal,
While the rotor position detection signal of each motor does not change, the rotor position detection signal used for generating the switching signal is switched between the motors, and the switching signal is generated using the rotor position detection signal of the motor after switching. In addition, in a parallel drive circuit including signal selection means for outputting a control signal to the switching signal generation means,
Means for passing a direct current through the stator coil of each motor for a certain period after startup;
A parallel drive circuit for a DC brushless motor, comprising: speed control means for accelerating each motor to a predetermined speed after the predetermined period has elapsed. In order to drive a plurality of DC brushless motors connected in parallel to each other at the same speed by a drive circuit having a plurality of semiconductor switching elements, a switching signal generating means uses the rotor position detection signal of each motor to A parallel drive circuit of a DC brushless motor that creates a switching signal,
While the rotor position detection signal of each motor does not change, the rotor position detection signal used for generating the switching signal is switched between the motors, and the switching signal is generated using the rotor position detection signal of the motor after switching. In addition, in a parallel drive circuit including signal selection means for outputting a control signal to the switching signal generation means,
Means for passing a direct current through the stator coil of each motor for a certain period after startup;
A parallel drive circuit for a DC brushless motor, comprising: a speed control unit that operates each motor at a low speed after a predetermined period of time, and thereafter accelerates to a predetermined speed. In order to drive a plurality of DC brushless motors connected in parallel to each other at the same speed by a drive circuit having a plurality of semiconductor switching elements, a switching signal generating means uses the rotor position detection signal of each motor to A parallel drive circuit of a DC brushless motor that creates a switching signal,
While the rotor position detection signal of each motor does not change, the rotor position detection signal used for generating the switching signal is switched between the motors, and the switching signal is generated using the rotor position detection signal of the motor after switching. In addition, in a parallel drive circuit including signal selection means for outputting a control signal to the switching signal generation means,
Means for passing a direct current through the stator coil of each motor for a certain period after startup;
Position signal error detection means for detecting an error in the rotor position detection signal of each motor;
Speed control means for operating each motor at a low speed after the predetermined period has elapsed, and accelerating each motor to a predetermined speed after an error detected by the position signal error detection means during the low speed operation falls below a specified value, A parallel drive circuit for a DC brushless motor, comprising:

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