JPH11164584A - Motor control device - Google Patents

Abstract

(57) [Summary] [Problem] A brushless DC that rotationally drives a fluctuating load
Provided is a motor control device that can smoothly accelerate a motor with a small starting current. SOLUTION: Before the brushless DC motor 1 is started, a voltage that does not exceed a peak of a load composed of a reciprocating compressor is controlled by a voltage control circuit 7 so as to prevent the motor 1 from operating.
And the motor 1 is rotated in the reverse direction, and when the motor 1 idles and stops just before the peak of the load, the motor 1 is synchronously started in the forward direction from this stop position.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a motor control device for controlling a brushless DC motor for rotating a variable load such as a reciprocating compressor.

[0002]

2. Description of the Related Art In general, a brushless DC motor is started by generating a rotating magnetic field by switching windings energized at a relatively low frequency, and synchronizing a rotor with the rotating magnetic field while gradually accelerating. If the energization phase of the winding and the phase of the induced voltage deviate, an overcurrent flows or vibrates according to the difference between the two voltages. Therefore, in order to prevent such a problem, conventionally, for example, as disclosed in JP-A-3-235694 and JP-A-61-1290, the positioning of the rotor by the fixed magnetic field before the start of the motor is performed. To prevent phase shift immediately after startup.

[0003] A curve shown in FIG. 4 shows a torque fluctuation during one rotation of a reciprocating compressor as a load (torque) with respect to a rotation angle. One reciprocating compressor
Since there is a compression step and a suction step during rotation, a large load fluctuation occurs as shown in FIG.

[0004]

When the DC motor is started, it is necessary to accelerate smoothly as described above. For this purpose, it is preferable to start the DC motor from the vicinity indicated by a circle with a small load in FIG. . That is, it is possible to accelerate by the next compression step. For example, when starting from the vicinity indicated by the cross mark in FIG. Cannot be synchronized with the rotating magnetic field. ○ in FIG.
In the case where the positioning is performed in the vicinity of the mark, there is a possibility that the positioning may be performed in the vicinity of the mark in FIG.
There is a problem that the acceleration is hindered by the compression load immediately after the start, and a smooth start cannot be performed.

[0005] The present invention has been made in view of the above,
An object of the present invention is to provide a motor control device capable of smoothly accelerating a brushless DC motor that rotationally drives a fluctuating load with a small starting current.

[0006]

According to one aspect of the present invention, there is provided a motor control device for controlling a brushless DC motor that rotationally drives a fluctuating load. Voltage applying means for applying a predetermined voltage to the motor to reversely rotate the motor; detecting means for detecting that the motor is reversely rotated by the voltage applying means and stopped at a predetermined position; And a synchronous starting means for synchronously starting the motor in the forward direction from the stop position when detecting that the motor has stopped.

According to the first aspect of the present invention, a predetermined voltage is applied to the motor before the motor is started to rotate the motor in the reverse direction, and it is detected that the motor has stopped at the predetermined position after the motor has rotated in the reverse direction. In this case, since the motor is synchronously started in the forward direction from the stop position, the rotor can be positioned at a position where acceleration is easy at the time of starting, and good acceleration can be performed.

According to a second aspect of the present invention, there is provided a motor control device for controlling a brushless DC motor that rotationally drives a fluctuating load, wherein the motor does not exceed a peak of the fluctuating load before the motor is started. Voltage applying means for applying a voltage to the motor to reversely rotate the motor, and the voltage applying means reversely rotating the motor, and the motor idles and stops just before the peak of the load. And a synchronous starting means for synchronously starting the motor in a forward direction from the stop position when the detecting means detects that the motor has stopped.

According to the second aspect of the present invention, before starting the motor, a voltage that does not exceed the peak of the load is applied to the motor and the motor is rotated in the reverse direction, so that the motor idles just before the peak of the load. When it is detected that the motor has stopped, the motor is synchronously started in the forward direction from this stop position.At the time of starting, the rotor is positioned at a position where acceleration is easy, and good acceleration can be performed. The phases of the applied voltages match, and starting can be performed with a small current.

Further, the present invention according to claim 3 provides the present invention as claimed in claim 2.
In the invention described in the above aspect, the gist of the invention is that the voltage applying means includes voltage increasing means for gradually increasing a voltage applied to the motor from a small value to a voltage not exceeding a peak portion of the load.

According to the third aspect of the present invention, since a voltage that gradually increases from a small value below a voltage that does not exceed the peak portion of the load is applied to the motor, the starting voltage at which the induced voltage is 0 is reduced. While preventing the occurrence of overcurrent,
The applied voltage can be set large, and positioning can be performed without perfect pressure balance.

According to a fourth aspect of the present invention, in the second aspect of the present invention, the voltage applying means reduces the voltage applied to the motor when the detecting means detects the stop of the motor. The point is to have means.

According to the fourth aspect of the present invention, when the stop of the motor is detected, the voltage applied to the motor is reduced, so that the operation of the overcurrent protection circuit can be prevented and the failure in starting can be reduced.

According to a fifth aspect of the present invention, there is provided a motor control device for controlling a brushless DC motor that rotationally drives a fluctuating load, wherein the motor controller includes a voltage lower than a voltage not exceeding a peak portion of the fluctuating load. Storage means for applying a gradually increasing voltage to the motor to rotate the motor in reverse, and for storing the motor applied voltage when the motor exceeds the peak of the load as a load point applied voltage; A voltage applying means for applying a voltage not exceeding the peak portion of the fluctuating load to the motor and rotating the motor in the reverse direction, and rotating the motor in the reverse direction by the voltage applying means; Detecting means for detecting that the motor has stopped running idling in front of the peak portion, and detecting that the motor has stopped by the detecting means. Synchronous start means for applying a voltage to the motor to start synchronously in the forward direction, applying a voltage equal to or higher than the load point applied voltage to the motor after one rotation of the rotor, and accelerating the motor with a large torque. Make a summary.

According to the present invention, the voltage applied to the motor when the motor exceeds the peak of the load is stored as a load point applied voltage, and the voltage applied to the peak of the load is exceeded before the motor is started. When the motor is rotated in the reverse direction by applying a voltage of an insignificant degree, and when it is detected that the motor has stopped idling before the peak of the load, a voltage smaller than the voltage applied to the load point is applied to the motor from this stop position. To start synchronously in the forward direction and apply a voltage higher than the load point applied voltage to the motor after one rotation of the rotor to accelerate the motor with a large torque, eliminating the need for a pressure sensor and shortening the restart prohibition period can do.

Further, according to the present invention, there is provided a motor control device for controlling a brushless DC motor for rotationally driving a fluctuating load, wherein the motor does not exceed a peak of the fluctuating load before the motor is started. Voltage applying means for applying a gradually increasing voltage from a small voltage not more than about the voltage to the motor to rotate the motor in a forward direction, and rotating the motor by the voltage applying means, and the motor After idling in front of the peak, the detecting means for detecting that the motor has exceeded the load peak by further increasing the motor applied voltage, and detecting that the motor has passed the load peak by the detecting means. From the point in time, apply a large voltage to the motor,
Synchronous starting means for synchronously starting the motor is provided.

According to the present invention, before the motor is started, a voltage gradually increasing from a small voltage is applied to the motor to rotate the motor in the forward direction, and the motor idles just before the load peak. Later, when it is detected that the motor has exceeded the peak of the load due to the increase in the motor applied voltage, a large voltage is applied to the motor and the motor is started synchronously, enabling rapid acceleration and prohibiting restart. The period can be shortened.

The present invention according to claim 7 provides the invention according to claims 1 to 6
In the invention according to any one of the first to third aspects, the load may be a reciprocating compressor.

[0019]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a diagram showing a configuration of a motor control device according to one embodiment of the present invention. The motor control device shown in FIG. 1 drives and controls a three-phase four-pole brushless / senseless DC motor 1 that does not use a sensor such as a Hall IC, and the load is varied by the brushless DC motor 1. For example, when rotating a reciprocating compressor, the drive can be controlled with a low start-up current and a low start-up trip (low start-up failure), and a DC voltage of 280 VDC is chopped by an inverter circuit 3 including a plurality of switching elements. Then, the torque is applied to the motor 1, and the torque of the motor 1 is controlled by the duty ratio of ON / OFF of the applied voltage.

An overcurrent detection circuit 5 is connected in series to one line for supplying a DC voltage to the inverter circuit 3, and the current supplied to the inverter circuit 3 by the overcurrent detection circuit 5 is larger than a predetermined value. When the value becomes a value, the overcurrent state is detected, and the overcurrent detection signal is supplied to the voltage control circuit 7, whereby the operation of the inverter circuit 3 is stopped.

A rotation detecting circuit 9 is connected to a connection line between the motor 1 and the inverter circuit 3, and detects the rotation of the motor 1 by the rotation detecting circuit 9. Supply to the circuit 7. The voltage control circuit 7 controls the reverse rotation torque or the forward torque of the motor 1 and has an energization switching circuit 7 therein.
a, and controls each switching element of the inverter circuit 3 so as to switch the winding through which the current of the motor 1 flows by the current supply switching circuit 7a.

The drive voltage pattern of the inverter circuit 3 in the motor control device configured as described above, that is, the voltage pattern applied to the motor 1 and the induced voltage waveform at the time of normal rotation of the motor 1 are shown by a bold rectangular in FIG. The phases of the two voltages coincide with each other so that the waveform indicates the applied voltage and the thin sine wave curve indicates the induced voltage. That is, since the torque T of the motor 1 is T = BIL (B: magnetic flux density, I: current, L: effective winding length of the motor), the induced voltage (= magnetic flux density of the rotor) and the phase of the applied voltage are Preferably, they match as shown in FIG.

More specifically, as shown in FIG.
The “applied voltage−induced voltage” is applied to the winding of the motor 1, and the “applied voltage−induced voltage” determines the current flowing through the winding of the motor 1. By supplying a current to the winding at a timing with a high density, a predetermined torque can be obtained with a small current.

As shown in FIG. 3 (b), when the timing of the phase of the applied voltage and the phase of the induced voltage are slightly shifted, since the voltage is applied at a timing with a low magnetic flux density, the same torque is output. Current increases. FIG.
As shown in (c), when the applied voltage and the induced voltage deviate greatly, a reverse rotation torque is generated in the motor 1 and a large vibration is generated.

That is, in order to reduce the starting current of the motor 1, it is necessary to match the phases of the induced voltage and the applied voltage as shown in FIG.

In general, a DC motor is started by generating a rotating magnetic field by switching windings to be energized at a relatively low cycle, and synchronizing the rotor with the rotating magnetic field while gradually accelerating. If the induced voltage phase shifts, an overcurrent will flow or vibration will occur, so if these become a problem, perform alignment using a fixed magnetic field as in the conventional example to prevent phase shift immediately after startup. I have.

FIG. 4 shows a variation in torque during one rotation of the compressor when a reciprocating compressor is driven by a brushless DC motor, in which the horizontal axis represents the rotation angle, and the vertical axis represents the load (torque). In addition to the curves shown in the graph, the directions of the currents flowing in the U-phase, V-phase, and W-phase windings of the three-phase four-pole DC motor in this case are shown. A reciprocating compressor has a large load fluctuation because of a compression step and a suction step during one rotation. In starting the DC motor,
As described above, it is necessary to accelerate smoothly. For this purpose, it is preferable to start the engine from the vicinity indicated by a circle in FIG. This is because it is possible to accelerate by the next compression step.For example, when starting from the vicinity indicated by the cross mark, acceleration is hindered by the compression load immediately after startup, and synchronization with the rotating magnetic field of the rotor is lost. Become. In FIG. 4, when the positioning is performed in the vicinity indicated by a circle, when the fixed winding is excited as in the conventional example, in the case of a four-pole rotor, the position is fixed in the vicinity indicated by a 180 ° shifted Δ. There is. FIG. 4 shows an example in which a current is applied to the U-phase to V-phase windings to perform DC excitation.

Therefore, in the motor control device of the present embodiment, a rotation torque not exceeding the maximum load point existing immediately before the vicinity indicated by the circle in FIG. 4 is applied to the voltage control circuit 7, and the reverse rotation is applied to the energization switching circuit 7a. By giving a command, a reverse rotation magnetic field is formed, and the rotor of the motor 1 is caused to idle by about 1 second near the point indicated by the circle immediately after the maximum load point, and then the rotor is rotated in the order of larger torque than the reverse rotation torque. The torque is switched to the rotation torque, and the motor 1 is synchronously started in the forward direction.

That is, in the motor control device of the present embodiment, a voltage that does not exceed the maximum load point, which is the peak of the variable load, which is a reciprocating compressor, is applied to the motor 1 before the motor 1 starts. After the motor 1 is rotated in the reverse direction to let the motor 1 idle for about one second before the maximum load point, the motor 1 is synchronously started in the forward direction from this position.

As a result, by positioning the rotor at a position where it is easy to accelerate at the time of startup, good acceleration can be obtained, the phase of the induced voltage matches the phase of the applied voltage, and the motor 1 can be started with a small current. Therefore, the current capacity of each drive element constituting the inverter circuit 3 can be reduced, and the cost can be reduced. In addition, starting failures are reduced, and vibration can be reduced.

FIG. 5 is a graph showing a change in load (torque) with respect to a rotation angle for explaining a motor control device according to another embodiment of the present invention, and a voltage applied to the motor 1 corresponding to the curve. FIG.

In the embodiment shown in FIG. 5, the reverse rotation torque is gradually increased from a small value to a value not exceeding the maximum load point of the compressor at the time of positioning the rotor. The applied voltage gradually increases from a small value to a large value as shown.

More specifically, the position of the rotor before starting is
It is undefined in the section P of FIG. For example, the torque required to move the rotor from the position in FIG.
As shown in FIG. 5, T1 <rotation torque <T2, and a corresponding voltage (Vst) is applied to the voltage control circuit 7. At this time, the voltage actually applied to the winding is “applied voltage−induced voltage” when the rotor starts to rotate in the reverse direction.
When Vst / R (R = winding resistance) is set and the voltage Vst is set high, an overcurrent flows and the overcurrent detection circuit 5 operates, so that the voltage Vst cannot be set high. Therefore, the applied voltage at the time of starting is set small, and is gradually increased to a value not exceeding the torque T2 at the maximum load point.

That is, in the motor control device according to the present embodiment, before the motor 1 is driven, when the voltage of the reciprocating type compressor is not higher than the maximum load point which is the peak portion of the fluctuating load as shown in FIG. A voltage gradually increasing from a small value as shown by a voltage is applied to the motor 1 to rotate the motor 1 in a reverse direction. After the motor 1 idles just before the maximum load point, the motor 1 is synchronously started in the forward direction from this position. I have.

As a result, in the present embodiment, the applied voltage can be set large while the rotor is rotating in the reverse direction, so that the positioning can be performed without perfect pressure balance.

FIG. 6 is a graph showing a change in load (torque) with respect to a rotation angle for explaining a motor control device according to still another embodiment of the present invention, and a voltage applied to the motor 1 corresponding to the curve. FIG.

In the embodiment shown in FIG. 6, when the rotor is rotated in the reverse rotation position, the arrival at the maximum load point is detected, and the reverse rotation torque is reduced to a level at which the overcurrent detection circuit 5 does not operate when the rotor reaches the maximum load point. Things.

More specifically, the rotor position before starting is shown in FIG.
Is undefined in the section of P in FIG. For example, when the rotor is moved from the position of FIG. 6 to the vicinity, when the rotor is rotating in the reverse direction, “applied voltage−induced voltage”. However, when the load reaches the maximum point and starts idling, the induced voltage becomes 0, The flowing current I is
I = V / R (R = winding resistance), and the overcurrent detection circuit 5
May operate. Therefore, it is possible to prevent the overcurrent detection circuit 5 from operating by reducing the applied voltage when the load reaches the maximum point.

That is, in the motor control device of this embodiment, before the motor 1 is started, when the voltage of the reciprocating type compressor shown in FIG. A voltage that gradually increases from a small value as shown by the voltage is applied to the motor 1 and the motor 1 is rotated in the reverse direction. After the motor 1 idles just before the maximum load point, the voltage applied to the motor 1 is reduced. Are started synchronously in the forward direction.

As a result, in the present embodiment, the operation of the overcurrent detection circuit 5 can be prevented, and starting failure can be reduced.

FIG. 7 is a graph showing a change in load (torque) with respect to a rotation angle for explaining a motor control device according to another embodiment of the present invention. FIG. 6 is a diagram illustrating the time of reverse rotation and the time of normal rotation.

In the embodiment shown in FIG.
The applied voltage when passing through the maximum load point is stored, the starting load is estimated from the stored value, and the required forward torque is calculated.

More specifically, the reverse rotation alignment is performed as described above. That is, the applied voltage at startup is small,
Then, the applied voltage is gradually increased until the rotor exceeds the maximum load point, and the applied voltage (Vmax) when the rotor exceeds the maximum load point is stored. This voltage Vmax is substantially the same as the forward torque required to exceed the maximum load when switching to forward rotation next time. Since the rotor is not rotating at the moment of switching to the forward direction, the applied voltage is reduced, and the forward torque is increased so that the voltage becomes equal to or higher than the voltage Vmax after one rotation of the rotor.

Generally, when a load torque before starting is required, a Pd / Ps sensor is provided. However, in this embodiment, a pressure sensor is not required, and cost can be reduced. Further, even when the pressure difference of Pd / Ps is high to some extent during the operation, the operation is stopped, and the operation is further started, the waiting time until the pressure is balanced can be reduced. That is, the restart prohibition time can be reduced.

FIG. 8 is a graph showing a change in load (torque) with respect to a rotation angle for explaining a motor control device according to still another embodiment of the present invention, and a voltage applied to the motor 1 corresponding to the curve. FIG.

In the embodiment shown in FIG. 8, at the time of startup, the vehicle is positioned at the maximum load point with forward torque, and when the passage of the maximum load point is detected, the vehicle is accelerated with a torque larger than the positioning torque up to that point. .

More specifically, the positioning is performed at the maximum load point with the forward torque. The applied voltage at the time of starting is reduced, and then the applied voltage is gradually increased until the rotor exceeds the maximum load point. From the time when the rotor exceeds the maximum load point, as shown by the applied voltage in FIG. Increase. That is, assuming that the voltage increase rate at the time of positioning until exceeding the maximum load point is S1 (S1 = Vstep / T), the voltage increase rate S2 after exceeding the maximum load point is S2 ≧ S1. During alignment, the rotor idles around the point of maximum load, but gradually approaches the point of maximum load by gradually increasing the applied voltage. At the moment when the voltage is further increased and exceeds the maximum load point, the phase of the induced voltage of the rotor and the phase of the applied voltage should be synchronized, so that even if the applied voltage is increased, an excessive current flows. And good induction characteristics can be obtained.

In this embodiment, the reliability can be improved even if the lubricating oil does not act sufficiently in reverse rotation due to the structure of the compressor. Further, according to the present embodiment, even when it is necessary to reach a certain number of rotations in a short time at the time of starting, rapid acceleration is possible because the phases of the induced voltage and the applied voltage are synchronized. Furthermore, at the time of starting, even if the pressure difference is high to some extent, it can be started, and the waiting time until the pressure is balanced can be reduced. That is, the restart prohibition time can be reduced.

[0050]

As described above, according to the first aspect of the present invention, a predetermined voltage is applied to the motor before the motor is started to reversely rotate the motor, and the motor idles at a predetermined position and stops. In this case, since the motor is synchronously started in the forward direction from this stop position, the rotor can be positioned at a position where acceleration is easy at the time of start, and good acceleration can be performed. In addition, the phases of the induced voltage and the applied voltage match. It can be started with a small current. Therefore, the current capacity of the switching element constituting the inverter circuit can be reduced, and the cost can be reduced. Further, the number of startup failures is reduced, and vibration can be reduced.

According to the second aspect of the present invention, before starting the motor, a voltage that does not exceed the peak of the load is applied to the motor and the motor is rotated in the reverse direction, so that the motor idles just before the peak of the load. When the motor stops, the motor is synchronously started in the forward direction from this stop position, so that at the time of starting, the rotor can be positioned at a position where acceleration is easy, and good acceleration can be performed. And can be started with less current. Therefore, the current capacity of the switching element constituting the inverter circuit can be reduced, and the cost can be reduced. Further, the number of startup failures is reduced, and vibration can be reduced.

According to the third aspect of the present invention, since a voltage that gradually increases from a small value below the voltage that does not exceed the peak of the load is applied to the motor, the starting voltage in which the induced voltage is zero is applied. In addition to preventing the occurrence of overcurrent at the time, the applied voltage can be set large while the rotor is rotating in the reverse direction, and positioning can be performed without perfect pressure balance.

According to the present invention, when the stop of the motor is detected, the voltage applied to the motor is reduced, so that the operation of the overcurrent protection circuit can be prevented and the failure in starting can be reduced.

According to the present invention, the motor applied voltage when the motor exceeds the load peak is stored as a load point applied voltage, and the load peak is stored before the motor is started. When the motor rotates in reverse by applying a voltage that does not exceed it, and when the motor stops running idle before the peak of the load, a voltage smaller than the applied voltage at the load point is applied to the motor from this stop position and forward direction. The motor is started synchronously with the motor, and after one rotation of the rotor, a voltage higher than the voltage applied to the load point is applied to the motor to accelerate the motor with a large torque. The period can be shortened.

According to the present invention, before the motor is started, a voltage gradually increasing from a small voltage is applied to the motor to rotate the motor in the forward direction, and after the motor idles just before the peak portion of the load. Further, when it is detected that the motor has exceeded the peak of the load due to an increase in the motor applied voltage, a large voltage is applied to the motor and the motor is started synchronously. Reliability can be improved when oil does not act sufficiently. Also, at startup,
Even when it is necessary to reach a certain number of rotations in a short time, the phase of the induced voltage and the applied voltage are synchronized, enabling rapid acceleration and starting even if the pressure difference at start-up is somewhat high. Waiting time can be shortened. That is, the restart prohibition period can be shortened.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of a motor control device according to one embodiment of the present invention.

FIG. 2 is a waveform diagram showing a voltage pattern applied to a motor and an induced voltage waveform at the time of normal rotation of the motor when the phases of the two voltages match.

FIG. 3 is an explanatory diagram for explaining a voltage applied to a winding of a motor.

FIG. 4 shows the fluctuation of torque during one rotation of the compressor when a reciprocating compressor is driven by a brushless DC motor, with the horizontal axis representing the rotation angle and the vertical axis representing the load (torque).
FIG. 7 is a graph showing the directions of currents flowing in the U-phase, V-phase, and W-phase windings of the three-phase four-pole DC motor in this case.

FIG. 5 is a graph showing a change in load (torque) with respect to a rotation angle for explaining a motor control device according to another embodiment of the present invention, and shows a voltage applied to the motor corresponding to the curve. FIG.

FIG. 6 shows a load (torque) with respect to a rotation angle for explaining a motor control device according to still another embodiment of the present invention.
FIG. 7 is a diagram showing the change of the curve in a curve and the voltage applied to the motor corresponding to the curve.

FIG. 7 is a graph showing a change in load (torque) with respect to a rotation angle for explaining a motor control device according to another embodiment of the present invention, and applying an applied voltage to the motor during reverse rotation according to the curve; FIG. 9 is a diagram illustrating a case of normal rotation.

FIG. 8 shows a load (torque) with respect to a rotation angle for explaining a motor control device according to still another embodiment of the present invention.
FIG. 7 is a diagram showing the change of the curve in a curve and the voltage applied to the motor corresponding to the curve.

[Explanation of symbols]

 Reference Signs List 1 brushless DC motor 3 inverter circuit 5 overcurrent detection circuit 7 voltage control circuit 7a conduction switching circuit 9 rotation detection circuit

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Mitsuo Takahashi 3-3-9, Shimbashi, Minato-ku, Tokyo Inside Toshiba AV EE Co., Ltd.

Claims (7)

[Claims] 1. A motor control device for controlling a brushless DC motor that rotationally drives a fluctuating load, comprising: voltage application means for applying a predetermined voltage to the motor before starting the motor to reversely rotate the motor. Detecting means for detecting that the motor is rotated in the reverse direction and then stopping at a predetermined position by the voltage applying means; and detecting that the motor is stopped by the detecting means, the motor is sequentially stopped from the stopped position. A motor control device comprising: synchronous start means for synchronously starting in directions. 2. A motor control device for controlling a brushless DC motor that rotationally drives a fluctuating load, wherein a voltage that does not exceed a peak of the fluctuating load is applied to the motor before starting the motor. Voltage applying means for reversely rotating the motor, and detecting means for reversely rotating the motor by the voltage applying means to detect that the motor has stopped idling before the peak of the load. And a synchronous starter for synchronously starting the motor in a forward direction from the stop position when the motor detects that the motor has stopped. 3. The voltage applying means includes voltage increasing means for gradually increasing a voltage applied to the motor from a small value below a voltage not exceeding a peak of the load. 3. The motor control device according to 2. 4. The motor control according to claim 2, wherein said voltage applying means has a voltage reducing means for reducing a voltage applied to said motor when said detecting means detects a stop of said motor. apparatus. 5. A motor control device for controlling a brushless DC motor that rotationally drives a fluctuating load, wherein a voltage that gradually increases from a small voltage that does not exceed a peak of the fluctuating load is applied to the motor. Storage means for storing the applied voltage of the motor as a load point applied voltage when the motor exceeds the peak of the load by applying the reverse rotation of the motor; and the peak of the fluctuating load before the motor is started. Voltage applying means for applying a voltage of not more than about to the motor to reversely rotate the motor, the voltage applying means causing the motor to rotate in the reverse direction, and the motor idles just before the peak of the load. Detecting means for detecting that the motor has stopped; and detecting, when the detecting means detects that the motor has stopped, a voltage smaller than the voltage applied to the load point from the stop position. To start synchronization in the forward direction, the motor control apparatus characterized by comprising a synchronization starting means for accelerating the motor with a large torque the load point applied voltage or more after one rotation of the rotor is applied to the motor. 6. A motor control device for controlling a brushless DC motor that rotationally drives a fluctuating load, wherein the motor gradually starts from a small voltage of a voltage not exceeding a peak of the fluctuating load before starting the motor. Voltage applying means for applying an increasing voltage to the motor to rotate the motor in the forward direction, and rotating the motor by the voltage applying means, and after the motor idles just before the peak of the load, Detecting means for detecting that the motor has exceeded the peak of the load by increasing the voltage applied to the motor; and applying a large voltage to the motor from the time when the detection means detects that the motor has exceeded the peak of the load. And a synchronous starting means for synchronously starting the motor. 7. The motor control device according to claim 1, wherein the load is a reciprocating compressor.