JP2003088158A - Motor controller - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a motor controller which can suppress the deterioration of an efficiency for supplying a power to a synchronous motor and the vibration of the synchronous motor and control a revolution of the motor, at a low cost. SOLUTION: During a half period from a zero-cross point (t0 ) to a next zero-cross point (t1 ) of an output voltage (a) of an AC power supply, the AC power supply is made to be electrically continuous with a motor to apply a voltage to a motor. After an interval of one period (t1 -t3 ) of the output voltage of the AC power supply during a half period from the zero-cross point (t3 ) to a next zero-cross point (t4 ), the AC power supply is made to be electrically continuous with the motor again to apply a voltage to the motor. After an interval of one period (t4 -t6 ) of the output voltage of the AC power supply, the AC power supply is made to be electrically continuous with the motor again. By repeating this procedure, an AC voltage (b) having a frequency (1/T1 ) one third of the frequency (1/T0 ) of the output voltage of the AC power supply is applied to the motor. With such a constitution, the motor can be turned with a revolution synchronizing with a frequency one third of the frequency of the output voltage of the AC power supply.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention changes the frequency of an AC voltage supplied from an AC power source to a motor,
The present invention relates to a motor control device that controls the rotation speed of the motor.

[0002]

2. Description of the Related Art Conventionally, as a motor control device for controlling the number of revolutions of a motor that rotates in synchronization with the frequency of the output voltage of the AC power source using the AC power source as a driving source, the AC voltage output from the AC power source is used. Is known as an inverter type motor control device that changes the rotation speed of the motor by generating a driving AC voltage of a desired frequency by switching the DC voltage to a DC voltage and applying the driving AC voltage to the motor. Has been.

However, in order to construct an inverter type motor control device, a relatively large capacity rectifying element and a switching element are required, so that the cost of the motor control device becomes high.

Therefore, in a device such as a blower fan of a warm air heater that does not require highly accurate rotation speed control, a current control element such as a triac is connected between an AC power supply and a motor to output the AC power supply. In each half cycle of the voltage waveform, the current control element is in the conducting state (from the start point of the half cycle (0 cross point) to the end point of the half cycle (the next 0 cross point) from the time point when the predetermined angle phase advances. ON state),
So-called phase control in which the average current supplied to the motor is changed by changing the predetermined angle is generally adopted.

However, in the case of the phase control, the rotation speed of the motor is controlled by changing the average current supplied to the motor to change the output torque of the motor. Therefore, when the motor is operated at a rotation speed lower than the rotation speed synchronized with the frequency of the output voltage of the AC power supply, the rotor of the motor rotates while slipping with a delay with respect to the phase of the output voltage of the AC power supply.

In this case, the lower the number of revolutions of the motor, the greater the degree of slippage of the motor, which deteriorates the efficiency of power supply to the motor, and the heat generation of the motor. Further, in the phase control, a relatively large inrush current is generated when the AC power supply and the motor are conducted by the current control element, so that it is necessary to use a current control element having a large allowable current. There is an inconvenience that vibration and noise from the motor are likely to occur.

[0007]

SUMMARY OF THE INVENTION The present invention solves the above inconveniences and suppresses the deterioration of the efficiency of power supply to a synchronous motor and the occurrence of vibration of the motor to control the rotational speed of the motor. An object is to provide a control device at low cost.

[0008]

The present invention has been made to achieve the above object, and an AC power source and a motor that rotates in synchronization with the frequency of the AC voltage supplied are connected to each other, and the AC power source is connected. The present invention relates to a motor control device including rotation control means for controlling the rotation speed of the motor by applying to the motor a motor drive voltage in which the frequency of the output voltage is changed.

The first aspect of the present invention comprises a zero-cross point detecting means for detecting a zero-cross point of the output voltage of the AC power source, and a connecting means for connecting the AC power source and the motor via a switching means. The rotation control means is configured to keep the switching means conductive until the half cycle of the output voltage elapses from the time when the zero-cross point of the output voltage of the AC power supply is detected by the zero-cross point detection means. Voltage is applied to the motor from the AC power source to the motor by turning off the switching means until a predetermined period, which is an integral multiple of one period of the output voltage, elapses after the half period elapses. And the switching means is turned on until the half cycle of the output voltage elapses after the predetermined cycle elapses. By applying a voltage from the power supply to the motor and repeating the process of prohibiting the application of the voltage from the AC power supply to the motor by turning off the switching means until the predetermined period elapses from the lapse of the half cycle. The motor drive voltage with the frequency of the output voltage changed is applied to the motor.

According to the present invention, as will be described in detail later, the rotation control means sets the output voltage of the AC power supply at intervals of the predetermined cycle which is an integral multiple of one cycle of the output voltage of the AC power supply. The voltage for the positive half cycle and the voltage for the negative half cycle are alternately applied to the motor. Therefore, when the predetermined cycle is 1 time, 2 times, 3 times, ... One cycle of the output voltage of the AC power supply, the cycle of the voltage applied to the motor is respectively the AC power supply. 3 times, 5 times, 7 times, ...

As a result, the rotation control means is 1/3 times, 1/5 times, 1 / times the frequency of the output voltage of the AC power supply.
The motor can be rotated at a rotation speed synchronized with the frequency of 7 times. In this way, when controlling the number of rotations of the motor by changing the frequency of the voltage applied to the motor, slippage at the time of rotation of the motor decreases, so It is possible to increase the efficiency of the power supply and suppress the heat generation of the motor.

A second aspect of the present invention is a zero-cross point detecting means for detecting a zero-cross point of the output voltage of the AC power supply, and a connecting means for connecting the AC power supply and the motor through a switching means. Polarity control means for inverting the polarity of the output voltage of the AC power supply, the rotation control means, half the output voltage from the time when the zero-cross point of the output voltage of the AC power supply is detected by the zero-cross point detection means. Until the cycle elapses, the switching means is turned on to apply a voltage from the AC power supply to the motor, and the switching is performed from the time when the half cycle elapses until a predetermined cycle that is an odd multiple of the half cycle of the output voltage elapses. The means is turned off to prohibit the application of voltage from the AC power supply to the motor, and a half cycle of the output voltage elapses from the time when the predetermined cycle elapses. Until the polarity reversal means reverses the polarity of the output voltage of the AC power supply and applies the voltage whose polarity is reversed from the AC power supply to the motor while the switching means is in a conducting state, from the time when the half cycle elapses. By repeating the processing of prohibiting the application of the voltage from the AC power supply to the motor by turning off the switching means until the predetermined period elapses, the motor drive voltage in which the frequency of the output voltage is changed is applied to the motor. It is characterized by applying.

According to the present invention, as will be described in detail later, the rotation control means sets the interval of the predetermined cycle obtained by multiplying the half cycle of the output voltage of the AC power supply by an integer multiple of the half cycle of the output voltage of the AC power supply. On the other hand, a positive half cycle voltage and a negative half cycle voltage of the output voltage of the AC voltage are alternately applied to the motor. Therefore, the predetermined period is 1 time, 3 times, 5 times the half cycle of the output voltage of the AC power supply,
.., the cycle of the voltage applied to the motor is 2 times the cycle of the output voltage of the AC power supply.
Double, 4 times, 6 times, ...

As a result, the rotation control means is 1/2 times, 1/4 times, 1 / times the frequency of the output voltage of the AC power supply.
The motor can be rotated at a rotation speed synchronized with the frequency of 6 times, .... Therefore, similarly to the above-described first aspect, the efficiency of power supply from the AC power supply to the motor can be improved and heat generation of the motor can be suppressed.

In the second aspect, the rotation speed control means may operate from the time when the zero cross point of the output voltage of the AC power supply is detected by the zero cross point detection means until a half cycle of the output voltage elapses. , Applying a voltage from the AC power supply to the motor with the switching means in a conductive state, and turning off the switching means until a predetermined cycle which is an integral multiple of one cycle of the output voltage has elapsed from the time point of the half cycle. Application of voltage from the AC power supply to the motor is prohibited, and voltage is applied from the AC power supply to the motor by turning on the switching means until a half cycle of the output voltage elapses after the predetermined cycle elapses. , The switching means is turned off until the predetermined period elapses after the half period elapses. By repeating the process of prohibiting the application of a voltage to, and applying the motor drive voltage changes the frequency of the output voltage to the motor.

According to the present invention, the rotation control means sets the frequency of the motor drive voltage applied to the motor to 1/2 times the frequency of the output voltage of the AC power supply, 1
Since it can be changed to / 3 times, 1/4 times, 1/5 times, 1/6 times, ..., It is possible to rotate the motor at the number of rotations synchronized with the frequency of more stages. it can.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described with reference to FIGS. 1 is a configuration diagram of a motor control device according to a first embodiment of the present invention, FIG. 2 is an explanatory diagram of an output waveform of a motor drive voltage, and FIG. 3 is a motor control device according to a second embodiment of the present invention. It is a block diagram.

First, a first embodiment of the present invention will be described with reference to FIGS. As shown in FIG. 1, the motor control device 1 (motor control device of the present invention) of the present embodiment is connected to an AC power supply 2 and a motor 3 that rotates in synchronization with the frequency of an applied voltage. By applying a motor drive voltage in which the frequency of the output voltage of the AC power supply 2 is changed, the rotation speed of the motor 3 is controlled.

The motor control device 1 is such that the output voltage of the microcomputer 4 (including the function of the rotation speed control means of the present invention; hereinafter referred to as the microcomputer 4) and the AC power supply 2 shifts from positive to negative or from negative to positive. The zero-cross point detection circuit 5 (corresponding to the zero-cross point detection means of the present invention) for detecting the zero-cross point, and the bidirectional thyristor 6 for switching the conduction / interruption state between the AC power supply 2 and the motor 3 (the present invention And a switch 7 for applying a trigger voltage to the gate of the thyristor 6. The connecting means of the present invention is constituted by the thyristor 6 and the lead wire connecting the AC power source 2 and the motor 3 via the thyristor 6.

With such a configuration, the microcomputer 4 inputs the zero-cross point detection signal output from the zero-cross point detection circuit 5 from the I / O port (I / O_0) and determines the zero-cross point of the output voltage of the AC power supply 2. Detect. Then, when the microcomputer 4 detects the zero-cross point, the output point (I
/ O_1) outputs a pulse signal to the switch 7 to turn the switch 7 on (conducting), thereby applying a trigger voltage to the gate of the thyristor 6 via the switch 7. As a result, the thyristor 6 is turned on (conducted) and the output voltage of the AC power supply 2 is applied to the motor 3 during a half cycle from the detected zero-cross point to the next zero-cross point.

Next, with reference to FIGS. 2A and 2B, a procedure for applying to the motor 3 the motor drive voltage in which the frequency of the output voltage of the AC power supply 2 is changed by the microcomputer 4 will be described.

FIG. 2A shows the output voltage waveform of the AC power supply 2.
The second terminal 11b is shown as a reference, and the horizontal axis is
Time (t), the vertical axis is voltage (V). AC power supply 2
Period T ₀AC voltage is output at time t₀, T₁, T₂, T₃，
... is the zero-cross point. In addition, FIG.
The waveform of the voltage applied to the motor 3 is based on the second terminal 12b.
As shown in FIG. 2A, the horizontal axis represents
Time (t), the vertical axis is voltage (V).

First, the microcomputer 4, when detecting the zero-cross point (t ₀ ), turns on the switch 7 to apply the trigger voltage to the gate of the thyristor 6. As a result, during the half cycle from the detected zero-cross point (t ₀ ) to the next zero-cross point (t ₁ ), the thyristor 6 is turned on and the positive-side output voltage of the AC power supply 2 is applied to the motor 3 and the zero-cross point is applied. The thyristor 6 is turned off by reversing the polarity of the voltage applied to the thyristor 6 at the point (t ₁ ), and the voltage application from the AC power supply 2 to the motor 3 is cut off.

[0024] The microcomputer 4, when (t ₃₎ of one cycle of the output voltage of the AC power source 2 (t ₁ ~t ₃₎ has elapsed,
The switch 7 is turned on again to input the trigger voltage to the gate of the thyristor 6. Thus, during the t ₁ ~t ₃ is cut off a voltage applied from the AC power source 2 to the motor 3, during a half period from t ₃ to the next zero-crossing point (t _4), the AC thyristor 6 is turned ON The negative side output voltage of the power supply 2 is applied to the motor 3, and the polarity of the voltage applied to the thyristor 6 is inverted at the zero cross point (t ₄ ) so that the thyristor 6 becomes OF.
Then, the voltage application from the AC power supply 2 to the motor 3 is cut off.

[0025] Then, the microcomputer 4, in one period of the output voltage of the AC power supply 2 when the (t ₄ ~t ₆₎ has elapsed (t _6),
The trigger voltage is again input to the gate of the thyristor 6 via the switch 7.

As described above, the voltage is applied from the AC power supply 2 to the motor 3 for a half cycle from the time point (t ₀ ) at which the zero-cross point is detected, and an interval of one cycle (t _{2 to} t ₄ ) (in the present invention). A voltage is applied from the AC power supply 2 to the motor 3 for a half cycle (t _{3 to} t ₄ ) after a predetermined cycle), and one cycle (t ₄ to
By repeating the process of setting the interval of t ₆ ), the microcomputer 4 can apply the AC voltage having the cycle (T ₁ ) three times the cycle (T ₀ ) of the output voltage of the AC power supply 2 to the motor 3. it can.

Similarly, the period (T ₀ ) of the AC power source 2 is 2
AC power supply 2 at intervals of 4 times, 4 times, 6 times, ...
By applying the output voltage of the AC power supply to the motor 3 for a half cycle from the zero cross point of, the microcomputer 4 causes the output voltage cycle (T ₀ ) of the AC power supply 2 to be 5 times, 7 times, 9 times,
An alternating voltage having a cycle of ... Can be applied to the motor 3.

As a result, the microcomputer 4 can rotate the motor 2 at a rotation speed synchronized with a frequency that is an even fraction of the frequency (1 / T ₀ ) of the output voltage of the AC power supply 2.
When the frequency of the voltage applied to the motor 3 is changed in this way to change the rotation speed of the motor 3, compared to the case where the output torque of the motor 3 is reduced by phase control to change the rotation speed of the motor 3. Since the slippage of the motor 3 during rotation is reduced, the efficiency of power supply from the AC power supply 2 to the motor 3 can be improved and heat generation and vibration of the motor 3 can be suppressed.

Next, a second embodiment of the present invention will be described with reference to FIGS. The same components as those of the motor control device 1 shown in FIG. 1 are designated by the same reference numerals and the description thereof will be omitted.

The motor control device 20 (motor control device of the present invention) shown in FIG. 3 has a configuration for inverting the polarity of the output voltage of the AC power supply 2 and applying it to the motor 3 in FIG. The motor control device 1 shown in FIG.

In the motor controller 20, the first terminal 11a of the AC power supply 2 is connected to the first terminal 12a of the motor 3 via the thyristor 23a, and the thyristor 23 is also connected.
It is connected to the second terminal 12b of the motor 3 via b. The second terminal 11b of the AC power supply 2 is connected to the second terminal 12b of the motor 3 via the thyristor 24a, and the first terminal 1 of the motor 3 is connected via the thyristor 24b.
2a is connected.

Then, the thyristor 23a and the thyristor 2
Switch 2 for applying a trigger voltage to the gate of 4a
2a, and a switch 22b for applying a trigger voltage to the gates of the thyristor 23b and the thyristor 24b. The switch 22a includes the microcomputer 21 (including the function of the rotation speed control means of the present invention.
1) is switched to conduction / interruption by a pulse signal output from the I / O port (I / O_2) of the switch 2
2b is switched on / off by a pulse signal output from the I / O port (I / O_1) of the microcomputer 21.

Therefore, the microcomputer 21 outputs a pulse signal from the I / O port (I / O_1),
From the output time point to the next zero-cross point of the output voltage of the AC power supply 2, the first terminal 11a of the AC power supply 2 is electrically connected to the first terminal 12a of the motor 3 via the thyristor 23a, and the 2 terminals 11b are thyristors 24a
A voltage having the same polarity as the output voltage of the AC power supply 2 can be applied to the motor 3 by conducting the second terminal 12b of the motor 3 via the.

Further, the microcomputer 21 has an I / O port (I
By outputting a pulse signal from / O_2), the second terminal 11b of the AC power supply 2 is connected to the first terminal 12a of the motor 3 via the thyristor 24b from the output time point to the next zero cross point of the AC power supply 2. While conducting, the first terminal 11a of the AC power supply 2 is conducted to the second terminal 12b of the motor 3 via the thyristor 23b, and a voltage with the polarity of the output voltage of the AC power supply 2 inverted can be applied to the motor 3. it can.

Next, referring to FIG. 2C, the microcomputer 4
The procedure for applying the motor drive voltage with the frequency of the output voltage of the AC power supply 2 changed to the motor 3 will be described.
FIG. 2C shows the waveform of the voltage applied to the motor 3 with the second terminal 12b of the motor 3 as a reference,
The horizontal axis represents time (t) and the vertical axis represents voltage (V).

First, the microcomputer 21 determines the zero-cross point (t
_{When 0} ) is detected, the trigger voltage is input to the gates of the thyristor 23a and the thyristor 24a via the switch 22a. As a result, during the half cycle from the detected zero-cross point (t ₀ ) to the next zero-cross point (t ₁ ), the thyristor 23a and the thyristor 24a are turned on, and a voltage having the same polarity as the output voltage of the AC power supply 2 is applied to the motor. is applied to 3, thyristors 23a and the thyristor 24 at the zero-crossing point (t ₁₎
By reversing the polarity of the voltage applied to a, the thyristor 23a and the thyristor 23b are turned off, and the application of the voltage from the AC power supply 2 to the motor 3 is cut off.

[0037] Then, the microcomputer 21, when the half cycle of the output voltage of the AC power source ₂ (t ₁ ~t 2) has elapsed, and inputs the trigger voltage through the switch 22b to the gate of the thyristor 23b and thyristor 24b. As a result, t ₁ ~
During t ₂ , the voltage application from the AC power supply 2 to the motor 3 is cut off, and during the half cycle from t ₂ to the next zero cross point (t ₃ ), the thyristor 23b and the thyristor 24b are turned on to turn on the AC power supply 2. the output voltage obtained by inverting the polarity of the voltage is applied to the motor 3, the thyristor 23b and thyristor 24b by the polarity of the voltage applied to the thyristor 23b and thyristor 24b at the zero-crossing point (t ₃₎ is reversed to OFF, the AC power source 2 The voltage application from the motor to the motor 3 is cut off.

Subsequently, the microcomputer 21 determines when the half cycle (t _{3 to} t ₄ ) of the output voltage of the AC power supply 2 has elapsed (t ₄ ).
Then, the trigger voltage is again input to the gates of the thyristor 23a and the thyristor 24a via the switch 22a.

In this way, from the time point (t ₀ ) when the zero-cross point is detected, a voltage having the same polarity as the output voltage of the AC power source 2 is applied to the motor 3 for a half period of time, and a half cycle (t _{1 to} t ₂ ) is reached. AC power supply 2 at intervals (corresponding to a predetermined cycle of the present invention)
The output voltage of the motor is inverted for half a cycle.
By repeating the process of applying a voltage to the motor 3 and placing an interval of a half cycle (t _{3 to} t ₄ ), the microcomputer 21 can apply a voltage having a cycle (T ₂ ) twice that of the AC power supply 2 to the motor 3. it can.

Similarly, at intervals of 3 times, 5 times, 7 times, ..., A half cycle of the cycle (T ₀ ) of the AC power supply 2, from the zero cross point of the AC power supply 2 to the half cycle, By alternately applying to the motor 3 a voltage having the same polarity as the output voltage of the AC power supply 2 and a voltage obtained by inverting the polarity of the output voltage of the AC power supply 2, the microcomputer 21 causes the cycle of the output voltage of the AC power supply 2 (T 4 times _0), 6-fold, 8-fold, can be applied.., a voltage having a period of the motor 3.

As a result, the microcomputer 4 can rotate the motor 2 at a rotation speed synchronized with a frequency that is an even fraction of the frequency (1 / T ₀ ) of the output voltage of the AC power supply 2.
Therefore, similarly to the motor control device 1 in the above-described first embodiment, slippage during rotation of the motor 3 can be reduced, and the efficiency of power supply from the AC power supply 2 to the motor can be increased.

According to the motor control device 20 of the second embodiment, the microcomputer 21 operates either the switch 22a or the switch 22b as in the motor control device 1 of the first embodiment. By turning on when the zero-cross point of the output voltage of the AC power supply 2 is detected at intervals of an integral multiple of one cycle of the output voltage, a voltage having an odd frequency of the output voltage of the AC power supply 2 is generated. Can be applied to the motor 3. Therefore, the motor control device 20 can rotate the motor 3 at the number of rotations synchronized with the odd-numbered fraction and the even-numbered fraction of the frequency of the output voltage of the AC power supply 2.

As described above, according to the motor control device 1 and the motor control device 20, the rotation speed of the motor 3 can be set in a wide range by changing the frequency of the voltage applied to the motor 3. .

Therefore, for example, like a fan motor for rotating a warm air blower fan provided in a clothes dryer having a heating function, when supplying warm air into the rotating drum at the time of drying clothes, high torque and high rotation are provided. The motor control is performed when it is necessary to secure a wide variable range of the fan motor speed in order to change the fan motor speed in a wide range from low speed to high speed during heating. It is advantageous to use the device 1 and the motor control device 20.

At the time of starting the motor, the output voltage of the AC power supply 2 is directly applied to the motor without any period for interrupting the voltage application from the AC power supply 2 in order to solve the torque shortage and shorten the start-up time. 3 may be applied.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a motor control device according to a first embodiment.

FIG. 2 is an explanatory diagram of a waveform of a voltage applied to a motor.

FIG. 3 is a configuration diagram of a motor control device according to a second embodiment.

[Explanation of symbols]

1 ... Motor control device (in the first embodiment), 2
... AC power supply, 3 ... Motor, 4 ... Microcomputer,
5 ... Zero-cross point detection circuit, 6 ... Thyristor, 7 ... Switch, 20 ... Motor control device (in the second embodiment), 21 ... Microcomputer, 22a, 22b ...
Switches, 23a, 23b, 24a, 24b ... Thyristor

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Shinji Yasue             2-26, Fukuzumi-cho, Nakagawa-ku, Nagoya, Aichi Prefecture             Inside Nanai Co., Ltd. (72) Inventor Koichi Ito             2-26, Fukuzumi-cho, Nakagawa-ku, Nagoya, Aichi Prefecture             Inside Nanai Co., Ltd. (72) Inventor Koji Asai             2-26, Fukuzumi-cho, Nakagawa-ku, Nagoya, Aichi Prefecture             Inside Nanai Co., Ltd. F term (reference) 5H560 AA10 BB11 DC13 DC20 GG03                       RR10 SS06 TT15 UA09                 5H576 AA08 BB10 CC05 DD01 DD05                       EE25 HA08 HB10 JJ03 KK04                       LL60

Claims (3)

[Claims] 1. An AC power supply is connected to a motor that rotates in synchronization with the frequency of the AC voltage supplied, and a motor drive voltage in which the frequency of the output voltage of the AC power supply is changed is applied to the motor. A motor control device comprising a rotation control means for controlling the rotation speed of the motor, wherein a zero cross point detection means for detecting a zero cross point of the output voltage of the AC power supply, the AC power supply and the motor via a switching means. The rotation control means, the switching means until the half cycle of the output voltage elapses from the time when the zero cross point of the output voltage of the AC power supply is detected by the zero cross point detection means. Where a voltage is applied to the motor from the AC power source and the one half cycle of the output voltage is multiplied by an integer when the half cycle elapses. Until the cycle elapses, the switching means is turned off to prohibit the application of voltage from the AC power supply to the motor, and the switching means is turned on until the half cycle of the output voltage elapses from the elapse of the predetermined cycle. As a process of applying a voltage from the AC power supply to the motor, and prohibiting the application of the voltage from the AC power supply to the motor by cutting off the switching means from the time point of the half cycle until the predetermined cycle elapses. A motor control device characterized in that the motor drive voltage with the frequency of the output voltage changed is applied to the motor by repeating the operation. 2. An AC power supply and a motor that rotates in synchronism with the frequency of the AC voltage supplied are connected, and a motor drive voltage in which the frequency of the output voltage of the AC power supply is changed is supplied to the motor. A motor control device comprising a rotation control means for controlling the rotation speed of the motor, wherein a zero cross point detection means for detecting a zero cross point of the output voltage of the AC power supply, the AC power supply and the motor via a switching means. And a polarity reversing means for reversing the polarity of the output voltage of the AC power supply. The rotation control means detects the zero-cross point of the output voltage of the AC power supply by the zero-cross point detecting means. From the time point until a half cycle of the output voltage elapses, the switching means is turned on to apply a voltage from the AC power supply to the motor, The application of voltage from the AC power supply to the motor is prohibited by turning off the switching means until a predetermined period, which is an odd half of the output voltage, elapses from the time when the half period elapses. Until the half cycle of the output voltage elapses, the polarity reversing means inverts the polarity of the output voltage of the AC power supply and makes the switching means conductive to apply the voltage with the reversed polarity to the motor from the AC power supply. Then, the frequency of the output voltage is changed by repeating the processing of prohibiting the application of the voltage from the AC power supply to the motor by turning off the switching means until the predetermined cycle elapses from the time when the half cycle elapses. The motor control device, wherein the motor drive voltage is applied to the motor. 3. The rotation speed control means keeps the switching means conductive until a half cycle of the output voltage elapses from the time when the zero cross point of the output voltage of the AC power supply is detected by the zero cross point detection means. As a voltage is applied to the motor from the AC power supply, the switching means is turned off from the AC power supply to the motor until a predetermined cycle, which is an integral multiple of one cycle of the output voltage, elapses after the half cycle has elapsed. Application of the voltage is prohibited, the switching means is turned on until a half cycle of the output voltage elapses from the time when the predetermined cycle elapses, and a voltage is applied to the motor from the AC power supply, and the time when the half cycle elapses. To prohibit the application of voltage from the AC power supply to the motor by turning off the switching means until the predetermined period elapses. By repeating, the motor control device according to claim 2, wherein applying the motor drive voltage changes the frequency of the output voltage to the motor.