JP2009062955A - Control device for compressor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve reliability of a compressor by not conforming the rotational speed of a DC motor to the rotational speed at which the compressor resonates. <P>SOLUTION: A rotational speed calculating means 109 calculates rotational speed. A rotational speed control means 108 controls to set the rotational speed of the DC motor 104 to a command rotational speed by comparing the rotational speed of the command rotational speed with the calculated rotational speed of the DC motor 104. When voltage applied to the DC motor 104 reaches the upper limit, the rotational speed control means selects rotational speed lower than the command rotational speed from a set rotational speed stored in a set rotational speed storage means 115 and controls to set the rotational speed of the DC motor to the selected rotational speed. When motor current is reduced, the motor is operated at the command rotational speed. Accordingly, the compressor 114 is not rotated at the resonance rotational speed, and reduction in cooling capability is minimized. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an inverter circuit that drives a motor by a switching element that is PWM-controlled, and is particularly suitable for driving a hermetic electric compressor for a refrigerator.

  Conventionally, in this type of compressor control device, there is one that detects the motor rotation speed and varies the duty ratio of the PWM signal to control the rotation speed of the motor (see, for example, Patent Document 1).

  The conventional compressor control apparatus will be described below with reference to the drawings.

  FIG. 4 is a circuit diagram of a conventional compressor control device described in Patent Document 1, and FIG. 5 is a flowchart of the operation of the conventional compressor control device. The DC motor of the conventional compressor control device is shown in FIG. It is a flowchart explaining the operation | movement which raises / lowers rotation speed.

  In FIG. 4, an AC / DC converting means 1 is connected to a commercial power source 2 and converts a commercial AC voltage into a DC voltage. The inverter circuit 3 is connected to the AC / DC converting means 1, and the output is connected to a DC motor 4 which is a motor.

  The DC motor 4 is incorporated in a compressor 14 that cools a refrigerator or the like.

  The inverter circuit 3 includes six switching elements T1, T2, T3, T4, T5, and T6, and the six switching elements are connected in a three-phase bridge.

  The control circuit 5 includes position detection means 6, commutation means 7, rotation speed control means 8, rotation speed calculation means 9, command rotation speed detection means 10, rotation speed comparison means 11, synthesis means 12, and drive means 13. ing.

  The position detection means 6 detects the position of the rotor from the back electromotive voltage of the DC motor 4 and sends a position detection signal to the commutation means 7, the rotation speed control means 8, and the rotation speed calculation means 9.

  The commutation means 7 sends commutation pulses driven by the synthesis means 12 in accordance with the output of the position detection means 6.

  The rotation speed calculation means 9 calculates the rotation speed of the DC motor 4 by counting the position detection signal of the position detection means 6 for a certain period or measuring the pulse interval, and the rotation speed comparison means 11 operates the DC motor 4. The number of rotations being sent is sent out.

  On the other hand, the command rotational speed detection means 10 detects the command rotational speed sent from the refrigerator or the like and sends it to the rotational speed comparison means 11.

  The rotation speed comparison means 11 compares the rotation speed of the DC motor 4 from the rotation speed calculation means 9 with the command rotation speed from the command rotation speed detection means 10, and if the rotation speed of the DC motor 4 is smaller than the command rotation speed, the duty is The output for increasing the ratio is output to the rotation speed control means 8, and the rotation speed control means 8 increases the duty ratio and increases the voltage applied to the DC motor 4 to increase the rotation speed.

  When the rotational speed of the DC motor 4 is larger than the command rotational speed, an output for decreasing the duty ratio is output to the rotational speed control means 8, and the rotational speed control means 8 decreases the duty and decreases the voltage applied to the DC motor 4. To lower the rotation speed.

  Here, the duty ratio is the ratio of the ON time to the total time of the ON time and the OFF time. However, the combined time of the ON time and the OFF time is constant.

  The synthesizing unit 12 outputs the logical product of the outputs of the commutation unit 7 and the rotation speed control unit 8 to the drive unit 13, and the drive unit 13 drives the switching elements T <b> 1 to T <b> 6 constituting the inverter circuit 3.

  With respect to the compressor control apparatus configured as described above, the operation of raising and lowering the rotational speed of the DC motor 4 will be described below with reference to FIG.

  When the control device of the compressor receives the change of the command rotational speed from the control device such as the refrigerator by the command rotational speed detection means 10 while the DC motor 4 is in operation, it controls the rotational speed of the DC motor 4 to be changed.

  In STEP 1, the rotation speed calculation means 9 calculates the rotation speed of the DC motor 4 from the signal of the position detection means 6. In STEP 2, the rotation speed comparison means 11 detects the command rotation speed and rotation speed calculation means detected by the command rotation speed detection means 10. The rotational speed calculation result of the DC motor 4 calculated in 9 is compared.

  When the rotational speed calculation result is smaller than the command rotational speed in STEP 3, the routine proceeds to STEP 4, where the rotational speed control means 8 increases the duty ratio, which is the ratio of the ON time to the combined ON time and OFF time. As a result, the ON time increases, so that the voltage applied to the DC motor 4 increases and the rotational speed of the DC motor 4 increases.

  If the rotational speed calculation result is not smaller than the command rotational speed in STEP3, the process proceeds to STEP5.

  When the rotational speed calculation result is larger than the command rotational speed in STEP5, the process proceeds to STEP6, and the rotational speed control means 8 decreases the duty ratio, which is the ratio of the ON time to the combined ON time and OFF time. As a result, the ON time decreases, so the voltage applied to the DC motor 4 decreases and the rotational speed of the DC motor 4 decreases.

  Therefore, when a command rotational speed greater than the current rotational speed is input, STEP 1, STEP 2, STEP 3, STEP 4, STEP 5 are advanced, and the rotational speed control means 8 gradually increases the duty ratio, thereby rotating the DC motor 4. When the number of rotations is increased and the calculation result of the rotation speed becomes the same as the rotation speed calculation result, the process proceeds to STEP1, STEP2, STEP3, and STEP5, and the rotation speed control means 8 does not change the duty ratio, and the rotation speed of the DC motor 4 is Maintain the speed.

When a command rotational speed smaller than the current rotational speed is input, STEP 1, STEP 2, STEP 3, STEP 5, and STEP 6 are advanced and the rotational speed control means 8 gradually decreases the duty ratio, thereby rotating the DC motor 4. If the command speed is decreased and the rotation speed calculation result is the same, the operation proceeds to STEP1, STEP2, STEP3, and STEP5, and the rotation speed control means 8 does not change the duty ratio, and the rotation speed of the DC motor 4 is commanded. Maintain the speed.
JP-A 62-66080

  However, in the above-described conventional configuration, when the load of the refrigerator or the like is in an overload state or the input voltage is reduced, the rotational speed control means 8 is supplied to the DC motor 4 if the maximum rotational speed is input as the command rotational speed. However, if the maximum applied voltage is reached, the applied voltage to the DC motor 4 can no longer be increased, and operation is performed at a certain rotational speed determined by the load and applied voltage without reaching the command rotational speed. Will be.

  Generally, the compressor 14 has a revolving speed at which resonance occurs, and in order to solve this, the operating speed of the DC motor 4 in the compressor 14 is not continuous, and is operated at a speed that avoids this resonant speed. However, if the applied voltage to the DC motor 4 reaches the upper limit, there is a risk that the compressor 14 will rotate at the resonance speed, and the discharge pipe may be broken.

  The present invention solves the above-described conventional problem, and controls the rotational speed of the DC motor 4 not to make the rotational speed at which the compressor 14 resonates in the case of an overload or when the input voltage is lowered. When the state or the input voltage drop is improved, it is quickly returned to the command rotational speed, and the purpose is to improve the reliability of the compressor 14 and prevent the cooling performance of the refrigerator or the like from being lowered. And

  In order to solve the above-described conventional problems, the compressor control device of the present invention commands a specific rotation speed that is lower than the command rotation speed and avoids resonance of the compressor when the applied voltage to the DC motor becomes maximum. Therefore, the compressor does not rotate at the resonance rotational speed, and has an effect of returning to the command rotational speed when the motor current detecting means detects the input and the motor current value decreases.

  According to the compressor control device of the present invention, the motor does not rotate at the resonance rotational speed at which the compressor resonates even when the applied voltage to the DC motor reaches the maximum. The discharge pipe can be prevented from being broken, and the reliability of the compressor can be improved. If the motor current value is reduced, the motor is returned to the command rotational speed, so that the cooling performance of the refrigerator or the like can be prevented from being insufficient.

  Further, since the current of the DC motor is constant regardless of the rotational speed and the input voltage when the load is constant, it can be accurately detected even if the rotational speed is changed or the input voltage is changed.

  According to the first aspect of the present invention, in the control device for the compressor that controls the operation rotational speed of the motor so as to change the voltage applied to the motor and operate at the command rotational speed, when the applied voltage reaches the upper limit, the command rotational speed A specific rotation speed that is low and avoids the resonance of the compressor is commanded, and the motor does not rotate at a resonance rotation speed that causes the compressor to resonate, thereby improving the reliability of the compressor.

  According to a second aspect of the present invention, in the first aspect of the present invention, an inverter circuit in which a plurality of semiconductor switches are bridge-connected, a position detecting means for detecting the position of the rotor of the motor and generating a position detection signal; A rotation speed calculation means for calculating the rotation speed of the motor from the output of the position detection means, a commutation means for outputting a commutation pulse of the inverter circuit based on the output of the position detection means, and a command rotation speed Is compared with the number of rotations of the motor calculated by the number-of-rotations calculation means, and the voltage is controlled to make the number of rotations of the compressor variable so that the number of rotations of the motor becomes the command number of rotations. Rotation speed control means, drive means for operating the inverter circuit by the output of the commutation means and the output of the rotation speed control means, and a set rotation that stores a specific rotation speed at which the compressor does not resonate Storage means; motor current detection means for detecting motor current; motor current storage means for storing motor current value; motor for comparing motor current value stored in input storage means with current value during motor operation When the rotational speed of the motor that is provided with a current comparison means and is calculated by the rotational speed calculation means does not reach the command rotational speed, a specific rotational speed lower than the command rotational speed is selected from the set rotational speed storage means. In the control device for the compressor that operates the motor at the rotational speed, when the applied voltage to the DC motor reaches the upper limit, the specific rotational speed lower than the command rotational speed is selected from the set rotational speed storage means. The motor does not rotate at a resonance speed that causes the machine to resonate, and the reliability of the compressor can be improved.

  According to a third aspect of the present invention, in the second aspect of the present invention, when the applied voltage to the DC motor reaches the upper limit, a rotational speed lower than and closest to the rotational speed calculated by the rotational speed calculating means is set. This is selected from the set number of rotations of the number storage means, and further the deterioration of the cooling performance of the refrigerator or the like is minimized, so that the compressor does not rotate at the resonance number of rotations.

  According to a fourth aspect of the present invention, in the invention of the second aspect, when a specific rotational speed lower than the command rotational speed is selected from the set rotational speed storage means and the motor is operated at the rotational speed, The motor current is stored, and if the motor current is lower than the stored motor current by exceeding a predetermined threshold value, it is provided with a rotation speed control means configured to return to the operation at the command rotation speed, The DC motor can be reliably operated at the command rotational speed.

  The content of Claim 5 drives a compressor with the compressor control apparatus as described in any one of Claim 1 to 4, and provides the drive method of a compressor with high quality and reliability. be able to.

(Embodiment 1)
1 is a circuit diagram of a control device for a compressor according to a first embodiment of the present invention, FIG. 2 is a block diagram showing a resonance system of a refrigeration apparatus including the compressor according to the first embodiment, and FIG. 3 is a diagram according to the first embodiment. It is a flowchart of operation | movement in the control apparatus of a compressor, and is a flowchart explaining the operation | movement which raises / lowers the rotation speed of the DC motor in the control apparatus of a compressor.

  In FIG. 1, an AC / DC converter 101 is connected to a commercial power source 102 and converts a commercial AC voltage into a DC voltage. The inverter circuit 103 is connected to the AC / DC converting means 101, and the output is connected to the DC motor 104.

  The DC motor 104 is incorporated in a compressor 114 that cools a refrigerator or the like.

  The inverter circuit 103 includes six switching elements T101, T102, T103, T104, T105, and T106, and the six switching elements are connected in a three-phase bridge.

  The motor current detection means 116 is connected between the AC / DC conversion means 101 and the inverter circuit 103, detects the motor current, and outputs the value.

  The control circuit 105 includes position detection means 106, commutation means 107, rotation speed control means 108, rotation speed calculation means 109, command rotation speed detection means 100, synthesis means 112, drive means 113, motor current comparison means 110, motor current. The storage unit 111 and the set rotation speed storage unit 115 are configured.

  The position detection means 106 detects the position of the rotor from the back electromotive voltage of the DC motor 104 and sends a position detection signal to the commutation means 107 and the rotation speed calculation means 109.

  The commutation means 107 sends commutation pulses driven by the synthesis means 112 in accordance with the output of the position detection means 106.

  The rotation speed calculation means 109 calculates the rotation speed of the DC motor 104 by counting the position detection signal of the position detection means 106 for a certain period or by measuring the pulse interval, and causes the rotation speed control means 108 to operate the DC motor 104. The number of rotations being sent is sent out.

  On the other hand, the command rotational speed detection means 100 detects the command rotational speed sent from the refrigerator or the like and sends it to the rotational speed control means 108.

  The rotation speed control means 108 compares the rotation speed of the DC motor 104 from the rotation speed calculation means 109 with the command rotation speed from the command rotation speed detection means 100, and when the rotation speed of the DC motor 104 is smaller than the command rotation speed, The rotational speed of the DC motor 104 is increased. This increases the duty ratio in order to increase the voltage applied to the DC motor 104 and outputs it to the combining means 112.

  When the rotational speed of the DC motor 104 is larger than the command rotational speed, the rotational speed of the DC motor 104 is decreased.

  This decreases the duty ratio in order to lower the voltage applied to the DC motor 104, and outputs it to the combining means 112.

  The synthesizing unit 112 outputs the logical product of the outputs of the commutation unit 107 and the rotation speed control unit 108 to the drive unit 113, and the drive unit 113 drives the switching elements T <b> 101 to T <b> 106 constituting the inverter circuit 103.

  When the rotation speed calculated by the rotation speed calculation means 109 is lower than the command rotation speed, the rotation speed control means 108 sets the rotation speed closest to the rotation speed lower than the rotation speed calculated by the rotation speed calculation means 109. The selection is made from the rotation speed storage means 115, the selected rotation speed is compared with the rotation speed calculated by the rotation speed calculation means 109, and the duty ratio is controlled so that the selected rotation speed is obtained.

  Then, the rotation speed control means 108 outputs to the motor current storage means 111 when the selected rotation speed and the rotation speed of the DC motor 104 become the selected rotation speed.

  The motor current storage means 111 is connected to the output of the motor current detection means 116, and stores the motor current value if there is an output of the rotation speed control means 108.

  The set rotational speed storage means 115 stores a plurality of rotational speeds that avoid the resonant rotational speed of the compressor 114.

  The motor current comparison unit 110 is connected to the outputs of the motor current storage unit 111 and the motor current detection unit 116, and the motor current detected by the motor current detection unit 116 is a constant value from the current value stored in the motor current storage unit 111. If it decreases, the rotation speed is output to the rotation speed control means 108, and the rotation speed control means 108 compares the command rotation speed with the rotation speed calculated by the rotation speed calculation means 109.

  In FIG. 2, a reciprocating compressor 114 houses a DC motor 104 and a compression element 120 driven by the DC motor 104 in a sealed container 130. A discharge pipe 122 communicating the compression element 120 and the discharge connection pipe 121 is connected to the discharge connection pipe 121 fixed to the hermetic container 130. The damping coil spring 123 is fitted at a predetermined position of the discharge pipe 122, and the resonance frequency of the discharge pipe 122 is adjusted and the vibration of the discharge pipe 122 is attenuated. The DC motor 104 is operated by the inverter circuit 103.

  In the reciprocating compressor 114 configured as described above, the compression element 120 is driven when the DC motor 104 is operated by the inverter circuit 103, and the compressed gas compressed by the compression element 120 passes through the discharge pipe 122. Derived from the discharge connecting pipe 121. At this time, vibration generated from the compression element 120 is transmitted to the discharge pipe 122, but the resonance is attenuated by the coil spring 123.

  However, the discharge pipe 122 has a unique resonance frequency and resonates at several operating speeds. The set rotation speed storage means 115 stores a plurality of rotation speeds at which the discharge pipe 122 does not resonate.

  With respect to the compressor control apparatus configured as described above, the operation of raising and lowering the rotational speed of the DC motor 104 will be described below with reference to FIG.

  When the control device of the compressor receives the change of the command rotational speed from the control device such as the refrigerator by the command rotational speed detection means 110 while the DC motor 104 is operating, it controls the rotational speed of the DC motor 104 to be changed.

  In STEP 101, the command rotational speed is stored as a reference rotational speed.

  In STEP 102, the rotation speed calculation means 109 calculates the rotation speed of the DC motor 104 from the signal of the position detection means 106. In STEP 103, the rotation speed control means 108 detects the command rotation speed and rotation speed calculation means detected by the command rotation speed detection means 100. The rotational speed calculation result of the DC motor 104 calculated in 109 is compared.

  If the reference rotational speed is larger than the rotational speed calculation result in STEP 104, the process proceeds to STEP 105, and in STEP 105, the rotational speed control means 108 determines whether the applied voltage is the upper limit. If it is not the upper limit, the routine proceeds to STEP 106, where the rotational speed control means 108 increases the duty ratio and proceeds to STEP 111. As a result, the ON time increases, so that the voltage applied to the DC motor 104 increases and the rotational speed of the DC motor 104 increases.

  If the duty ratio is the upper limit in STEP 105, the process proceeds to STEP 107. In STEP 107, the reference rotational speed is set lower than the command rotational speed and lower than the rotational speed calculated by the rotational speed calculation means 109, and closest to the rotational speed. Selection is made from the rotation speed storage means 115, setting is performed, and the process proceeds to STEP110. Then, the motor current storage unit 111 stores the motor current and proceeds to STEP 111.

  If the reference rotational speed is equal to or lower than the rotational speed calculation result in STEP 104, the process proceeds to STEP 108.

  If the reference rotational speed and the rotational speed calculation result are not the same in STEP 108, the process proceeds to STEP 109, and the rotational speed control means 108 decreases the duty ratio. As a result, the ON time decreases, so that the voltage applied to the DC motor 104 decreases and the rotational speed of the DC motor 104 decreases.

  If the rotation speed calculation result is the same as the reference rotation speed in STEP 108, the process proceeds to STEP 111.

  In STEP 111, if the reference rotational speed is equal to or higher than the command rotational speed, the process returns to STEP 101, and the command rotational speed is stored again as the reference rotational speed. If not, the process proceeds to STEP 112. At STEP 112, the motor current comparing means 110 compares the values of the motor current detecting means 116 and the motor current storing means 111, and the value of the motor current detecting means 116 is 90% of the value of the motor current storing means 111. If it is not less than 90%, the process returns to STEP102.

  If the value of the motor current detection means 116 is 90% or less of the value of the motor current storage means 111 in STEP 112, the process returns to STEP 101, and the command rotational speed is stored again as the reference rotational speed.

  Therefore, when a command rotational speed greater than the current rotational speed is input and the duty ratio does not reach 100%, the process proceeds to STEP 101, STEP 102, STEP 103, STEP 104, STEP 105, STEP 106, STEP 111, and the rotational speed control means 108 sets the duty ratio. By gradually increasing the rotation speed of the DC motor 104, if the rotation speed calculation result is the same as the reference rotation speed, the process proceeds to STEP 101, STEP 102, STEP 103, STEP 104, STEP 108, STEP 111, and the rotation speed control means 108. Does not change the duty ratio, and maintains the rotational speed of the DC motor 104 at the reference rotational speed.

  When the applied voltage reaches the upper limit, the process proceeds to STEP 101, STEP 102, STEP 103, STEP 104, STEP 105, STEP 107, STEP 110, STEP 111, and the reference rotational speed is lower than the rotational speed calculated by the rotational speed calculating means 109, and the rotational speed. , The motor current is stored, the process returns to STEP 102, and the process proceeds to STEP 103. Since the reference rotation speed is lower than the rotation speed calculated by the rotation speed calculation means 109 in STEP 104, the flow returns to STEP 108. In step 109, the duty ratio is decreased, and the rotation speed control means 108 gradually decreases the duty ratio to decrease the rotation speed of the DC motor 104.

  If the reference rotation speed and the rotation speed calculation result are the same, the process proceeds to STEP 102, STEP 103, STEP 104, STEP 108, and STEP 111, and the rotation speed control means 108 does not change the duty ratio, and the rotation speed of the DC motor 104 is set to the reference rotation speed. maintain.

  If such an operation is continued, the inside of the refrigerator is cooled, the overload state is gradually eliminated, and the rotational speed can be maintained even if the voltage applied to the DC motor 104 is decreased, and the motor current is reduced. It will decrease.

  Therefore, if the reference rotational speed is set to a rotational speed that is lower than the rotational speed calculated by the rotational speed calculation means 109 and is closest to the rotational speed in STEP 111, the process proceeds to STEP 112, where the motor current value is stored in the motor current storage means. If it is 90% or less of the value stored in 111, the process proceeds to STEP 101, the reference rotational speed is set to the command rotational speed, and normal control is performed.

  In this embodiment, the motor current decrease value is 90% or less, but this value is a decrease value of the current value that allows the DC motor 104 to operate at the command rotation speed even if the reference rotation speed is returned to the command rotation speed through experiments or the like. May be calculated.

  Further, when a command rotational speed smaller than the current rotational speed is input, STEP 101, STEP 102, STEP 103, STEP 104, STEP 108, STEP 109, STEP 111 and the advance rotational speed control means 108 gradually decreases the duty ratio, thereby reducing the DC. If the rotation speed of the motor 104 is lowered and the rotation speed calculation result becomes the same as the reference rotation speed, the operation proceeds to STEP 101, STEP 102, STEP 103, STEP 104, STEP 108, STEP 111, and the rotation speed control means 108 does not change the duty ratio. The rotational speed of the DC motor 104 is maintained at the reference rotational speed.

  Therefore, in normal times, the duty ratio is controlled to be increased or decreased so that the rotational speed of the DC motor 104 becomes the same as the command rotational speed. When the applied voltage reaches the upper limit, the deterioration of the cooling performance is minimized, and the compressor 114 Is controlled so as not to rotate at the resonance rotational speed, breakage of the discharge pipe can be prevented and the reliability of the compressor 114 can be improved.

  Further, if the value of the motor current is detected and it is determined that the DC motor 104 can be operated at the command rotational speed, the DC motor 104 can be reliably operated at the command rotational speed.

  As described above, the compressor control device according to the present invention has a function of preventing the compressor from rotating at the resonance speed, preventing breakage of the discharge pipe, and improving the reliability of the compressor. Since it has, it is useful for the inverter drive device and refrigerator control of a compressor.

The circuit diagram of the control apparatus of the compressor in Embodiment 1 of this invention The block diagram showing the resonance system of the freezing apparatus containing the compressor in the embodiment Flowchart of operation in compressor control apparatus in same embodiment Circuit diagram of conventional compressor control device Flowchart of operation in a conventional compressor control device

Explanation of symbols

DESCRIPTION OF SYMBOLS 103 Inverter circuit 104 DC motor 106 Position detection means 107 Commutation means 108 Speed control means 109 Speed calculation means 110 Motor current comparison means 111 Motor current storage means 113 Drive means 114 Compressor 115 Set rotation speed memory means 116 Motor current detection means

Claims (5)

  In a compressor control device that controls the motor to operate at a command speed by changing a voltage applied to the motor, the motor speed is lower than the command speed when the motor speed does not reach the command speed. And a control device for the compressor which instructs to operate at a specific rotational speed while avoiding resonance of the compressor.   An inverter circuit in which a plurality of semiconductor switches are bridge-connected, position detecting means for detecting the position of the rotor of the motor and generating a position detection signal, and rotation for calculating the rotational speed of the motor from the output of the position detecting means Number calculation means, commutation means for outputting a commutation pulse of the inverter circuit based on the output of the position detection means, the rotation speed of the motor calculated by the rotation speed of the command rotation speed and the rotation speed calculation means A rotational speed control means for controlling the voltage to make the rotational speed of the compressor variable so that the rotational speed of the motor becomes the command rotational speed, and the output of the commutation means and the rotational speed Drive means for operating the inverter circuit by the output of the number control means, set rotation speed storage means for storing a specific rotation speed at which the compressor does not resonate, and a motor for detecting motor current A current detection means; a motor current storage means for storing the value of the motor current; and a motor current comparison means for comparing the motor current value stored in the motor storage means with the current value during operation of the motor. When the rotational speed of the motor calculated by the means does not reach the command rotational speed, a specific rotational speed lower than the command rotational speed is selected from the set rotational speed storage means, and the motor is operated at the rotational speed. The compressor control device according to claim 1.   When the rotation speed of the motor calculated by the rotation speed calculation means does not reach the command rotation speed, a rotation speed lower than and closest to the rotation speed calculated by the rotation speed calculation means is set in the set rotation speed storage means. The compressor control device according to claim 2, which is selected from a number.   When a specific rotational speed lower than the command rotational speed is selected from the set rotational speed storage means and the motor is operated at the rotational speed, the motor current at that time is stored, and the motor current is stored in a predetermined range from the stored motor current. 3. The compressor control device according to claim 2, wherein when the pressure drops beyond a threshold value, the operation is resumed at the command rotational speed.   The compressor control device according to any one of claims 1 to 4, which controls a refrigerator.