CN1307369C - Compressor frequency conversion control device - Google Patents

Abstract

The compressor frequency-changing control device is combined with an arithmetic unit and a logic controller, and is used for controlling said frequency converter and compressor, and according to the feedback signal of compressor controlling output frequency of frequency converter and making relative frequency change so as to change the motor speed of compressor at proper time.

Description

Compressor frequency conversion control device

【Technical field】

The invention relates to a compressor frequency conversion control device, in particular to a compressor frequency conversion control device which can precisely control the compressor motor speed and effectively save energy by controlling the output frequency of the frequency converter.

【Background technique】

There are not only many types of compressors, but also a wide range of uses. Various air compressors, gas compressors or refrigeration compressors are all applications of compressors. As far as the refrigeration compressor (hereinafter referred to as the compressor) used in general small air conditioners is concerned, it usually uses a DC motor to drive the compressor, so the AC voltage (mains) must be converted into a DC voltage before it can be driven. motor. However, when the traditional air conditioner is running, the way to control the room temperature is that when the room temperature drops below a preset value, for example, when the room temperature is 3 degrees lower than the preset value, the compressor will stop running until the room temperature rises above The preset value, for example, the compressor starts to run again when it is 3 degrees higher than the preset value. , not only easily damaged but also consumes more power. Therefore, a kind of so-called " variable frequency compressor " arises at the historic moment. However, since the variable frequency compressor uses a DC motor, it is not actually frequency conversion, but the speed of the DC motor is changed in time by changing the duty cycle (DutyCycle) of the DC voltage sent to the DC motor, so that When the room temperature is lower than a preset temperature, the motor speed is slowed down, and when the room temperature is higher than the preset temperature, the motor speed is increased, thereby controlling the room temperature to change within a small range, so that the motor can The effect of continuous operation and timely power saving, so the above-mentioned compressor can only be said to be a "variable pressure compressor". Moreover, since the above-mentioned compressor uses a DC motor with only a small horsepower, it cannot be used on a large industrial machine. Therefore, it cannot be used on large machines for industrial use. Therefore, AC compressors with higher horsepower are usually used on large machines. However, the current AC compressor does not have the function of frequency conversion, so that the above-mentioned traditional compressor cannot effectively reduce the electric energy consumed by the motor when the system demand changes.

In addition, in the current compressor control method, no matter whether it is an inverter compressor or a non-inverter compressor, when an abnormal situation occurs, there is no other way to respond except to send out a warning signal or trip the machine, so that the user must jump In the event of a compressor failure, emergency survey and maintenance should be carried out so that the compressor can resume operation as soon as possible in a short period of time.

【Content of invention】

The object of the present invention is to provide a frequency conversion control device for a compressor that can precisely control the motor speed of an AC compressor and save energy in proportion by controlling the frequency output of the frequency converter.

The invention provides a compressor frequency conversion control device, wherein the compressor is an AC compressor, which is connected with a frequency converter, and its motor speed is controlled by the output frequency of the frequency converter. In particular, the frequency conversion control device for the compressor is combined with an arithmetic unit and a logic controller to control the frequency converter and the compressor. It controls the output frequency of the frequency converter according to the feedback signal of the compressor to change the frequency of the compressor in time. The motor speed of the compressor can be precisely controlled, thereby achieving the purpose of precisely controlling the motor speed of the compressor and saving energy in proportion.

【Brief description of the drawings】

The compressor frequency conversion control device of the present invention will be described in detail below through the best embodiment and accompanying drawings. In the accompanying drawings:

Fig. 1 is the circuit block diagram of the frequency conversion control device of an embodiment of the compressor frequency conversion control method and device thereof of the present invention, compressor and frequency converter;

Fig. 2 is the frequency conversion control device and wiring diagram thereof of the present embodiment;

Fig. 3 is the schematic diagram of the control panel of the frequency conversion control device of the present embodiment;

Fig. 4 is a control flow of the frequency conversion control device of this embodiment.

【Detailed ways】

Referring to Fig. 1, it is a circuit block diagram of an embodiment of the compressor frequency conversion control device of the present invention, wherein the compressor 1 is an AC compressor, which is mainly used on large machines and has an AC that can generate high horsepower motor (not shown), in this embodiment, the compressor 1 is connected to a frequency converter 2, the frequency converter 2 can change its output frequency according to an input control signal, and control the output frequency of the compressor 1 with the output frequency The motor changes speed relatively. And since the working principles of general AC compressors and frequency converters should be known to those skilled in the art, no further description will be given here. And the compressor frequency conversion control device 3 of the present invention (hereinafter referred to as the frequency conversion control device 3) is used to control the frequency converter 2, so that the frequency converter 2 can change its output frequency relatively according to the control signal output by the frequency conversion control device 3, and then control The motor of the compressor 1 changes rotational speed relatively.

Referring to Fig. 2, the frequency conversion control device 3 of the present invention can be said to be a kind of FUZZY (fuzzy) controller, which mainly combines (or integrates) an arithmetic unit and a logic controller. The calculator is actually composed of a composite PID (proportional-integral-derivative) controller. A general PID controller is usually composed of one or two proportional units, integral units and differential units. It is jointly driven by the system command and the feedback signal of the controlled device, and the output signal of the system is formed by the superposition of their respective outputs to control the controlled device. However, because the general PID controller is prone to overshoot (overshoot) phenomenon, the system It oscillates around the target value, causing the system to be unstable, and the speed of the controlled motor to be unstable, so that it cannot effectively save power. Therefore, the compound PID controller used in the present invention is different from the above-mentioned general PID controller in that it is composed of multiple proportional controllers, multiple integral controllers and multiple differential controllers, and the compound PID controller The characteristic of the controller is that when the input feedback signal passes through the multiple calculations of these proportional, integral and differential controllers, the system will finally reach the target value accurately and stabilize without overshoot (overshoot) phenomenon. Therefore, when the composite PID controller compares a feedback signal output by the controlled equipment with an expected value, the control signal it generates can accurately control the controlled equipment to reach an expected value.

The logic controller contains control logic functions required by various compressors to detect and measure various states of the compressors, so as to control the operation of the compressors in a timely manner. And as shown in Figure 2, in this embodiment, the frequency conversion control device 3 is made in a single chip, and implanted in a circuit board (not shown) to become a frequency conversion control module, and the composite PID control The controller and the logic controller can be respectively implemented in a software program or hardware logic mode and built into the single chip, that is, the frequency conversion control device 3 . Furthermore, as shown in FIG. 2, the frequency conversion control device 3 is provided with some external contacts in actual application, and the purpose of each contact is as follows:

JP1:

R: AC, 1ph, 220V, input terminal.

T: Form a loop with R.

FG: ground wire.

JP2:

ALM: COMMON TRIP output contact, a contact.

CM1: common point.

OIL_SV: oil stop valve, cooling water solenoid valve action contact, a contact.

CM2: common point.

NULL: reserved contacts.

CM3: Reserved contact.

FAN_RUN: fan running contact, a contact.

CM4: common point.

INV_RUN: Inverter running contact, a contact.

CM5: common point.

SV1: Empty and heavy vehicle solenoid valve, discharge valve, return valve contact, a contact.

CM6: common point.

Input connection:

1. Reserved.

2. Reserved.

3. Reserved. 1, 2, 3 are used for chiller or heat pump PT100Ω temperature feedback.

4. TC1_C: Exhaust high temperature tripping contact PT100Ω.

5. TC1_B: Exhaust high temperature tripping contact PT100Ω.

6. TC1_A: Exhaust high temperature tripping contact PT100Ω.

7.0V1: 0V power supply.

8. Reserved.

9. Reserve. 8 and 9 are for RS485 use.

10.OUT: output voltage. Connect with 7 to the frequency analog input contact of the inverter.

11.IN2: Feedback 4~20mA.

12. Reserved.

13.+24V: Feedback 4~20Ma input supply voltage.

14. COM: common point.

15.S_LOW: Inlet pressure drop pressure switch pressure too low contact.

16.MOTOR_HT: Motor coil temperature is too high temperature switch contact.

17.NO_WATER: Loss of water switch contact.

18.RESET: Light reset contact.

19.A/O_SEP: Oil-air separator differential pressure switch contact.

20.OIL_FILTER: oil filter differential pressure switch contact.

21.S_FILTER: Intake filter differential pressure switch contact.

22.INV: Inverter abnormality, signal input.

23.FAN: abnormal fan, signal input.

24.OFF: Remote stop signal contact.

25.ON: Remote control operation signal contact. No. 15 to No. 25 contacts all take No. 14 as the common point.

As shown in Figure 3, the frequency conversion control device 3, that is, the frequency conversion control module, is usually provided with an external control panel for the user to operate and set, and a plurality of control buttons electrically connected to the internal circuit of the frequency conversion control device 3 are provided on it. And the function names of these control buttons can be changed according to the application requirements. Taking the application of controlling the compressor as an example, the functions of each control button on the control panel are listed as follows:

Indicator lights:

1. RUN (running): running indication, after pressing the run button, the running indicator light is on, indicating that the unit is in the running state.

2. STOP (stop): stop indication, the system has been powered but is in a stop state.

3. ELECTRIC FAIL (electrical failure): When the inverter or motor trips due to failure, it will be indicated by the electrical failure light and the system will stop at the same time.

4. AUTO STOP (automatic stop): In the fully automatic operation mode, if the unit stops due to being empty for a long time, this indicator light will be on.

5. FAN FAIL (fan failure): When the cooling fan trips, this indicator light is on, and the system shuts down at the same time.

6. AIR HIGH TEMPERATURE (exhaust high temperature): If the compressor exhaust temperature is too high, the indicator light will be on.

7. AIR FILTER (intake filter element): When the air filter element is blocked and the pressure difference is too large, the indicator light will start to flash to provide a warning function, but the system can still operate without tripping.

8. COOLING WATER FAIL (water loss indication): If the cooling water supply is interrupted, the indicator light will be on and the system will stop at the same time.

9. OIL FILTER (oil filter): When the oil filter is blocked and the pressure difference is too large, the indicator light will start to flash to provide a warning function, but the system can still operate without tripping.

10. MOTOR HIGH TEMPERATURE (motor high temperature): When the temperature of the motor coil exceeds the set value, the indicator light is on, and the unit trips at the same time.

11. AIR/OIL SEPARATOR (oil-air filter element): When the pressure difference of the oil-air separator is too large due to blockage, the indicator light will start to flash to provide a warning function, but the system can still operate without tripping.

12. SUCTION PRESSURE ABNORMAL (inlet pressure drop): When the inlet pressure of gas compressors such as refrigeration compressors or superchargers is abnormal, the light will flash and the system will stop at the same time.

13. SPARE (spare indicator light): used when providing other control logic.

14. DECREASE LOADING (load reduction): When the system trips due to reaching the limit of load reduction, this indicator light will be on.

button:

1. RUN (operation): press this button, the system starts to run.

2. STOP (stop): press this key, the system stops.

3. The SET, UP▲, DOWN and <P> keys can be used to modify the set pressure, timer time and operating parameters.

LED digital display board:

1.PV: actual operating pressure of the system.

2.SP: The pressure value set by the system.

Of course, in addition to the above-mentioned start and stop functions, the operation parameters required by various compressors can also be input into the frequency conversion control device 3 through buttons, so as to achieve the best operation effect.

Next, referring to Fig. 4, it is a control flowchart of an AC compressor controlled by the frequency conversion control device 3. As shown in the figure, the control procedure is as follows: If there is an "abnormality" occurs, the machine will trip, otherwise, it will run at constant pressure according to the set value of the PID controller.

At the same time, the compressor will output a feedback signal to the frequency conversion control device 3 during operation, and in this embodiment, the feedback signal is a pressure signal, and the pressure signal is sent to the PID controller (that is, the arithmetic unit ) to perform comparison and calculation actions, and generate a control signal to the inverter to control the compressor to operate at constant pressure according to the set value of the PID controller. And when the compressor is empty, let the compressor run at the lowest speed when it is empty, and then judge whether the pressure at the lowest speed is "lower than the set value of the PID controller"? If so, perform constant pressure operation according to the set value of the PID controller, otherwise, judge whether the compressor has "exceeded the set time for emptying"? If yes, the compressor will stop automatically, and when the pressure is "lower than the set value of the PID controller", the compressor will start automatically and return to the empty running state. In addition, when the constant pressure operation is performed according to the set value of the PID controller, when the compressor receives a "demand increase" signal and needs to increase its motor speed, the PID controller will send a control signal to increase the speed to the inverter , to change the output frequency and control the motor of the compressor to increase the speed. Conversely, when the compressor receives a signal of "decrease in demand" and needs to reduce its motor speed, the PID controller sends a control signal to reduce the speed to the inverter to reduce the output frequency to control the motor of the compressor to reduce the speed. In addition, in the case of constant pressure operation according to the set value of the PID controller, when there is an "abnormal" situation, it will trip, otherwise, the compressor will stop when it is empty after pressing the stop button. In addition, when the system is abnormal and the abnormality is not caused by the failure of the compressor, but the motor of the compressor is fully loaded at the highest speed, such as 3000rpm, the load current of the motor is too high, which meets the preset of the frequency conversion control device 3 When the condition of "reaching the setting value of reduced speed", the frequency conversion control device 3 promptly sends out a control signal to set the maximum speed of the motor to a lower speed, so that the maximum speed of the motor is reduced, so that the speed of the motor will "return to To the "set value" (that is, the lower speed) and the load current is reduced accordingly. At this time, unless the abnormal value of the system "reaches the set upper limit" (that is, reaches the set upper limit for reducing the maximum rotational speed), the compressor will trip. For this load reduction function, a more detailed description will be given below.

Therefore, when a traditional AC compressor without frequency conversion function is equipped with a frequency converter and the frequency conversion control device 3 of the present invention is installed, it becomes a motor that can change its motor speed according to the feedback signal of the system and can accurately control its output pressure or temperature. inverter compressor.

Therefore, it can be seen from the control flow of the above-mentioned frequency conversion control device 3 that the frequency conversion control device 3 receives a feedback signal (maybe a pressure signal or a temperature signal, etc.) output by the compressor, and this signal comes from the pressure sensor at the discharge end of the compressor Or the chilled water temperature sensor of the chilled water machine, and convert the pressure (or temperature) into a feedback signal and send it to the frequency conversion control device 3 . The frequency conversion control device 3 uses the composite PID controller to compare the feedback signal with an expected value, and the expected value is usually the user expects the compressor to reach the expected value for comparison operation, and the expected value is usually the user expects the compressor to reach the expected value. set value. Therefore, according to the working principle of the composite PID, the feedback signal will be subtracted from the expected value to generate an error signal, and then the error signal will be multiplied by a constant through the proportional controller, and then passed through multiple differential controllers and integral control After the operation of the inverter, a control signal is generated to the frequency converter to relatively change its output frequency according to the control signal, so that the motor speed of the compressor is changed to meet the requirements of the expected value. Therefore, the output of the AC compressor can be precisely controlled, so that the energy consumed by the system can also increase or decrease in almost equal proportion with the change of the motor speed under the condition of maintaining the precise pressure or temperature of the compressor, so as to achieve the purpose of saving energy.

In addition, as mentioned above, in addition to the function of controlling the frequency conversion of the AC compressor, the frequency conversion control device 1 also provides a load reduction function for the unit. And the way of warning is given, so that the user can respond more calmly and carry out maintenance without jumping out of the machine. The following special examples illustrate it.

Example 1: When the frequency conversion control device 3 is applied to the constant pressure of compressor exhaust

One of the biggest advantages of the inverter compressor is that under the condition of constant exhaust pressure, the motor speed of the compressor can be adjusted according to the change of the outlet air volume. Assuming that the speed range is set at 1000rpm-3000rpm, then 3000rpm is the full-load speed. This means that under normal conditions, the variable frequency compressor can change speed between 1000rpm and 3000rpm according to the outlet air volume. However, when the intake pressure of the compressor is full-loaded at 3000rpm due to system abnormality and the load current is too high, the existing compressors generally respond by tripping until the system returns to normal. Can be turned on again. However, since the change in the system inlet pressure is not caused by the failure of the compressor itself, it does not need to be shut down immediately. Therefore, the frequency conversion control device 3 of the present invention is designed with a load reduction function, which can be used according to the change in the pressure at the inlet end of the compressor. An intake air pressure sensor senses the pressure and returns a feedback signal to the frequency conversion control device 3. The PID controller compares the difference between the preset parameters and the feedback signal, and generates a control signal to reduce the maximum speed of the compressor. For example, the speed range is adjusted from 1000rpm to 3000rpm to 1000rpm to 2800rpm, and the reduced maximum speed can be fixed or adjusted according to the change of the intake pressure. Therefore, since the motor speed is reduced, the load current will also be reduced accordingly, thereby avoiding the excessive current caused by the change of the intake air pressure. Unless the intake pressure of the system changes too much after the speed is reduced, the compressor will only trip when the maximum value of the change (that is, the difference between the intake pressure and the standard value reaches the maximum set value), and when it is less than this maximum value, Maneuverability control can still be performed between a lower rotational speed and the standard maximum rotational speed, and a warning signal is sent to remind the user while the maximum rotational speed is reduced.

Example 2: The frequency conversion control device 3 is used for constant temperature at the outlet of the chiller

The ice water machine uses a refrigeration compressor to compress the refrigerant, and the refrigerant is cooled and liquefied by the condenser, and then passes through the expansion valve to the evaporator, and the refrigerant is exchanged with water or other media in the evaporator, and then Send the cooled ice water to the occasions that need to be air-conditioned or cooled. And whether it is an inverter chiller or an inverter air conditioner, a certain operating value is fixed, such as the temperature of the ice water outlet or the gas outlet temperature, and then the frequency conversion is performed according to this operating value. As mentioned above, assuming that the speed of the inverter compressor is 1000rpm-3000rpm, the speed of the refrigeration compressor can be between 1000rpm-3000rpm, which can be automatically increased or decreased depending on the temperature of the ice water. However, since the condensation needs to use external cooling water or air for cooling, when the condenser fouling increases the temperature of the cooling water and air, the condensation temperature will also increase, and the increase in the condensation temperature will increase the pressure, and the refrigeration compressor will The outlet temperature rises. When the temperature rises to a certain value, in order to protect the compressor, the current practice is to trip the compressor. However, the fouling of the condenser is not caused by the failure of the compressor itself, and it does not need to be shut down immediately. Therefore, the frequency conversion control device 3 of the present invention can use a temperature sensor to convert the sensed temperature into a feedback signal according to the discharge temperature at the outlet of the compressor, and then send it back to the frequency conversion control device 3, so that it can be compared with the pre-input After setting the parameters, generate a control signal to reduce the maximum speed of the compressor, and adjust its speed to 1000rpm ~ 2800rpm, and the maximum speed can be fixed or adjusted according to the exhaust temperature or pressure. After the rotating speed is reduced, the load current of the motor is also reduced, and the amount of refrigerant at the outlet is reduced, so the condensing pressure or condensing temperature can be reduced. Unless the condensing pressure or condensing temperature continues to rise to a higher value, the system will not be tripped.

Similarly, when the frequency conversion control device 3 is used in a frequency conversion heat pump, the outlet temperature of the hot water or hot air of the condenser can be used as a reference, and other abnormal conditions can be used as a reference value for reducing the maximum speed.

Therefore, the frequency conversion control device 3 uses the function of reducing the maximum speed to reduce the full load or other characteristics, which is to make the variable frequency drive unit, under the operating conditions based on a certain output characteristic, if it encounters an abnormal situation, and this abnormal situation It does not come from the compressor itself, and the frequency conversion unit can be relieved by reducing the maximum speed. According to the user's needs, according to the preset parameters, some abnormal conditions can be selected as the target for the function of reducing the maximum speed. Only jump out of the plane when the situation is more serious.

In addition, although the function of reducing the maximum speed of the frequency conversion control device 3 has the characteristic of possibly reducing the full load of the frequency conversion unit under normal conditions, it can provide users with a longer time for abnormal response processing. As far as the frequency conversion host of the air conditioning system is concerned, it is better to reduce the load of the system due to an abnormality and increase the temperature, rather than turning the office into a large oven due to a system trip.

Furthermore, the frequency conversion control device 3 of the present invention has been realized and can be practically applied to traditional compressors. The internal settings of the frequency conversion control device 3 for different application examples will be listed below.

(1) When applied to a screw air compressor

1.SV: 7.00----The air compressor will output constant pressure at 7Kg/cm ² G

14.SRC: 2---LED display feedback pressure value

15.L-Hz: 0---The lowest frequency value of the inverter

16.HIP1: 0.5---The air compressor will be empty at 7.5Kg/cm ² G

17.SSR: 16---Pressure transmitter specification 0～16Kg/cm ² G

18. LOAD: 0---Intake pressure is atmospheric pressure

19.L-HI: 0---do not use load reduction

20.IDP: 0---When L-HI is set to 0, this setting value is not used

21. T1: 20 --- stop 20 seconds after pressing the stop button

22.T2: 20---After pressing the start button, the car will be empty for 20 seconds

23.T3: 300 --- The car will automatically stop when it runs empty for more than 300 seconds

24.T4: 0---do not detect oil pressure

25.T5: 10---The water loss switch is not activated for more than 10 seconds, and the machine will trip

26.T6: 300---Intake pressure drop switch action exceeds 300 seconds and trips

27. TC: 90 --- Compressor discharge temperature exceeds 90 ℃ trip

28.TCBA: 0---no preset temperature correction

29.AOUT: 100---output voltage 100%

30.TC-R: 100---Temperature feedback 100%

31. YUNN: 0---compressor application software

(2) When applied to a gas booster

1. SV: 15.00---The booster output constant pressure is 15Kg/cm ² G

2.～15. The parameters are the same as (1) ——

16.HIP1: 1.0---The supercharger will be empty at 16Kg/cm ² G

17.SSR: 16---Pressure transmitter specification 0-16Kg/cm ²

18. LOAD: 6.0---Intake pressure is 6Kg/cm ² G

19.L-HI: 2.0 --- reduce the load to 2Kg/cm ² G lower than 6Kg/cm ² G, and jump the machine

20.IDP: 1---The intake pressure is lower than 6Kg/cm ² G

21.～23. Same (1)

24.T4: 5---The machine will trip if the oil pressure is lower than the set value within 5 seconds

25.26. Same (1)

27. TC: 140 --- compressor discharge temperature exceeds 140 ℃ trip

28.～31. Same (1)

(3) When used in ice water machines

1.SV: 6.00 The temperature of the ice water outlet is set at 6°C

2.～13. The parameters are the same as (1)

14.SRC: 4---LED display feedback ice water temperature

15. Same (1)

16.HIP1: 2---The chiller will be empty at 4°C

17.SSR: 0 --- pressure transmitter not used

18. LOAD: 4---Intake pressure is 4Kg/cm ² G

19.L-HI: 3---Drop the load to 3Kg/cm ² G lower than 4Kg/cm ² G, jump the machine

20.IDP: 1---The intake pressure is lower than 4Kg/cm ² G starts to reduce the load

21.～23. Same (1)

24.T4: 5---The machine will trip if the oil pressure is lower than the set value within 5 seconds

25.26. Same (1)

27. TC: 110 --- compressor discharge temperature exceeds 110 ℃ trip

28.～30. Same (1)

31. YUNN: 1 --- chiller application software

To sum up, the frequency conversion control device 3 of the present invention not only has the precision of High, fast response and product reliability, and the frequency conversion control device 3 can control the output frequency of the frequency converter according to the feedback signal of the controlled system to change the motor speed of the compressor of the controlled system to achieve precise control of pressure or temperature, etc. Purpose, and its accuracy can be adjusted according to the needs of use. At the same time, due to the change of the motor speed, under the condition of maintaining accurate pressure or temperature, the energy consumed by the system can also increase or decrease in almost equal proportion with the change of the speed, so as to achieve the purpose of saving energy. In addition, using the load reduction function of the frequency conversion control device 3, when the unit encounters a non-faulty abnormality, it can first reduce the performance and speed of the unit and issue a warning, so that the user can respond more calmly without tripping the machine .

Claims (5)

1. A frequency conversion control device for a compressor, wherein the compressor is an AC compressor, which is connected to a frequency converter, and its motor speed is controlled by the output frequency of the frequency converter, characterized in that: The frequency conversion control device for the compressor is a combination of a compound proportional-integral-derivative controller and a logic controller, wherein the compound proportional-integral-derivative controller is used to receive the feedback signal from the compressor, and is first compared with the expected value The subtraction produces an error signal, and then the error signal is multiplied by a constant through the proportional controller, and after the operation of multiple differential controllers and integral controllers, the output frequency is controlled to change to the motor speed that meets the requirements of the expected value. 2. The compressor frequency conversion control device according to claim 1, characterized in that: the arithmetic unit and the logic controller can be realized by software. 3. The compressor frequency conversion control device according to claim 1, characterized in that: the arithmetic unit and the logic controller can be realized by hardware. 4. The variable frequency control device for compressors according to claim 1, characterized in that: a lower speed can also be preset in the arithmetic unit of the variable frequency control device for compressors, so that when the compressor is abnormal due to non-failure And when it is fully loaded at the highest speed and the load current is too high, the calculator sends a control signal to set the maximum speed of the compressor to the lower speed, so that the load current of the compressor can be reduced accordingly. 5. The compressor frequency conversion control device according to claim 4, characterized in that: the lower rotational speed can be adjusted as a fixed value or dynamically depending on the discharge temperature or pressure of the compressor.

Images