Introduction to inverter application

The frequency converter is a power control device that uses the on-off function of the power semiconductor device to convert the power frequency power supply to another frequency. It integrates high-voltage high-power transistor technology and electronic control technology and is widely used. The function of the frequency changer is to change the frequency and amplitude of the power supply of the AC motor, thus changing the cycle of its moving magnetic field and achieving the purpose of smoothly controlling the rotation speed of the motor. The appearance of the frequency converter makes the complex speed control simple, and replaces most of the work that can only be done with a DC motor with a combination of an inverter + AC squirrel-cage induction motor, which reduces the size, reduces the maintenance rate, and makes the drive The technology has developed to a new stage. In use, the inverter mainly performs functions such as soft start, frequency control, improved operation accuracy, energy saving, power factor change, over-current/overvoltage/overload protection.
The application scope of the frequency converter covers all applications applied to AC motors, including production, municipal, commercial and other purposes. According to the application and load characteristics, the technical and performance requirements of the frequency converter will also be different. See the application classification and selection guidelines below for details.
Here is a summary of common questions and answers.

1. What are the differences between PWM and PAM?
PWM is English Pulse Width Modulation (Pulse Width Modulation) (abbreviation of pulse width modulation), according to a certain rule to change the pulse width of the pulse train to adjust the amount of output and waveform adjustment method.
PAM is the abbreviation of English Pulse Amplitude Modulation (Pulse Amplitude Modulation). It is a modulation method that changes the pulse amplitude of a pulse train according to a certain rule to adjust the output value and the waveform.
2. What is the difference between voltage type and current type?
The main circuit of the frequency converter can be roughly divided into two kinds: The voltage type is the frequency converter which converts the direct current of the voltage source into alternating current, the filter of the direct current loop is the electric capacity; The electric current type is the frequency converter which transforms the direct current of the current source into alternating current, Its DC loop filter is an inductor.
3. Why does the inverter's voltage change proportional to the frequency?
The electromagnetic torque of any motor is the result of the interaction of the current and the magnetic flux. The current is not allowed to exceed the rated value, otherwise it will cause the motor to generate heat. Therefore, if the magnetic flux decreases, the electromagnetic torque must also be reduced, resulting in a reduction in the load carrying capacity.
It can be seen from the formula E=4.44*K*F*N*Φ that in the frequency conversion speed regulation, the magnetic circuit of the motor changes with the operating frequency fX in a relatively large range, and it is very easy to make the magnetic circuit of the motor serious. Saturation causes the waveform of the excitation current to be severely distorted, resulting in a very high spike current.
Therefore, the frequency and voltage must be changed proportionally, that is, the inverter output voltage should be controlled while changing the frequency, so that the magnetic flux of the motor can be kept constant to avoid the generation of weak magnetic and magnetic saturation phenomena. This kind of control method is mostly used in fans, pumps and energy-saving inverters.
4. When the motor is driven by the commercial frequency power supply, the current increases when the voltage drops. For the inverter drive, if the voltage drops when the frequency drops, does the current increase?
When the frequency drops (low speed), the current increases if the same power is output, but the current does not change substantially under the condition of constant torque.
5. When using the inverter to run, what is the starting current and starting torque of the motor?
With the inverter running, the starting current is limited to 150% or less of the rated current (125% to 200% depending on the model) as the motor speeds up the frequency and voltage accordingly. When using industrial frequency power supply to start directly, the starting current is 6~7 times of the rated current. Therefore, mechanical and electrical impact will occur. The inverter drive can be started smoothly (the starting time becomes longer). The starting current is 1.2~1.5 times the rated current and the starting torque is 70%~120% of the rated torque. For the inverter with the torque automatic enhancement function, the starting torque is more than 100%, and it can be started with full load.
6. What does the V/F mode mean?
When the frequency drops, the voltage V also decreases proportionally. This problem has been explained in answer 3. The proportional relationship between V and f is predetermined in consideration of the characteristics of the motor. Usually, there are several characteristics in the controller's storage device (ROM) that can be selected with a switch or a dial.
7. When the V and f are proportionally changed, how does the torque of the motor change?
When the frequency is decreased, the voltage is completely proportionally reduced. Therefore, since the AC resistance becomes smaller and the DC resistance does not change, the torque generated at a low speed tends to decrease. Therefore, given V/f at low frequencies, the output voltage should be increased somewhat in order to obtain a certain starting torque. This compensation is called enhanced starting. Can use various methods to achieve, there are automatic methods, select V / f mode or adjust the potentiometer and other methods.
8. In the description of the speed range of 60 ~ 6Hz, that is 10:1, then there is no output power below 6Hz?
Below 6Hz can still output power, but according to the motor temperature rise and starting torque size and other conditions, the minimum frequency of use to take about 6Hz, then the motor can output the rated torque without causing serious heating problems. The actual output frequency of the inverter (starting frequency) is 0.5~3Hz depending on the model. .
9. For the general motor combination, the torque is also required above 60Hz. Is it OK?
Normally it is not possible. In the above 60Hz (also has the mode above 50Hz) the voltage does not change, basically is the constant power characteristic, when requests the identical torque in the high speed, must pay attention to the choice of the electrical machinery and the frequency changer capacity.
10. What does the so-called open loop mean?
A speed detector (PG) is provided for the motor device to be used, and the actual speed is fed back to the control device for control. This is called "closed loop", and the operation without PG is called "open loop." General-purpose inverters are mostly open-loop, and some models use options for PG feedback. The speed sensor-less closed-loop control method is based on the established mathematical model to calculate the actual speed of the motor based on the magnetic flux, equivalent to using a virtual speed sensor. Form closed loop control.
11. What happens if the actual speed deviates from a given speed?
In the open loop, even if the inverter outputs a given frequency and the motor is running with a load, the motor speed changes within the range of the rated slip (1% to 5%). For the requirement of higher speed control accuracy, even if the load change requires operation near a given speed, a frequency converter (option) with PG feedback function can be used.
12. If using a motor with PG, can the speed accuracy be improved after feedback?
The inverter with PG feedback function has improved accuracy. However, the speed accuracy value depends on the accuracy of the PG itself and the resolution of the inverter output frequency.
13. What does the stall prevention feature mean?
If the given acceleration time is too short, the output frequency of the inverter will change far beyond the change of the speed (electrical angle frequency). The inverter will trip due to the overcurrent and the operation will stop. This is called stalling. In order to prevent the stall from continuing to operate the motor, it is necessary to detect the magnitude of the current for frequency control. When the acceleration current is too large, the acceleration rate is appropriately slowed down. This is also true during deceleration. The combination of the two is a stall function.
14. What is the meaning of models with acceleration time and deceleration time that can be given respectively, and the type of acceleration/deceleration time given together?
Acceleration and deceleration can be given for each type of machine, for short-time acceleration, slow deceleration occasions, or for small machine tools need to strictly specify the production of the tact time is appropriate, but for fan drive and other occasions, acceleration and deceleration time is longer, Acceleration time and deceleration time can be given together.
15. What is regenerative braking?
When the motor is running, if the command frequency is reduced, the motor becomes an asynchronous generator state operation and operates as a brake, which is called regenerative (electrical) braking.
16. Is greater braking power available?
The energy regenerated from the motor is stored in the filter capacitor of the inverter. Due to the relationship between the capacity of the capacitor and the withstand voltage, the regenerative braking force of the general inverter is about 10% to 20% of the rated torque. If using a braking unit, it can reach 50% to 100%.
17. Please explain the protection function of the inverter.
Protection features can be divided into the following two categories:
(1) Corrective actions are automatically performed after detection of abnormal conditions, such as overcurrent stall prevention and regenerative overvoltage stall prevention.
(2) Block the power semiconductor device PWM control signal after detecting the abnormality, so that the motor will automatically stop. Such as over-current cut-off, regenerative over-voltage cut-off, semiconductor cooling fan overheating and instantaneous power failure protection.
18. Why does the inverter's protection function operate when the clutch is continuously loaded?
When the clutch is used to connect the load, the motor rapidly changes from the no-load state to the area where the slip rate is large at the moment of connection, and the large current that flows causes the inverter to trip overcurrent and cannot operate.
19. In the same factory, a large motor moves together and the inverter stops during operation. Why?
When the motor starts, the starting current corresponding to the capacity will flow. The transformer on the stator side of the motor will generate a voltage drop. When the motor capacity is large, the pressure drop will also have a large effect. The inverter connected to the same transformer will make undervoltage or instantaneous stop. As a result of the judgment, the protection function (IPE) may occasionally act to stop the operation.
20. What is the frequency resolution? has no meaning?
For digitally controlled inverters, even if the frequency command is an analog signal, the output frequency is given a step. The smallest unit of this differential is called the frequency resolution.
The resolution of the frequency conversion is usually 0.015~0.5Hz. For example, if the resolution is 0.5Hz, then the upper side of 23Hz can be changed to 23.5, 24.0Hz, so the action of the motor is also followed step by step. This causes problems for the use of a continuous coil control. In this case, if the resolution is about 0.015 Hz, one step for a 4-stage motor is 1 r/min or less, and it can be sufficiently adapted. In addition, some models have different resolutions and output resolutions.
21. Is there any limit to the mounting direction when installing the inverter?
The structure of the inside and the back of the inverter considers the cooling effect. The relationship between the upper and the lower is also important for ventilation. Therefore, take the longitudinal position of the unit type in the tray and hung on the wall, and install it as vertically as possible.
22. Is it possible to use a soft start without putting the motor directly into a fixed frequency inverter?
It is possible to operate at very low frequencies, but if the given frequency is high then the conditions for direct start with the commercial frequency power supply are similar. A large starting current (6 to 7 times the rated current) will flow, and the motor cannot be started because the inverter cuts off the overcurrent.
23. What should you watch out for when the motor is running at over 60Hz?
When operating over 60Hz, please note the following:
(1) Machines and devices must be fully operational at this speed (mechanical strength, noise, vibration, etc.).
(2) The motor enters the constant power output range, and its output torque must be able to maintain the work (shaft, pump and other shaft output power increase in proportion to the cube speed, so speed should be paid attention to when it rises slightly).
(3) The bearing life problem should be fully considered.
(4) For motors of medium-capacity and above, especially 2-pole motors, it is necessary to carefully discuss with the manufacturer when operating above 60Hz.
24. Can the inverter drive the gear motor?
According to the structure and lubrication of the reducer, there are a number of issues that need attention. The maximum limit of 70~80Hz can be considered in the structure of the gear. When oil lubrication is used, the continuous operation at low speed is related to the damage of the gear.
25. Can the inverter be used to drive a single-phase motor? Can I use single-phase power?
Basically can not be used. For single-phase motors with governor switch start-up type, the auxiliary winding will be burned when the speed range is below the operating point; for capacitive start-up or capacitor operation, capacitor explosion will be induced. The frequency converter usually has a 3-phase power supply, but for a small capacity, it also has a single-phase power-running type.
26. How much power does the inverter itself consume?
It is related to the model, operating status, frequency of use of the inverter, but it is difficult to answer. However, the efficiency of inverters below 60Hz is about 94% to 96%. Based on this, the losses can be inferred. However, if a built-in regenerative braking inverter is incorporated, the power consumption will be increased if the loss during braking is also taken into consideration. Operation panel design must pay attention.
27. Why can't I use it continuously in the whole area of ​​6~60Hz?
General motors use blades mounted on the outer fan or rotor end ring of the shaft for cooling. If the speed is reduced, the cooling effect is reduced, so that the same heat generation as the high speed operation cannot be endured, the load torque at low speed must be reduced, or A large-capacity frequency converter is combined with a motor or a special motor is used.
28. What should you watch out for when using motors with brakes?
Brake excitation circuit power should be taken from the input side of the inverter. If the brake is operating when the inverter is outputting power, the overcurrent will be cut off. Therefore, the brake must be operated after the inverter stops outputting.
29. If you want to use a frequency converter to drive a motor with capacitors for power factor improvement, the motor does not move. Please explain why.
The inverter's current flows into the capacitor for improving the power factor. Since the inverter's overcurrent (OCT) is caused by the charging current, it cannot be started. As a countermeasure, remove the capacitor and operate it. Improve the power factor at the input side of the inverter. Access to AC reactors is effective.
30. How long is the life of the inverter?
Although inverters are stationary devices, they also have consumer devices such as filter capacitors and cooling fans. If these inverters are regularly maintained, they are expected to have a lifetime of more than 10 years.
31. The inverter contains a cooling fan. What is the direction of the wind? What happens if the fan is broken?
Kechuang power inverter adopts an independent wind tunnel design. The direction of the wind is from the bottom up. Therefore, where the inverter is installed, do not place mechanical devices that impede suction and exhaust on the upper and lower parts. Also, do not place heat-sensitive parts above the inverter. When the fan fails, it is protected by fan stop detection or overheat detection on the cooling fan
32. The filter capacitor is a consumable, so how to judge its life?
Capacitors used as filter capacitors gradually decrease their electrostatic capacity over time, and periodically measure the electrostatic capacitance. The life is judged on the basis of 85% of the rated capacity of the product.
33. Is there any limit to the mounting direction when installing the inverter?
Should be basically stored in the disk, the problem is that the use of fully enclosed structure of the disk size, space, cost is relatively high. The measures are:
(1) The design of the disk should be based on the heat dissipation required by the actual device;
(2) Increase the cooling area with aluminum heat sinks, airfoil coolants, etc.;
(3) Use a heat pipe.
In addition, a type in which the reverse side of the inverter can be exposed has been developed.
34. Want to increase the speed of the original conveyor belt and operate at 80Hz. How to choose the capacity of the inverter?
The power consumed by the conveyor belt is proportional to the rotation speed. Therefore, if you want to operate at 80Hz, the power of the inverter and the motor must be increased in proportion to 80HZ/50HZ, that is, increase the capacity by 60%.
35. Energy-saving effect of frequency converter The energy-saving effect of frequency converter is mainly reflected in the application of fans and water pumps. In order to ensure the reliability of production, all kinds of production machinery are left with a certain margin when designing and using power drives. When the motor cannot be operated at full load, the excess torque increases the consumption of active power in addition to the power drive requirements, resulting in a waste of electrical energy. The conventional speed regulation method for fans, pumps and other equipment is to adjust the amount of air supply and water supply by adjusting the baffle and valve opening of the inlet or outlet. The input power is large, and a large amount of energy is consumed in the closure process of baffles and valves. in. When using variable frequency speed regulation, if the flow requirements are reduced, the requirements can be met by reducing the pump or fan speed.
According to fluid mechanics, P (power) = Q (flow) × H (pressure). The flow Q is proportional to the primary speed of the speed N, the pressure H is proportional to the square of the speed N, and the power P is proportional to the cube of the speed N. If the efficiency of the pump is constant, when the flow rate needs to be adjusted to decrease, the speed N can decrease proportionally, and at this time, the output power P of the shaft decreases. That is, the relationship between the power consumption of the water pump motor and the rotational speed is approximately established. When the required flow rate Q of the team is reduced, the inverter output frequency can be adjusted so that the motor speed n is reduced proportionally. At this time, the power P of the motor will be greatly reduced according to the relationship of the third power, saving 40% to 50% of the power of the baffle and the valve, thereby achieving the purpose of saving electricity.
The energy saving example of the P5 series blower and water pump special frequency converter of Kechuang power source is: The power of a centrifugal pump motor is 55kw. When the speed drops to 4/5 of the original speed, the power consumption is 28.16kw. 48.8% of the electricity, when the speed is reduced to 1 / 2 of the original speed, its power consumption is 6.875kw, saving 87.5%.
Power factor compensation energy. Reactive power not only increases the line loss and heat generation of the equipment, but more importantly, the reduction of the power factor leads to the reduction of the active power of the power grid. A large amount of reactive power is consumed in the line. The equipment is inefficiently used and the waste is serious. The frequency conversion speed controller is used. Later, because of the internal filter capacitor of the frequency converter, thus has reduced the reactive power loss, has increased the active power of the electrical network.
Soft start energy. The hard start of the motor has a serious impact on the power grid, and it also imposes excessive demands on the capacity of the power grid. The large current and vibration generated during start-up will cause great damage to the baffle and valve, and it will be extremely unfavorable to the service life of the equipment and piping. After the inverter energy-saving device is used, the use of the soft-start function of the inverter will start the starting current from zero, and the maximum value will not exceed the rated current, reducing the impact on the power grid and the requirements for the power supply capacity, and prolonging the use of equipment and valves. life. Save equipment maintenance costs.