FAQs about inverters

1. What is a frequency converter?

The frequency converter is an electric energy control device that uses the on-off function of power semiconductor devices to convert the industrial frequency power supply into another frequency. It can realize soft starting, variable frequency speed regulation, improve operation accuracy, change power factor, overcurrent/overvoltage/overload protection and other functions of AC asynchronous motors.

2. Differences between PWM and PAM

PWM is the abbreviation of Pulse Width Modulation, which is a modulation method that changes the pulse width of a pulse train according to a certain rule to adjust the output value and waveform. PAM is the abbreviation of Pulse Amplitude Modulation, which is a modulation method that changes the pulse amplitude of a pulse train according to a certain rule to adjust the output value and waveform.

3. Why does the voltage of the inverter change in proportion to the frequency?

The electromagnetic torque of any motor is the result of the interaction between current and magnetic flux. The current is not allowed to exceed the rated value, otherwise it will cause the motor to heat up. Therefore, if the magnetic flux decreases, the electromagnetic torque will also decrease, resulting in a decrease in load capacity.

From the formula E=4.44*K*F*N*Φ, we can see that during variable frequency speed regulation, the magnetic circuit of the motor changes within a fairly large range with the operating frequency fX, which can easily cause the magnetic circuit of the motor to be severely saturated, resulting in severe distortion of the waveform of the excitation current and generating a very high peak current.

Therefore, the frequency and voltage should be changed proportionally, that is, the inverter output voltage should be controlled while changing the frequency, so that the magnetic flux of the motor remains constant and the occurrence of weak magnetic field and magnetic saturation is avoided. This control method is mostly used in energy-saving inverters for fans and pumps.

4. How does the motor torque change when V and f are changed proportionally?

When the frequency decreases, the voltage is reduced in full proportion. Since the AC impedance decreases and the DC resistance remains unchanged, the torque generated at low speed tends to decrease. Therefore, when the V/f is given at low frequency, the output voltage should be increased to obtain a certain starting torque. This compensation is called enhanced starting. It can be achieved by various methods, such as automatic method, selecting V/f mode or adjusting potentiometer.

5. The manual states that the speed range is 60 to 6 Hz, or 10:1. Does that mean there is no output power below 6 Hz?

Power can still be output below 6Hz, but according to the motor temperature rise and starting torque, the minimum operating frequency is about 6Hz, at which the motor can output rated torque without causing serious heating problems. The actual output frequency (starting frequency) of the inverter is 0.5~3Hz depending on the model.

6. What does open loop mean?

The motor device used is equipped with a speed detector (PG) to feed back the actual speed to the control device for control, which is called "closed loop", and the operation without PG is called "open loop". Most general inverters are open loop, and some models can use options to perform PG feedback. The speed sensorless closed loop control method is to calculate the actual speed of the motor based on the magnetic flux according to the established mathematical model, which is equivalent to using a virtual speed sensor to form a closed loop control.

7. What should I do if the actual speed deviates from the given speed?

In open loop, even if the inverter outputs a given frequency, the motor speed will change within the rated slip range (1% to 5%) when the motor is running with load. For applications requiring higher speed regulation accuracy and requiring the motor to run at a speed close to the given speed even if the load changes, an inverter with PG feedback function (optional) can be used.

8. If a motor with PG is used, can the speed accuracy be improved after feedback?

The inverter with PG feedback function has improved accuracy, but the value of speed accuracy depends on the accuracy of PG itself and the resolution of inverter output frequency.

9.What does the stall prevention function mean?

If the given acceleration time is too short, the output frequency change of the inverter far exceeds the change of the speed (electrical angular frequency), and the inverter will trip due to overcurrent, and the operation will stop. This is called stalling. In order to prevent stalling and keep the motor running, the current size must be detected to control the frequency. When the acceleration current is too large, the acceleration rate should be appropriately slowed down. The same is true for deceleration. The combination of the two is the stall function.

10. There are models in which the acceleration time and deceleration time can be given separately, and models in which the acceleration and deceleration time can be given together. What is the significance of this?

Machines for which acceleration and deceleration can be given separately are suitable for situations where short-time acceleration and slow deceleration are required, or for small machine tools where the production cycle time needs to be strictly given. However, for situations such as fan drives, the acceleration and deceleration times are relatively long, so the acceleration time and deceleration time can be given together.

11. What protection functions does the inverter have?

Protection functions can be divided into the following two categories:

(1) Automatically perform corrective actions upon detecting an abnormal condition, such as overcurrent stall prevention and regenerative overvoltage stall prevention.

(2) After detecting an abnormality, the PWM control signal of the power semiconductor device is blocked to automatically stop the motor, such as over-current cut-off, regenerative over-voltage cut-off, semiconductor cooling fan overheating and instantaneous power failure protection.

12. Why does the inverter's protection function activate when the load is connected using a clutch?

When the load is connected with the clutch, at the moment of connection, the motor changes sharply from the no-load state to the area with large slip rate. The large current flowing through causes the inverter to trip due to overcurrent and cannot operate.

13. When large motors in the same factory are started together, the inverter stops during operation. Why is this?

When the motor starts, a starting current corresponding to the capacity will flow through it, and the transformer on the stator side of the motor will produce a voltage drop. When the motor capacity is large, the impact of this voltage drop is also large. The inverter connected to the same transformer will make a judgment of undervoltage or instantaneous power failure, so sometimes the protection function (IPE) will be activated, causing it to stop running.

14. Is it possible to directly connect the motor to a fixed-frequency inverter without using soft starting?

It is possible at very low frequencies, but if the given frequency is high, the conditions are similar to those of direct starting with industrial frequency power supply. A large starting current (6 to 7 times the rated current) will flow, and the motor cannot start because the inverter cuts off the overcurrent.

15. Can the inverter be used to drive a single-phase motor? Can it use a single-phase power supply?

Basically, it cannot be used. For single-phase motors with speed regulator switch start, the auxiliary winding will be burned in the speed regulation range below the working point; for capacitor start or capacitor operation, it will induce capacitor explosion. The power supply of the inverter is usually 3-phase, but for small capacity, there are also models that use single-phase power supply to operate.

16. I want to use a frequency converter to drive a motor with a capacitor to improve the power factor, but the motor does not move. Please explain the reason.

The inverter current flows into the capacitor used to improve the power factor. Due to its charging current, the inverter overcurrent (OCT) occurs, so it cannot start. As a countermeasure, please remove the capacitor and operate it. As for improving the power factor, it is effective to connect an AC inductor on the input side of the inverter.

17. There is a cooling fan inside the inverter. What is the direction of the wind? What will happen if the fan breaks down?

For small capacity, there are also models without cooling fans. For models with fans, the wind direction is from bottom to top, so the place where the inverter is installed, do not place mechanical equipment that hinders the suction and exhaust above or below. Also, do not place heat-sensitive parts above the inverter. When the fan fails, protection is provided by the fan stop detection or the overheat detection on the cooling fan.

18. What is the function of the sine filter as an accessory of the inverter?

Sine filters allow the inverter to be operated with longer motor cables and are also suitable for circuits with an intermediate transformer between the inverter and the motor.

19. Why can’t the inverter be used as a variable frequency power supply?

The entire circuit of the variable frequency power supply is composed of AC-DC-AC-filtering, so the voltage and current waveforms it outputs are pure sine waves, which are very close to the ideal AC power supply. It can output the voltage and frequency of the power grid in any country in the world. The frequency converter is composed of AC-DC-AC (modulation wave) and other circuits. The standard name of the frequency converter should be variable frequency speed regulator. The waveform of its output voltage is a pulse square wave, and there are many harmonic components. The voltage and frequency change proportionally at the same time and cannot be adjusted separately, which does not meet the requirements of AC power supply. In principle, it cannot be used as a power supply, and is generally only used for speed regulation of three-phase asynchronous motors.