Basic knowledge of inverter and troubleshooting

01. 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.

02. What are the 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.

03. What is the difference between voltage type and current type?

The main circuit of the inverter can be roughly divided into two categories: the voltage type is an inverter that converts the DC of the voltage source into AC, and the DC circuit filter is a capacitor; the current type is an inverter that converts the DC of the current source into AC, and its DC circuit filter is an inductor.

04. 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.

05. When the motor is driven by industrial frequency power supply, the current increases when the voltage decreases; for inverter drive, if the voltage decreases when the frequency decreases, does the current increase?

When the frequency decreases (low speed), if the output power is the same (constant power), the current increases, but under the condition of constant torque (constant torque), the current remains almost unchanged.

06. When the inverter is used for operation, what are the starting current and starting torque of the motor?

When using a frequency converter to operate, the frequency and voltage are increased accordingly as the motor accelerates, and the starting current is limited to less than 150% of the rated current (125%~200% depending on the model). When starting directly with an industrial frequency power supply, the starting current is 6~7 times the rated current, so there will be mechanical and electrical impacts. Using a frequency converter drive can start 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 frequency converters with automatic torque enhancement function, the starting torque is more than 100%, and it can start with full load.

07. What does V/f mode mean?

When the frequency decreases, the voltage V also decreases proportionally. This problem has been explained in answer 4. The proportional relationship between V and f is predetermined by considering the motor characteristics. Usually, several characteristics are stored in the controller's storage device (ROM) and can be selected using a switch or dial.

08. 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.

09. The manual says the speed range is 60~6Hz, i.e. 10:1. So is there no output power below 6Hz?

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.

10. Is it possible to require a certain torque for general motor combinations above 60Hz?

Normally, this is not possible. Above 60Hz (there are also modes above 50Hz) the voltage remains constant, which is roughly a constant power characteristic, requiring the same torque at high speeds.

11. 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 machines 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.

12. 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%~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.

13. 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.

14. 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.

15. There are models where the acceleration time and deceleration time can be given separately, and models where 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.

16. What is regenerative braking?

If the command frequency is reduced during the operation of the motor, the motor will change to an asynchronous generator and work as a brake. This is called regenerative (electrical) braking.

17. Can I get more braking force?

The energy regenerated from the motor is stored in the filter capacitor of the inverter. Due to the relationship between the capacity and withstand voltage of the capacitor, the regenerative braking force of a general inverter is about 10%~20% of the rated torque. If an optional brake unit is used, it can reach 50%~100%.

18. Please explain the protection function of the inverter?

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, regeneration over-voltage cut-off, semiconductor cooling fan overheating and instantaneous power failure protection.

19. Why does the inverter protection function work when the load is connected by 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.

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

When the motor starts, a starting current corresponding to its 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.