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Answers to basic questions about inverters

Oct 08, 2020

Answers to basic questions about inverters

1. What is an inverter?

  Inverter is an electric energy control device that uses the on-off action of power semiconductor devices to transform industrial frequency power into another frequency.

  2, what is the difference between PWM and PAM?

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

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

The main circuit of the frequency converter can be roughly divided into two categories: the voltage type is the frequency converter that converts the DC of the voltage source into AC, and the filtering of the DC loop is the capacitor; the current type is the frequency converter that converts the DC of the current source into AC. Its DC loop filter stone inductance.

  4. Why does the voltage of the inverter change in proportion to the current?

The torque of a non-synchronous motor is generated by the interaction between the magnetic flux of the motor and the current flowing in the rotor. Under the rated frequency, if the voltage is constant and the frequency is only reduced, the magnetic flux will be too large and the magnetic circuit will be saturated. The motor will be burned out at a time. Therefore, the frequency and the voltage should be changed proportionally, that is, the output voltage of the frequency converter is controlled while changing the frequency, so that the magnetic flux of the motor is kept constant, and the phenomenon of magnetic field weakening and magnetic saturation is avoided. This control method is mostly used for energy-saving inverters such as fans and pumps.

  5. When the motor is driven by a commercial power supply, the voltage drops and the current increases; for inverter drives, if the voltage drops when the frequency drops, does the current increase?

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

  6. What is the starting current and starting torque of the motor when the inverter is used for operation?

  Using frequency converter operation, with the acceleration of the motor, the frequency and voltage are correspondingly increased, and the starting current is limited to 150% of the rated current (depending on the model, it is 125%~200%). When the power frequency power supply is used for direct starting, the starting current is 6 to 7 times, therefore, mechanical and electrical shocks will be generated. Using frequency converter drive can start smoothly (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 automatic torque enhancement function, the starting torque is above 100%, and it can be started with full load .

  7. What does V/f mode mean?

   When the frequency decreases, the voltage V also decreases proportionally. This question has been explained in answer 4. The proportional relationship between V and f is determined in advance in consideration of the characteristics of the motor. There are usually several characteristics stored in the storage device (ROM) of the controller, which can be selected with a switch or a dial.

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

   When the frequency drops, the voltage is reduced in full proportion, then the AC impedance becomes smaller and the DC resistance remains unchanged, which will cause the torque generated at low speeds to decrease. Therefore, given V/f at low frequencies, the output voltage should be increased to obtain a certain starting torque. This kind of compensation is called enhanced starting. It can be realized by various methods, such as automatic method, selecting V/f mode or adjusting potentiometer.

  9. It is stated in the manual that the speed range is 60~6Hz, which is 10:1, then there is no output power below 6Hz?

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

   10. For the combination of general motors, a constant torque is required even above 60Hz, is it possible?

   It is not always possible under normal circumstances. Above 60Hz (there are also modes above 50Hz), the voltage remains unchanged, and the characteristic is generally constant power. When the same torque is required at high speed, attention must be paid to the selection of motor and inverter capacity.

  11. What does the so-called open loop mean?

   Set a speed detector (PG) for the motor device used, and feedback the actual speed to the control device for control, which is called "closed loop", and the operation without PG is called "open loop". General-purpose inverters are mostly open-loop mode, and some models can use options for PG feedback.

  12. What should I do when the actual speed deviates from the given speed?

  When the loop is open, even if the inverter outputs a given frequency, when the motor is running with a load, the speed of the motor will vary within the range of rated slip (1%~5%). For occasions that require relatively high speed regulation accuracy and require operation at a speed close to a 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?

  Inverter with PG feedback function, the accuracy is improved. But the speed accuracy depends on the accuracy of the PG itself and the resolution of the 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 angle frequency), the inverter will trip due to the overcurrent, and the operation will stop, which is called a stall. In order to prevent the motor from stalling, it is necessary to detect the magnitude of the current and control the frequency. When the acceleration current is too large, slow down the acceleration rate appropriately. The same is true when decelerating. The combination of the two is the stall function.

  15. There are models for which acceleration time and deceleration time can be given separately, and models for which the acceleration and deceleration time are given together. What's the point?

Acceleration and deceleration can be given for each model. It is suitable for short-time acceleration and slow deceleration, or for small machine tools where the production tact time needs to be strictly specified. However, for fan transmission, the acceleration and deceleration time are both Longer, acceleration time and deceleration time can be given together.

   16. What is regenerative braking?

When the motor is running, if the command frequency is reduced, the motor will run as an asynchronous generator and work as a brake. This is called regenerative (electric) braking.

China Zhejiang SAFESAVE Electric Technology Co., Ltd. specializes in manufacturing inverters. The existing general-purpose inverter 200G series, mini inverter 200mn series, and solar energy dedicated inverter 360 series, focusing on inverter research and development for 20 years, welcome all major customers to come to consult products .