How Does the Variable Frequency Converter Control the Speed of the Motor?

2022-07-14

1. Why can the rotation speed of the variable frequency converter be freely changed?

The rotational speed of an induction AC motor is approximately determined by the number of poles and frequency of the motor. The number of poles of the motor is fixed by the working principle of the motor. Since the pole value is not a continuous value, it is generally not suitable to adjust the speed of the motor by changing this value. In addition, the frequency can be adjusted outside the motor and then supplied to the motor, so that the rotation speed of the motor can be freely controlled. Therefore, the frequency converter for the purpose of controlling the frequency is the preferred device as the motor speed regulating device.

If you only change the frequency without changing the voltage, the motor will be in overvoltage when the frequency is reduced, which may cause the motor to burn out. Therefore, the frequency converter inverter must change the voltage at the same time when changing the frequency. When the output frequency is above the rated frequency, the voltage cannot continue to increase, and the maximum can only be equal to the rated voltage of the motor.

2. When the rotational speed and frequency of the variable frequency converter change, what will happen to the output torque?

The starting torque and maximum torque when driven by a frequency converter are smaller than those driven by a direct power frequency power supply. When the motor is powered by a power frequency power supply, the start and acceleration shocks are large, but when the frequency converter is used for power supply, these shocks are weaker. The direct starting of the power frequency will generate a large starting current, and when the inverter is used, the output voltage and frequency of the inverter are gradually added to the motor, so the starting current and impact of the motor are smaller. Usually, the torque generated by the motor should be reduced with the reduction of the frequency, and the actual data of the reduction will be explained in some inverter manuals. By using the inverter controlled by the magnetic flux vector, the insufficient torque of the motor at low speed will be improved, and the motor can output sufficient torque even in the low speed region.

3. When the frequency converter inverter is adjusted to a frequency greater than 50Hz, the output torque of the motor will decrease

The usual motor is designed and manufactured according to 50Hz voltage, and its rated torque is also given in this voltage range. Therefore, the speed regulation below the rated frequency is called constant torque speed regulation. When the output frequency of the inverter is greater than 50Hz, the torque generated by the motor should decrease in a linear relationship that is inversely proportional to the frequency. When the motor runs at a frequency greater than 50Hz, the size of the motor load must be considered to prevent insufficient motor output torque.

4. Application of variable frequency converter above 50Hz

For a particular motor, its rated voltage and rated current are constant. If the rated value of the inverter and the motor are both: 15kW/380V/30A, the motor can work above 50Hz. When the speed is 50Hz, the output voltage of the inverter is 380V and the current is 30A. At this time, if the output frequency is increased to 60Hz, the maximum output voltage and current of the inverter can only be 380V/30A. Obviously, the output power remains unchanged, so we call it constant power speed regulation.

5. Other factors related to the output torque of the variable frequency converter

The heating and cooling capacity determines the output current capacity of the inverter, thus affecting the output torque capacity of the inverter.

(1) Carrier frequency: Generally, the rated current of the inverter is the value that can guarantee continuous output at the highest carrier frequency and the highest ambient temperature. Reduce the carrier frequency, the current of the motor will not be affected, but the heating of the components will be reduced.

(2) Ambient temperature: The inverter protection current value will not be increased when the ambient temperature is detected to be relatively low.

(3) Altitude: The increase in altitude will affect both heat dissipation and insulation performance. Generally, the capacity below 1000m can be ignored, and the derating of 5% per 1000m above is enough.