Calculation formula for variable frequency speed regulation

The calculation formula of variable frequency speed regulation includes the following aspects:

1. Frequency: The frequency of the inverter output can be calculated as follows:

Output frequency = input frequency x speed ratio

The speed ratio is usually set through the inverter control panel or program, usually between 0.1 and 1. For example, if the input frequency is 50Hz and the speed ratio is 0.5, the output frequency is 25Hz.

2. Speed: The speed output by the inverter can be calculated as follows:

Output speed = input speed x speed ratio

The input speed refers to the speed of the motor, which is usually calculated based on the rated power and rated speed. For example, if the rated power of the motor is 7.5kW and the rated speed is 1440rpm, the input speed is:

Input speed = 60 x (60 x 50) / 2 = 1500rpm

If the speed ratio is 0.5, the output speed is:

Output speed = 1500 x 0.5 = 750rpm

3. Voltage: The voltage output by the inverter can be calculated as follows:

Output voltage = input voltage x modulation index

The modulation index refers to the amount of change in the inverter output waveform. Generally speaking, when the modulation index is 1, the output waveform is closest to a sine wave. When the modulation index is greater than 1, the output waveform becomes sharper and may have an adverse effect on the motor. For example, if the input voltage is 380V and the modulation index is 1, the output voltage is also 380V.

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

Usually the motor is designed and manufactured according to 50Hz voltage, and its rated torque is also given within this voltage range. Therefore, speed regulation below the rated frequency is called constant torque speed regulation. (T=Te, P<=Pe)

When the inverter output frequency is greater than 50Hz, the torque generated by the motor decreases in a linear relationship inversely proportional to the frequency.

When the motor runs at a frequency greater than 50 Hz, the size of the motor load must be considered to prevent insufficient motor output torque.

For example, the torque produced by a motor at 100 Hz is reduced to about 1/2 of the torque produced at 50 Hz.

Therefore, speed regulation above rated frequency is called constant power speed regulation. (P=Ue*Ie)