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# Can variable frequency speed regulation and industrial frequency fixed speed switch operation?

Nov 26, 2020

Can variable frequency speed regulation and industrial frequency fixed speed switch operation?

The answer is yes, yes. But what I want to introduce here is the situation of switching to industrial frequency operation when the inverter fails.

The basic principle of the motor is based on the relationship between the speed of the motor and the input frequency of the working power supply: n = 60 f(1-s)/p, (where n, f, s, p represent the speed, input frequency, and motor slip respectively Rate, motor pole pairs). The asynchronous speed is equal to the synchronous speed minus the slip. According to the relational formula, the speed is related to frequency and the number of pole pairs, and the number of pole pairs of the motor is fixed, so the speed has a direct relationship with frequency.

Power frequency fixed speed: Power frequency is the general mains frequency, in China it is 50Hz, and it is fixed without frequency change measures. Therefore, if the power frequency is not changed, and the number of magnetic pole pairs is not changed, the speed will not change.

Variable frequency variable speed: According to the formula, as long as the frequency f is changed, the speed of the motor will also change accordingly.

In actual production, some equipment is not allowed to stop during operation, such as the cooling water pump of the central air conditioner and the blower of the boiler. Once the frequency converter trips due to a fault, these machines must be able to automatically switch to industrial frequency operation during the frequency conversion operation process, and at the same time, they can perform sound and light alarms. The main circuit of the switch is shown in the figure below. The function of KM1 is to connect the power supply to the power input of the inverter, the function of KM2 is to connect the output of the inverter to the motor, and the function of KM3 is to connect the industrial frequency power supply to the motor. Because the inverter cannot protect the motor from overload when the power frequency is running, the thermal relay FR must be used to protect the power frequency running motor.

The switching between motor frequency conversion operation and industrial frequency operation should be automatic.

If the motor trips due to a fault of the inverter during variable frequency operation and needs to be switched to power frequency operation, the control circuit should first release the contactors KM1 and KM2, disconnect the motor from the inverter, and close KM3 after a proper delay. The motor is connected to the power frequency power supply and starts working at the power frequency. Contactors KM2 and KM3 must not be connected at the same time. In addition to using electrical locking between the two, it is best to use mechanical locking at the same time.