What is the Difference Between VFD for Synchronous Motor and VFD for Asynchronous Motor?

VFD electric motors are divided into VFD for synchronous motors and VFD for asynchronous motors. VFD for asynchronous motors is derived from ordinary asynchronous motors. Due to the characteristics of the output power of VFD electric motors, the motors have made great changes in the rotor slot, insulation technology and electromagnetic design verification, especially the ventilation and heat dissipation of the motor. In general, an independent forced cooling fan is added to adapt to the efficient heat dissipation of the motor at low speeds and to reduce the wind friction of the motor at high speeds. The output of the inverter generally displays the output frequency of the power supply, and the speed output is displayed as the number of poles of the motor and the calculated value of the output frequency of the electrical power supply unit, which is very different from the actual speed of the asynchronous motor. Generally, asynchronous frequency conversion is used.

Since the slip rate of an asynchronous motor is determined by the manufacturing process of the motor, its dispersion is very large, and the change of load directly affects the speed of the motor. To accurately control the speed of the VFD compatible motor, only the photoelectric encoder can be used for closed-loop control. The accuracy of the speed during control is determined by the number of pulses of the encoder. When multiple machines are controlled, the speed of multiple motors cannot be strictly synchronized. This is determined by the asynchronous motor.

The rotor of VFD for synchronous motor is embedded with permanent magnets. When the motor is started instantly, the motor turns into a normal operation. The stator rotating magnetic field drives the rotor with permanent magnets to operate synchronously. At this time, the speed of the motor is based on the number of poles of the motor. It has a strict corresponding relationship with the input power frequency of the motor, and the speed is not affected by load and other factors. Similarly, VFD for synchronous motor is also equipped with an independent forced cooling fan to adapt to the efficient heat dissipation of the motor at low speed and reduce the wind friction of the motor at high speed. 

Due to the strict correspondence between the speed of the motor and the frequency of the power supply, the speed accuracy of the motor mainly depends on the accuracy of the output power frequency of the inverter. The control system is simple. One inverter controls multiple motors to achieve the same speed of multiple motors. There is also no need for expensive optical encoders for closed-loop control.