The Speed of Induction Motor Under VFD Control

Time:2022-10-26 Category:Technical Knowledge

Induction motor has been widely used in most of industry fields. And In many cases, the motor is required to be used with a variable frequency drive, which is also called VFD to control the speed, save energy, protect the motor system and realize various action functions of motors. And speed adjustment of motor is the key in most of applications.

In this passage, And we will shortly introduce the speed of Induction motor under VFD control.

1. Why does VFD can change the speed of asynchronous motor?

      VFD supply voltage with adjustable frequency to run the motor and can change the speed of motor by varying its frequency.

  Here’s question: how does this work?

The great scientist and engineer Nicola Tesla discovered the rotating magnetic field in a park in Budapest 1882 and later invented the alternating current motor. And the modern induction motor evolved from Tesla’s genius work.

      In induction motor, three phase winding as stator can generate rotating magnetic field (RMF) when AC current flows in, and the RMF will cut the rotor bar to generate current and motor’s rotor will start rotating due to the torque caused by the interaction between current and magnetic fields.

      And VFD can supply the AC power and change the output frequency, which will change the the rotation speed of RMF, which is also called synchronous speed (Ns), to control the motor’s speed, which is also called rotational speed (N or RPM).

2. What’s the speed of three phase asynchronous motor under VFD control?

     The Speed of motor (N), or its RPM (revolutions per minute) is related with the number of motor’s poles and the frequency of alternating current that VFD supply.

     To understand this well, we need to introduce a concept of slip, it is used to describe the relative difference between synchronous speed and rotational speed:

     And in asynchronous motor, s≥0, that means the synchronous speed and rotational speed are different, and former is larger then latter. That’s why we call it asynchronous motor sometimes. In this situation, the RMF has different speed from rotor and this causes RMF to cut the rotor bar, so the rotor can have the current and rotate due to the torque caused by interaction between current and magnetic fields. And averagely, s≈4%.

Now let’s calculate the asynchronous speed first then we can get the motor’s speed.

For 2 to 14 poles of motor, the asynchronous speed Ns can be calculated by the following equation:

Here, f represents frequency, and P represents numbers of poles. And you can get the motor’s speed N by the following equation:

For example, if you are using 50Hz frequency to run a 4 poles motor with slip of 4%, you will get the its N is about 1440 RPM.

4 poles motor runs at the speed of 1440RPM at the rated power of 50Hz, and the 2 poles motor runs at 2880RPM at the rated power of 50Hz.

3. How to adjust motor’s speed by VFD?

   We can adjust motor’s speed by rotating the potentiometer of VFD to change speed directly or using analog input & digital input of VFD to give electronic signal to change the speed, or maybe we can use communication protocol like Modbus, Profibus or other industry communication protocol to integrate the VFD into control system and control the speed.

4. How to set multi-stage speed control of VFD to run motor at different stage of speed?    

Here we presents also a very commonly used control mode on USFULL VFD: multi-stage speed control. It is a very common control mode used in many industry cases.

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