Is it ok while vfd output terminal connects with contactor?

- Oct 11, 2018 -

1. Some friends like to connect contactor with the output terminal of VFD,Is this suitable?

Answer: Under normal circumstances, The contactor is not allowed to be connected with the output terminal of vfd . Because when the contactor at the output of vfd is in open status,the contactor will only suck closed when the vfd starts and reaches a certain frequency. And then it will bring a large overload current, to cause the vfd trips as over-current or damage the vfd. 

If the contactor must be connected to the output of the vfd during use, the control loop must be ensure that the contactor is suck closed before the vfd started. It is not permissible to use contactor as a stop or starter component.


2. What is the application of the vfd above 50Hz?

As you know, for a particular motor, its rated voltage and current rating are constant.

If the vfd and motor are rated: 15kW/380V/30A, the motor can work above 50Hz.

When rotating speed is at 50Hz, the output voltage of the vfd is 380V, and the current is 30A, At this time, if the output frequency is increased to 60Hz, the maximum output voltage of the vfd can only be 380V/30A. Obviously, the output power is unchanged. So we call it constant power speed regulation.

What is the torque situation at this time?

Because P = wT (w: angular velocity, T: torque). P does not change, w increases, 

so the torque will decrease accordingly.

from another angle:

The stator voltage of the motor U = E + I*R (I is the current, R is the electronic resistance, and E is the induced potential)

You can see that when U, I are unchanged, E does not change.

However, E = k*f*X, (k: constant, f: frequency, X: flux), so when f is from 50-->60Hz, X will decrease accordingly.

For the motor, T = K * I * X, (K: constant, I: current, X: flux), so the torque T will decrease as the flux X decreases.

Meanwhile, when it is less than 50 Hz, since I*R is small, when U/f=E/f is constant, the magnetic flux (X) is constant, The torque T is proportional to the current. 

That is why we describe vfd's overload (torque) capability with the overcurrent capability and called it constant torque speed regulation (rated current is not changed -> maximum torque is constant).

Conclusion: When the vfd output frequency increases more than 50Hz, the output torque of the motor will decrease.


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