Is it appropriate to connect a contactor to the output terminal of a frequency converter?
A: Generally, contactors should not be connected to the output terminals of a frequency converter . This is because if the contactor at the inverter's output terminal is not engaged, and the inverter only engages after starting and reaching a certain frequency, a large overload current will occur, causing the inverter to trip due to overcurrent or be damaged. If a contactor must be connected to the inverter's output terminal during operation, the control circuit must ensure that the output contactor is engaged when the inverter starts. The inverter's output contactor must not be used as a stop or start element.
What are the applications of frequency converters with a frequency of 50Hz or higher?
As we all know, for a specific motor , its rated voltage and rated current are constant.
If both the frequency converter and the motor are rated at 15kW/380V/30A, the motor can operate at frequencies above 50Hz.
When the speed is 50Hz, the inverter's output voltage is 380V and the current is 30A. If the output frequency is increased to 60Hz, the inverter's maximum output voltage and current will still be 380V/30A. Obviously, the output power remains unchanged. Therefore, we call this constant power speed regulation.
What is the torque situation at this point?
Because P = wT (w: angular velocity, T: torque), and since P remains constant while w increases, the torque will decrease accordingly.
We can also look at it from another angle:
The stator voltage of the motor is U = E + I * R (where I is the current, R is the electronic resistance, and E is the induced electromotive force).
It can be seen that when U and I remain unchanged, E also remains unchanged.
Since E = k * f * X (k: constant, f: frequency, X: magnetic flux), X will decrease accordingly when f changes from 50 to 60 Hz.
For a motor, T = K * I * X (K: constant, I: current, X: magnetic flux), therefore the torque T will decrease as the magnetic flux X decreases.
Meanwhile, below 50Hz, since I*R is very small, the magnetic flux (X) is constant when U/f=E/f remains unchanged. The torque T is proportional to the current. This is why the overload (torque) capability of a frequency converter is usually described by its overcurrent capability, and it is called constant torque speed regulation (rated current remains constant --> maximum torque remains constant).
Conclusion: When the inverter output frequency increases from above 50Hz, the motor output torque will decrease.
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