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V/HZ relationship and more


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Hello everyone,


I'm new to this board. I have two questions.


1. I understand that VDF keeps a constant voltage v.s. frequency ratio in order to prevent the core from saturating.

I vaguely recall from school that if the motor has load, then you can safely increase the voltage without saturating the core.

Is this true and does some VFD have encoder feedback input to allow for load sensing?


2. Are there VFD that can drive single phase 120V capacitive starter induction motor?

I would think that the centrifugal switch may pose problems.


Thanks for you help.






[Edited on 10/9/02 by Rotate]

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Hello Rotate - Welcome to the discussions.


1. Yes, the voltage to frequency ratio must not exceed that of rated voltage and frequency in order to prevent saturation. Even if th motor is fully loaded, you must not exceed this ratio for any period of time. If you refer to the equivilent circuit of the motor, you will see that the magnetising current (which produces the flux) is a shunt component accross the incomming supply and is not really affected by load. It is common however, to operate the motor at reduced flux under reduced load conditions and that way to reduce energy consumption and motor heat.


2. Standard VFDs can not be applied to single phase motors. A single phase motor is actually a poly phase machine with a second phase beng generated by the capacitor and start winding. Changing the frequency would alter the current vector in this winding and would affect the torque. Additionally, the high frequency carrier would have bad effects on the capacitor!!

It is technically possible to replace the capacitor with a second output phase from the invertor, i.e. to have a two phase output inverter with a reduced voltage on one phase and drive the single phase motor that way, but in reality, it is better just to use a standard three phase inverter and motor. You can get single phase input, thre phase output inverters for this purpose.


Best regards,

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