Basic VFD questions

[sry110]

I'm considering using a VFD to softly engage a lightly loaded system driven by an AC motor. I have a couple basic questions regarding VFD:

1) Can the VFD be set to control motor speed even if the motor is unloaded?

2) After the speed ramp period expires, can the VFD allow full line voltage to the AC motor so that it can produce its full Locked Rotor Torque? (i.e. like a soft starter)

Thanks in advance!
Steve

 

[Skogsgurra]

Yes, the speed is controlled via frequency and motor speed is quite load independent (the droop is some few percents from no load to full load). I recently published a check list for VFD drives that shows a by-pass arrangement. A by-pass contactor will give you the DOL characteristic of the motor if you need it.

See thread237-282147

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[pavan16182]

What is a difference between A.C choke and D.C choke?

 

[DickDV]

An AC choke is normally called a reactor and, in the AC drive world, would be placed in series with each phase of the incoming AC supply.

Some drives have an inductor between the input rectifiers and the DC bus capacitors to improve the filtering. This would be called a bus choke or DC bus choke. It also improves the input current harmonics somewhat by keeping the rectifier diodes conducting longer.