I have built Variable frequency drive (VFD) for a three-phase induction motor using V/F constant control. while I was testing 15HP three-phase induction motor without load with my VFD, the motor draws current with a spike at low frequency. Low frequency means (10 to 20 HZ), and the current value is about to 1A to 3A. This only happens when I start the motor from 0 HZ to 20 Hz. when the motor reaches to above 20 HZ, the current value becomes low (1A) and smooth.
My question is to you what will be the factor that draws the current with variation at low speed or at low frequency. ? what should I have to notice?
is there any winding arrange of the three-phase induction motor for starting torque that is drawing current more also at low frequency?
or I should not run motor below 20HZ?
Electrical – why three phase induction motor draws current with spike at low frequency by using VFD
induction motorvf-fv-convertervfd
Related Solutions
Now, how can i further simplify the equivalent circuit of the three phase motor
Try this one: -
Taken from here
Yes of course, you can limit the current in VFD. With VFD actualy you will have a soft start, the VFD is more sophisticated device than a soft starter. If the load is a fan (VFD for HVAC?) , then it's even more simple because the load is very small at low rpms, so at the start the all load is basicly the acceleration of rotor and blades inertia, you can set large ramp time - slow acceleration and there will be no problems.
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Best Answer
You have to understand an induction motor is basically a transformer with an odd rotating secondary. It has all the properties of the transformer.
What beats you here is magnetic saturation. The voltage you may apply to a given transformer primary depends linear on frequency (within the bounds of the used magnetic material). For your motor/transformer, as soon you go below 20Hz, you have to reduce the primary voltage to avoid saturation of the core.
Saturation does exactly this, during one half of the AC cycle the magnetic material cannot "eat" more voltage-time and so the current increases as it wasn't there. That's why you see those spikes. The lower your frequency drops, the longer the time per AC cycle in which the magnetic material is saturated.