why three phase induction motor draws current with spike at low frequency by using VFD?

Question

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?

Answer 1

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.

Answer 2

I suspect the PWM is not behaving like is should or it is providing boosted V/Hz below 20 Hz. If you provide constant V/Hz programmed to close to zero V at zero Hz, the motor should run fine without a load from 1 or 2 Hz and above. It should also work well with a fan or centrifugal pump load. To get enough torque to start a constant torque load, you need some increase in V/Hz below 10 Hz or so. You need to find out by trial and error how much the motor will tolerate without drawing too much current.

Answer 3

Assuming you are using a commercially available VFD, most of them provide a "Torque Boost" function at low speeds when using V/F control, because of the reasons stated by the previous responders. Most manufacturers ship their drives with this as an optional feature that you must program to turn on, some ship with it turned on as the default setting and you must turn it off in programming. It could also be that someone else before you turned it on. Look through your programming parameters.