Motor shows high current once run through VFD

Thread Starter

tahir mehmood

Joined Jun 28, 2013
27
Have a VFD of 45 kw, 380/480 volts. motor connected to it is of 37 kw, 380/400 volts. the FLA for the motor is 66 amperes. But once it runs through VFD it shows 76 amperes on the VFD display. similarly checked through clamp meter at motor JB, still the same readings appear.

The question is why the motor shows high current on vfd display and clamp meter though its running smoothly. kindly assist.
 

MaxHeadRoom

Joined Jul 18, 2013
26,286
For one the nature of the three phase output signal. is quite different in nature to the normal transformer fed of the same value.
The wave form varies in both amplitude and frequency, of which the sine wave has a PWM signal superimposed on it, I would motor the temperature and disregard the reading if it shows normal.
What is the current limit set to in the VFD?
Max.
 

shortbus

Joined Sep 30, 2009
9,762
Just a guess and some one may say different. Motors run by a VFD are sometime plagued by electrical shorting in the shaft ball bearings. This applies more to old motors made before VFD's became popular. So could it be your getting a short in the bearings causing a higher load showing when running? Like I said just a guess.
 

MaxHeadRoom

Joined Jul 18, 2013
26,286
One positive method to see if the motor is actually in over current is to measure the rpm against the indicated plate rpm at the rated frequency, if the same or lower then there is nothing to worry about, if there is any kind of over current it will be indicated by rpm as this is the only way a induction motor current can be exceeded.
Max.
 

Thread Starter

tahir mehmood

Joined Jun 28, 2013
27
For one the nature of the three phase output signal. is quite different in nature to the normal transformer fed of the same value.
The wave form varies in both amplitude and frequency, of which the sine wave has a PWM signal superimposed on it, I would motor the temperature and disregard the reading if it shows normal.
What is the current limit set to in the VFD?
Max.
The rated motor current set in the VFD is 81 amperes.
 

MisterBill2

Joined Jan 23, 2018
13,797
Just a guess and some one may say different. Motors run by a VFD are sometime plagued by electrical shorting in the shaft ball bearings. This applies more to old motors made before VFD's became popular. So could it be your getting a short in the bearings causing a higher load showing when running? Like I said just a guess.
The problem might be arcing in the bearings. But for a standard induction motor it is indeed more likely a waveform issue.
 
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shortbus

Joined Sep 30, 2009
9,762
The problem might be arcing in the bearings, if there are windings on the rotor. But for a standard induction motor it is indeed more likely a waveform issue.
Then all of the information on the web about this problem is wrong? And there is a winding analog in an induction motor rotor, it's what allows them to work.
 

MisterBill2

Joined Jan 23, 2018
13,797
Then all of the information on the web about this problem is wrong? And there is a winding analog in an induction motor rotor, it's what allows them to work.
The winding in the rotor of an induction motor is not connected except by magnetic coupling. So any currents induced are not flowing in the power lines to the field. Bearing arcing is simple to cure by adding a shaft grounding brush, now sold by several companies. Yes, inverter drive can be hard on motors not designed for it. Mostly because of waveform distortion and harmonic content of the drive power.
 

MaxHeadRoom

Joined Jul 18, 2013
26,286
The winding in the rotor of an induction motor is not connected except by magnetic coupling. So any currents induced are not flowing in the power lines to the field. Bearing arcing is simple to cure by adding a shaft grounding brush, now sold by several companies. Yes, inverter drive can be hard on motors not designed for it. Mostly because of waveform distortion and harmonic content of the drive power.
A typical squirrel cage AC induction motor at the point of switch on is a transformer with a shorted turn secondary, the current in the shorted 'secondary' coils depend on RPM (decreasing slip frequency).
Using a 3 phase inductor/choke in between the VFD output and motor reduces harmonic content and makes it easier on both motor and VFD.
Max.
 
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