While running a induction motor at 5 Hz, When i measure current in one line, its 0.1A and other line its 0.01A.Winding resistance all same.
Generally, why imbalance in each line occurs in induction motor, if it is small variation means, its ok...In the case of 5.5kw i found morethan 5A variation.why?

 

1)Check the loose connection at all the ends
2)If supply voltage (Between the phase)itself have variation then the you have variation of load in each pahse
Other than this we dont have any reason,Best of luck

 

> While running a induction motor at 5 Hz, When i measure current in one line, its 0.1A and other line its 0.01A.Winding resistance all same.


I think it is problem of your meter. The digital multimeters usually integrate the value over the line frequnecy's period. So if this is 20 ms, it samples one tenth of the period of a 5Hz signal. And this may occur near zero crossing or near peek of the sine wave.


> Generally, why imbalance in each line occurs in induction motor, if it is small variation means, its ok...In the case of 5.5kw i found morethan 5A variation.why?


Also, expect strange effects in case of VFD driven motors, even when they run on line frequency, because the output is not a sine wave and it's harmonics present problems to digital meters.
When the motor is directly connected to a threephase lin, all currents should be equal and any meter should show that correctly.

 

I assume following things :

a) You are using VFD for running motor on 5Hz.
b) You are getting deviations in measurements when checking with hand held ammeter.

Checkout :
Accuracy of meter you are using for current measurement. Is meter designed to know such low current.

Since u hv mensioned that you are getting difference of 5Amps when 5.5kW is connected then I think VFD should trip immediately by such vast difference in currents of phases.

Check amp indications in VFD generally it shows accurate reading than hand held meters

Are you getting these readings under loading condition or no-load? if loaded then what is load applied to motor??

do reply to me on [email protected] or call 7230021-26 extn: 502 at mumbai.

Regards,
yogesh

 

> I assume following things :
>
> a) You are using VFD for running motor on 5Hz.
> b) You are getting deviations in measurements when checking with hand held ammeter.
>
> Checkout :
> Accuracy of meter you are using for current measurement. Is meter designed to know such low current.

IT IS POWER METER..IMPORTED FROM JAPAN.AND ITS OK WITH ACCURACY.


> Since u hv mensioned that you are getting difference of 5Amps when 5.5kW is connected then I think VFD should trip immediately by such vast difference in currents of phases.
>
> Check amp indications in VFD generally it shows accurate reading than hand held meters
>
> Are you getting these readings under loading condition or no-load? if loaded then what is load applied to motor??

LOAD IS =100KG
A ROTATING DRUM CONNECTED TO THE MOTOR WITH TIMING BELT. THE TYPE OF LOAD IS JUST "COTTON OF
100KGS"

 

If your applied voltage is (almost) equal then it is always the instrument that causes the error. Try with a different type of instrument and measure at a point where it does not have a interference with stray magnetic fields from the motors or transformers. This has really nothing to do with the motor.

Best regards,
Sekar

 

> While running a induction motor at 5 Hz, When i measure current in one line, its 0.1A and other line its 0.01A.Winding resistance all same.
> Generally, why imbalance in each line occurs in induction motor, if it is small variation means, its ok...In the case of 5.5kw i found morethan 5A variation.why?

The problem is with the meter. The meter does not read correctly when reading the current after the drive caused by the carrier frequency. This is generally 2500 to 3800 Hz on older drives, around 5000 Hz on modern drives and as high as 10000 Hz on some newer drives. Even true RMS meters do NOT read accuratly in this application. You could simply read the current INTO the drive with a true RMS meter.