i want to explain more ..
a sqirel cage induction motor 400v , delta winding, 110kw.
this motor protected by an earth fault relay with a core balanced CT. when measuring the isolation resistance of the motor, it founds high (>1 G ohm for 1 min),but at the start of the motor the earth fault relay tripped (I=0.6A , delay time=0.6sec). i have to increase the delay time to 2.4sec to prevent the trip at the start of the motor. what is the cause of the appearance of the residual cauurent at the start of the motor. i knew that the motor spend about 6 monthes without operation .. the motor doesn't have a space heater.
Sometimes can happen like this, with large low voltage motors. Try to have the wiring in the middle of the earth leakage CT. During the start because of the flux is not uniform can give false earth fault trips. If is posible, try to adjust the current as well (try 1A OR 2A set point). 600 mA it's a little bit to low.
As well check the contactor of the motor if is ok (must have good electrical contacts)
Good luck!
the motor was working normally before 6 monthes ... it was working on this setting normally .. and there are similar motors and others with greater power working on setting I=0.3A ... t=0.3sec without any problems ...
maintenance was done on this motor ... and a calibration check for the earth fault relay ... but nothing new ... the setting of the time must be increased to prevent the tripping ... what i want to know is what may happened to make this change ... this phenomena appears on some others motor with less power ... (note: all the motors have no space heaters)
I have seen this type of performance a couple of times.
In one instance, the motor was being started by a soft starter and the harmonic current during start were affecting the earth leakage relay, and in then other, there was moisture forming in the motor which was causing some initial leakage but being vapourised off before damage occured.
Does the problem occur at a restart son after the motor has been running (Still warm) or is it only first thing in the morning.
Try doing a megger test on the motor before you start it for the first time in the morning.
hi Marke
ur analysis is what i was thinking for this problem .. as i was say that when a motor spends a long time out of service, this may form a moister inside it ... specially when there is no space heater inside it.
when restarting the motor after running for a long time it doesn't tripped, and it seems normal ..
when measuring the isolation resistance it seems good, more than 1G ohm ... but at measuring the megger gives a small value (1 - 2 Mega ohm) for about 4 or 5 sec and then increased.
if we want to explain what happened .. do we say the this moisture for a capacitance that affected by an AC sourc while doesn't detected by a DC source which is the Megger.
is this right or how can we explain it?
at the end ... thank u very much .. u r the second one who analys what happend like this ... no one agree with me when i say that there is a moisture inside the motor.. thank u
One way to check moist winding is to take its Polarization Index (P.I.). Measure one minute and ten minute insulation resistance (IR) values of the winding. P.I. is 10 minute IR value / 1 minute IR value.
If the winding is wet, P.I. will be less than 1.5 for class B machine and less than 2.0 for class F machine. A P.I. of more than 2.0 indicates a dry winding.
You would need a motorized or digital megger and a stop watch to do this test.
I agree, since "when restarting the motor after running for a long time it doesn't tripped, and it seems normal ..".
Seems it is likely moisture. Can the motor be heated for several hours before starting after being idle for a long time? At least attempting this once may prove your suspicions in addition to the other suggestions.
if the is unbalance in the current of the motor (delta) for any reason and there is no ground fault .... the summation of the current will be zero and the output of the core balanced CT will be zero ... isn't it?
