Motor protection 1

[marges]

When using a GE multilin 469 relay to protect a motor which has power factor capacitor installed where must the CT be installed.

 

[dpc]

Ideally, you would measure the actual current into the motor.

If your protection CT is upstream of the capacitors, the measured current will be less than the actual current into the motor. You will need to compensate for this somehow, such as by adjusting the motor full load current that is entered into the relay. The relay's overload protection model depends on the measured current being equal to the current into the motor.

 

[davidbeach]

What dpc said. You will get much better protection of the motor if the motor is the only thing the protection is protecting.

 

[aolalde]

The difference between the line current (I2) feeding the capacitors+motor assembly and that on the motor alone (I1) is the reactive current.
When the line power factor is corrected to 1.0 (maximum, but not the ideal), the total line current I2 reaches the minimum value:
I2 = I1*PF.
Were;
Il = Motor rms current without capacitors.
PF = Motor power factor.

 

[jraef]

dpc is correct, but I find it curious that GE has for some reason left this information out of the 469 manual. I found no reference to this issue whatsoever. I would think that if one were to decide that your motor is important enough to spend this much on a relay, that they would want to assist the user in making it as accurate as possible. It can't be that they just expect someone to know this, because they are very explicit about other issues that I would expect most everyone to know.

Accidentally dropped in a manual revision maybe? I purged myself of all my out-of-date manuals unfortunately (I told my wife that I would need them!). It would be interesting to see if that information was in there pre-GE.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[waross]

Check the Canadian Electrical Code for reduced setting of the overload device when capacitors are connected after the overload devices. Warning, this a trick question. The answer is not under Motors and Generators, Protection and Control but under Installation of Electrical Equipment, Capacitors.
yours