Large AC motor with PF Correction capacitor connected at its terminals 1

[RRaghunath]

Motor protection relay (MPR) is provided current input from the CTs in the switchgear panel and the thermal overload setting determined based on the rated current of the motor.
This is alright when the PF correction capacitor is connected to the motor terminals.

But, when the capacitor is connected at its terminals, the current seen by the relay would be different from the rated current of the motor (as the reactive component is being supplied by the capacitor).

How do we adjust the setting to compensate for the same.

In case the capacitor is taken out of service later on, what is to be done with the settings - recalculate?

Any experiences!

Thanks in anticipation.

 

[busbar]

The US National Electrical Code is not detailed in overload protection calculations. Where a motor installation includes a capacitor connected on the load side of the motor overload device, the rating or setting of the motor overload device shall be based on the improved power factor of the motor circuit.

Consider connecting capacitors upstream of the overload-sensing components, so capacitor current does not affect relay current.

 

[cbarn24050]

Hi, the relay was designed to work without pf caps, therefore any caps must be conncted on the line side of the relay otherwise the reactive current will not be monitored.

 

[jbartos]

Suggestion to the previous posting: It appears that the content is in a contradiction of NEC requirement posted in busbar posting.

 

[cbarn24050]

Hi Jbartos, it only apears that way because you haven't read it properly.

 

[jbartos]

Suggestion to the original posting marked ///\\faulty (Electrical) Jun 20, 2003
Motor protection relay (MPR) is provided current input from the CTs in the switchgear panel and the thermal overload setting determined based on the rated current of the motor.
This is alright when the PF correction capacitor is connected to the motor terminals.
///Please, would you clarify this in view of your posting below. Namely, should not the above sentence read: "This is alright when the PF correction capacitor is not connected to the motor terminals."?\\
But, when the capacitor is connected at its terminals,
///To which terminals? To the motor terminals?\\ the current seen by the relay would be different from the rated current of the motor (as the reactive component is being supplied by the capacitor).

 

[jbartos]

Suggestion to cbarn24050 (Industrial) Jun 22, 2003 marked ///\\Hi Jbartos, it only apears that way because you haven't read it properly.
///It is not quite clear from the original posting how the MPR is designed to operate since the sentence below the first sentence reads: "This is alright when the PF correction capacitor is connected to the motor terminals." This introduces an ambiguity since the capacitor is alegedly connected to the motor terminals and the MPR overload settings is determined on the motor rated current. This is not in agreement with the busbar posting:
""Where a motor installation includes a capacitor connected on the load side of the motor overload device, the rating or setting of the motor overload device shall be based on the improved power factor of the motor circuit.""\\

 

[cbarn24050]

Hi jbartos, clearly the op dosen't know whats what, thats why he's asking the question. You should be clearer as to whos posting you'r critisising.

 

[jbartos]

Suggestion: Often, the industrial expertise is more on the operation side than on the actual electrical engineering and design side.

 

[bigamp]

If the MPR CT's are upstream of where the capacitor is connected you have to do a calculation to determine the relationship between current as seen by the CT's and current going into the motor and base MPR settings on the calculated current. This is easy enough to do as you can take the capacitor current as being constant. Calculate the capacitor current based on normal voltage at the capacitor.

If you remove the capacitor, or connect it upstream of the CT's, you have to change the relay settings because the CT's will now see pure motor current.

 

[busbar]

A suggestion of using another set of electrically equal CTs to ‘offset’ capacitor current at the relay current inputs [thread238-60894] may work.

 

[dpc]

What a big fuss over a simple question! Bigamp is correct - overload settings must be adjusted if capacitors are downstream of overload device. If caps are taken out of service, overload settings must be adjusted. I think this question was on my PE test.

This is one reason why capacitors are often connected at the motor starter ahead of the overload relay.

 

[jbartos]

Suggestion: The capacitors connected upstream of the overload relay can cause transient oscillations passing through the overload relay elements.

 

[RRaghunath]

Thanks everyone.

I plan to calculate overload setting based on net current (vectorial sum of motor plus capacitor) seen by the relay at rated voltage.

With the overload pickup chosen at 110% as above, the pickup with capacitor out of service drops to about 100%. This I guess would be acceptable to our client considering that the capacitor outage is only a contingency.

Thanks everyone once again for the responses.

 

[GusD]

Hi faulty

Motor Protection Relays can only work if they are calibrated and set to protect a specific current equivalent value.
It does not matter to the relay if the motor has a capacitor,no capacitor or what have you.The only thing the relay looks at is the "Total" current/heat being drawn by the drive .If future conditions change due to Cap removal or other,relay will have to be reset to the expected new value.That is the only way a MPR will be able to do its job properly.

GusD

 

[Shortstub]

Jbartos,
This is somewhat removed from the original query (effect of pf cap on protection), but "transients" caused by the cap, would not be "seen" by the "overload" device. As was said before, pf improvement caps are sized to offset most of the no-load inductive reactance! More than that would be considered poor engineering.