Mechanically held coil for motor MCC 1

[Mila15]

Are motor MCCs with Mechanically held coils (one close, and one trip) common? Vendor said those are mainly used on breakers, or starters feeding transformers. The only reason I can think of is safety concern, as you need to be able to stop motor remotely if needed, but that can be solved. Any thoughts?

 

[waross]

Many application are prohibited by code from restarting automatically after a power failure. A trip col would have to have an independent source of energy and automatically trip on loss of supply power.
Yes, it can be done but the cost and complexity and possible unsafe failure modes dictate an electrically held contactor for motor starting.
And to answer your main question:
No, trip coils are not common in MCCs. Actually very rare unless a power circuit breaker has been repurposed as a motor starter.
Even then the trip coil is often replaced with a trip coil that is energized when the motor runs and which will trip the breaker on loss of power.
AKA Shunt trip.
Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Mila15]

Thanks for the help! In our MV application, we have control power suppliesd by external UPS panel. The reason why we're not using a CPT configuration is because we have suffered multiple sag incidents that caused MCCs to drop, losing very critical loads. But that also created an issue when technicians tripped the CB feeding this control power, when working on panel. This could've been avoided if mechanically held coils were used. We still need to make sure protection is always ON when motor is ON, so we could use a CPT circuit just for that, while keeping other control on UPS. The relay could be used to trip MCC when power is lost to satisfy requirement Bill indicated above.

 

[waross]

But that also created an issue when technicians tripped the CB feeding this control power, when working on panel.

A word with your technicians may be in order.
Really, the tech's should be aware of the loads on a breaker before tripping it.
You are trying to fix a symptom rather than the root cause of the problem.
Once should be enough to learn. If it ever happens again, don't change the circuit, change the tech. There is where the problem is.
Changing to mechanically held coils may cause more serious problems than you have now, particularly with that level of technical support.
The UPS is a good solution to power dips, but you should add a relay with a very low drop out voltage to shut down the critical motors in the event of a real power failure.
A power failure lasting for a short time (less than a few seconds) may do serious mechanical damage to large motors, including bent or broken shafts.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[FreddyNurk]

For context, as I've not heard the term 'mechanically held coil' before, what does it refer to?

Is it similar to the coil on a contactor, which needs to stay energised to keep the contacts closed?
Is it what I know to be an 'undervoltage release coil' to be fitted to a circuit breaker (Schneider Electric call theirs an 'MN' release coil, as opposed to an 'MX' shunt release that requires the coil to be energised to open the breaker).

EDMS Australia

 

[davidbeach]

Contactor, not coil. A contactor with both a close coil and an open coil. Much less common than the electrically held contactor that only has a close coil. More likely to be used for lighting control applications than motor control applications. In lighting it is highly desirable for the contactor to stay where it was through a power out unlike motor applications where any attempt at a mechanically held contactor needs an undervoltage relay and a separate power supply (battery) to hit the open coil during an interruption of the normal supply.

 

[LionelHutz]

The equipment you're describing with UPS control power and latching circuit breakers is very common in medium and high voltage systems. I don't really see any need to "fix" it. You just need to figure out the correct operating procedures to ensure it works as intended.

In theory, you could put an under-voltage trip in each breaker and power it from the UPS circuit so the breaker would trip if control power is lost. But, I don't see that normally being done in practice.

"Even then the trip coil is often replaced with a trip coil that is energized when the motor runs and which will trip the breaker on loss of power.
AKA Shunt trip."

Bill, that is an under voltage trip coil. The normal trip coil that is powered to cause a trip is an shunt trip.

 

[waross]

I stand corrected.
Thank you for the clarification Lionel.
Yours
Bill

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

Yes, I’ll confirm that this is done quite often in MV (and I suppose HV) motors because the loads are usually very large and a momentary loss/reapplication of control power can create massive torque transients and/or voltage spikes. I have seen it shear off large motor shafts. So the contactor is made to be mechanically held with either a UPS for control power, or a Capacitor Trip Unit for the unlatch coil.

The entire concept is generally avoided on low voltage and smaller machines because of the inherent safety concerns that require a lot more attention and money to be paid on the control circuit.


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