Motor Circuit Protection 3

[microaid]

I have been browsing this forum to gain an appreciation of motor circuit protection in an attempt to broaden my knowledge.

It appears to me that motors are supplied from a MCC (Motor Control Cubicle/Centres?) which may be up to say 100m from the motor. The motor has a thermal overload protection device physically located in the motor and there would be a thermal magnetic device in the MCC to protect the motor circuit under overload (thermal) and short circuit (magnetic) conditions. There would also be OC and EF protection in the form of a ACB, MCCB or relays on the circuits feeding the MCC.

Would you be kind enough to correct or expand on my understanding of this area.

 

[dpc]

Motor protection will vary depending on motor type/size and your geographic location and local codes.

In general, integral horsepower motors are NOT provided with built-in overload protection as a standard. OL protection is generally provided by an overload relay at the motor starter.

In some instances, some type of embedded temperature detector may be provided in the motor (thermistor, RTD, thermocouple) and wired back to the MCC to shut down the motor on high temperature. This is generally provided in addition to the overload relay at the starter.

Short circuit protection can be provided by thermal-magnetic circuit breakers, magnetic-only circuit breakers (a.k.a. Motor Circuit Protectors or MCP) or by fuses.

Ground (earth) fault protection is generally NOT required or provided in most cases, at least in the U.S, although it is becoming more common, especially for larger motors.

The other common protection that you don't mention is protection against loss of one phase since motors will quickly overheat when one phase is lost.

Phase reversal protection is sometimes provided when necessary.

Its always good to keep in mind the cost of the motor versus the cost of the protection. For smaller motors, the costs for elaborate protection can easily exceed the cost of a replacment motor or re-wind.

Fire pump motors are a special case and do not have any overload protection.

Likewise, motors to be located in hazardous locations have special requirements.

 

[d23]

microaid

DPC gave you a great explanation!!!

I would like to add a couple things about control devices that you may want to consider. It seems a customer is willing to spend thousands or ten of thousands of dollars buying production equipment, but when it comes to controls they would prefer electro-mechanical to micro processor control in order to save a few dollars. This saves money????

I would recommend that you should consider a MCC being within 100m to be the norm, but the exception is out there. All codes that I have seen require a means of disconnect to be located within X meters of the motors. For protection of your personnel this is a good “control” circuit.