Today is...
Saturday, June 15, 2019
Welcome to, the global online
community of automation professionals.
Featured Video...
Featured Video
A tutorial introduction to programming using the QuickBuilder Programming Environment.
Our Advertisers
Help keep our servers running...
Patronize our advertisers!
Visit our Post Archive
Motor Protection Questions
MPCB's, Contactors and Overload relays

Hi Everyone,

I am fairly new to automation and controls, control panels, PLC world. I am trying to better understand the protection for motors.

What I am trying to understand is, we typically use a MPCB (Motor Protection Circuit Breakers), Contactor and an Overload relay to protect the motor. Can someone explain to me in layman's terms the functions of each of these devices? For instance, I know the overload relay keeps the motor from overloading. But I also understand the MPCB can also provide overload protection for the motor. So why have both? And what is the contactors main function? Is it to basically manually and electrically a shut off?

Any advice is much appreciated.

You've given a perfect description of why they are used. Fuses can fail, breakers can fail, and you may not be able to get to the manual disconnect quick enough.

Some manufacturers even require an additional "safety disconnect" within arms reach of the motor to insure maintenance safety, but most companies now consider the disconnect a half-mile away is enough, at least for the boss....

Any motor requires both overload and magnetic protection, because the first is related to the thermal state of the motor at any given moment while in operation. If the motor overloads, it will overheat and should be shut off after a certain period of time vs. overload, usually this being 150% for one minute. Larger overloads trip quicker, overloads up to 115% (called service factor or SF) will not trip the breaker. The magnetic protection will work in case of overvoltage or overcurrent. If you relied only on thermal protection. by the time the overload breaker trips, the motor may be damaged severely. These electrical conditions require for a much quicker trip to prevent further damage, which usually relies on a curent sensing solenoid which will trip the breaker. There are combined thermal/magnetic breakers that do both jobs in one package. Please note that there are different magnetic trip current vs. time curves. Google AC motor protection to find more literature on this.

I hope this helps.

The breaker protects the power circuit from faults. The contactor is there for control of the motor (energizing/deenergizing); the overload protects the motor from loading related problems.


How big is the motor (KVA or KW), and the use?