Motor Overloads and Service Factors

what do you mean by service factor ?

Normally, over load relay is current operated. Motor should not draw the current more then its rated continuous current. But, we normally set the relay at the actual maximum load current, which may be less then the motor rated current. This protects indirectly the mechanical equipment / malfunction in the process also.

Service factor is the amount of overload the motor is designed to tolerate. A service factor of 1.15 means that the motor can be safely run at 1.15 times the rated amps.

Temporarily

For those not in the US or Canada, Service Factor is a NEMA motor terinology related to a percentage of short duration additional load that can be applied to a motor. Although there is no "official" definition of "short duration", it is understood that service factor is meant to be left alone so that the motor can withstand temporary low voltage conditions or unusually heavy loading. That said, many OEMs ignore this and design motors into their equipment such that the SF is used continuously, and if challenged will point out that the NEMA definition does not define a time limit. Brinksmanship is generally considered bad engineering practice but is not unheard of.

So with that in mnd, the BEST practice is to set OLs to the motor FLA and no more. IF YOU MUST USE THE SF, then set the OL relay to the absolute lowest amount you intend on using. For example even say your motor is labeled as 1.15SF, if you can do the job with 1.10, then set the OLR to 110% of FLA. For every 10 deg. C rise in motor temperature over t's rated temperature, you cut the life of the insulation in half. So anything that raises the core temperature is lowering the motor life. Running into the SF is acknwledged to be purposefully raising the temperature, thus removing any "fudge factor" you may have had.

Actually i want to make myself understand by provide you an example

Suppose a motor is having service factor of 1 and the full load amps are 20 what ll b my setting on the overlaod ll it be 20 Amps, 18ampsor 22amps.And if the same motor is having service factor of 1.15 what ll be my setting on overload ll it be 20 Amps, or 22 Amps\

Pls guide me accordingly

thanks

For you, I recommend setting the overload to it's maximum for all the motors in your facility. Please also remove all the safey guards from the prime movers as they will interfere with proper operational maintenance. If you have dangerous chemicals, store them in a heavily populated area such as a cafeteria or break room.

While you're at it, replace all the fuses with peices of conduit or dowel rods wrapped in aluminum foil.

Dowel rods, if made from wood, make terrible fuses.

I recommend to read the NEC Hand book Article 430 may be it can help you as a reference guide.