motor overloads standards 2

[ilian]

I am looking for any standardized information on overloads motors should withstand relatively to their power rating.
Is there any norm or admitted standard about that ?

Thank you

 

[electricpete]

NEMA MG-1 only required rated temperature rise at rated load (or service factor rise at service factor load). That doesn't tell us anything about overload capability... except if it has a service factor... in which case the rise at service factor load is approx 10C above the normal required rise at rated load... equating roughly to a halving of insulation life. But there is a lot more that can be said about service factor... search this forum.

The fact that NEMA also allows the standard motor to operate at a voltage up to 10% low would seem to imply to me that NEMA is suggesting motors contain at least 10% overload capability.

Motor overload protective settings for 49 device are typically in the range of 115%-125%FLA and higher for service-factor rated motors. Some info in IEEEC37.96 section 7.2.9.1. I'm not sure exactly where these numbers originated, it may be the NEC.

The motor purchaser can request from the manufacturer a thermal damage curve IAW IEEE620-1996 which will define exactly how much time is allowed for any given level of overload. Alternatively if the motor is required to have a known short-time duty above rated, the user can make this part of his specification.

 

[dpc]

I agree with electricpete and would only add that you should not expect any overload capability beyond what is provided by the motor's service factor and duty specifications.

 

[ctolbert]

Electricpete is on the money as always

I would also like to add that the report from GAMBICA for vfd's is really good as far as insulation stresses for motors including NEMA MG1 pt 31 and IEC 60034-17 (the other motor standard that I see more and more these days.)

http://www.gambica.org.uk/newsite/public.html#g1

Carl