LRC Time for Motors and Compressors
- Thread starter fifty60
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- Illinois
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- retired electrician
The standard overload relay in a NEMA starter is Class 20 which will permit a current of 6 times full load to run for 20 seconds before the overload relay will trip.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Well, it must trip no later than 20 seconds, but it CAN trip sooner.
Even an IEC Class 10 OL can take 10 seconds to trip. The thermal damage curve of the motor is typically based on these "Class" values and they are the same for NEMA and IEC as far as the OL trip times. The motors themselves are however different. NEMA (North American) design motors are typically built to a thermal damage curve characteristic based on the Class 20 overload relay, "Mill Duty" motors are (were) built to a Class 30 standard, most IEC motors are built to a Class 10. Submersible pumps and other specialty motors are not built to either standard but will tell you the LR time, for example many ESP (Electric Submersible Pump) motors are only able to take 5 seconds at LRC.
How long LRC lasts is not a fixed value. The LRC will remain the same up until the motor and load is accelerated to roughly 80% speed. So that time frame is based on the inertia in the motor and the load, along with any added accelerating torque requirements of the load.
weressl
Esteemed Member
My understanding was that the classes of overloads were tailored to the NEMA design characteristics of A, B, C & D motor design that in turn are characteristically designed for smaller to larger inertia acceleration in increasing order. Consequently one would apply class 10 for A&B, class 20 for B&C and class 30 for C&D.
I am trying to size circuit breakers for hermetic refrigeration compressors. With class CC fuses, I would normally take 175% of the RLA and round down. My supervisor has advised me to size the breakers at 125% and round up. The breakers are D-curve with instantaneous trip levels set at 10-20 percent.
My LRA's are coming out to about 5-6 X's the RLA as expected. At 6 times the rating of a D curve breaker I have a trip time of about 3 seconds. The motors are internally thermally protected, and if the thermal protection is designed to NEMA class 20 then they will take 20 seconds to trip at 6X RLA.
I know 430.55 allows me to combine short circuit protection and overload protection into one inverse time breaker. I have read that as long as the LRC is less than 3x's the rating of the inverse time breaker it should be fine.
I like the process of sizing the OCPD at 125% and working my way up to 175% if needed, but what are some items I can pull from the Curves and ratings that will help me size it right the first time, without just "going big".
My LRA's are coming out to about 5-6 X's the RLA as expected. At 6 times the rating of a D curve breaker I have a trip time of about 3 seconds. The motors are internally thermally protected, and if the thermal protection is designed to NEMA class 20 then they will take 20 seconds to trip at 6X RLA.
I know 430.55 allows me to combine short circuit protection and overload protection into one inverse time breaker. I have read that as long as the LRC is less than 3x's the rating of the inverse time breaker it should be fine.
I like the process of sizing the OCPD at 125% and working my way up to 175% if needed, but what are some items I can pull from the Curves and ratings that will help me size it right the first time, without just "going big".
Do all internal thermal overloads of motors/compressors fall into the NEMA classes(10,20,30)? Do all overloads fall into the NEMA classes?
Are there any rules of thumb for sizing dual element fuses and IT breakers for motors? One that I have seen is that if the LRC is 60A, for example, the the IT breaker should be sized at 20A (which is 1/3). If I knew the average time the compressor maintained the LRC I could size them from the curves. 20 seconds at 6 times rating is definitely outside the range of an IT breakers sized at 125% Rated Load Amps.
Are there any rules of thumb for sizing dual element fuses and IT breakers for motors? One that I have seen is that if the LRC is 60A, for example, the the IT breaker should be sized at 20A (which is 1/3). If I knew the average time the compressor maintained the LRC I could size them from the curves. 20 seconds at 6 times rating is definitely outside the range of an IT breakers sized at 125% Rated Load Amps.
weressl
Esteemed Member
Internal thermal motor protection are closer calibrated to the motor thermal withstand characteristics.
In the case of motors approved for Class I, Div. 1 application the thermal element is designed to cut out early enough so that external motor temperature does not exceed 80% of the stamped T-rating.
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