Finding LRC from NEMA MG 1 "Code Letter"
Finding LRC from NEMA MG 1 "Code Letter"
(OP)
How do I determine the locked rotor current of a motor if I know the Code Letter from the nameplate? The motor is a 460V, 10 HP motor with code letter "H".
Thanks
Thanks





RE: Finding LRC from NEMA MG 1 "Code Letter"
Maybe I'm wrong, but that 'H' is the insulation class is it not? I would think the full load amps (FLA) rating would be quite close, if not exactly what you need. I have no idea what your motor is, but I looked at 3 misc. 10HP 460V motors and the FLA's range from 12.3 to 13.8A at 460V.
RE: Finding LRC from NEMA MG 1 "Code Letter"
From here, I am lost or missing something. The motor is 460V with a PF of 76%. I think this should leave me with a LRC of 136A, but that is much higher than the manufacturer specified (80A LRC). Where did I goof up?
RE: Finding LRC from NEMA MG 1 "Code Letter"
Gherian, might you mean 63kVA locked-rotor? {63,000 voltamperes} A range would be 63-71kVA for code-H / 10 horsepower.
RE: Finding LRC from NEMA MG 1 "Code Letter"
So for 10 hp motor, maximum would be 70.9 kVA. At 460V, this is about 89 amps. This is a symmetrical rms value. The asymmetrical current could be a lot higher than this. On the other hand, if you have a lot of voltage drop, the current will be less.
This assumes a full-voltage start, of course.
RE: Finding LRC from NEMA MG 1 "Code Letter"
The "code leter" gives a range of starting kVA per each HP.
As you mentioned H corresponds to 6.30 to 7.09
So the starting current will be:
Imin = 6.3*10(HP)*1000/(1.732*460)= 79.074 Amperes
Imax = 7.09*10*1000/(1.732*460) = 88.89 Amperes
Note that here the PF is not a factor since you are interested in the total power, active and reactive.
The manufacturer gave 80 amperes which comply with the standard.
RE: Finding LRC from NEMA MG 1 "Code Letter"
The Code letter is an indicator of starting inrush while the Insulation Class is mostly a measure of insulation temperature tolerance.
Both can have an H marking on the nameplate but it means something completely different depending on where it is.