Finding LRC from NEMA MG 1 "Code Letter" 2

[trobgherian]

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

 

[snaggletto]

Hello,
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.

 

[trobgherian]

The "Code Letter" comes from NEMA MG 1-10.37.2. It is also called the "locked-rotor KVA per horsepower at rated voltage and rated frequency". Code letter "H" corresponds to a locked-rotor KVA per horsepower of minimum 6.30 to 7.09. So, for my 10 HP motor, the locked-rotor KVA=63000.

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?

 

[busbar]

Gherian, might you mean 63kVA locked-rotor? {63,000 voltamperes} A range would be 63-71kVA for code-H / 10 horsepower.

 

[dpc]

See NEC Article 430. Starting Code Letter H = 6.3 to 7.09 kVA per hp at locked rotor.

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.

 

[aolalde]

Gherian:

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.

 

[DickDV]

To Snagletto. It is important to distinquish between Insulation Class on a motor nameplate and NEMA CODE.

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.