How do you find the no load amp rating of a motor?

Answer 1

Full load amps is found on the motor's nameplate, and is unique to that particular brand and model number. A Magnetec 2hp motor will draw different amps than a Baldor 2hp, etc. There are high efficiency motors, and not so high as well. Consult the nameplate. If you want a "rule of thumb" to estimate the RLA (Running Load Amps - this is the term most often found on the nameplate - same as full load amps), then use this: Figure about 1400 watts per horsepower. Divide Watts by volts to get Amps, for example: * 3hp motor, 120V * 3hp X 1400 W = 4200 W * 4200 W / 120 V = 35 A This formula is only for single-phase motors! I looked up a typical 3hp 120 V Baldor motor in the Grainger's catalog, and the RLA for it was 32.0, so this will get you in the ballpark. I have seen 3hp 120 V motors as low as 25 A and as high as 40, so DO NOT use the rule of thumb to size branch circuit conductors or overcurrent protection! For 3-phase motors: Figure about 1100 watts per horsepower (3-phase is a little more efficient). Divide Watts by Volts, then divide the answer by 1.73 to get Amps, for example: * 3hp motor, 208V 3Ph * 3hp X 1100 W = 3300 W * 3300 W / 208 V = 15.86 * 15.86 / 1.73 = 9.16 A This formula is only for 3-phase motors! My Grainger's catalog motor with the same Voltage and HP had a nameplate rating of 8.34 A.

Answer 2

First, calculate the load on the motor in watts, kW, or MW, depending on the size of the motor. Then compare to the motor curves provided by the manufacturer.

If you really want to calculate this, you'll need significant information from the manufacturer about the motor - characteristic impedances, time constants, etc. It's not a simple plug and chug operation. That's why they provide the curves.

Answer 3

Answer :

Power = 1.73 x Voltage x Current x Power factor.

Current = Power / (1.73 x Voltage x Power factor)

If you want to add efficiency factor you may do so, say 90% efficiency for example.

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Motors are rated in horse power in North America. Different size HP motors have different voltages and full load amperages.

If you know the HP of the motor then use the following formula to find the motors full load amperage; Amps = HP x 746/1.73 x Volts x % eff x pf.

A standard motor's % eff increases as the HP increases. From 10 to 100 HP the scale increases from .86 to .92.

A standard motor's power factor increases as the HP increases. From 1 to 100 HP the scale increases from .84 to .91

Answer 4

It should be in the manufacturers' data sheet, but it is not a very important parameter for a motor that is usually run with a mechanical load.

Answer 5

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