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Electrical Motor Shaft Power

Electrical motors are rated in horsepower or watt

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Power is in general rated in watt (W) or horsepower (hp). The old imperial unit horsepower is equal to 746 watts (0.745 kW) or 33000 lb ft per minute (or 550 lb ft per second).

The unit of electric power - 1 watt - is equal to the power produced by an electrical current of 1 amp at a potential difference of 1 volt.

  • 1 watt = 1 / 746 hp
  • 1 hp = 746 watts = 0.76 kW

Shaft Power in watts

Direct Current - DC

Shaft power produced from a direct current (DC) electrical motor:

Pshaft_kW = ηm U I / 1000                      (1)

where

Pshaft_kW = shaft power (kW)

ηm = motor efficiency

U = voltage (V)

I = current (A, amps)

Alternating Current - AC

Shaft power produced from an alternating current (AC) electrical motor:

Single Phase

Pshaft_kW = ηm U I PF / 1000               (1b)

where 

PF = Power Factor

Two Phase Four Wire

Pshaft_kW = ηm 2 U I PF / 1000                  (1c)

Three Phase

Pshaft_kW = ηm 1.73 U I PF / 1000                  (1d)

Shaft Power in hp

Shaft power as horse power:

Pshaft_hpPshaft_kW / 0.746                      (2)

or for a DC motor

Pshaft_hp = (ηm U I / 1000) / 0.746 

      =  ηm U I / 746                    (2b)

where

Pshaft_hp = shaft power (hp)

Example - Shaft Power Electric Motor

The shaft power produced by an electric direct current motor with 36 V, 85% efficiency and 5 amps - can be calculated in watts as

Pshaft_kW = 0.85 (36 V) (5 amps) / 1000

    = 0.153  kW

    = 153 W

The shaft power as hp

Pshaft_hp = (0.153 kW) / 0.746

    = 0.21  hp

Standard Horsepower Ratings

Standard horsepower ratings for electrical motors - 1 to 4000 hp are indicated below:

  • 1, 1.5, 2, 3, 5, 7.5, 10, 15, 20, 25, 30, 40, 50, 60, 75, 100, 125, 150, 200, 250, 300, 350, 400, 450, 500, 600, 700, 800, 900, 1000, 1250, 1500, 1750, 2000, 2250, 2500, 3000, 3500, 4000

Nominal voltage of not more than 600 V AC and nominal frequency 50 or 60 Hz.

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