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Electrical Motors - Shaft Power vs. Voltage and Current

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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:

P shaft_kW = ηm U I / 1000                      (1)


P shaft_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

P shaft_kW = ηm U I PF / 1000               (1b)


PF = Power Factor

Two Phase Four Wire

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

Three Phase

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


Shaft Power in hp

Shaft power as horse power :

P shaft_hp = P shaft_kW / 0.746 (2)

or for a DC motor

P shaft_hp = (ηm U I / 1000) / 0.746

= ηm U I / 746 (2b)


P shaft_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

P shaft_kW = 0.85 (36 V) (5 amps) / 1000

= 0.153 kW

= 153 W

The shaft power as hp

P shaft_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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