Power and Horsepower in Electrical Motors

Electrical power is rated in Horsepower or Watts

Electrical power is in general rated in Watts or Horsepower. A horsepower is a unit of power equal to 746 watts or 33,000 lb.ft per minute (or 550 lb.ft per second).

A watt is a unit of measure equal to the power produced by a current of 1 amp across the potential difference of 1 volt. A watt is 1/746 of 1 horsepower.

Even if the watt is the base unit of electrical power, its common to rate motor power in either horsepower or watts.

Power in Watts

Direct Current

Electric power of a motor can be expressed as:

Pw = ηm U I / 1000        (1)

where

Pw = power (kW)

ηm = motor efficiency

U = voltage (V)

I = current (A, amps)

Single Phase

Pw = ηm U I PF / 1000        (1b)

where 

PF = Power Factor 

Two Phase Four Wire

Pw = ηm 2 U I PF / 1000        (1c)

Three Phase

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

Power in Horsepower

Horse power of a motor can be expressed as:

Php = Pw / 0.746         (2)

or

Php = ηm U I / 0.746         (2b)

where

Php = horsepower (hp)

Example - The Horsepower of an Electrical Motor

electric motor

The horse power of an 230 V electrical motor with 85% efficiency pulling 10 amps can be calculated as:

Php = 0.85 (230 V) (10 amps) / 0.746

    = 2.62 (hp)

Standard Horsepower Ratings

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

  • 1, 1 1/2, 2, 3, 5, 7 1/2, 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

Related Topics

  • Electrical - Amps and electrical wiring, AWG - wire gauge, electrical formulas, motors and units

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