Electrical Motors - Shaft Power vs. Voltage and Current
Calculate electric motors shaft power.
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_hp = Pshaft_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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