Electrical Motors - Shaft Power vs. Voltage and Current
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 = η m U I / 1000 (1)
where
P shaft_kW = shaft power (kW)
η m = 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 = η m U I PF / 1000 (1b)
where
PF = Power Factor
Two Phase Four Wire
P shaft_kW = η m 2 U I PF / 1000 (1c)
Three Phase
P shaft_kW = η m 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 = (η m U I / 1000) / 0.746
= η m U I / 746 (2b)
where
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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