An electrical motor is rated 1.6 hp for normal operation off 120V AC. a.) How mu

Question

An electrical motor is rated 1.6 hp for normal operation off 120V AC.

a.) How much current will the motor draw when connected to 120V AC?

b.) The resistance of the motor winding is 3. What is the back emf produced in the motor?

c.) How much mechanical power will the motor deliver? What is its efficiency?

d.) The motor is now loaded so it slows down to half its normal operating speed. What are now the back emf, current, electrical power drawn, mechanical power and efficiency?

Answers:

a.) 9.95 A

b.) 90.2 V

c.) 897 W = 1.20 hp, 75% efficiency

d.) 45.1 V, 25 A, 2996 W = 4.02 hp, 1123 W = 1.51 hp, 37.5% efficiency

Explanation / Answer

A) P = V*I

Current I = P/V = 1.6*746/120 = 9.95 A

B) voltage is V = i*R = 9.95*3 = 29.85 V

back emf = 120-29.85 = 90.15 V

C) power delivered is P = 90.15*9.95 = 897 W = 897/746= 1.2 hp

efficiency eta = (1.2/1.6)*100 = 75.15 %

D) for half of the speed

back emf 90.2/2 = 45.1 V

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