Question

A 120-V motor has mechanical power output of 3.00 hp. It is 88.0% efficient in converting power that it takes in by electrical transmission into mechanical power.

(a) Find the current in the motor.
A

(b) Find the energy delivered to the motor by electrical transmission in 3.90 h of operation.
MJ

(c) If the electric company charges $0.110/kWh, what does it cost to run the motor for 3.90 h?
$

Answer 1

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A 120-V motor has mechanical power output of 2.10 hp. It is 80.0% efficient in convertingpower that it takes in by electrical transmission into mechanicalpower.

(a) Find the current in the motor.
1 A
(b) Find the energy delivered to the motor by electricaltransmission in 2.50 h of operation.
2 J
(c) If the electric company charges $0.160/kWh, what does it costto run the motor for 2.50 h?
$ 3

answer

We know that 1 hp = 746W

Since P_out = 2.10 x 746= 1567 W

Pin = P_out / efficiency = 1567 / 0.80=1958W

a) Find the current in themotor.
Since P=VI

I=P/V= 1958 / 120

I=16.3A

(b) Find the energy delivered to the motor byelectrical transmission in 2.50 h of operation.

E= P_int= 1958 x 2.5 x 3600 =

E= 17.6 x10⁶ J or

E= 17.6 MJ

(c) If the electric company charges$0.160/kWh, what does it cost to run the motor for 2.50h?
E= P_in t=1958 x 2.5= 4896W or 4.896kW

$=E x rate

$=4.896 x 0.160

$ = $0.783