Question

could someone help me with these problems I would be very grateful :)

Problem

a) a small factory uses a 750W compressor, a 5.2kW air conditioning unit and a 1250J / s rotor for its production
What will be the capacity of the fuse (in A) to support the load of the 3 devices connected in parallel at a voltage of 480V?

b) Referring to the previous problem, what is the cost (in $) of using the 5.2 kW air conditioning unit for 8 hours a day, in a month of 30 days, if the price of electric power is $ 1.12 / kWh

Answer 1

Part A.

Given that all three devices are in parallel, So Voltage across each of them will be same and equal to 480 V

Also we know that

Power = Voltage*Current

P = V*I

I = P/V

So for compressor

I1 = P1/V1 = 750/480 = 1.5625 Amp

for AC unit

I2 = P2/V2 = 5200/480 = 10.8333 Amp

for rotor

I3 = P3/V3 = 1250/480 = 2.6042 Amp

So total current in circuit will be:

Ieq = I1 + I2 + I3

Ieq = 1.5625 + 10.8333 + 2.6042 = 15 Amp

So capacity of fuse should be equal to 15 A

Part B.

Power Used by AC unit = 5.2 kW

time for which AC is running = 8 hr/day for 30 days = 240 hr

So Amount of power used

Total power used = P_net = 5.2 kW*240 hr = 1248 kWh

Now price per kWh = $1.12

So total cost per month will be:

Total cost = (1248 kWh)*($1.12 per kWh) = $1397.76

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