ELECTRIC MOTOR POWER CONSUMPTION QUESTION|Online electrical engineering & Electrical Interview Questions

Saturday, July 27, 2013


Here is an Electrical Engineering question from one of our friend.Really this is a good question and sometimes some small things would cause us a big question.The doubt of our friend is "let's say you have an electric motor that operated on 110vac at, say, 3 amps. What would be the outcome if you put a 1:1 isolation transformer (110 in - 110 out) between the source and the load (motor). Would the power consumption at the source be the same or would it be measured only as the consumption of the transformer?"

Let the motor have a load of 3A and it runs with the input voltage of 110V.The power drawn by the motor would be 330 Watts(P=V*I). So if you connect a 1:1 isolation transformer between the source and the motor for the same amount of load,will the power consumption at the source be same? The answer is a big NO.

The source has to supply the power to the isolation transformer and also to the motor load via the isolation transformer.So the transformer losses such as eddy current loss,heat loss etc would also consume power form the source.Therefore the power consumption wont be the same at the source.

If you are not convinced I will give you a simple analogy.Say you have a 100 litre water tank(100 V battery).There are two locations pointA and pointB.PointA is at 10 meters from the tank and the pointB is at 110 meters away from the tank.

SCENARIO 1 : Tank is at full capacity and you fill a bucket of 20 litres at pointA.Remaining capacity in the tank would be 100-20-the amount of water in the 10 meter pipeline.
SCENARIO 2 : Tank is at full capacity and you fill the bucket 0f 20 litres at pointB.Remaining capacity in the tank would be 100-20- the amount of water in the 110 meter pipeline.

Will the capacity of tank after the two scenarios will be the same?Not at all.This also applies to the electrical circuit.Hope the answer is clear.Please post you valuable comments and views.Cheers !!!

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