# Electrical Principles Help!Watch

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Thread starter 5 years ago
#1
Hi guys One of my teacher put down this question as a homework but i don't get it. Help please!

A 500d.c motor has an armature resistance of 1.67ohms. The manufacture states that the armature current is 35A. Explain why there appears to be a discrepancy and provide a solution for a suitable method of starting this motor along with a description of the current operation.
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5 years ago
#2
(Original post by richu31)
Hi guys One of my teacher put down this question as a homework but i don't get it. Help please!

A 500d.c motor has an armature resistance of 1.67ohms. The manufacture states that the armature current is 35A. Explain why there appears to be a discrepancy and provide a solution for a suitable method of starting this motor along with a description of the current operation.
Hello and welcome to TSR.

Whilst we can help you by pointing you in the right direction, you need to know that TSR is not here to do your homework for you.

Having got that straight you need to look up the operation of a d.c. motor.

Specifically: Back e.m.f.

The motor works by generating an electromagnetic field in a coil of wire working in opposition to a permanent magnetic field, which immediately brings Faradays and Lenz's laws into play.

The force generated between the two magnetic fields causes the armature to rotate and in doing so, will induce a back e.m.f. across the coil which opposes any change to the magnetic field. i.e., the back e.m.f. tries to maintain the status-quo of the magnetic field in the coil.

The net voltage driving the motor is therefore produced by the supply voltage less that produced by the back e.m.f.

Hence the current stated for the armature is calculated from the net voltage together with the armature resistance.

You have enough information to work out the back e.m.f. from the information stated in the question.
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