Dumb Question on DC Motors!

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dave12

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• 31 Aug 2008

• #1

I know this isn't House Electrics, but I thought someone might be able to help.

When checking the resistance between the +ve & -ve Busbars on a large DC Motor, is it right to have a short Circuit?

The DC Motor is a Brush Motor.

Many thanks

 

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plugwash

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• 31 Aug 2008

• #2

Please define what you mean by "short circuit" and tell us some specs for this motor.

The stalled resistance of a DC motor is very low. That is why it is vital to have overload protection on motors. A stalled motor will almost certainly cook itself if there is nothing to stop the current.

 

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GaryMo

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• 31 Aug 2008

• #3

Field winding will probably be high but armature winding may be very low and seem like a short circuit (less than 1ohm).

 

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dave12

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• 1 Sep 2008

• #4

Hi Thanks for the replies!

The Motor is a 550V 1280KW Ansaldo DC Motor. It is fed from an SCR Unit.

The tests I have carried out are accross the in coming + & - (Armature?) the resistance is 0.7ohm.

I have also IR Checked each to ground and got >50Mohm.

Io my mind it seems ok. But my understanding of these systems isn't great. Any thoughts?

 

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tim west

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• 2 Sep 2008

• #5

The unit will limit the current (usually through an H-Bridge) until the motor has gained sufficient speed for the back EMF to counter the supply voltage and current flow drops to an operating level.