Sizing cables for Dc Motor application

[topire]

Hi i am re-installing a large machine at the moment
the dc mator is 570 amp F.L.C (300vdc)
the problem is this motor had its feed cables replaced
about 6 months ago and they went on fire about a month ago. I am reluctant to use the same cable again ,there are 2 terminals on the motor each terminal has 3 of 70sq cables terminated.(the cables have only about 1mm of insulation) any assistance would be appreciated

 

[DickDV]

Size the armature and field cables according to NEC or IEC rules, whichever is most common in your area. Then make sure the DC drive is set up to properly protect the motor at it's nameplate currents and voltages---no cheating here.

If done in this way there should be no particular hazard even for a full ground fault. I've seen those happen as when brush springs come loose and ground the armature circuit and, will it results in a huge flash and usually a damaged drive, the cables are undamaged.

 

[ScottyUK]

Depending on your installation method, insulation type, and ambient conditions, 3x 70sq.mm does not have a lot of margin for 570A. Although it is hopefully obvious, keep parallel cables the same length to ensure equal current sharing. You are probably close to the rating of the cables assuming perfect current sharing, so you do not need a big imbalance between conductors before one cable will get into trouble leading to a cascade failure.

DickDV has offered some good advice. You need to take a closer look at your cable sizing calcs, as the existing arrangement appears to be marginal.



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If we learn from our mistakes,
I'm getting a great education!

 

[mc5w]

If your motor has 4 or more sets of brushes, you should consider putting each brush set on its own cable(s) back to the motor controller and then putting a supplemental motor overload relay in series with each brush set. This is in case brushes pop loose causing only part of the armature to carry current. The other brushes will carry an abnormal amount of current under this condition which will trip the supplemental overloads before armature or cable damage occurs.

For the size of motor that you have this would be cheap insurance.

I onetime went to clean a 4 pole motor and discovered that one of the brushes had popped loose causing only 1/2 of the armature to carry current. I put in a work order to the effect that the motor had been in slow cook mode and needed to be replace Real Soon Now. Sure enough, about 1 month later the motor burned up.