Earthing a Motor..?

[jimyoc]

i work on a pharmaceutical site in instrumentation and i am vey concerned about the way motors are earthed. In our tank farm, which is an explosive enviroment, the motors are earthed using long coils (18inches) of earth wire going from the motor to the frame to the conduit etc etc.. cant this coil pick up from the moving magnetic fields in the motor and induce a voltage across which could lead to a spark in the tank farm???
Thanks for any help on this...

 

[UKpete]

jimyoc, firstly I'm certainly not an expert on protection in explosive environments, but ... for a spark to form you would need an airgap between two conductors with a voltage difference between them (e.g. an opening or closing contact, a wire or connection in the process of burning out).

For a dangerously large induced voltage you will need a lot of turns and/or a high amplitude alternating field, I don't see how you will get this from what you describe.

Certainly the earth connection should be sized to cope with the worst case fault current. But I wonder if the problem will be nuisance noise in nearby instrumentation rather than any safety problem.

I re-stress my original disclaimer!

 

[Skogsgurra]

The magic coil again!

This is something that seems to pop up with irregular intervals. A coiled wire normally doesn't pick up more voltage than a straight wire of the same length. Probably less. It is only when the magnetic flux is aligned along the axis of the coil - and when the flux is very strong - and when the frequency of the flux is high that you can induce any appreciable voltage in the coil. None of these conditions is met if you have an 18 inch coiled wire in the leakage field of a motor. I doubt if you even get 10 millivolts, worst case. And that is not a voltage that can cause any dangerous sparks.

Is it the old Frankenstein movies that makes folks so nervous about coiled wires?

Gunnar Englund

http://www.gke.org

 

[aolalde]

The magnetic field of an electric motor is confined to circulate into the core lamination of the stator and rotor (magnetic circuit). Some small amount (around 2%) of that flux leaks out of the magnetic circuit and circulates through the frame, end brackets and shaft. The flux leakage outside of the frame is less of 5% of that 2%.
If the magnetic field (F) is close to zero and you have 1 turn, the induced voltage is in the order of few millivolts. e = -N* dF/dt
Static charges, due to friction and windage could develop more dangerous voltages.