AIR GAP MEASUREMENT

[powerjunx]

hi..
i just want to share this idea of mine..
we had rewinded the armature of a DC motor, it was then brought back since it was found it doubles its current drawn (running without sparking)as compared to the operational and existing spare units(the same capacity rating, same load and location).
we had recheck the winding data and it was found that the repair was correct, shunt field polarity,netral point setting, connections and all other neccesary test was undergone.. but still the problem persist..

we got now the last resort.. we plan to adjust the air gap of all shunt pole face, do this has the possibility to reduced the current drawn to 1/2 from before? could anyone explain to me further the flux-field current relation to reduce the Armature current(load current)..


all suggestions are welcome..and very much appreciated, thanks!!

fbjavier

 

[electricpete]

Let's say you had a shunt motor.
Et = Ia*Ra + Eg
where
Et = terminal voltage
Ia = armature current
Ra = armature resistance
Eg = generated voltage

Solve
Ia = (Et-Eg)/(Ra)

As flux goes up, Eg goes up (for a given speed), Ia goes down. Eg is proportional to speed times flux.

 

[jbartos]

Comment: It must be relatively small motor where the commutator and brushes are overrated.
Note: Eg is also proportional to If, i.e. the field current:
Eg=(Rf+Re+Ra)If + dFieldflux/dt
at no load for the assumed shunt dc motor. The correction or troubleshooting will be more involved.
Depending on the voltage, there may also be a winding insulation voltage concern.
However, if you rewound the motor only, there should not be magnetic circuit problem except premagnetization of the magnetic path iron.