DC motor winding resistance

[Vestel]

Hi, I need the winding resistance on this motor for calculations related to a turbine development:

24 volt 0.5 amp at 375 RPMs DC Indiana general motor
I assume that the steady state resistance is 48ohms and includes the back EMF.

But the startup resistance is the actual winding resistance i believe. so i assume the startup current will be higher than 1/2Amp and Resistance is really smaller. is it likely half? a third? 10% less?

Can i measure this or estimate it somehow. It does not have to be perfect...and an estimate would be great!

Motivation: the output of the generator for the turbine is based on the resistance in the windings and affects my blade design.

thanks.

 

[zog]

Use a winding resistance test set, or a microhmeter.

 

[gar]

110526-2329 EDT

Vestel:

What you want to accomplish is not clear. You need some background theory on DC machines. Find a suitable book.

You can attempt to measure the DC resistance of the armature of a DC motor with an instrument as Zog mentioned. Two other ways are:

(1) Lock the rotor, apply a known direct current of about full rated current via the brushes thru the armature, and with a DC low voltage meter measure the voltage drop across the armature. Position the rotor so that the brushes have commutator segments centered under the brushes. Put the voltmeter probes directly on the centered segments and not touching the brushes to measure the armature voltage drop. If you want to include the brush and brush contact resistance, then make the voltage measurement between the wires embedded in the brushes.

(2) Lock the rotor, apply full rated voltage for a very short time, and measure the current, and applied voltage.

Basically the initial starting current from 0 RPM is what you measure in (2) above.

You do not describe the motor as having a stead-state resistance of 48 ohms because it is not an informative measure.

Rather you model the motor (PM magnetic field motor) as Applied armature voltage = Ia*Ra + K*RPM.

K*RPM is the counter-emf and Ia*Ra is the internal voltage drop of the armature current, Ia, at whatever operating load you are at and the assumed constant armature resistance, Ra.

So you are using a PM motor as a generator. The above equation still applies, but Ia is negative. There are mechanical losses from motor windage and friction which are part of your energy equation and could be viewed as a reduction of Ia, but maybe better treated separately. You can determine the windage and friction losses by operating the motor as a motor at your desired speed and measuring the electrical power input with no external load on the motor.

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