Is it TRICKY to measure armature winding resistance?

[onemilimeter]

I've a brushed DC shunt wound machine with the following specs:
Rated Speed: 3000rpm
Rated Power: 0.25kW
Armature Voltage: 180V
Armature Current (full load): 1.6A
Field Voltage: 210V
Field Current (full load): 0.2A

I did some calculation as follows:
w = n*2*pi/60 = 3000*2*pi/60 = 314.1593 rad/s
Rated torque, Torq = P/w = 250/314.1593 = 0.7958 Nm
K = Torq/Ia = 0.7958/1.6 = 0.4974
E = K*w = 0.4974*314.1593 = 156.2628 V
Armature resistance, Ra = (Va - E)/Ia = (180 - 156.2628)/1.6 = 14.8358 ohm
Field resistance, Rf = Vf/If = 210/0.2 = 1050 ohm

Then, I carried out some measurements. I measured the resistance with 4-wire method using Agilent 34410A. The measured field winding resistance is about 976 ohm, which is not too far from the calculated 1050 ohm. However, I found out that the measurement of armature winding resistance is not consistent (8~100 ohm). May I know why? Is it because of the 'brush/commutator' assembly? Any good approach to measure armature winding resistance?

Thank you very much

 

[onemilimeter]

Has anyone such experience before?

 

[BobM3]

I've measured it on a series wound motor (actually a universal motor). I didn't have any problems. With a series wound motor there are windings in series and parallel to each commutator segment so your will get different resistances if you measure between two adjoining commutator segments or between segments separated by 1 or more other segments. I don't know if a shunt wound motor has the same "multiple winding" armature structure.

 

[onemilimeter]

Hi BobM3,

Thanks for your reply. I have no access to the brush/commutator assembly. There are two pair of leads out of the motor. One of them is for field winding, whilst another pair is for armature winding. I place the 4-wire probe across the armature winding and I find out the measurement is not consistent, ranged from 8-100ohm when the rotor of the motor is turned manually by hand. Is that normal?

 

[ScottyUK]

Brushgear does introduce some peculiar effects at low currents and low voltages. The brute force solution would be to apply a higher voltage passing more current and use a four terminal Kelvin resistance measurement. The Agilent meter is a nice instrument but the source current and the voltage driving it are too small to overcome the non-linear behaviour of carbon at low voltages. Use the Agilent meter as the voltage measuring device, then look for a reasonably accurate ammeter to measure the current you're passing and apply Ohm's Law. Inject the current from the outer pair of connections and measure voltage at the inner pair.


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