# HELP ! DC MOTOR QUESTION

• posted

There was no response to my earlier post. Hence reposting again.

The Temperature rise by resistance method needs accurate measurement of armature resistance.

We want to test a small - sub - fhp ( around 50 w) motor having very few com bars ( around 12)

The problem is that the resistance at motor terminals changes as we rotate the motor shaft through a fraction of a turn.This is possibly due to the brushes covering more than one segment at some angular position and covering only one segment at some other position. The resistance measurement is therefore not accurate enough for the calculation by rise of resistance method.

One of the methods suggested in Kenjo and Nagamori's 'PM and Brushless Dc motors' is to measure the volts and amps at locked rotor at two points and determine the armature resistance as Ra = (V2 - V1)/(A2 - A1). This method is time consuming and also may not lead to correct resistances for temperature rise.

Is there any better method ? Is there any STANDARD method as per AIEE or IEC etc?

KASTURY rn snipped-for-privacy@vsnl.com

• posted

in article snipped-for-privacy@posting.google.com, KASTURY at rn snipped-for-privacy@vsnl.com wrote on 2/7/04 5:40 PM:

You are asking for something that is difficult to do. If you must, use the motor winding in an ayrton shunt arrangement. Make sure that you are on only one pair of commutator bars. Run a known current through the armature. Measure the voltage across the armature with a high resistance meter. The meter should load down the circuit so lightly, that contact resistance does not have effect on the measurement.

I said it would not be easy.

Bill

• posted

On 08/02/2004 KASTURY opined:-

I would suggest that to be almost impossible to do. The resistance of the brushes contact with the commutator will be so high and so variable, when compared to the much lower resistance of the windings, that the figures derived would be meaningless.

I'm not sure what you are trying to achieve here, however....

Could you not simply work out the cold resistance of the windings, measure the temperature of the motor, then calculate the resistance change from that?

• posted

Could you try English next time? Thanks.

• posted

The Lord alerted my mind to the presence of this EVIL article by Harry Bloomfield, and I thusly replied:

What were you trying to achieve when you went a-netKKKoping?

A spank? Several spanks? Chased off Usenet?

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