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Anonymous Poster

Rotor Voltage Drop Test

01/11/2011 2:59 AM

What voltage to apply ac or dc to test the voltage drop in rotor pole coil in a brushless alternator? What should be the allowable difference in voltage drop in each pole?

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Guru

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Posts: 1686
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#1

Re: Rotor Voltage Drop Test

01/12/2011 11:16 AM

You have not given the alternator size! So have to be careful not to apply too much and cause damage. Make sure winding is disconnected from diodes when measuring.

1.

A DC test should be easy with a portable ohmmeter or resistance bridge. The current will be small enough not to damage anything. Difference between windings may be done by using two windings in one bridge arm and reference resistors/box in other arm.
2.

This should give you an idea of resistance R =volts/amps for application of a voltage.
3.

Look at the size of wire in the field windings. If it is 0.4 mm diameter or more, 1 amp will be safe, it will not overheat anything quickly. Safe Amps is proportional to cross-section area of wire.
4.

But you are better off with a bridge/ohmmeter, because the current will not heat up the winding, changing its resistance and making comparison more difficult.

It is useful to get an idea of the field ampere-turns to get rated voltage :=

You need the following dimensions - (a) radial air gap between rotor and stator (centimetres)
(b) distance [crossing the axis of revolution] between opposite poles cm.
(c) Distance around half the circumference of the stator, near to the rotor cm (as you will see, the accuracy of this is not critical).

Assuming a flux density of 1 Tesla, in the magnetic circuit at rated volts (a usual value), the air gap requires 8000 ampere-turns per cm,
the rotor about 9 A-t/cm and the stator about 2 A-t/cm. In the usual case, the air gap takes most of A-t.
The total A-t needed is 2 x a x 8000 + b x 9 + c X 2 e.g. for a = 0.2 cm, b = 30cm, c = 60 cm, N = no. of turns I current in amps


F = NI = 2x0.2x8000 + 30x9 + 60x2 = 3200 + 270 + 120 = 3590 Ampere-turns (1)



To do AC testing, to compare windings on each pole, it will be necessary to avoid saturation. I am assuming the rotor is out of the machine and on a wooden stand well away from any magnetic materials (so comparison pole to pole is not affected - turning rotor so each pole is same position as it is measured is a useful cross-check)

Taking, as example, the figure of 0.4 mm diameter in 3. a current of 1 amp could be tried without over-stressing the pole winding.

  1. Apply an AC voltage at 50/60 Hz, which would not exceed 1 amp, with just the winding resistance measured in circuit. V = R/I = R (for 1 amp)
  2. Measure voltage across winding V and current into it I. Most likely, current will be a lot less than expected from DC resistance. Calculate V/I, the impedance Z. If it is say 10 times the DC resistance or more, the impedance measurement will be little affected by resistance.
  3. Approximately halve the voltage applied. Calculate Z again, as per 2. If it is within a percent or so of the first value, the voltage now applied is small enough not to give saturation error.
  4. You may have to raise voltage, because current is too small to measure accurately. But same care to avoid measuring at saturation values!
  5. Putting a few turns of insulated wire around the winding may give a measurable voltage - allowing a rough estimate of volt/turn and turns on whole winding.

I am not sure what differences are allowable - much less on big machine. Just a percent or two difference in numbers of turns would affect waveform of one pole vs another - with consequent half cycle to half cycle or cycle to cycle effect on voltage waveform. On bigger M/C I would expect number of turns to be exact on each pole. Very small wires have much more fluctuation of resistance because wire diameter is more difficult to control.

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Guru
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#2

Re: Rotor Voltage Drop Test

01/20/2025 7:07 AM

Call the manufacturer of the <...brushless alternator...> for advice over the telephone.

As for <...allowable...>, it is for the test protocol to state this BEFORE carrying out the test, otherwise all one is doing is gathering a measurement.

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