Strange electric motorfault.

[Andrew TinsleyParticipant @andrewtinsley63637]

Yesterday, My central heating boiler began squeaking and then stopped . The 90 watt Remco motor would not run. This unit is a straightforward capacitor start type. The only unusual feature being a fourth wire which provided approx 60 volts AC for other purposes (a straight tap from the main run winding). Apart from this, the motor drives a fan  (to provide air for the oil fired burner and an oil pump, again to the main burner.

I had a spare motor and once both motors were on the bench, I did a check on the resistance between all 4 wires. The defective motor gave a very jittery readings (using a DVM) The new motor gave rock steady readings! The strange thing is, that the readings on the faulty motor were giving a mean value which agreed with the new motor values, although the jittery readings were around plus or minus 30% of the mean value!

The motor bearings were not brilliant Small grumbling on rotating by hand, although there was virtually no radial play and only a little end float. The capacitor checked out at 3.7 microfarads for a nominal 4 microfarads. The oil pump was free and the fan was not catching anything

I rigged up a supply to the new motor to check all was well. The motor ran as expected. The faulty motor would not run when tried in the same test circuit, no hum or start when the motor shaft was rotated by hand.

I have never seen anything like this in 50 years of playing with motors. I am totally at a loss to figure out what is wrong. Can anyone explain what is gong on?

Andrew.

[noel shelleyParticipant @noelshelley55608]

faulty connection or faulty windings ? Noel.

[Andrew TinsleyParticipant @andrewtinsley63637]

Hello Noel.

Like you I suspect a faulty internal connection giving open circuit and then normal connection. But I am not vey convinced! I assume the fault had something to do with the noise heard before its demise, but can’t think what on earth it could be.

Andrew.

[Martin CargillParticipant @martincargill50290]

Its easy for a motor to have shorted turns in its windings, but very difficult to measure or find it.

 

Martin

[Andrew TinsleyParticipant @andrewtinsley63637]

Shorted turns are a possibility. I can’t see that it would cause the jittery resistance readings.

Andrew.

[Colin WhittakerParticipant @colinwhittaker20544]

Iffy contact on centrifugal switch? Is the capacitor RUN or START? The latter implies the presence of a centrifugal switch.

(I wouldn’t put much weight to a jittery eye averaged resistance on a DVM.)

Colin

[Andrew TinsleyParticipant @andrewtinsley63637]

Hi Colin,

I think the Remco motor is capacitor start and run. I am not putting a lot of weight on the jittery resistance values, but they are certainly of the same order as the stable values that I get from the replacement motor.

Andrew.

[Andrew SkinnerParticipant @andrewskinner94774]

Could the crimps that join the enamelled winding wires to the flexible ones in the connection box have loosened, perhaps due to overheating?

I could see how the resultant jittery connection, combined with the inductive nature of the windings, could cause reading fluctuations either side of normal.

On a similar note, I can’t use my Fluke MFT to check motor winding resistance, since it uses some kind of pulsed waveform on that setting that throws out wild readings.

[SillyOldDufferModerator @sillyoldduffer]

Sounds like an intermittent insulation fault, my guess causing shorted turns.

Ordinary bench meters, whether analogue or digital, aren’t very good at detecting this type of fault.   The test voltage applied to measure resistance is low, and may not be enough to jump the gap at all, making a bad circuit look good,  Or it might be just enough to open the gap briefly, causing jittery readings.

A similar problem occurs when testing Run/Start capacitors.  These operate at greater than 240Vac, and their insulation often fails briefly at voltage peaks.   Misleading to test them with a basic resistance meter that only applies 1.5Vdc.

Plain meters are excellent most of the time, but it’s not difficult to get misleading results by taking them out of their comfort zone.   Then an appropriate meter is needed, except they too cause confusion if applied inappropriately.  Andrew Skinner mentions his Fluke outputs a pulsed waveform,  which is excellent when appropriate, but perplexing if used on an inductive or capacitive circuit.

To test a motor properly, I guess the best instrument would be a Time Domain Reflectometer, an expensive toy without which no hobby workshop is complete!   They work by sending an adjustable voltage pulse into the winding, and looking for an echo.    Short and open circuits produce visibly different echos, and measuring how long it takes for the echo to return gives the location of the fault.    It’s a form of radar.  As a TDR is much more expensive than a multimeter, no surprise I make do with ordinary kit,  accepting that results may be peculiar!    But one can be reasonably confident that getting jittery readings out of a motor means the motor is sick.

Dave

[Russell EberhardtParticipant @russelleberhardt48058]

The Fluke MFT measures the winding resistance with a 10 mA current source and measuring the voltage, if there is an intermittent open circuit the current source will fail to provide any current to an open circuit.  A normal multimeter is more appropriate in that case.

Russell

[Andrew TinsleyParticipant @andrewtinsley63637]

I have a home built set up using my oscilloscope to do what is termed a “ringing test” to check for shorted turns. So I at last dug this out and checked the motor windings. Result, no shorted turns.

Andrew.

[Howard LewisParticipant @howardlewis46836]

Is it possible to repeat the tests, with the capacitor out of circuit?

It could be that the capacitor has gone short, or the value has changed and the varying capacitance is causing the jittery readings,

Howard

[Andrew TinsleyParticipant @andrewtinsley63637]

Hello Howard,

The ringing test was done without the capacitor! I checked the capacitor and it is near enough 4microfarad, the stated value. I also checked it with a 500 volt megger (a bit naughty as the volts are really a bit high!) and that showed zero leakage.

Andrew

[Andrew TinsleyParticipant @andrewtinsley63637]

Hello Howard,

The capacitor checked out at 3.7 microfarads, not 4 as I stated above! Not that it makes much difference, in the past I have used a 2 microfarad capacitor in this motor circuit.

Andrew.

[Andrew Tinsley]

On Andrew Tinsley Said:

I have a home built set up using my oscilloscope to do what is termed a “ringing test” to check for shorted turns. So I at last dug this out and checked the motor windings. Result, no shorted turns.

Andrew.

That sounds good, can you describe how it works please?

If it’s like a TDR, it should be able to distinguish visually between: an open circuit; short circuit, and intermittent connection.  What does the scope display?

The resistance readings and motor not heating up both suggest an intermittent connection rather than shorted turns.   Could be corrosion, damaged wire, faulty thermal cut-out, or a dirty start/run switch.    Probably more!

As a good spare exists, and it’s certain the motor is defective, I suggest a post-mortem.  Open it up and have a thorough look inside.  Lots of gory pictures please!

Dave

[SillyOldDuffer]

On SillyOldDuffer Said:

On Andrew Tinsley Said:

I have a home built set up using my oscilloscope to do what is termed a “ringing test” to check for shorted turns. So I at last dug this out and checked the motor windings. Result, no shorted turns.

Andrew.

That sounds good, can you describe how it works please?

If it’s like a TDR, it should be able to distinguish visually between: an open circuit; short circuit, and intermittent connection.  What does the scope display?

The resistance readings and motor not heating up both suggest an intermittent connection rather than shorted turns.   Could be corrosion, damaged wire, faulty thermal cut-out, or a dirty start/run switch.    Probably more!

As a good spare exists, and it’s certain the motor is defective, I suggest a post-mortem.  Open it up and have a thorough look inside.  Lots of gory pictures please!

Dave

I built a simple one that pulses the windings and uses a shift register to count the rings, displaying the result on LEDs. It’s better at comparing two windings, since you may not know how many rings to expect.

You can also scope the output. I think this is an SMPS transformer primary.

[MacolmParticipant @macolm]

It is quite possible that winding cross contact that results in shorted turns can be intermittent. Indeed, I had a VHS recorder where the playback signal varied from normal to a washed out picture to none and back again. The fault was found to be in the rotary transformer for the signal from the head. So a motor could similarly have intermittent and variable shorted turn effects.

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