Testing a three phase supply

15 Dec 2012
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United Kingdom
Hi there,

I have a washing machine that's on the blink, the motors not running at all. It is has a small three phase induction motor and I'm trying to work out if it's the motor or the control unit that's buggered. Sadly I'm not great with three phase and I'm unsure how to test the supply to the motor, I've already tested the winding's for a break or short.
There are three wires going to the motor and from my limited knowledge I'm assuming they are all live and there isn't a neutral.
So if I want to test to see whether the motor is being given a current with a multimeter do I put both probes on a different live? Can I disconnect the motor and test directly from the connector, or should the motor remain connected?

Any help much appreciated.
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You'll have a big bang if you try touching two lives with a meter set to current! Can you upload some photo's?
First check on a 3 phase motor is to check the windings are ballanced with an ohm meter.
A 3phase AC motor will be a balanced load so no neutral. Check each phase voltage with respect to earth and between each phase, if somethings amiss there I would bet that a contactor somewhere in the control circuit has burnt out
Cheers for the replies guys,

I've already checked the windings with an ohm meter and they look fine, and I've had the control board off and examined it and it looks brand new, no burnt spots, no discolouration and no burnt smell.

The socket that plugs into the motor has three red wires and two white, the white go to a plastic housing at the rear of the motor which I presume is a speed sensor - So there are no neutral wires.
In single phase electrics you'd never test between two lives obviously, you'd put the red probe to live and the black probe to neutral to test for a current.... Simples! So where do I test to when there are three lives and no neutral?
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Between phase and earth you should see a nominal 230v.

Between two different phases you should see a nominal 400v.

I'm not sure where you've got the idea of sticking the meter between live and neutral to measure current, that'll (hopefully) just blow your meter fuses! You need to break into the circuit to measure current and nothing changes for a three phase supply. The alternative is a clip on ammeter.

If you test between two lives on a single phase circuit, nothing 'bad' will happen, your meter should show zero volts as there is no potential difference.
Sounds like you could do with a set of drawings, what is the make and model, maybe they are available online.

Presumably there is a contactor responsible for this motor, if so have you verified that the contactor is operating when it should be?

With a voltmeter check both sides of the contactor to earth, I.e. each pole individually to earth, if you have a supply one one side and not the other the coil of the contactor may not be pulling in when it should which would indicate a control related problem.

All of this is total speculation though, more details required really.
Where does the three phase come from? I have a three phase fan on my PC but as far as most people are concerned it is 24vDC. Inverters can produce three phase at any voltage or frequency often they vary.

You seem to have voltage and amperage mixed up. I use a clamp on meter in interest of safety there is no way I can connect it in a manor to cause a big bang.

However where there is an amp range using wandering leads specially if not to GN38 connecting across where one wants to measure voltage when set to amps can cause ionisation of the atmosphere. In other words a Big Bang.

I have seen three phase in Algeria in the home which I would think was a French installation but not seen it in France. In Algeria it was 110 volt three phase.

So may be better to ask on French forum?
hi what make and model is it, does the machine do anything when switched on ie fill, pump out etc if so and the windings meter out ok is it induction or brushed (are brushes worn) after that check the motor module connections and for dry joints if still no joy then look at programmer..
The 3 phase will come from a small inverter (variable frequency) on the control board. The two white wires will be from a tacho used for speed feedback to the controls.

I would start the testing thus :
Set on AC VOLTS, test across the 3 combinations of phases (ie 1-2, 2-3, and 3-1) which should all be the same and non-zero.
Then check phase-neutral (that's supply neutral) which should also be equal and non-zero.
That's about all you can do with a multimeter, unless it has a frequency measurement option in which case you could also check that.

Another simple test would be to unplug the motor and connect three equal light bulbs (in delta) across the phases. The lights should be equal in brightness - if (for example) one phase was dead then you'd see two lights dimmer than the third.

Also, don't forget the test features built into most humans. Does the motor hum at all ? If it does, then that would indicate power getting to it. Giving the drum a spin (in such a way that you don't leave bits of finger behind in the mechanism !) might make it spring into life - that too would be an indication of a missing phase (if the load is light enough, it'll run on single phase, but not start).

EDIT: Also, with the power off, unplug the motor and check across the wire wires (of the motor) while you spin the drum. You should detect a small AC voltage which will increase with speed. If you have frequency measurement, the frequency will also increase with speed.
A failed tacho shouldn't stop the motor staring at all - though if the control board monitors for a failed input it may have shut down on a fault.

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