Motor question

Is it possible for a three phase induction motor to change directions while running without changing two of the incoming phases? I have a motor that changed direction on a piece of equipment supposedly without anyone changing the wiring. It is not connected to a drive, using a standard motor starter. Could the load possibly change the direction of the motor? I have never seen this happen before, but it has happened on this particular setup three times. Any help would be appreciated.

Thanks

I don’t have any answers - but I’ve sure got some questions.

(1) Is the 3-phase power coming straight from a utility company - or from a generator set? If from the utility is there a transformer in the line?

(2) Is the motor just running along at its normal speed - and then “WHAM” it just reverses and then continues to run at normal speed - but now in the wrong direction? Or is the machine stopped - and then the motor starts up in the wrong direction?

(3) Are any other motors connected to the same electrical service? If so, do they reverse direction too?

(4) As a troubleshooting step, do you have a spare motor that you could temporarily connect across the same lines - just to see if it reverses too?

(5) You said this has happened three times? Over how long a period - minutes, hours, days, weeks?

(6) What kind of load is this? Does it have enough muscle to spin the motor in reverse?

(7) When the motor reverses - does it run for a long time before someone gets excited and shuts it down? If it’s a long time, does the motor get extra hot?

(8) No offense, but are you absolutely SURE that this is a 3-phase motor? Single-phase motors can be reversed just by spinning the shaft in the wrong direction at start-up. But I’ve never heard of a 3-phase motor acting like this.

Thanks for sharing. This one is probably going to end up in my little notebook.

Ron Beaufort, loved your little relay diagram at the bottom of your post. If you don't mind a further imbellishment, why not show the relay coil off to the right and put a ? in it!

As far as rotation on a three phase motor, assuming the incoming phase rotation is the same, the motor will always motor in the same direction. The load could conceivable overhaul it and force it backward but that wouldn't go on for long without the overloads tripping or the motor roasting.

The conclusion almost has to be that the AC phase rotation in the supply has been changed to produce a reversal of shaft rotation. Keep in mind that this change can occur anywhere in the supply system. That includes the substation, distribution transformers on site, etc. I think it is safe to assume that the utility would not reverse rotation--one of you power guys can verify this but, if I'm not mistaken, they are obliged to provide a common rotation by national spec.

Just a thought

May be your 3 phase motor is being fed with a single phase supply

If you strape out you motor in delta.( So you now have 3 terminals )

Place the neutral to the centre terminal, Place a capacitor across the other 2 terminals. Place a phase wire to one side of the capacitor, the motor will spin in 1 direction.

If you stop the supply to the phase wire, and place a phase wire to the other side of the capacitor it will change direction.

Maybe you have come across this type of set up.


Lightning uk

Something smells a little fishy here - my experience coincides exactly with DickDV's. If this is causing you a problem (and I suspect it is) I suggest you spend a few hundred bucks and get a phase rotation monitor/alarm relay from Cutler HAmmer, A-B, Time Mark, or many other suppliers. The relay can trip an alarm and a counter so you know when and if phase rotation occurs.

Lightninguk, your arrangement sounds to me similar to what used to be called a "red leg" three phase. I haven't seen it for years, and then only in remote parts of the Southern US.

Maybe the motor is not in a good shape. I´ve experience once a motor that change in direction without anyone do anything. If one of the three circuits is corrupt than it´s really possible that once the motor stops, when it starts again, start in other direction. That´s because the third circuit is corrupt and the alternate magnetic field is done only by the other´s two. If you have only two alternate circuits working , then no direction it´s possible to define. In fact it depends of the position of the rotor inside the stator when it stops.

1, 2, 1, 2 ---- your situation

1, 2, 3, 1, 2, 3 ---- the correct situation

I know that if you have a 3-phase motor stopped - but then power up only two of the three line wires - then the motor will NOT rotate but it will hum very loudly and heat up very quickly.

(Electricians call this "single-phasing the motor" but I don't want to debate the guys who are going to say it should be called "two-phasing the motor" so the jury is instructed to disregard my last remark.)

Now while the motor is humming, you can start it rotating in EITHER direction by simply turning the shaft manually. The motor will now continue to rotate in whichever direction you started it spinning. But it still runs noisy - slower than normal - and it will quickly overheat like DickDV said.

Until we get some more information, I’m going to guess that the problem occurs when the motor is powered down but coasting BACKWARDS due to the load. Then when the power is reapplied, one of the power lines is not feeding properly. The motor starts to rotate in the wrong direction as described above. Then someone notices the “running in reverse” error and throws the disconnect switch before the motor has time to overheat.

So my best guess so far: the starter has an intermittently bad contact - or the circuit breaker has an intermittently bad section - or one of the line wires is intermittently bad - or SOMETHING - SOMEWHERE is interrupting the power through one of the three feed wires.

But I’m not going to bet the rent ...

And while I was typing, Agostinho Macedo just posted something along the same lines. If it turns out to be an intermittent lead inside the motor, then he wins the big prize.

I'm betting on a burned/high restance contact in the starter, although I have seen what the motor rebuilders call an "open rotor " do simalar things......please keep us informed as you investigate .

David beerchug

You don't really give much information as to what this motor is used for etc. but here is just a thought ...

If the motor is on a portable device, i.e. a mixer machine etc., and if that machine is feed from a 3-phase welding outlet, you can pretty much garantee that whenever you plug it into a new welding plug it will spin in the other direction. Unless of course all you welding outlet are check for phasing.

As others have pointed out, most probably it's a single phasing situation. However the motor shouldn't be able to drive much load in this condition, and should stall. The protection device, usually a thermal O/L relay with single phasing preventer should do its job and disconnect the supply. Else it could only be a acse of phase reversal in the supply chain.

Biswanath Dutta

You say a standard starter? No chance of it being a reversing starter?

If your motor is a large synchronous motor it will have a dramatic effect on your Vars and a benifical effedt on your power factor )but I digress). It is possible to slip a pole if your Vars get too out of hand. If you are using a synchronous motor you are looking in the wrong place for information. You will need an Engineer with power generation knowledge. Power generation is the only place that I know of that really deals with Vars. To everyone else they only exist in class room theory class.

Good Luck

And yet another possibility is that the motor does not change direction at all but something fed from the shaft does.
A 'reversing clutch set up' or something similar. We are all guessing without further information.

A Three phase motor rotor will rotate in the direction of the rotating electromagnetic force set up by the 3 phases...

1) I would advise that you check check the current drawn by the motor, to determine if the 3 phases are balanced.(Thus confirming the state of the windings)
2) A open rotor circuit will cause the motor to loose its strength ( low torque).
3) Check the motors RPM and see if it matches the number of poles that the motor has.
4) This is a question ???? Are there any other motors in the plant that behave in the same manner ??

There is a possibility that you may have two feeders that are not phased in.. (perhaps an old drum reverse switch)

Do keep us in informed

One more question ... has this particular motor been rewound or sent for repairs prior to this problem commencing ??

I agree the problem is in the starter, and also
think a BACKSTOP should be installed or the one
you have in use now has FAILED and your operators
just THINK the motor is running backwards.