Thread: Electric motor question

It is hard to offer good advice without a circuit diagram. Can you post a circuit diagram?

A few points:

• When you measure "2M ohms" are you touching the probes? Readings at this level are quite susceptible to body resistance and are often a open circuit.
• I'm guessing the machine has a "soft start" in the sense that the switch configures the winding to star (240 V per winding) for starting and is then switched to delta (415V per winding) for running.
• Because it is three phase, you should be getting the same resistance reading on each winding (I think you already know this).
• If the motor has burnt out, try using your nose - smell is a good (bad) give-away that the motor has been damaged. You may have to open it up first if it is a TEFC motor.

Chris,
thanks for replying.
I don't have a circuit diagram and I'm not sure I can get one. I can probably get the markings off the terminals and post the readings between each one if that helps. I know not to touch the probes when measuring so these should be actual readings. You're right about the soft start, I can just never remember which is star and which is delta. I figured I should be getting the same readings, but I thought that for the breakers to trip the way they did I should be getting a dead short some where. The 6 - 7 ohms is between pairs of star and delta windings. There's no smell, but it is a TEFC motor. I was hoping that I wouldn't have to resort to dismantling it.

I'll try to get more details tommorow and hopefully catch the sparky next door for him to look. I'm hoping it isn't the motor and if it is that it's somehow been caused by the phases being swapped (thereby making the electricity provider responsible).

Mick

G'day Mick,

I work a bit with 3 phase machines however I am by no means a sparky or an authority on things electrical. In my experience of connecting 3 phase to machines is that there are 3 wires, red, blue, and white. One of these wires is always the middle position between the other two coloured wires. My thopughts, get a sparky to look at it first and foremost from the original switch side inside the saw where you have not touched yet. I'm guessing, and please don't get upset but I'm thinking maybe you have swapped the middle wire with one of the others. Kinda like fitting a spark plug lead to the wrong cylinder of a car engine.

As to the breakers not tripping when you disconnected the motor, you now have no motor windings to provide resistance or overloading to be measured back at the breakers.

And lastly I have got a circuit diagram of a star/delta switch for a dust extractor buried in my files somewhere at work if you need them, I can scan and email them to you.

Hope this helps you out, and please don't get upset by my comments as my knowledge is very limited on the subject.

Cheers

Bazzy

Mick,
Found this diagaram on me pc with all the other junk I keep

The power diagram on the left is what you should have in your saw. Forget the control diagram on the right as it sounds like you have a rotary switch instead of control cct and contactors.
The contacts KM1,2 and 3 on the power diagram are on your rotary switch.

Sounds to me like you may have got the winding tails mixed up when reconecting it? maybe? Hard to say without looking at it
Hope this helps anyway.
Ben

Operation

Bazzy,
thanks, no offence meant and none taken. I haven't swapped the wires over, the electrical supplly authority did that when they rewired the supply out on the road. I was hoping that the breakers would trip with the motor disconnected as it would point to a short in the cabling which has some mechanical damage.

Mick

Originally Posted by journeyman Mick

Bazzy,
thanks, no offence meant and none taken. I haven't swapped the wires over, the electrical supplly authority did that when they rewired the supply out on the road. I was hoping that the breakers would trip with the motor disconnected as it would point to a short in the cabling which has some mechanical damage.

Mick

This all sounds a little odd, Mick. Do the electricity authorities swap phases over when they do work? It strikes me that they shouldn't. Surely they use phase sequence testers?

Anyway, if all else fails, i do have a couple of good three phase motors here. One's 5.5 KW and the other is around 7.5 KW from memory.

Originally Posted by Bendog

Mick,
Found this diagaram on me pc with all the other junk I keep

The power diagram on the left is what you should have in your saw. Forget the control diagram on the right as it sounds like you have a rotary switch instead of control cct and contactors.
The contacts KM1,2 and 3 on the power diagram are on your rotary switch.

Sounds to me like you may have got the winding tails mixed up when reconecting it? maybe? Hard to say without looking at it
Hope this helps anyway.
Ben

Operation

Mick, as you look at the above diagram you see the motor terminals marked u v & w.
With the wires removed from the motor you should have a circuit between u1 to u2 , v1 to v2 , w1 to w2. With that size motor you should have around 5 - 20 ohms resistance. You should have an open circuit on any other combination.
As you said you had 2mohms between some and kohms between others I would say you have a dud motor.
At that size motor you may be able to get a generic motor to fit unless it is some weird shape. On the motor nameplate they usually have a long number which tells a motor company the motor frame orientation and type.

Hope the above is useful

Shane.

Just to make it really clear to everyone: I haven't swapped the phases over the supplier stuffed it up!

The supply lines here ran down the middle of people's back yards and they couldn't access them to replace the termite eaten poles as people had very selfishly erected houses on their blocks in the fifty or so years since the lines were run.

They've run new lines down both streets and have been gradually hooking houses up. I haven't touched anything between the last time I ran the saw and it went round the right way and the weekend when it ran backwards. The only thing that changed was that my supply no longer came from the pole in my neighbour's backyard but now runs from the pole across the road.

Unless the **** up faries have gotten linesman's tickets or electrical contractor's licenses then, yes, the electricity supplier has stuffed up the phase sequence. Might have to give them a call tommorrow. Good thing I don't have a 3phase air conditioner, I can imagine that swapping the phase sequence on one of them might prove to be spectacularly expensive.

Mick

Mick

Originally Posted by wattlewemake

Mick, as you look at the above diagram you see the motor terminals marked u v & w.
With the wires removed from the motor you should have a circuit between u1 to u2 , v1 to v2 , w1 to w2. With that size motor you should have around 5 - 20 ohms resistance. You should have an open circuit on any other combination.
As you said you had 2mohms between some and kohms between others I would say you have a dud motor.
At that size motor you may be able to get a generic motor to fit unless it is some weird shape. On the motor nameplate they usually have a long number which tells a motor company the motor frame orientation and type.

Hope the above is useful

Shane.

Thanks for that Shane, is there any chance that the motor failure was due to the phase swapping? I'm hoping that I might be able to get them to shout me a new motor.

Mick

Mick,

A bit of a long short here. Initially the machine was okay but running backwards due to the swap in the phase sequence of the incoming mains. Therefore you, rightly, swapped two phases to change the direction.

Does the machine have a neutral? An issue I've come across every now and again with 3-phase wiring and 3-phase plugs. The old flex cable used to have red, white and blue for the 3 actives and black as the neutral (green/yellow as earth). Newer cable (IEC) has brown, black & gray (or white) as the 3 actives and blue as the neutral (green/yellow as earth). Note the difference: black is used as a neutral in the old stuff but is used as an active in the new stuff. I have seen the new cable wired using the black as a neutral (yep, and by a licenced electrician). When I see the IEC cable used, and I have to fit a 3-phase plug, I open the equipment up and check which conductor has been used as the neutral. I wonder if you may have accidentally swapped a phase and a neutral rather than a phase and a phase due to confusion with the colour code?

Mick I hate to say it but buckleys had more luck than you will have. Energex will say to you no they didnt swap phases and at worst they will deny that that would cause your motor to blow up. It would of just run backwards. They will tell you most likely that they will send someone around to look at it but if they reckon its not your fault they will charge you for the call out.
I have been standing in a house with no power and rang them and said there is no power in this small country town and been told there is no problems with the power in the town and that it must be in my place. After telling them that I dont like being fed horse poo and to send someone out to find the problem. Well they sent out thier linesman who looked for a whole day and couldnt find it. I took great pleasure in ringing them up asking for the bloke who told me the horse poo and asking him did he want to know where the fault was? It was an old power pole which had rotted and fell over. Grudgingly apologised and was horrified when I asked that my parents power bill be discounted because they couldnt fix it within the garanteed time limits. I got a chuckle out of it anyway. The power problems of a few years ago in qld are not dead and buried I can tell you...

I think it would be easier to get a sparky out and get him to say your motor died of fusion and claim it on your insurance.

Shane.

Thanks for the help, there's no neutral on the machine and I swapped the phases on the sub-board breaker. I did this because I've got four seperate 3 phase outlets and decided I'd rather swap 2 wires than 8. Not sure what I'll do as I'm still on a pension and not sure when I'll be ready to go back to work, so money is very tight. I'll give the supplier a call tomorrow and take it from there.

Mick

Mick the drawing above is for a standard Star/Delta starter. The motor only has one set of windings. The windings are connected in star to start and then switched to delta once the motor is running. This is done by a timer that is started when you push the start button.
I'm not convinced that the motor is damaged. Can you take a reading between U1 and U2, V1 and V2 and W1 and W2 and let me know what you get.
You could have a fault in the control circuit which will trip the breaker.
With no neutral the control voltage is 415 for the contactors and timer.
(the timer may be pneumatic and attached to the K1 contactor.)
The phase switcheroo may have have inadvertantly shorted two phases together (this may be impossible but is worth considering before paying to have it checked out.)

I put everything back the way it was this morning and called the supplier. They sent a crew out who checked the supply and found it had been connected the wrong way (left hand rotation). They climbed up the pole and swapped the phases over so now everything is as it should be, with all the machines running the way they were before the power supply was changed. They tested the saw's circuit and it was okay. They also tested the saw's wiring with a megger and they couldn't see why it was tripping the breaker.

I'm going to see if I can get the sparky next door to check it out for me. Don't have time just now to test the readings on those circuits as I've got a couple of BAS statements I need to do, I don't think they'll give me a second extension. I'll post results when I can.

Mick