3 phase six lead two speed motor burnning down the house! 3

[gristlehead]

Let me start by introducing myself. I have worked as an elecrician for the last 17 years, primarily in industrial maintenance. Last year I moved more into a managment role and have started working on my EE.

Now for the nightmare. I have a motor that my electricians have managed to burn up three times in the last week. I'm pulling my hair out. This motor is an SEW gear drive motor, 4.5/5.0 hp. 1750/3600 rpm. It has six post to connect wires to and they are labeled U1, V1, W1, U2, V2, W2. There is also a 480v brake. This motor moves a pallet lift up and down and utilizes both speeds. It is controled by a very old S5 plc. So old that you cannot go online with it to trouble shoot (It needs a ram chip to replace the eprom to do so) The machine has no home run program and about a year ago I put in a push button station to manually fire the contactors to get parts of the machine back to basic position. this particular motor utilizes 6 contactors. A lift contactor, a lower contactor, a fast contactor, a short contactor that shorts U1,V1,W1 when the fast contactor is energized, a slow contactor that puts voltage to U1,V1,W1 and finally a brake contactor that releases the brake.
Combinations to run the motor are as follows, (Lift, slow, brake; lift, fast, short, brake exchange lower for lift in the above two for lowering the pallet.
I've set here and looked at the drawing, It appears to me that there are a couple of combinations that could cause the motor to burn up. If the slow contactor sticks or shorts across a contact while the motor is in fast operation it would introduce 480 into the windings that should be shorted. The next idea I have is that mabe the short contactor has a bad contact in it and one of the legs is not getting connected durring the fast run cycle.

I was told tonight that the motor ran fine on the slow setting using my push button controls. It was only when they switched the machine into auto did it fry itself.

Do you see any other ways this motor could self destruct?

 

[waross]

The first thing to check is to see if a contact has welded closed in one of the contactors.
The second thing to check is the brake.
The next thing is to verify that the PLC outputs are getting to the contactors. As a matter of course, I would check across each pole of the energized contactors with a voltmeter. Any voltage across a closed contactor is a sign of contact failure.
I have found that information such as "It works on the push button" to often be misleading.
Don't let this information distract you from complete trouble shooting.
I would check the circuit to see if the push button may be bypassing something that is in the circuit when the PLC is controlling.
I would also look for other problems.
The problem may be mechanical. Something dragging, or a lack of lubricant.
Check the currents. High, balanced currents may indicate a mechanical problem. Problems with the contactors may show up as high and unbalanced currents.
respectfully

 

[MikeHalloran]

I doubt that any PLC could be much help with diagnosis; it has to be just automatically "pushing the buttons" you would find on a manually controlled hoist, and your problem seems to be well downstream of the actual control signals.

It happens that I have drawn and modified wiring diagrams for two speed hoists with brakes. I don't pretend to understand them when they are working right, much less when they fail, but I overhear our service techs talking, and the fast speed circuits and their failure modes are often a subject of discussion.

I do note that our controls have a lot of electrical interlocks, e.g. auxiliary contacts that prevent both contactors of a complementary pair from being energized at once, and some controls have time delays in places you wouldn't expect.

As a start, in my extreme ignorance, I'd suggest squeezing in some extra fuse blocks so you don't keep using the motor as a fuse, and reviewing the extra possibilites for damage collateral to a contactor failure provided by your wiring modifications.



Mike Halloran
Pembroke Pines, FL, USA

 

[gristlehead]

Thanks for the reminders, I keep having to tell myself that even though I've asked that all these things be checked, they may not have been. I'm going in in the morning. It really pisses me off that I have to go in and chack things myself. The really bad part of this is this last motor came from Ohio.... I don't know where I'll find another. If every thing checks out contactor wise ( no stuck contactors, and no Contactors being pulled in when their not suppose to ) I'll have to wait on another motor to check currents.

 

[stardelta]

First thing that crosses my mind is that the motor is running against the brake so check that the rectifier is ok and that the brake is coming off cleanly in each speed but something is not right about the lead markings U1 V1 W1,U2 V2 W2 suggest a single speed winding. A 2 speed 6 lead motor should have the following markings, 1U 1V 1W for slow 2U 2V 2W for fast. Does the motor have 2 seperate windings, one for each speed or does it have a tapped winding? Just check with a Fluke for circuits between the starts and finishes. A seperate winding for each speed is easy, just treat it as 2 sperate motors and feed each winding , however if its a tapped winding ie a singlw winding for both speeds its a bit more complicated. For slow speed operation you feed 1u 1v 1w, for fast speed operation you feed 2u 2v 2w but MUST short out 1u 1v 1w at the same time. If this is not done the winding is not properly configured and burns up in no time.

 

[gristlehead]

Thanks Stardelta, when the motor is using the slow speed the contactors only put 480 to the U1 V1 W1; When using the fast speed the contactors short U1, V1, W1 and apply 480 to the U2, V2, W2; Keep in mind I'm writing this from home and I am working off of memory, I could have gotten my 1's and 2's confused. I'm about to head in now to see what I can find.

 

[BJC]

Did the loads change? Do the pallets weigh more now than they use to?

 

[gristlehead]

I've gone down and studied the contactors, the motor,.... took ohm readings all around and found that the wires going to W2 and U2 are inverted, but that is the only thing jumping out at me. All the coils are good on all seven contactors.

There is one contactor that is puzzling as to what it does.. on the U1,V1,W1 side of the motor K323 drops out one single leg.... A phase.... This is the slow side, is this suppose to be open or closed durring slow speed operation? It is in between the star contactor and the slow contactor...

 

[stardelta]

Gristle?
Bear in mind I have limited knowledge with regard to control gear but what you say doesnt sound right, the way I read it K323 is opening and as a result the slow speed is only getting fed 2 phases and not 3. The motor winding wont last long in this situation, like any 3ph motor it needs all 3 phases to run correctly. After you mentioning the fact W2 and U2 were reversed another thought occurs, if during operation the motor runs in the same direction and switches between speeds you need to ensure correct phase rotation otherwise it will instantly change direction as it switches from one speed to another, this is not generaly accepted without a time delay for breaking or for rundown. What has been done to the motor since the original fault, has it been rewound?, if so I am thinking the lead markings have been incorrectly replaced. Is there a chance of you taking the motor off the machine and bench-testing it with a seperate supply. Basically this is what should happen:

Feed 1u 1v 1w with R Y B phase respectively, it should run clockwise in the slow speed with balanced phase currents

Feed 2u 2v 2w with R Y B phase respectively while shorting out 1u 1v 1w, it should run clockwise in the fast speed with balanced currents.

While its off you can check the brake too. SEW brakes have a good "clunk" to them when they come off and you should be able to turn the cooling fan by hand. The euro spec SEW brake motors have a black rectifier with 5 terminals on it and the brake coil will have 3 leads from it, white, red and blue, there is a diagram embossed on the top of the rectifier which shows how it connects.

 

[gristlehead]

Thanks StarDelta ! I think you have just figured it out for me. Sometimes it really takes another set of eyes. I had never considered what would happen if it changes speeds mid stream. The brake appears to be connected correctly. What had me worried with K323 was that when I designed the push button panel I had the same thoughts (its a three phase motor and needs all three for slow speed and all three for fast speed) and I have this contactor pulling in when using the slow speed....I only use the slow speed from the pushbutton panel, so I was worried that something I had done a year ago was causing this problem. In normal operation the push button cabinet is locked out. Another concern is that the K323 contactor is not being picked up from the PLC durring slow speed operation, Thus single phasing the slow speed windings. With the slow speed being 1760 ussually being equal to 4 poles this may not be enough to burn up the motor.
The High speed being 3600.. 2 poles... well a switch in direction durring speed changes will cause exactly what is happening... Again thanks, Hopefully, we'll be getting another motor soon.

 

[gristlehead]

UPDATE

On Monday, I tried my best to describe to the electricians what I had found. Spread sheet and all with all the ohm readings. They didn't believe me. Said the motor was hooked up properly. I did convince their boss "not me". He sent the motor for repair and the repair shop called back and said it was load that took the motor out. Between He and I we convinced the electricians that it would be in their best interest when the repaired motor arrived to hook it up and put the lift in the mid position with the slow speed. We all agreed that the slow speed functioned properly. The motor came in on second shift last night. They installed it and moved the lift to the middle position. This morning the day shift guys went down and low and behold the motor ran backwards in the fast mode. They made the swap at the motor and put it back in service and as I write this we have discharged 7 pallets. Thats about 14,000 lbs. Thanks again for helping through the fog.