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Old 05-01-2018, 04:33 AM   #1
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Default 100hp compressor doesn't start

I have just recently joined Electrician Talk and this is my first post so please forgive me if I am doing something wrong. I am from the DC world but I have just completed a course on Industrial Maintenance (circuits, motor controls, PLC, etc). I have a decent grasp on circuit design. Being fairly new to the A/C world, feel free to correct me on my terminology.
I work at a facility that employs only 2 maintenance men, a seasoned electrician and myself, on 2 different shifts.
About 2 weeks ago, I was told that one of our air compressors, a 100hp screw compressor, wasn't working. Our electrician tells us it needs a new motor. I megged it out, winding to winding and all windings to the frame and no shorts. I left it disconnected.
I disconnected the motor and isolated the main lines with the rubber boots that were already on the lines.
Here is the rundown: 480v into a 100A fused disconnect to a transformer down to 208v- then through a contactor then a motor starter (not sure why both) straight to the motor (not connected, but was delta).
I fed power back to the transformer and thus 208 back to the contactor. NO, I saw normal 208 voltage to contactor. I closed it and it went almost immediately to crap (40v and lower) . Then of course I saw smoke billowing from the disconnect on the wall. I shut it off as fast as I could but it was too late. Some of the insulation was burnt, but oddly enough, the center phase fuse was melted, and I still have 480v before the disconnect and no breaker was tripped.
We have had a disconnect panel single phase somewhat recently. What are the chances it just happened again? (different machine)
What are the chances that that the contactor is bad? I seriously appreciate any input so that I can gain some of your knowledge,
Thank you.
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Old 05-01-2018, 05:35 AM   #2
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It's a fairly normal failure mode. Nothing too exciting there, but I want to tune you up about something else....

Unless someone specifically asked you to re-do all that troubleshooting, you NEVER go behind someone and try to figure out if they were wrong or not. That's, well... uncool. Maybe he is wrong about the motor. Maybe you were just trying to learn something, but this is a serious faux paus. You might have to work with that guy for the next 20 years, so don't make him look bad until you have demonstrated sufficient competence (years from now). If he's wrong with his diagnosis, it will just cost the company a little more downtime and maybe buying parts they don't need. Keeping everyone in the shop on good terms is more important than that.
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Old 05-01-2018, 01:30 PM   #3
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...
Here is the rundown: 480v into a 100A fused disconnect to a transformer down to 208v- then through a contactor then a motor starter (not sure why both) straight to the motor (not connected, but was delta).
I fed power back to the transformer and thus 208 back to the contactor. NO, I saw normal 208 voltage to contactor. I closed it and it went almost immediately to crap (40v and lower) . Then of course I saw smoke billowing from the disconnect on the wall. I shut it off as fast as I could but it was too late. Some of the insulation was burnt, but oddly enough, the center phase fuse was melted, and I still have 480v before the disconnect and no breaker was tripped. ...
First off, I totally agree with MDShunk, you overstepped your boundaries by second guessing the more experienced guy. Even if you might suspect he was wrong, you could (should) have framed it as a "Hey, help me learn how to troubleshoot what's going on here; how did you determine the motor was bad?"

Now as to what happened. If all you did was to close the contactor on the 208V side, and the compressor motor was NOT connected, then the only thing that could have blown that fuse was a cable going to ground, which should have been eliminated ion your megger testing (if done right) or the contactor having melted into a shorted condition, which is VERY rare. So something tells me you are leaving something out of this story, probably unintentionally because you don't see all aspects of this equipment. Two contactors in series (or as you said, contactor + motor starter, which would include a contactor) is HIGHLY unusual, so I have to believe there was something else connected to this circuit, like maybe an oil pump or air dryer.
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Old 05-01-2018, 02:38 PM   #4
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Default How many wires going to motor from contactor

Were there only three wires going to the motor?

It could of been a reduced voltage starter, that is two starters and if only one set of leads were disconnected or wrong leads tied together= smoke.
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Old 05-01-2018, 03:14 PM   #5
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I have left some things out to keep the story shorter. Firstly, I never intended to try to prove my electrician wrong just to prove him wrong. We are under a very tight budget right now and if I did find a separate problem I would only tell him. I did talk to him about megging the motor already. He is trying to help me with my troubleshooting skills but I thought I would get an outside opinion as I haven't talked to him yet. I am not trying to make him look bad, and as a result, I will be the one to look foolish, with obvious reason. I should also note that the company had to do a $9000 conversion on one of our machines to run on A/C with a VFD. I took the old (two weeks old) DC motor to school to find out that it only needed brushes. I never told anyone about that diagnosis (done by our electrician) but the school got a shiny new motor.
I will ask him about it more today and let him know what I screwed up. When asked by my supervisor, I will just present it as a learning experience and take whatever scolding he wants to deal out. They were going to replace the motor, so that had to be disconnected anyway.
As JRaef points out, there is a bigger picture. It also has a cooling fan with its own starter. And yes, the "contactor" and "starter" are really just two contactors, the second having an overload. But the fan has a smaller starter.
As I stated, I may find out more today. I will take the advice though and make it clear that I would like to check out any diagnosis that he finds, with his consent. If he doesn't want me to touch it, I will leave it alone.
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Old 05-01-2018, 10:40 PM   #6
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I
About 2 weeks ago, I was told that one of our air compressors, a 100hp screw compressor, wasn't working. Our electrician tells us it needs a new motor. I megged it out, winding to winding and all windings to the frame and no shorts. I left it disconnected.

Thank you.
There had better be shorts if you Meggered winding to winding.
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Old 05-01-2018, 11:55 PM   #7
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I have left some things out to keep the story shorter. Firstly, I never intended to try to prove my electrician wrong just to prove him wrong. We are under a very tight budget right now and if I did find a separate problem I would only tell him. I did talk to him about megging the motor already. He is trying to help me with my troubleshooting skills but I thought I would get an outside opinion as I haven't talked to him yet. I am not trying to make him look bad, and as a result, I will be the one to look foolish, with obvious reason. I should also note that the company had to do a $9000 conversion on one of our machines to run on A/C with a VFD. I took the old (two weeks old) DC motor to school to find out that it only needed brushes. I never told anyone about that diagnosis (done by our electrician) but the school got a shiny new motor.
I will ask him about it more today and let him know what I screwed up. When asked by my supervisor, I will just present it as a learning experience and take whatever scolding he wants to deal out. They were going to replace the motor, so that had to be disconnected anyway.
As JRaef points out, there is a bigger picture. It also has a cooling fan with its own starter. And yes, the "contactor" and "starter" are really just two contactors, the second having an overload. But the fan has a smaller starter.
As I stated, I may find out more today. I will take the advice though and make it clear that I would like to check out any diagnosis that he finds, with his consent. If he doesn't want me to touch it, I will leave it alone.

I would relax about replacing a DC motor. They require regular maintenance blowing out the carbon and replacing brushes, average life between rewinds is about half to two thirds of an AC induction motor, and DC drives go through technological obsolescence almost as soon as the warranty expires. Although it is making a comeback in the form of BLDCs and other electronically commutated motors in small sizes, it’s practically dead technology.

And the cost...typically the cost of conversion is less than a single rebuild. For example one customer recently needed two 100 HP DC motors used to rotate a rail car unloader worked on.

One just requires a basic cleaning, dip, bake, and replace bearings and brushes. The other one had a commutator problem and needed the commutator rebuilt which was $23,000 over and above the rest of the work. The entire conversion including VFDs and motors was around $25,000 and that’s with encoder feedback since this is a load sharing and virtual gearing application. I usually tell customers don’t be shocked at prices ten times higher than AC and they’re rarely pleasantly surprised that I’m wrong.

DC motors are barely taught in school any more. Electrically they’re about as simple as it gets but mechanically they’re a bear and the extra addition of the commutator and the need to avoid for instance excessive field weakening or outright field loss complicates an otherwise very simple control scheme. There are a lot of moving parts in the system. I get a lot of business just because of these things. Misdiagnosis is very common, even with experienced DC techs.



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Old 05-02-2018, 12:05 AM   #8
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There had better be shorts if you Meggered winding to winding.
From one winding to another, it should be clear.

From one end of a winding to the other end, it should fail a megger test.
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Old 05-02-2018, 01:51 AM   #9
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I agree about the DC motor. I was glad to get the VFD setup. The DC drives that we use are not very dependable and changing brushes is no fun, especially when you have to pull the motor every time. A $23,000 commutator repair would definitely justify a conversion. Thanks for the feedback Paul.
Also, thanks to micromind for clearing that megger situation for me. I just used an ohmmeter to find the winding ends first, then used the megger to test from there.
Cowboy, there are 3 wires going to the motor. The wires go in series through the two contactors. I was informed though that the second "contactor" is not a contactor but just an overload. It looks almost exactly like the contactor, with an overload and 2 separate wires that I thought were coil wires going to the base. I appreciate the feedback. These are the types of responses I was hoping for.
So I went to work a little early today to talk to our electrician. The first thing I said is "I did some very bad things last night." I showed him the carnage (a blown fuse and some melted insulation) and he just laughed and gave me advice on other things to check. I left him a note to see if he will double check my work.
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Old 05-02-2018, 12:04 PM   #10
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This makes little sense to me.
You had a DC motor they replaced with AC? Correct? AC Drive?
If the answer is yes, why do you have a contactor and why did you have a contactor before with the DC?
Do you mean disconnect?
So you guys have a TEBC motor with two starters? The blower may need the starter but the VFD motor does not unless you mean disconnect?

What size motors? Your costs sound through the roof.
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