Motor problem 1

[Rick9]

Hello..

We have a small single phase motor which operates a dust collector. The motor is 3kw x 240V x PH-1
This motor has an FLA rating of 19 amps. It is run on its own 30amp circuit and all wiring is appropriate in size .. less than 25 feet from the panel.

The problem is that the motor has been shutting off (magnetic starter) due to over current.

The motor is made by ABT in Germany and is only 3 years old. The starter is made by Klinger and Born also in Germany.

Its a bit of an odd motor. In order to start it, you hold the power button for 3-4 seconds. There is no centrifugal switch. The start and run capacitors are mounted inside the dust collector cabinet, they are separate from the motor as well.

I put a meter on a single wire between the starter and the motor and left 1 x 6" Blast gate open in the system. When the motor reached full speed, it was drawing 13.1 Amps..

I opened a second gate and the amperage increased to 13.8 Amps.

This all seemed normal so I shut it off.

I then opened 6 gates and started the motor again. This time the amperage settled at 23.4 Amps . Almost 20% over current .. I shut it down before it tripped the starter.

I then closed all the gates, expecting the current to be 13 amps or a bit lower, instead when I started it up, it settled at 17.4 Amps. At this point I am confused.

So I opened that original gate, the one where the current was 13.1 amps. The motor jumped to 20.5 Amps.

***************

The machine is in a shop where most all the tools are 3 phase, the compressor and DC are Single phase because they assist other machinery and the entire shop is run off a Digital Phase converter. The DC and compressor would use up 3 phase power.

Could the run capacitor failing cause the motor to draw excessive current ?
What else should I consider ?

 

[MikeHalloran]

Look for sources of excessive torque load.
E.g. something fouling the impeller, or failing bearings.

Make sure the contactor for the start winding is not stuck on.



Mike Halloran
Pembroke Pines, FL, USA

 

[jraef]

If you have a phase converter running the whole shop, that means you have a single phase utility source. Are you sure that the phases you used to power this motor are the "real" lines from the utility? If not, the "digital" phase may be interacting with your capacitors in the motor, so when they heat up, they may be failing. Some caps have a thermal device that opens inside the cap to prevent swelling and leaking of the electrolyte.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[MiketheEngineer]

One way to lower load is clean it and re-lube. Often seems to work wonders - usually

 

[waross]

Something may be failing as it heats up. Check the caps, and check the capacitor switching.
What jraef said also. Also take this opportunity to clean the accumulated dust out of the motor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Rick9]

Thanks for the replies.

This motor will shut down without the Digital Phase converter operating at all. Its a simple 30 amp breaker running it 220V ..

It has been on that circuit for 3 years and ran fine until about a week ago..

I accessed the motor and spun it by hand, it spins freely with no noise at all. To check the shaft would be tough, its attached to the fan. The motor is a C-Face style .. I really don't think its bearings. This machine is not really that old and has not been used much. If it where bearings, it would have to be a factory flaw rather than wear..

I think I will get at the capacitors and test them next ..

 

[DRWeig]

1. When you say you closed six gates, did you issue a command to a controller, or did you visually have a look to see if they were closed?

2. Are you sure you don't have a torn bag in one of the streams with the 5 gates you opened after it behaved OK with only one gate open? Or multiple torn bags?

3. The slightly higher amps at "all-gates-closed" is odd. With a centrifugal blower, shutoff current should be lower than at minimum flow. That's another thing that makes me think a gate might be sticking or there's some other sort of leak (a pressure relief damper that opens if you run the fan with all discharge gates closed?)

If the other folks' suggestions about the motor itself and drive check out, look for extra flow. In a six-gate system, a torn or missing bag can overload the motor nicely.

Good on ya,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[Compositepro]

A bad run capacitor will cause the problems you are seeing.

 

[Rick9]

Thanks for the replies.

The unit is in tact, no leaks, no torn bags.. its air tight ..

The Gates are self cleaning and manually operated, there is no question that the gates where all closed..

99% sure its electrical. How can I tell if the start contactor is sticking ? I am good with testing the cap's ..

 

[waross]

Change it for a new capacitor. Do some shopping, don't buy the first one you see. I have seen a 10:1 price range for motor capacitors.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

I'm really curious about this starting sequence. It may be something common that I've just never seen before, I don't do a lot with single phase stuff. But it sounds a bit odd to me.

If there is no centrifugal switch (and presumably no voltage relay) for the start caps, holding down the start button until it accelerates sounds as though you are manually providing that function. It's likely that you are engaging the start caps directly with a separate set of contacts on the start button at the same time as the other set of contacts closes the contactor to energize the stator and run caps, the contactor then seals itself in around the start button through the stop button. Then when you release the start button, it is dropping out the start caps and since the contactor has sealed itself in, the motor continues to run. If that's how it works, you might have a bad set of contacts on that start button, i.e. the ones going to the start cap are welded shut. Capacitor switching is hard on contacts, it may be that this button was originally designed with heavier duty contacts than normal but has worn out or it happened a while ago and they were replaced with standard push button contacts that can't take the abuse. It's also possible that this start cap circuit is interlocked through a set of aux contacts on the contactor, which also may have welded. If for any reason all sets of contacts in that part of the circuit weld, the start caps never disengage even though the motor is at speed and no longer needs them. That could manifest as the symptoms you are experiencing.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[electricpete]

Perhaps it is possible to to clamp on to the lead connected to each capacitor (with suitable approved safety precautions) to see which carries current when the motor is running.

=====================================
(2B)+(2B)' ?

 

[Rick9]

The starter has 2 contactor's as described by Jraef .. When you push in the start button, one contactor engages and energizes the motor and start capacitor ..

When the motor reaches speed, the second contactor energizes and closes the run circuit automatically. Then when you release the start button, the start capacitor is taken out of the circuit.

Its actually a pretty simple system, but its odd to me..

In this case, both caps are identical, uf-80 x 400 V .. That seems large for a 3kw motor but it is what it is.

The start capacitor is fine .. tested okay .. The run capacitor is dead. Today we fired up the motor and tested the amperage again, its higher than it was last time .. When we discharged the capacitors, the run cap had no charge.. Its also swollen on one side ..

I had a motor electrician come look at it and he said he has seen it before, the motor is starting because of the start cap, your holding the start cap in the circuit till its at speed .. After the motor is at speed, the fan acts as a flywheel .. very hard on the motor to run it with no run cap but he said in most cases, a new run cap will fix the problem .. He suggested that with the system they use, a panel ammeter would be a great add-on because relying on an overload to tell you there is a problem, in this application, is weak protection..

 

[waross]

Sounds as if your electrician knows where his towel is. That is not always the case. grin

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Rick9]

He is a fella I know who is retired. 35 years of diagnosing, rebuilding and upgrading electric motors in saw mills and pulp mills..

He dislikes single phase motors and believes every motor should be controlled by a VFD .. lol ..

 

[jraef]

... knows where his towel is...

Is that an obscure "Hitchhikers Guide to the Galaxy" reference?

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[waross]

Yes. But obscure? I thought that reading Hitchhiker's was a right of passage for engineers. Grin.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[zeusfaber]

Reading? I saw it on stage on Friday night with most of the original radio show cast (and Phil Jupitus as the book)

Very interesting listening to the audience on the way out (because we had the kids with us, we were listening rather than making equal fools of ourselves).

A.

 

[waross]

I envy you zeusfaber.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LiteYear]

Interesting read. Looking forward to the resolution. Had to give Compositepro a star for zir understated prescience.