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Any motor experts on the forum?

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Ed Duffner04/03/2020 22:02:03
800 forum posts
91 photos

Hello guys,

At work I am trying to restore a Scabbler (concrete de-chipper) machine. Problem is, the motor makes a weird noise when running.

The motor is an induction type with permanent magnet rotor and a wound field-coil. There is no centrifugal switch and starting is handled by two 80uf capacitors wired in parallel.

It's switched on via a cylinder type, multi-pole switch which energises a contactor coil in a start position then springs back to a run position.

In the start position the motor sounds very smooth but when in the run position it sounds rough and the motor speed drops somewhat. It's like there's a choking effect or field being introduced.

I'm pretty sure I have wired the motor and controls according to the circuit diagram. Please excuse the use of an earth wire in one of the below photos, it's just a link, not an earth connection.

In the video I start the motor and release it twice so I can let you hear the 'choking' sound. It doesn't quite work on the first start.


I'd appreciate any ideas and your input as always.

Thank you,








Motor connections.


On-Off-Start Switch.


Circuit Diagram


Edited By Ed Duffner on 04/03/2020 22:18:57

Edited By Ed Duffner on 04/03/2020 22:27:33

Simon Williams 304/03/2020 22:58:41
508 forum posts
80 photos

I think the clue is in the fact that it started OK the first time, and only made the straining noise when you moved the switch. That's a faulty switch. No I would definitely not be happy to let that go out of my shop behaving like that, but I think the fault is in the rotary switch.

Looks like it's a pretty standard sort of switch - the internals and the spring return will be specially designed for this application but you should be able to obtain a direct replacement.

If you would like to put up a photo of the labels on the switch which hopefully will show a manufacturer (Kraus and Naimer?) and a type number we can probably locate a supplier.

You may even be able to get a replacement from Marcrist as it seems to be their name on the drawing.

Worth spending the money for the right thing - should be £20 - 30 or thereabouts. While you're at it replace the contactor as well at the same time and have done with it. That looks to me like a Siemens contactor (edit - sorry the clue's on the picture it's Eaton if you blow it up big enough - but any similar contactor will do) - should be possible again to buy a direct replacement for about the same again. If you can get a picture of the label on the side of the contactor it will show manufacturer and part number.

HTH Simon

Edited By Simon Williams 3 on 04/03/2020 23:03:26

Ed Duffner05/03/2020 00:07:17
800 forum posts
91 photos

Hi Simon,

Thank you very much indeed for your expertise. It's always good to get another pair of eyes on a problem.

I can contact Marcrist to see about a replacement switch. The contactor was actually a replacement itself, some years ago but the machine was left in disrepair because it would not start at all. I traced that problem to an open-circuit thermal overload in the motor. I've bypassed that for now just to get the motor running. It'll be another required part, if its available.

Thanks again,

Brian Morehen08/03/2020 08:28:07
81 forum posts
6 photos

Is this a single phase motor or a 3 Phase one that has been altered..


J Hancock08/03/2020 11:00:16
406 forum posts

Good point, details on nameplate worth showing here.

duncan webster08/03/2020 11:18:17
2591 forum posts
34 photos

I'm confused by the description as an induction motor with a permanent magnet rotor. Can anyone clarify?

Les Jones 108/03/2020 11:58:02
2130 forum posts
146 photos

I too have never heard of " an induction motor with a permanent magnet rotor ". I think Brian's question about it being a 3 phase motor that has been modified needs to be answered.


Brian Morehen08/03/2020 13:33:30
81 forum posts
6 photos

The motor name plate and details will teel us all a lot , This looks like a 5hp or more size motor. Are you running this through a 3ph Inverter .The motor sound chages when running I wonder if this is running on 2 phases because the inverter has brocken down and lost a phase .

Brian M

Steviegtr08/03/2020 15:18:32
1233 forum posts
115 photos

Look at the wiring diagram, lower right portion. It shows a single phase supply.


Paul Barter08/03/2020 16:25:00
103 forum posts
7 photos

Sounds like a dud bearing, damaged ball or balls skidding rather than rotating.This may only be happening when the motor has run at full speed for a few seconds and a lack of lubrication and or roundness starts the music.


Mike Poole08/03/2020 19:30:43
2575 forum posts
60 photos

It is a single phase 110v machine, looks to be identical to a Von Arx machine.


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