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Motor Problem


jim clark

Question

hi there, wonder if anyone can help, I bought a motor for my lathe its 1hp single phase 1425 rpm cap start 50hz cycle windings have been tested and are fine motor runs perfectly but every now and then when I press the button to turn it on I get a pop and a flash and it trips the rcd can anyone tell what it could be please.

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18 answers to this question

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where is the pop and flash coming from?

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RCD or MCB ????

 

john..

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its the rcd and its coming from the starter switch, I have taking the motor apart and cleaned the contacts on the switch and it makes no difference its not all the time I can stop start the motor twenty times and then pop it trips out but I reset the rcd and it starts straight up no problems

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You have an RCD in the motor starter?

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Sorry that sounds a bit strange no the rcd is not in the starter switch

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So what you are saying, is that the "pop and flash" is coming from the motor starter and at the same time the RCD trips????

 

What happens to the MCB while this is happening?? How have you got this thing connected to the power supply??

 

Photos be good!! We like lathes and things on this forum!!

 

john...

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Ive got it connected to a dewhurst drum switch run windings connected to 5 and 7 start windings to 6 and 2 with the start capacitor in series with the starter switch if that makes any sense lol

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Forgot to say that nothing happens to mcb

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I thought Dewhurst drum switches were "off load devices" ie. the supply should be off before they are operated, as they have no means of dealing with the arc.  Were they not used for reversing the direction of motors?  It has been a while since I have seen one.

 

I could be wrong here but to me it does not sound like the correct gear for doing this.  I am sure someone else will be along to confirm or say it can be done like this, I have just never seen a drum switch used for this purpose.

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I have got it connected to a nvr switch before the dewhurst I dont use the dewhurst to switch it on and off sorry should of mentioned this before

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Ahh OK Jim, so where is the pop and flash coming from?

Four more posts and you can post a couple of pics up which will help a lot.

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The pop and flash is coming feom the starter switch it dont happen all the time I can turn it on and off twenty times with no problems and I can do this both directions then all of a sudden it will happen I reset the rcd and it starts up fine with no issues

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Have you had a look inside the starter to see if there is any flash over "scars" or other signs of damage?

Do you have a camera you can take a couple of pics and upload.  You will be able to do this once you get to ten posts which you are almost there.

Just to help you get to the 10 post mark. what type of lathe do you have, what do you intend to use it for?

I have a Colchester Master lathe Mk 11/2 which is the round top.

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Iam at work at the moment so cant take any photos of the motor my lathe is a 9 in model A southbend with the screw cutting gearbox and 3 1/2 foot bed shes flatbelt 1947 I do all kinds of bits and pieces iam currently I the middle of converting a straight six engine to take six individual throttle bodies

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Forgot say that when I got to motor I striped it down gave it a good clean out cleaned the contacts on the starter switch checked the windings and all looks ok someone said to me it could be the resistor in the capacitor but then it would do it every time

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Not trying to be picky Jim, but when you are referring to the starter switch are you referring to a starter that is mounted away from the motor or are you referring to the centrifugal switch which is part of the motor and used for switching the start winding in and out of the circuit.?

The way you are describing it in direct relation to the motor clean, it sounds like you are talking about a centrifugal switch.

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I've read about set ups with start and run capacitors whereby you can get sparking when the second capacitor is brought in and in effect you connect a charged capacitor across an uncharged. You get a high peak current that can cause sparking and sometimes welding of the contacts. To help they sometimes fit small resistors in series with either the start or run cap and use about 3 ohms per half horse power, 10W wirewound.

 

Does it have a centrifugal switch is that working properly and dropping out the start cap?

 

Big dose of contact cleaner!   :lol:

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Yes all seems fine switch is working ok its just a capacitor start motor there aint a run cap on it ive wired a few of these up and this one has got me stumped

Sorry roys yes iam referring to the centrifugal switch

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