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How many poles in a motor?

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TFrench

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I picked up a powered work head from a surface grinder today. The motor has been cracked, and the name tag is missing. Need to know what speed motor to get to replace it and everything I can find online says "look at the nameplate" or "run it and find out". As both of these options are unavailable, how do I check how many poles it has? (It's 3 phase)
Thanks!
 

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Inspector

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Not being an electrical wizard I would take it to a motor repair/rewinder. They can look in it and tell you in a few seconds what it is. It is also possible they might have another motor or at least parts from one to fix yours.

Pete
 

HamsterJam

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Been a while since I did any motor theory but you will need to have a peek inside too see whether you can work out how many sets of coils there are per phase. The approximate speed can be calculated from that.
Assuming 50Hz, I think two coils per phase gives approx 3000rpm, four per phase 1500rpm, eight per phase 750rpm,
Actual speed is a few percent lower than the theoretical speeds due to ‘slippage’ (so typically 2750, 1450, etc). This a function of how the magnetic fields interact in the motor to produce the rotational force.
 

jimmy_s

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As above, not that I've had to check poles before but I recon you would need to take the ends of and get the rotor out of the way and have a good look at the stator to see if you can work out how many poles its got. Judging by the size of the pulley and the speed those things go at I would have guessed its 2 pole. Failing looking at the stator windings visually, I dunno if you could use iron filings or something with the stator laid out on a bench wired up with a bit of paper under it to see how many pole pairs you have as I'd think the filings would line up with the fields. Would be 3 pole pairs if 2 pole and 6 pairs if 4 pole I recon.
 

heimlaga

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It is fully possible to rebuild that motor. Find a way to repair the hole in the end bell and rewind the motor. Though counting the windings and figuring out the speed and replacing it may be more cost effective.
 

TFrench

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It is fully possible to rebuild that motor. Find a way to repair the hole in the end bell and rewind the motor. Though counting the windings and figuring out the speed and replacing it may be more cost effective.
I know it's possible to repair, but given the cost of a second hand motor I'm not even going to think about it. Needs to be a sealed motor as well due to the grinding environment it's for.
I opened it up and just ended up going round in circles (literally) and confusing myself trying to work out how many windings there were. Then I had a brainwave - I entered the pulley sizes in an online calculator and tried 1425rpm and 3600rpm - 1425 came out around 400 rpm at the work head which sounds about right to me. More than one way to skin a cat...
 

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HamsterJam

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I know it's possible to repair, but given the cost of a second hand motor I'm not even going to think about it. Needs to be a sealed motor as well due to the grinding environment it's for.
I opened it up and just ended up going round in circles (literally) and confusing myself trying to work out how many windings there were. Then I had a brainwave - I entered the pulley sizes in an online calculator and tried 1425rpm and 3600rpm - 1425 came out around 400 rpm at the work head which sounds about right to me. More than one way to skin a cat...
Looking at those windings I can believe that it’s has four-poles per phase (so just below 1500rpm).
This is based on it looking to have about a 4-way symmetry to it and 12 coils (4-per phase). Don’t take that as definite though.
 
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