Electrical Experts - ????

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bobleibo

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I have a question for you electrical experts. I have an older bench top grinder that has a grinding wheel on one side and a wire wheel on the other. The grinder itself is nothing special, 1/2 hp, 3.5 amps so if it ends up being toast no big loss. Might be time to replace anyway.
Before, whenever I would use it and really get after something on it, no problem, it kept going full speed ahead. Suddenly if I put the same pressure on either wheel it slows the motor down to almost a complete stop. Checked to make sure nothing is loose (shafts, wheels, nuts, etc.), nothing. Everything is tight as can be.
Time to replace or easy fix?
Thanks~
Bob
 
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magpens

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I was going to say - check the brushes. And also check the capacitor (which I don't think would be the problem if the motor starts and runs up to operating speed just as it always did).
 

maxwell_smart007

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Some of those smaller motors have capacitor-run setups (rather than just capacitor-start), I think. So if it's spinning freely by hand, then it's definitely possible that it's the capacitor (assuming it has one). If it has brushes, then that's likely the culprit.
 

low_48

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Sorry I'm not an expert, but would be shocked if an old bench grinder had brushes! They are basically the same type of motor that is on the washing machine, or furnace blower. NOT like a router or drill. But since I'm not an expert, what do I know!
 

Magicbob

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yep, most grinders are an induction motor (thanks Mr Tesla)
High load motors like grinder often have start/run capacitors.
If it starts up good but wont kieep speed under load, it is most likely the run cap.
not too pricey, look in your local phone book for a motor repair shop and call them.
You should be able to take the grinder in to them and they can sell you a new part, easy fix.
 

bobleibo

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Thank you everyone for your input. It sounds like it could be one of a few things which are definitely above my pay grade. I think I'm going to buy a new grinder.
In the mean time, I'm going to take this one apart and try out the different suggestions you each had. It will be a great opportunity for me to learn a little bit about electric motors. Who knows, I might even end up fixing it....but don't bet the farm on that one!
Thank you again!
Bob
 
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Magicbob

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I stopped by our local Woodcraft the other day and they had a Rickon low speed grinder on sale
 

jttheclockman

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It could be a single phase induction motor which is a better workhorse of a motor,, no brushes. With an induction motor they are much quieter than a (universal motor such as used on hand tools and shop vacs. These have brushes.) Depending on the size and the age of the motor it can have both a start and run capacitor, or it can have a combo of the 2. If it is one then that is simple enough to get the same size or slightly larger. Never smaller. If it has 2 you need to know which one is the run capacitor. Run capacitor is the one with the lowest microfarad (mfd) The start capacitor will have the highest capacitance rating.

There is danger in replacing them in that they can have a built up charge left in them. If the capacitor does not have a resistor built into it for drainage than you have the possibility of having a harmful DC voltage sitting at the terminals. DC is much more harmful so do not be fooled. Needs to be drained. I did not tell you to short the terminals.

Now if the run capacitor has broken down which sounds like the case there can be internal problems. Could be that heat has broken it down. Age could be the problem but those capacitors are designed to run a long time. More-so than a start capacitor which has to work harder.

Now after I mentioned all this, this is not the only option. This can also be a single phase split phase motor. This has 2 sets of windings, a start winding and a run winding and uses a centrifugal switch. It may also have a thermal switch that protects the motor from overheating. Being you said it was old it could be a salvaged motor to build this.

So just to show you there is no easy answer to your question. The name plate if it still has one will supply you with some valuable information. It could be a learning experience. :) Good luck.
 
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bobleibo

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It could be a single phase induction motor which is a better workhorse of a motor,, no brushes. With an induction motor they are much quieter than a (universal motor such as used on hand tools and shop vacs. These have brushes.) Depending on the size and the age of the motor it can have both a start and run capacitor, or it can have a combo of the 2. If it is one then that is simple enough to get the same size or slightly larger. Never smaller. If it has 2 you need to know which one is the run capacitor. Run capacitor is the one with the lowest microfarad (mfd) The start capacitor will have the highest capacitance rating.

There is danger in replacing them in that they can have a built up charge left in them. If the capacitor does not have a resistor built into it for drainage than you have the possibility of having a harmful DC voltage sitting at the terminals. DC is much more harmful so do not be fooled. Needs to be drained. I did not tell you to short the terminals.

Now if the run capacitor has broken down which sounds like the case there can be internal problems. Could be that heat has broken it down. Age could be the problem but those capacitors are designed to run a long time. More-so than a start capacitor which has to work harder.

Now after I mentioned all this, this is not the only option. This can also be a single phase split phase motor. This has 2 sets of windings, a start winding and a run winding and uses a centrifugal switch. It may also have a thermal switch that protects the motor from overheating. Being you said it was old it could be a salvaged motor to build this.

So just to show you there is no easy answer to your question. The name plate if it still has one will supply you with some valuable information. It could be a learning experience. :) Good luck.


Huh ???? This proves once again that I am right, all of this is WAAAYYYYYYY above my pay grade. I understood about 20% of these words. So, I'm gonna' buy the new grinder (wallet is saying ouch) and this old one will become my educational guinea pig. Time for me to do some electric motor learning and if it actually ever runs again, I'm gonna' do some very loud bragging!
Cheers~
 
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