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I've been reading some of the threads over the last couple of days about electric motor wiring, and I am hoping some of you experts might give me some clues.
I've been given an oldish GMF 8" double ended bench grinder (1/2 HP, 2850 rpm, single phase) that has the switch missing. It has a square metal capacitor attached to the base plate.
It has three wires coming out of the motor body - blue, yellow and black. From reading Gerhard's information in a post in August, I am guessing that one of these wires is the common active to both windings, and the other two are the neutral returns from the windings. I also understand that the capacitor needs to be wired in series with the auxiliary winding.
The first question is, how do I find out which wire is which?
I have checked the resistance between the three wires as follows:
blue to black - 6.3 ohms
Blue to yellow - 63 ohms
black to yellow - 70 ohms.
The wires all look to be the same thickness.
Can anyone offer any suggestions, please?
Originally Posted by colhu
Can anyone offer any suggestions, please?
Yes, Don't fool around with it yourself, take it to a qualified electrical type person and pay them to do it.
The yellow wire from the aux run winding (63 ohm) goes to the cap.
The blue wire (common from run & aux) goes to switched neutral.
The black wire from the run winding (6.3 ohm) goes to switched active, and to the other terminal of the cap.
The switch is double pole.
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Geoff's advice is sound and the safest way to go. Doing this on your own poses two risks; a safety risk for yourself and a certain risk of damage to the machine. If you are knowledgeable with electric stuff and know the dangers and how to avert them, you can make a safe test set-up and have some quick tries. You can judge this by yourself, but to me you sound savvy enough.
This story is much the same as the previous stories of motor wirings, notably that of the sewing machine motor. The coil sets with the lowest DC resistance measured in the motor, are usually the main coils. The motor stator and its coils are often visible from underneath, after unscrewing the base plate. In many cases the main coils are the first sets to be inserted in the pole slots, the aux coils are fitted as stage two, so they are a sort of "inner layer", which is often not visible from the stator's "outer circle" and therefore equally unlikely to be visible from the grinder's exposed stator section as viewed from the base plate hole. That may explain why you have only seen one apparent wire gauge.
For myself is use a little low DC voltage trick, if i'm absolutely clueless about the connections. On DC, an induction motor acts as a powerful brake. If connected to a sturdy 12 Volts battery charger, main coils will absorb a few Amps and the rotor cannot be turned around by hand. Aux coils on DC will absorb a fraction of the main coil Amps and often it's possible to turn the rotor. With this established, you know at least the difference between sets of coils.
Then connect the black and blue wire with a mains cord, give the wheel arbour a whizz in the right direction by hand and plug and unplug the cord only shortly. Two or three seconds are enough to establish if the motor sounds reasonably normal and if it wants to pick up speed, before you can do serious overheating damage to the coils. With the motor fully disconnected again, sniff inside the base hole to detect warmth (the nose is very apt as a heat sensor at close proximity, besides the smell of heated dusty air is distinguishable enough), and feel the coil wires. If there is no heating from these few seconds, give the rotor another whizz and let it run a bit longer; see if it reaches full rpm.
If all has gone well thus far, test the capacitor first for proper resistance. The meter's needle will at first swing a bit the the right (loading current) and then should return to the far left (equalling very high resistance). Then connect the yellow aux wire with the capacitor and the cap's other wire with the black wire. The assembly of the aux coils and the cap (both connected in series) is now connected in parallel to the main coils. When connected to the mains power, the motor should now spin up by itself. Disconnect immediately after spin-up and check again for coil wire heat through the base hole. During acceleration, the main noise should be the typical whirring with increased pitch (rotor bars resonating while passing the stator pole slots). If the mains AC hum dominates and remains strong, the current input is too high and something in the wiring layout is not yet right.
But i think you'll be okay with the suggested layout. Important: keep all tries short and watch closely for heat. Only heat can destroy the copper wire insulation and thus damage the motor.
Since this is a forum with many readers, you could also try to ask around for other owners of this particular machine. Ask them to unscrew the base plate and to have a look inside, how all leads are connected to each other and to the switch. That's probably the best way to be absolutely sure you can't go wrong.
I underwrite Big Shed's disclaimer statement, you must decide for yourself if you take the risks involved. Information posted by me is tried and tested from my own experience, but every single motor wiring casus can be unique and my posts must not be regarded as universally applicable. Furthermore my information is based on the supposition that theoretical and practical experience should be present with the readers, wanting to experiment with the pointers in my story. I try to minimise risk in the procedures i describe, but i can not possibly rule out risk for the full 100%. If in any doubt, indeed have the job done for you. The forum's administrators and its sponsors are not in anyway responsible or accountable for my posts.
Thank you Geoff, Chas, Big Shed and Gerhard - I acknowledge all of your comments, including the disclaimer. I am a Mechanical Engineer by training and an experienced home handyman. Rest assured I will not be rushing in to this without making sure I understand what I am doing.
Gerhard, thank you for the technical description and your circuit diagram - you have obviously spent quite some time putting your reply together. Unfortunately the motor is totally enclosed, ie it has no openings through which I can see the windings, only the three wires coming through a grommet.
As it happens, a tradesman friend of mine with whom I discussed it today has a 6 inch GMF model which looks very similar externally. Next weekend we will pull the base cover off it and see if the wiring, cap etc look the same, check the resistances across the circuits, etc.
If anyone reading this happens to have an 8" GMF Utility bench grinder with a 1/2 HP 2850 rpm motor, and feels inclined to pull the base cover off and post a description of the wiring, I would greatly appreciate it.
Attached is the wiring from my GMF Grinder.
that's great, thank you. The wire colours match exactly. I now need to visit Jaycar for a double pole switch.
thanks again, everyone.
just a quick post to wrap up this issue - yesterday I wired up the grinder with a new double pole switch according to Keith's wiring diagram and it ran perfectly.
So thank you again for all your advice.
I think it will end up with a cloth buffing wheel on it for pieces that I can't finish on the lathe.
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