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Old 11-21-2013, 12:35 AM   #1
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attempting to rewire old electric motor


Hi all: This is my first post ,so please bear with me. I am in the process of setting up a machine shop in my garage. I am attempting to rewire a old 1/2 hp GE electric motor on a 1945 circa Bridgeport milling machine. It is currently wired for 230 volts. I opened up the drum switch and found the insulation in extremely poor shape. I removed the cover plate on the motor, and there is a schematic on the back of the plate, but it is barely legable. I can make out that the motor was manufactured for GE by Century Electric. The thing I am confused by is it says the motor can be run at high and low speed, even though the nameplate lists the rpm as 1140. The machine runs, I want to replace the wiring to and from the drum switch, but my question is this. Is this a typo, or can a single phase motor be wired to run at more than 1 speed? If not, its not really a big deal, but if it can,how is it done? It would be nice to have the flexibility as far as machining is concerned. Thank you in advance. Karl

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Old 11-21-2013, 07:57 AM   #2
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attempting to rewire old electric motor


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Originally Posted by kephilipp View Post
Hi all: This is my first post ,so please bear with me. I am in the process of setting up a machine shop in my garage. I am attempting to rewire a old 1/2 hp GE electric motor on a 1945 circa Bridgeport milling machine. It is currently wired for 230 volts. I opened up the drum switch and found the insulation in extremely poor shape. I removed the cover plate on the motor, and there is a schematic on the back of the plate, but it is barely legable. I can make out that the motor was manufactured for GE by Century Electric. The thing I am confused by is it says the motor can be run at high and low speed, even though the nameplate lists the rpm as 1140. The machine runs, I want to replace the wiring to and from the drum switch, but my question is this. Is this a typo, or can a single phase motor be wired to run at more than 1 speed? If not, its not really a big deal, but if it can,how is it done? It would be nice to have the flexibility as far as machining is concerned. Thank you in advance. Karl
I'm not too familiar with these machines, but I'd assume that it was gearing and not a motor speed up or down? If so there are ways to speed up and down a motor, that rating is most likely the max?

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Old 11-21-2013, 09:12 AM   #3
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I'm not too familiar with these machines, but I'd assume that it was gearing and not a motor speed up or down? If so there are ways to speed up and down a motor, that rating is most likely the max?
You are correct, there is a step pulley system on the machine, and spindle speeds are changed by that means. I was wondering if the actual speed of the motor itself could be changed. The schematic on the motor plate seems to indicate a high and low speed circuit in the motor. It appears that the position selected on the drum switch determines which circuit is energized. Currently both circuits are wired on at the same time. I can't ascertain the rpm the motor is running at now. I just don't want to try and wire the motor from the switch and cook the motor. I can make out part of the schematic, but not clearly enough to be comfortable. Thanks for the input. Karl
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Old 11-21-2013, 09:39 AM   #4
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attempting to rewire old electric motor


Yes some motors can change speed. There is usually a plate on the motor that shows the wring for the two speeds. What the speed change is doing is changing the motor between two pole to four pole thus doubling or halfing the speed.
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Old 11-22-2013, 11:07 PM   #5
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The vast majority of motors that have more than one speed are single voltage. Some are reversible, some are not.

How many wires in the motors terminal box?

If there are 3, then it's most likely single voltage, two speed and not reversible. If 5, then it's single voltage, two speed and reversible.

Most single phase two speed motors are 1140/1725 RPM.

The vast majority or washing machines that have a normal and a delicate setting have two speed motors, almost always 1140/1725.
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Old 11-23-2013, 10:23 AM   #6
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attempting to rewire old electric motor


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Most single phase two speed motors are 1140/1725 RPM.
Actually most of the ones I have seen are 1725/3450
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Old 11-23-2013, 10:58 AM   #7
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Unless this is different then then hundreds of Bridgeport's I've ran and worked on that drum switch is to just switch it from forward, stop and reverse.
Look on the motor it's self and see if there's a metal plate to better ID it.
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Old 11-23-2013, 12:33 PM   #8
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attempting to rewire old electric motor


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The thing I am confused by is it says the motor can be run at high and low speed, even though the nameplate lists the rpm as 1140. The machine runs, I want to replace the wiring to and from the drum switch, but my question is this. Is this a typo, or can a single phase motor be wired to run at more than 1 speed? If not, its not really a big deal, but if it can,how is it done? It would be nice to have the flexibility as far as machining is concerned. Thank you in advance. Karl
You sure it didn't say HIGH/LOW voltage instead? meaning it can be wired at 220 or 110?
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Old 11-25-2013, 12:33 AM   #9
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Thanks all for all the info. The motor is 230 volts only.Upon closer examination, it appears that the cover plate I removed was not the original one on that motor, as the number of wires on the schematic and in the motor did not match. I wired it back the way it was, and it is working fine now. Apparently, if the motor installed matched the wiring diagram, itindicated it could be wired to run high or low speed, in addition to being reversed as well. Not going to worry about it right now. If I need to vary the motor speed I found a company that makes VFD's for sinle phase motors. I will try to get a decent picture of the wiring diagram and post it. Interesting if nothing else. Thanks. Karl

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