Double-checking the wiring on an old induction motor

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Forum topic by Thuzmund posted 12-04-2016 07:28 PM 846 views 0 times favorited 7 replies Add to Favorites Watch
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151 posts in 1776 days

12-04-2016 07:28 PM

Hi everyone,

I have been googling this for about a week so I thought I might as well see what you all think. I know that electricity is super dangerous stuff, so I haven’t run the motor due to my concerns and I agree in advance with your important warnings. However I do know some very basics about wiring induction motors, at least at a consumer level, and I have taken apart motors, rewired them for reverse switches and kill switches, etc. Basic stuff.. Before any project, I always spend a couple days reviewing old notes and looking online to double-check things.

So here goes: very old, very large 7/14 amp single phase induction motor (no capacitor) bought from the widow of a woodworker. It is running a jointer. It is wired for 220V (which I’m less familiar with than 110V) and the wiring appears to be in the correct “series” connection for 220V upon first glance.

However, I have two questions:

1) First and foremost I notice that the green ground wire coming from the power cord is disconnected, just out there hanging. Big red flag—is this normal for 220V wiring? As I look at the switch it would likely be wired to, there’s no obvious place where it used to live (and fell out).

2) The colors of the wires coming from the motor windings is also odd. We have a green and black pair and a white and black pair.

The green wire is odd to me. Maybe in a 110V configuration this means “neutral” or “return”? But of course I think this can depend on which direction you want the motor to spin. Now, I really don’t care about the color on a very old motor as much as the nagging thought that I can’t find a ground strap anywhere on this motor, so I keep thinking about this green wire. The motor isn’t connected to ground!

I did a bit of researching the differences between 110 and 220 circuits, and here’s my understanding:

My best guess is that since this is 220V series, it’s a giant loop, from the hot wire on one pair to the white wire connection, then the white wire to the neutral wire, which takes it back to the receptacle. This loop then reverses, so the roles of the wires reverse, 60 times per second. So in effect we have two hot wires (or more accurately, a rapid switch between the 1 hot and 1 neutral wire). So there is a return (neutral), but I don’t see any protection against shorts without an additional ground wire. In other words, I expect this motor to work, but to also be unsafe.

Now, because there is no separate ground wire, what do you do with the green ground from the power cord? I think the previous user just left it hanging, almost like an old two-prong appliance with no separate ground. I do not think it would be possible (or wise) to simply connect the hanging green wire to the switch (e.g., connect it to the green wire from the motor). I think I may attach that dangling green ground to the chassis of the motor instead, directly (bypassing any connections on the switch).

Sorry for yet another electricity question from a non-electrician! Thanks in advance for any experience you can share. I promise I have been digging for answers for many days.

-- Here to learn

7 replies so far

View MrUnix's profile


6943 posts in 2346 days

#1 posted 12-04-2016 08:29 PM

If that is wired for 240v, unplug it, walk away, and ask someone who knows what they are doing to look at it. First problem I see is that it’s only switching one of the two hot legs, so one is live all the time, which isn’t good. As for the ground wire (green) coming from the input source, it should be hooked up to the metal frame of the motor somewhere. A ground is not actually required for the machine to work, but is a safety connection in case there is a short somewhere – better to have it short to ground through the wire than your body, and should never be switched.

Does the motor have a wiring diagram on it?


-- Brad in FL - In Dog I trust... everything else is questionable

View Thuzmund's profile


151 posts in 1776 days

#2 posted 12-04-2016 08:39 PM

Hey Brad,

I was thinking the same thing regarding the switch—if both wires can be hot, then placing a switch in that configuration only applies half the time (i.e., effectively never). There is a wire directly into the wall with no switch interrupting it!

Unfortunately, the wiring plate is lost on this old motor. I am in the process of emailing the manufacturer on the off chance they can help.

I may end up rewiring this to 110V…To stay at 220 I would need to get a new power plug anyway (wrong shape).

-- Here to learn

View Lee's profile


122 posts in 1025 days

#3 posted 12-05-2016 01:33 AM

It looks like that switch is a double pole, witch means it can switch both legs of the 240. The one side might have gone bad witch is why he bypassed it. The green wire coming out of the motor is not a ground wire. Any way you go either 120 or 240 the ground needs to be connected to the motor frame, just run a separate wire down to the motor and find a bolt to attach it to. Also, stranded wire should not be under a terminal screw, you should get some crimp on lugs to make a better connection. hope this helps.

-- Colombia Custom Woodworking

View runswithscissors's profile


2843 posts in 2172 days

#4 posted 12-05-2016 01:39 AM

If you have a 240 outlet, plugs are available in several configurations, are cheap, and are easy to install. And I have never heard of green being anything other than ground. I doubt if there is a neutral wire in that motor (but there would be if you converted it to 120).

I was baffled by an old induction motor (with a capacitor that had gone up in smoke) that had numbered wires rather than color coded. Took it in to a local motor shop, and the woman behind the counter hooked it up correctly in about 45 seconds, and put in a new capacitor, all for about $17.

-- I admit to being an adrenaline junky; fortunately, I'm very easily frightened

View Thuzmund's profile


151 posts in 1776 days

#5 posted 12-05-2016 01:57 AM

Man I wish there were more motor shops in the world. I live in an old industrial area (Albany/Schenectady/Troy NY, home of GE, and once about a million machine shops and other industrial bones). But there are only 2 shops in the area and both charge a $90+ bench fee to take a peek (they will politely tell you that only people with multiple HP motors or very specific form factors ever bring them motors).

-- Here to learn

View Thuzmund's profile


151 posts in 1776 days

#6 posted 12-06-2016 03:52 AM

I will have it brought up to correct spec and share the resulting configuration for future reference/posterity!

-- Here to learn

View Thuzmund's profile


151 posts in 1776 days

#7 posted 12-16-2016 09:11 PM

I often use these forums as search engines , so I’ll post the solution that worked for me in case anyone runs into similar issues.

The pic below shows my final setup on the left, just a regular 110V circuit. I chose 110 over 220V because I am more familiar with it and the colors of wires on my motor made me extra wary. After testing it I am more than happy with the results.

By using a multimeter it’s easy to tell which wires are connected. Just check for very low resistance (ohms) or mine had a continuity tester. I also checked for continuity between all wires and the motor housing, and between the housing and my new ground screw and strap. I also installed a switch and forward/reverse (not shown) so I ran ground to the switch ’s metal box housing.

During testing, I quickly found issues with the switch, including a missing inner back plate—think of a orange inside a box, the orange was missing it’s peel to the fruit (wiring) was touching the metal box walls. I found a replacement part for the switch though , plus the metal box is connected to ground.

I verified the switch’s operation using continuity, double-checked my ground attachments with the ground prong all the way at the end of the cable, and then plugged it in and have been happy ever since. It runs very smooth and I just have to be sure to keep the fabric-filled lubrication cauldrons well-fed. :)

In retrospect a big lesson from this is NEVER BLINDLY TRUST THE COLOR OF WIRES. The fact that one winding used a green wire threw me for a loop from the beginning. After triple-confirming every step, I am amazed how simple it was. Also, I will always inspect metal switch housings from now on!

-- Here to learn

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