LumberJocks Woodworking Forum banner

Building a flat grinder, but question about wiring motor...

9868 Views 15 Replies 5 Participants Last post by  TopamaxSurvivor
So I've decided to build myself a sharpener. Lately I've been restoring some old tools and after flattening and sharpening a set of old chisels and a plane iron I decided that I've had enough with the sharpening by hand. So I found a great plan for a flat grinder by Nick Engler from his Sharpening book.

The best thing about it was that I have practically all the parts I need just laying around.

The motor I have is from an old evaporative cooler. It's perfect too, because it's a 2 speed, so I can grind slow and extra slow. 1725/1140 rpm. The problem with this motor is: 1. The plug is weird and 2. Because of the 2 speed option, there's an extra lead in the back.
Wood Gas Metal Cylinder Household hardware

Here are the leads in the back: I circled each one in yellow.
Motor vehicle Bag Automotive tire Electrical wiring Bicycle handlebar

My questions are how can I wire this so that I can utilize the two speed option during use; and also how do I convert this to a regular 3 prong plug?

I was thinking I can run the 4 wires (3 leads: [L=low C=common H=high] 1 ground) to a switch control box of some sort and then from there run a regular 14/3 cord to any outlet. But I seem to have an extra wire that needs to be eliminated somehow to go from 14/4 to 14/3. Is there some "magic" in the switch box that can take care of that?

Here's a pic of the wiring diagram:
Rectangle Handwriting Sleeve Font Material property

Here are the motor specifications:
Font Material property Gas Rectangle Metal

Thanks for any help you can provide me. I'm proficient in wood only and get easily confused when it comes to electrical…but I'm working on that.


See less See more
1 - 16 of 16 Posts
You could use a 3 postion switch with a center off.
I did a quick search for you on grainger. TopamaxSurvivor has the right idea.
Just cut that weird plug off of the the cord that already exists on the motor. You ought to verify this with an ohmeter, but from your picture it looks like the colors in that cord are:
Green - frame ground
Red - "L"
Black - "H"
White - "C"

Wire the leads from a regular 3-prong 110V cord so that:
Green - ground, goes directly to frame ground on the motor
White - neutral, goes to "C" terminal on the motor
Black - line, goes to the center terminal of the switch that ScottN suggested.

The "H" and "L" terminals of the motor should go to the outside terminals of the switch. This way, center position of the switch will be OFF, flip one way for HI speed and flip the other way for LO speed.

Sounded like you were struggling with the electrical issues; is that enough detail for you?
See less See more
You guys are awesome. Thanks for the collaborative effort! And EEngineer I really appreciate the detail. I wouldn't have been able to figure out how to use that switch properly without your description. And I thought this was going to be at least more difficult or more expensive! $2.99, I can handle that.

I'll post pics from the build as soon as I finish. I've got to wait a couple days 'cause I've got some family visiting now, but once they're gone…
Ok, so I went to the electrical supply store to buy the switch and the guy there told me that that switch wasn't rated for motor use and that the in-rush current would be too high for that switch. The motor rated switches he had there were very expensive, so I decided to look around on the net for something cheaper. But as I'm searching, I'm finding thousands of different switches and I'm not sure what I need exactly.

What I do know:
I need:
1. Motor rated
2. 20A @ 115v
3. 3 position (On-Off-On) I think this is called "double throw" ?

What I don't know:
1. What is the difference between the poles. Double pole vs. single pole and does it matter which one I use for this application?
2. Some switches I've found have 6 connections in the back instead of 3. If I get one of those can I just ignore the other extra 3 and hook it up like EEngineer said, or do I need a different switch altogether?

Thanks for any insight you may have.
See less See more
If you don't use a switch that is motor rated, it will destroy the contacts starting and stopping it in short order.

If you don't want to spend the money for a good switch, you need to decide if you really need 2 speeds. the single pole switch will be a lot cheaper. Or, you could use a couple small open faced relays that are rated for the motor load. They will be about $12 each. How much do you want to spend on getting this 2 speed operatiion going?
You only need single pole for 120 volt. Double throw will not have a center off.
You want a single pole - double throw switch. You could use the double pole - double throw, but those are usually for 208/220v applications. Connect the black from the 14/3 supply cord to the common terminal of the switch and connect the black and red wires from the motor to each of the outside "poles" of the switch. Wirenut the white wires together and wirenut the green wires together. Don't forget a green lead to your switch box if it's metal. You need to ground that too.

The 6 connector switch is the double pole - double throw. You can use it to switch the neutral (white) connection if you want or leave them unused. If you do leave them unused, just tighten the screws up so they don't vibrate loose on you.

Here's a link to what you're looking for. It's rated for 3/4 HP, 20A 125V. It would mount in a metal switch plate with a 1/2" round hole in it
Hi TSurvivor,
The 2 speeds aren't absolutely necessary, but I figured it would be a nice luxury to have as long as I already have a 2 speed motor…

I didn't want to spend more than $15 bucks for a switch, but if I have to to do this safetly I guess I will.

I found this switch that is motor rated, DPDT and has 3 (on-off-on) positions.
Will that work? If so, what about the extra connections in the back? Can I still connect it all as EEngineer suggested?

Sorry if these are extremely elementary questions, but this is all new territory for me.
That switch should be fine. It says 1 1/2 hp rated.
Didn't see the other question ;-) Just use them to beef up the contacts or don't worry about them. It is 2 switches in one housing.
FYI, Double throw switches do not necessarily have a center off unless the switch functins says they do. Normally a double throw will just be one position or the other.
Thanks for the clarification RTim and TSurvivor.

RTim, that switch looks great. I'm ordering it. It's amazing how that one is $50 cheaper than what the guy at the store was trying to sell me.

Now back to the shop…
TSurvivor, the info on the link of that switch says it has on/off/on…will that not work?
Sorry, looks like I misread it. Either one will work just fine.
1 - 16 of 16 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.