Homebrew Magnetic Motor Starter




About: I was pfred1 but moved, changed my email address, and lost my password. I suppose worse things could happen.

This is one of those things that I always wanted, and wanted to do besides. Well now I have it, and have done it too. I will share how I managed to do it with you in this article if you bear with me for a bit.

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Step 1: My Motivation

When I got this saw it had no motor or switch. I put a motor on it and rigged up a switch using a scrap circuit breaker. That setup was hardly ideal but I said to myself while I was doing it this is temporary, all the while also thinking is this going to be temporarily permanent? We all know how those temporary things can go don't we?

I used that switch and it worked, but every time I flipped it all I could hear in my head was my inner voice echoing, "this is temporary ..." Stuff like that can really throw me off my game let me tell you. I wanted my push button start and stop buttons!

Step 2: How It Happend

I didn't have my push button switches because there is more to them than just the switches. I have 3 sets of push button switches in fact. It was an agonizing decision which I was going to use too. Ultimately I went with the ones that were in a box already. They were another yard sale purchase I made in a previous season, 25 cents if I recall correctly. Was under a dollar, I remember that.

What I didn't have was the motor contactor to hook them up to. Those don't come cheap either. One day I was out doing yard sales and I ran across a definite purpose contactor new in a box that looked like it'd do the job to me, and the price was right, $5 so I picked it up.

I picked up a few other knick knacks that day as well but forget about all of that, this article is about the little dark blackish brownish thing in front of the little brown box on the right. The big boxes in the back are just backdrop.

It says on the box here Definite Purpose Contactor Power Poles 2, Amp Rating 30, Coil Voltage 24V 50/60 Hz That little beastie ought to do the trick.

Now it is that coil voltage rating that is going to make this a bit more complicated. But we're going to address that shortly.

Step 3: Schematic

Isn't it great that we live in the modern age of digital information? Sometimes it just seems like the information we can access on the Internet is limitless doesn't it? I know it does to me. So I jumped into my favorite search engine with what I thought was a good phrase to use to look for what I wanted, a schematic for motor contactors, and lo and behold tens, nay hundreds of thousands of returns! A veritable deluge of information was inundating me, so I dove right in and waded through about 28 or so pages of search results but didn't see what I was looking for.

Yeah well maybe we're living in the digital age but this might not exactly be the golden era of it yet? Anyhow a bit discouraged I decided to make my own. Seemed easier than finding a needle in a hay stack to me. I'm sure it is out there somewhere, but where I've no idea. Anyhow here is mine.

Step 4: What Makes Mine Different

Sure there are hundreds of thousands of similar circuits on the net but all the ones I saw had the same coil as output voltage so they could latch themselves. My contactor didn't run off the output voltage I was planning on running through it, not even close. So I had to add the "Latching Relay" to my schematic to get around this little detail. It makes things a bit more complicated, but it makes things work the way I want them too so it is a fair trade off.

I also added an additional relay because I use an external fan to cool my motor, long story why that I won't get into here. But you could use that to say run a vacuum dust collector, or eliminate it if you decide you want one of these too. So the relay named "Fan Relay" is there but it doesn't have to be.

As simple as all of this looked on paper I'm still quite the doubting Thomas when it comes to original circuits of my own design so before I invested too much time making a final copy I made a quick bench lash up of it to make sure it worked as I'd desired. It works like a champ too!

Step 5: Finalizing the Project: an Enclosure

That thing I had going on my desk I couldn't mount in any permanent fashion. No, no, no! That would be an easy way to get on a first name basis with my fire department in fairly short order. Line voltage driven electrical equipment needs to be securely mounted in sturdy enclosures.

I didn't have a suitable box in my vast collection of junk so I made my own. I could write a whole article about how I struggled to do this but instead I'll just put a series of pictures here. This isn't really what this project is about. We all like action shots taken in the heat of battle and I got a number of them.

Step 6: Piercings Are Sexy?

Some seem to think so. For me they are more of a utilitarian nature. This project needs a number of them. I have a knock out kit but oddly it doesn't include quite the right size I need so I used a step drill. More action shots of the box getting pierced. Having just typed that sentence I think I am going to be driving hits to this site from a strange crowd when crawlers get a hold of that line!

I hope they're not too disappointed. Heavy Metal rules!

Step 7: Parts Mounted and Wired Up

Keeping in the double entendre theme I seem to be going with here. Now that the machining is done on the box it it time to put all the bits inside it and hook everything together. It isn't even a bad idea to test out work at this stage either. I did.

Step 8: Install in Control Box in Location

Finally! Well not quite. I got to this point and walked the box over to my machine and I was like where the heck am I going to put this thing? So I ripped out what was there and made some rough shelving it could sit on. I'll also include a picture of the bracket I made rigged up before I welded it. I wanted to make sure it was just so.

To do that I had to take a bit longer a view of it, and try to access the hand cranks around them and all. I actually took the bracket picture as a construction aid for when I needed to layout the pieces to weld them together. But when I was looking at the mock up I noticed two planes that needed to be parallel with one and another so I used that as my alignment reference. Oh, and some pencil marks I'd put on the pieces too.

The final picture is what I'd made previously that this replaced. I guess you can view it as the before picture.

Step 9: Wiring in Location

Are we there yet? Not quite. Now it is time to wire everything to everything.

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    11 Discussions


    3 years ago

    I realize this is an old post - but it looks like you didn't really get an answer about the difference between your circuit and the 'common' circuit posted by raheen. The main difference from your circuit is what happens when both the ON and OFF buttons are activated simultaneously, or the ON switch gets stuck ON, or the relay contacts fuse together. In your circuit, the contactor and fan remain energized via the ON switch or fused relay contacts. In the 'common circuit' the OFF switch takes precedence: OFF means OFF. Just a slightly safer circuit overall.

    raheen was trying to follow JIC convention.  Remember, just because something works, doesn't make it right.  There are a few things missing that should be discussed:
    The drawing doesn't show but your panel and motor must be grounded.
    Does this motor starter have an overload heater w/N.C. contact?  If so, this contact should be wired in series with your 'Stop' button. 

    Does your motor have a 'reset' button or an internal thermal switch?

    We usually show the transformer at the top of the drawing with a small fuse on the secondary LH line (L1).  The RH line (L2) is normally grounded to the panel (in case the transformer shorts across to the sec. side).  (So far we protected the transformer, the relays and the internal wiring.)

    Plugs and receptacles are disconnects and should be used on the MOTOR cord for operation and ease of maintenance.  In fact, consider using three different receptacles on your control panel (one for each device).  An "on" LED should show your contactor is energized.  The only other 'improvement' might be a raised (off the bottom) terminal strip.

    1 reply

    7 years ago on Introduction

    There is an error in your circuit diagram.
    It should look like this for non-latching push-buttons.

    3 replies

    Reply 7 years ago on Introduction

    Thank you for the different schematic. Maybe you can explain to me what the advantage to your circuit is? Also why you say mine has an error. Because mine does in fact work fine here.


    Reply 7 years ago on Introduction

    Don't know if you ever got your answer from raheen. But this a common Circuit used in commercial application. on his diagram is a 2 push button switch. when pushing the on button activates latching relay. if you look at the line between switch circuit and the out circuit to relays. cause a continuous Circuit unit broken by the off push button.


    Reply 6 years ago on Introduction

    The common circuit uses the same voltage for the coil as is being switched. Using different coil, and switched voltages is less common. Uncommon enough that I did not find a single schematic depicting it in over 30 pages of search engine links. Believe me, I looked! I wasted more time looking than it took me to design this circuit myself in fact. I always prefer to use professionally designed circuits over my own. I figure perhaps the pros know more than I do so I might as well avail myself. In this case they were all mute though.

    Since I've posted this the pros have come out of the woodwork to criticize what I've done. Too bad for them what I've done still works flawlessly to this day though. Duh, winning!


    7 years ago on Introduction

    Nifty idea. The description is mostly story (and fairly lengthy), maybe a tiny more focus on the topic and a little less life-story would turn your good instructable to a great one.

    Good job.

    1 reply

    Reply 7 years ago on Introduction

    This article was featured so I suppose it meets this site's criteria. As rtarvin points out in their comment, "this a common Circuit used in commercial application.", so it isn't my nifty idea, just my build of it. I was surprised I couldn't find a suitable schematic online, which is what prompted me to post my article. Now I know there is a schematic online of this particular kind of contactor.


    7 years ago on Introduction

    I enjoyed this one. This is precisely the type of thing that I love spending weekends with my father doing in his shop. It seems he always finds little projects like this to make the "barn" better !! Looks good !