Variable Speed Controller for AC Motors




Introduction: Variable Speed Controller for AC Motors

About: I've always liked pulling things apart - it's the putting back together again that I have some issues with!

I wanted some way to control the speed of a Jigsaw and decided to use a light dimmer. It’s probably not the right way to do this but it does the job and I haven’t done any damage to the jigsaw so I’m going to assume it’s perfectly fine to do!

You could also use this to control the speed of a drill or any electric tool that has a motor. It actually works really well so if you do need some way to control speed, then this is a simple and dirty way to go about it.

There is a clip of it in action but you will need to skip to the 1 minute mark to see it in action.

Lastly, this uses AC mains so if you are unsure of how to correctly wire AC or are worried about the polarities, then I would probably get someone in to help you. You really don’t want to go playing around with AC unless you know what you are doing.

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Step 1: Parts and Tools

You can easily get all of these parts from any hardware or electrical store. I’ve also put links to eBay as well.


1. Light Dimmer Controller – eBay

2. Extension Cord – eBay

3. Junction box / Electrical box- eBay


1. Pliers

2. Screwdriver

3. Wire cutters

Step 2: Cut the Extension Cord

The premise of this speed controller is to add the male and female ends of an extension cord to a variable controller (light dimmer). All you are really doing is splicing in the controller between the extension cord. If you really wanted to, you could just add the dimmer to the live wire on the extension cord and that would be that. However, I wanted to be able to turn it off as well so got a dimmer with a switch


1. Find the mid-way point on the extension cord and cut with some wire cutters

2. Remove some of the outer plastic and strip the wire ends

3. Push the ends through the holes in the junction box on the sides (if you don’t have any just drill some.

Step 3: Adding the Wires to the Dimmer

The dimmer that you buy should come with instructions on how to wire up the power. There are all different types of dimmers out there so make sure that you ready the instructions and understand which wires go where.


1. First connect the neutral wires together from the extension cord and add these to the dimmer where indicated

2. Next you will need to add the hot wires to the dimmer. In my one, a wire from the dimmer and the extension cord was connected and then secured together in the dimmer. The other hot wire was secured by itself and so was the wire from the dimmer. So there was a total of 4 connectors in the dimmer for the wiring.

3. I have also included a basic, generic schematic so you can understand better how this needs to be wired up.

4. Once you have wired everything up, screw on the dimmer to the junction box, grab a drill and test to make sure it works.

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    4 Discussions


    2 years ago

    Gday lonesolesurfer. Good instructable. Simple. Concise and easy to follow. Nice.

    Just about your dimmer and its rating. The dimmer can only handle 400VA...

    VA is Volt Amperes (Volt Amps). Basically this is the amount of load the dimmer can handle IF the load was purely resistive. Like a kettle element or a hot glue gun.

    Inductive loads like motors (loads that have coils of wire) are a bit more tricky... Only way I know how to explain this is maths... Sorry!

    VA = Volts × Amps; transposed is,
    Amps = VA ÷ Volts.

    For your dimmer:
    Amps = 400VA ÷ 240Volts = 1.67Amps max load.

    Remember, that is a purely resistive load. If the load is a motor, the dimmer will only be good for between 80% to maybe 90% of that meaning it could be as low as 1.33Amps (effect called power factor). On the flip side, this is basically how ceiling fan speed controllers work however their motors are usually only rated to 150VA making a 400VA device perfect.

    This percentage can get worse too depending on motor type and the decreased voltage. Working current could be as low as 50 or 60%.

    Its also important to know that motors can commonly draw 2 to 3 times (even up to 7 times) their running current at startup. My jointer is rated at 9 Amps and will draw about 45amps when I start it causing my shed lights to dim.

    One other point, the extension lead is 10 rated meaning someone could run a 2400watt (10 amp) kettle through the dimmer or worse some fool could use it to try and control the output current of their cheap stick welder.

    All I'm saying is this instructable is great however anyone doing this needs to be aware of the dimmers limit and only use it for application of very low power and it is dependent on the dimmer used. It would work wonders on a soldering iron to drop temp for marking plastics, speed control of a little on/off desktop fan or on a small blower for a DIY coal furnace. Just don't use it on a router, or a table saw or anything like that.

    I've had an old apprentice of mine try this to give him temperature control on a plug in 25L hot water urn to make an alcohol still. Blew the dimmer up very quickly.

    Again, the instructable is great - I've just had some experience with mains power and have seen the adverse effects it can have on property and people.


    Reply 2 years ago

    Great comments RussellR8. Nice to have some facts and numbers behind this 'ible.


    Reply 2 years ago

    this should handle about 2000W


    Reply 2 years ago

    You can't beat that price!