Induction Motor Wiring

Remember, electricity can kill. Proceed with caution and respect.
Consult a licensed electrician if you are unfamiliar with the electrical codes in your area.

Table of Contents
Wiring a Power Switch
A little about wires and cords
A little about extension cords
A little more about NEMA plugs
Theory of Motor Speeds
Changing Voltage or Rotation
Three Speed Motor Control
X10 Remote Control for Dust Collectors
120V/240V Dual Voltage Power Bar
Links for more electrical information



It's not difficult to wire a switch on a woodworking machine.   If it's a 120V motor then either a SPST or a DPST switch will work.  If it's using 240V then you must use a DPST switch.  Of course to make things more complicated, some machines use 4 wires instead of 3 wires and some switches are grounded and others are not.
A Quick Note About Switches
There are different types of switches that are capable of doing certain things. Here are some of the basics:
SPST (single pole, single throw)
2 terminals
On/Off for one circuit
STDT (single pole, double throw)
3 terminals
On/Off/On for one circuit
DPST (double pole, single throw)
4 terminals
On/Off for two circuits
DPDT (double pole, double throw)
6 terminals
On/Off/On for two circuits
Of course you must use a switch that is rated properly to handle the voltage and amps required in a given situation.


Most machines come with black plastic covered cord which I simply don't like. I prefer a rubber cord since it's easier to coil up, lasts longer and feels nicer. When I replace the cords I get a cord with the next larger gauge wire. If the machine had a 14/3 cord, I will replace it with a 12/3. Cords have letters which indicated it's charateristics. I choose SJOW which signifies Stranded, Junior (300V instead of 600V insulation), Oil and Water resistant.



One way to kill a machine is to starve it's motor of electrical current. Yes, too little current can be just as bad as too much. A motor that isn't able to draw the amps that it requires can overheat and, in worse case, burn out. No one does this intentionally, but using an extension cord that is too long or too light gauge will restrict the current flow and cause the motor to overheat or burnt out. It happens more often that it should. Tool manuals always warn of this and include a chart of the different lengths and gauges of extension cords that are required. But be honest, how many people read the manuals. In fact there are some appliances, like small air conditioners that simply state to never use an extension cord. Its easier for them to say that then it is to expect the consumer to figure out the proper type of extension cord. For tools its reasonable to expect extension cords to be used, but when they are, they MUST be the right type. Using a $80 extension cord may seem excessive, but ruining a tool using a lightweight extension cord is a bad idea.

To determine the correct extension cord to use, look at the amp rating of the tool, usually located on the motor name plate, and then refer to this chart. The gauge of any extension cord will be written is small numbers along the length of the cord: 16/2 means 16 gauge 2 wire cord, 14/3 means 14 gauge 3 wire, 12/3 means 12 gauge, etc. The smaller the number, the heavier the gauge the wire will be, so 14 gauge cable is thinner and has less capacity than 12 gauge cable.



0 - 4 16 16 16
4 - 6 16 16 14
6 - 8 16 16 12
8 - 10 16 14 12
10 - 12 16 14 10
12 - 14 16 12 10
14 - 16 16 12 10
16 - 18 14 12 8

Always use the shortest extension cord possible, and when in doubt use a heavier gauge (thicker) extension cord that necessary.



The National Electrical Manufactures Association sets the standard for plug and recepticals.
Here are some of the ones you will mostly likely find on newer woodworking equiment.

If you didn't realize this already, you will notice that a 5-15P plug will fit into either a 5-15R or a 5-20R receptical, but a 5-20P plug will only fit in a 5-20R receptical. Likewise, a 6-15P plug will fit into either a 6-15R or a 6-20R receptical but a 6-20P plug will only fit in a 6-20R receptical. These guys at NEMA know what they are doing.



People often ask about reducing the speed of a motor. Typically they've found an old motor from somewhere and they want to use it for a woodworking machine like a disk sander. Or they have a single speed tool and want to reduce it's speed. Those light dimmers you see hardware stores always look so tempting.

First of all let me explain that there are two type of AC (alternating current) motors. DC (direct current) motors are more complicated and not as common so I won't discuss these here. The two most common type of AC motors are universal and induction.

We all know universal motors even if we didn't know their name. They are lightweight, come in all sorts of speeds, some as fast as 30,000RPM, and are relatively noisy.   Due to their design, they are used intermittantly and don't last forever. Most portable power tools use this type of motor. Electric drills, routers, sanders, shop vacs, etc. You rarely see these motors by themselves as they are almost always built into the tool itself.

The other type is an induction motor. Those are the motors that you see on most larger machines. Tablesaws, bandsaws, jointers, etc. They are relatively heavy, come in a  limited number of speeds, are pretty quiet, can be run continuously for hours and day on end, and last a long time. The most common are 3250RPM or 1725RPM.  It's speed is set when it's built based on it's design.   So fractional horsepower induction motors are multispeed, like 3 speed fan motors on furnace blowers. But they are designed and built with 3 different circuits inside. Think of it like 3 different motors sharing one shaft and body.

Portable tools with universal motors often are variable speed, meaning they have the internal electronic circuity that allows it to run at different speeds. For some tools like single-speed routers you can get aftermarket speed recducers that are quite effective. Inductions motors can't have their speed reduced the same way. Many induction otors can be wired to turn clockwise or counter closkwise, and can be wired to use 120V or 240V, but an induction motor can only turn the speed it was designed.  You can't make it run slower by reducing it's voltage. Well, you can, sort of. Lower the voltage, with a light dimmer switch. In a few minutes, the motor will heat up. After the fire in the motor is extinquished you will have reduce the speed to zero, permanently.  That's a sure fire way to "let the smoke out of a motor".

The only way to reduce the speed of an single speed induction motor is to use pulleys.   If you were thinking about using that old 3250RPM motor for a disk sander by mounting the disk directly to the motor shaft, you quickly find that 3250 is too fast for that.   There are many sources of pulleys and precison ground shafts and bearings like McMaster-Carr

If you have a machine that uses an induction motor and pulleys, and you want to use a different speed induction motor, you can usually play around with pulleys sizes to get the speed you are looking for.   Of course you don't ever want to change the speed that the machine operates.  If a tablesaw has a 3250RPM motor it would be unwise to change the pulleys to make the blade go faster.  A machine is designed to operate at a certain speed.  But likewise if you replaced the motor with a 1725 motor, or if you replaced a 1725RPM bandsaw motor with a 3250RPM motor, you'll have to change the pulleys to get the blade speed the same as it was originally.   If you are familiar with the way a 10-speed, or an 18-speed or 21-speed bicycle works, this is easy.


It's not that complicated to change the voltage or reverse the direction on most induction motors. I'm not an electrician but I'm familiar with the standard do-it-yourself electrical tasks like replacing wall switches and installing GFCI outlets in the house.

An induction motor will have a 3-wire cord (assuming it's grounded) coming into it. There's a metal plate on the motor near where the wires come in. If you open that cover you'll see the wires that come from your electrical cord and you'll also see a few other wires that go further into the motor. These wires get connected together on four posts or terminals under that cover. By switching the combinations you can change the voltage of the motor or change the rotation.

Normally, you just take the motor as it comes from the factory. Some will venture as far as changing the voltage or rotation. I do often. When you do that you make the change and close the cover and that's it. But if you'd like to be able to change the rotation without opening up the cover and changing the wires each time, you can use an ordinary DPDT switch available at most hardware stores. The back of the switch will have 6 terminals. The middle pair will be connected inside to either the upper pair or the lower pair depending on which way the switch is set. By cross wiring a DPDT switch the way I show in the diagram you are basically switching the red and black wires when you flip the switch and Voila! you are changing the rotation of the motor.



You may find a 3 speed motor in an old furnace blower.  You can get a 3-speed fan switch from the local hardware store for under $5 but chances are it's for a ceiling fan and can't handle the amps on a furnace fan motor.  You could plop down $20 or more on a 3-position switch with a higher amp rating, or you could do it with a few 15A switches that you would use in your house for lights.  Basically the first switch is your simple SPST light switch that you can get for about $1. That controls ON/OFF, then you need two SPDT (or often called 3-way) light switches for a few dollars each. Those two switches will let you select Low, Medium or High speeds.   When the first one is on Low the second switch does nothing, but when the first switch is on Medium/High then the second switch will select between Medium and High speeds.



I bought some X10 system components at Lowes.
The X10 system is used in a house to control lights and appliances, but can also be used to control your dust collector. The X10 outlet was about $15 and the Receiver/Keyfob remote was $20 at Lowes.

My Delta 50-850 dust collector came pre-wired for 120V.  I replaced the switch will an X10 outlet. It works very well.
Soon, I will rewire the dust collector to use 240V, when I do, I must either get an X10 compatible 240V outlet for $50-$80, or, I think I've found a 120V-240V relay at  McMaster-Carr for much less money that should work.



You may find yourself with a garage workshop with only one dedicated 240V and several 240V machines. This is fine since you probably never need to run more than one machine at the same time. But what if you hate having to unplug and re-plug each machine every time you want to use it? That would be a real hassle.

And what if you only have 120V outlets that are on the same circuit as your lights and you're tired of your lights dimming everytime you turn on your compound miter saw or other 120V tool.


Perhaps you could make a power bar.... like the the surge protected power bar strips we use with our computers. But since a 240V circuit has two separate HOT leads, you could also use this power strip to power your 120V machines. This would also help prevent the lights dimming when you use your 120V tool, or worse yet: tripping a circuit breaker and being left in the dark. Plus it could be usful if you have some 240V machines with different type plugs (3 or 4 prong). You could use outlets to match each of your machines.

For example, you could plug your machines like this:
Outlet-A: Drill Press
Outlet-B: Compound Miter Saw
Outlet-C: Tablesaw
Outlet-D: Jointer
Outlet-E: Planer
And as long as you don't run more than one machine at the same time you'd be fine. The breaker switch would give you the ability to turn off power to all your machines at once when you leave the shop for the night. Remember, you couldn't include your Dust Collector on this since it would run at the same time as your other machines and needs it's own separate circuit.

I can't imagine that it would be difficult or costly to make something like this. It certainly would be less expensive that re-wiring your garage or basement shop to have more 240V circuits and outlets. And it would save lots of time since you wouldn't have to unplug and replug your machines each time you want to use a different one. When you use a tablesaw, jointer and planer to prepare wood, that can mean alot of time savings.



Here are some interesting sites relating to electric code and stuff:
NEC Code Online (electrical power)
Dan's Wiring Page

© 2008 Mark Goodall