Question regarding electric motors

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Blog entry by richgreer posted 01-28-2010 01:54 AM 1108 reads 0 times favorited 4 comments Add to Favorites Watch

In various woodworking catalogs you will see a device that allows you to vary the speed of your router by turning a dial. I assume it is like a rheostat on a light fixture. I also assume that lower speed also means lower power.

I have never seen a device like this promoted for other tools. Could this device, marketed for routers, also be used on a drill press or a lathe? On the drill press, it would be nice to make a change in speeds without the hassle of moving the belts and, on the drill press, a reduction in power may be acceptable in some situations. On my lathe my lowest possible speed is 500 rpm. When turning a really big piece (16” in diameter) I would like a slower speed. I realize the reduction in power would force me to go slower, but it would make it possible to turn some big turnings.

Of course, the last thing I want to do is damage my motors.

Any thoughts and advice are welcomed.

-- Rich, Cedar Rapids, IA - I'm a woodworker. I don't create beauty, I reveal it.

4 comments so far

View lew's profile


12056 posts in 3751 days

#1 posted 01-28-2010 02:37 AM

Routers, hand drills, sanders are basically DC motors so a variable speed circuit is somewhat easy to make. Larger motors- drill press, table saw are AC motors. Changing the speed of an AC motor is not as easily accomplished and therefore a much more expensive venture.

Here is a web site that can get you started

-- Lew- Time traveler. Purveyor of the Universe's finest custom rolling pins.

View Don Butler's profile

Don Butler

1092 posts in 3392 days

#2 posted 01-28-2010 02:21 PM

The router is a universal motor, just like the little motor on a sewing machine. Bigger motors are NOT universal motors. They should not be put in line with variable speed controls like the ones used for routers.

-- No trees were damaged in posting this message, but thousands of electrons were seriously inconvenienced.

View 8iowa's profile


1580 posts in 3757 days

#3 posted 01-29-2010 05:52 AM

Routers have universal type electric motors. They can be designed to run on either AC or DC. They have brushes, run at very high speeds, with low torque, and are the noisest motors in your shop. When you “turn the dial” to lower the speed you are also getting less HP and torque.

Most of the stationary tools in our shops are powered with AC induction motors. In this type of motor only the field coils are energized. The motor is wound around a fixed number of poles. This, and the frequency of the current dictates the speed. Thus at a frequency of 60 HZ, a two pole motor has a base speed of 3600 rpm, a four pole motor, 1800 rpm. The only practical way to control speed of an induction motor is with a variable frequency controller. Even here, there is no free lunch, as you reduce speed you also reduce power.

-- "Heaven is North of the Bridge"

View ashahidan's profile


64 posts in 3095 days

#4 posted 01-31-2010 12:56 PM

Speed controllers wouldn’t work on induction motors i.e motor without carbon brushes. In a universal motor speed controller the voltage peaks are chopped off. Normally a thyristor is used in the speed controller. I have seen a few electronic circuits to control the induction motors but I wouldn’t know if they would work as I never built one. Here the mains is 230volts ac so the component values for the controllers cannot be the same as for 115volts ac.


-- asm

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