Dimmer Switch to Slow Down Sander

[sean]

Ok this may be a really stupid question....... but here we go, I have a combo belt/disk sander that spins at 3340 and it's a bit fast for my comfort. Would it be possible to attach a voltage dimmer switch (like one you would attach to a light) that would lower the voltage and therefore lower the spinning speed of the motor? Or would I die in a giant electrical fire as a result?

thoughts?

[MrJeff]

I'm sure the experts will chime in, but it seems to me that if you did that you would lose torque. Not quite the same as changing the gear ratio. That's just my initial impression, though.

[Sasquatch]

I suspect you could do it - make sure the switch is rated for higher amperage than your motor is drawing.

The issue with this is that electric motors are designed for a certain voltage, and that's what you are choking off with this, so if you load up the machine at all, what will happen is it will be running too slow to cool itself off, and the whole thing can burn out. So you'd have to be careful using it. I'm really not sure if there would be negative long term effects on the motor or not.

[KurtHuhn]

Please to be reading and utilizing search function. This was covered not too long ago. See this link for warnings:
viewtopic.php?p=47094#p47094

AC motors are not like DC motors where you can vary the input voltage and get reduced speed at consistent torque. Dropping intput voltage on an AC motor makes it do stupid things, like not run at all.

Keep in mind, an AC motor that is rated for running at a given speed (RPM) at a specific voltage, will not necessarily run slower if you drop input voltage. These things want to run at one speed, and will resist changes unless you fundamentally alter the construction of the motor.

Keep in mind that most AC induction motors you find have starter windings and centrifugal switches. That centrifugal switch only disengages the start windings when you reach about 75% of the rated speed of the motor, and keeping those engaged generates a crapload of heat, and can bun out the motor very quickly. The run windings on induction motors run much cooler at a lower amperage (less heat) and cooling is not typically an issue. These start windings are also fed by a big hungry capacitor, and keeping them engaged keeps the capacitor engaged, possibly burning it out with actual explosive force.

The induction motor exception, Fan motors, are not horribly affected by this since they use a permanent capacitor setup rated for continuous use, and switching speeds is done by switching the windings to a set rated for a different speed. If you leave the switch set for the fastest windings, you can use a triac (not necessarily a dimmer) to vary the speed. Just be sure you get a quality triac, and not a cheapo, so you don't burn your workshop down. You do lose a crapload of torque, however, from a motor that hardly generates any to begin with.

Brushed motors, like those found in circular saws, dremels, drills, portable bandsaws, etc can be controlled with a dimmer because they do not rely on an EM field of a given strength to run. Instead they're specifically designed to be variable.

If you want infinitely variable speed in an electric motor with only minor loss of torque, find yourself a 90V or 180V DC motor and pick up an analog controller from Dart Controls. That combo is about the least expensive you'll be able to get.

If you have some bucks, a 3-phase AC motor in combination with a phase convertor and variable frequency drive (VFD) will give bulletproof performance. This combo is expensive, however.

Or, take your existing motor, and outfit it as the drive unit for a shaft setup. This will give you the ability to change speeds, and costs next to nothing relative to the two choices above.

[sean]

[quote="KurtHuhn"]Please to be reading and utilizing search function. This was covered not too long ago. See this link for warnings:
viewtopic.php?p=47094#p47094

Opps sorry about that Kurt. Thanks for the link and the great info!