Speed reduction help needed please.

[Oldmanandhistoy]

Hi

I have a rotary tool (dremel type) and would like to reduce the minimum rpm from around 9000 rpm to 1000-2000 rpm. It has a built in speed control of what type I am unsure. If I said, if the tool was running at max rpm and then I quickly reduce the speed, it drops quickly and then speeds up a little and kind of settles down it will probably tell you electronics guys what type it is.

It’s 240Vac @140Watts

What would happen if I reduced the Voltage down to 110Vac would that work to reduce minimum rpm and I would loose most of the torque?

If the above idea is no good then is there any other simple method of doing this?

As always advice and comments welcome and appreciated,

John

[Al_The_Man]

It is most certainly a Universal motor type, many dremels come with a Triac type control that reduces rpm down quite low.
If you want to run off 110 the easiest without any electronic control is a 240v/110 transformer.
Otherwise you could do a Google search, there is probabally lots of Triac controllers out there you can build for a couple of $'s.
They are not that expensive to buy.
Al.

[Oldmanandhistoy]

Hi Al,

Thought you might help with this one

I already have a 240v/110 but would be a PITA to wire it up if it is not going to be any good.

So if I use it will it reduce my minimum speed and what will happen to the torque if anything?

John

[Al_The_Man]

Torque is dependant on current, so if you reduce the voltage, the available torque is reduced.
Without some kind of feedback, universal motors have notoriously bad speed regulation.
Al.

[hixfixit]

A ceiling fan speed controller can be used as a basic speed controller. Model Railroad guys use them to feed the transformer for the same effect.

BTW, Torque is proportional to voltage squared. Don't bite yet - Current is dependant on voltage applied in this type of circuit (fixed resistance/impedance). V=IR.

So without going too far off track, the SCR/Triac route is the best option as full voltage is applied (therefore full current) but the duty cycle reduced to reduce speed.

[Oldmanandhistoy]

Just a quick thank you for the replies and to let anyone else know who might be interested.

I have wired my rotary tool (Dremel) to a 240Vac to 110Vac transformer and it has had the desired effect that I was after. I now am getting 0- maybe around 5000(just a guess) rpm. The torque has dropped off but it is still more than enough for my needs.

The only thing I may have to worry about now would be how it will affect things long term but they are cheap enough to replace so no problem.

Just because I have an inquisitive mind I am wondering how it would have gone the other way (110Vac to 240Vac) but would imagine it would not be good.

Thanks again,
John

[sigma relief]

I know I'm dredging up ancient history, but for sake of technical accuracy for future readers, I felt compelled to post. I am an engineer for the company that invented the electronic solid state dimmer using a triac and as such can speak on the subject. At one time(70's?) we did sell a belt clip speed controller for drills, but have long since gotten out of the speed control business. While I myself have used a dimmer as a speed controller, I must say that you need to be careful doing this as it can wear the motor out much quicker than an un dimmed circuit.

A triac (or the inferior SCR) based dimmer (essentially the only style still no the market for lighting control) does not reduce the instantaneous voltage to the load, it simply turns the load on and off at 120 Hz (100Hz in Europe and other areas). Much like PWM, the power to the load is varied by increasing or decreasing the time that the load is on. Standard dimmers use forward phase dimming, they wait a predetermined amount of time after the AC voltage signal crosses 0V and then "turn on" until the voltage drops back down to 0V at which point a Triac naturally stops conducting. If you look at an oscilloscope, it is the rapid change in voltage at the turn on point that can cause problems with motors, especially ones that already have speed controllers built into them. At best, the dimmer and or motor will hum, at worst, it can overheat and die. The overheating problem has to do with an asymmetry of voltage going in to load between the positive and negative half cycles, causing a net DC current, which is bad mojo for an AC motor. This asymmetry is easily solved by adding (or properly selecting) a few electrical components internal to the dimmer, but most cheaper non name brand dimmers don’t do this.

Be warry of fan speed controllers, there are 2 types, souped up dimmers which have the nasty tendancy of making your fan hum, and "quiet" speed controls which use capacitors as a phase shift. The "quiet" versions WILL NOT WORK on some motors and may cause serious damage to certain motor designs. They are intended for paddle based ceiling fans only (the speed pull chain on the fan itself simply switches capacitors in and out as well). Fortunately the "quiet" controllers are easy to identify as they will have a discreet number of speeds (usually 3 or 4 with "On"). Back to the real dimmer based speed controls, which are only re-badged MLV (Magnetic Low Voltage) dimmers. Low voltage (12V or 24V) lighting using a magnetic transformer is, like most motors, susceptible to a DC current so the Magnetic low voltage dimmers are built to keep the DC component below 1V thru even control of the positive and negative cycles. For the technical minds, the hum is produced by the high dV/dt (sharp change in voltage when turning on) going in to a large inductor. Stay away from Electronic low voltage dimmers, first, they are much more expensive, second, they use FETs and reverse phase control to turn the dimmer off, this trailing edge dV/dt is incredibly hard on an inductive motor, but is just what the capacitive electronic transformers like.

If you decide to use a dimmer, stay out of the middle of the range (actually ~2/3s of the way up in the dial, light perception is nonlinear so the dials and slides are often calibrated to appear linear) as it essentially turns the load on when the instantaneous voltage is at its peak (~40% above the listed RMS 120V/240V) in lights, this can produce a hum, it motors, it can hum or worse. Additionally, most dimmers have a minimum load rating around 40W. This is because the Triac will not work properly at its low range if it can not pass enough current, this results in spotty low end performance which can be hard on your motor. If you want full low range operation, just add a small light bulb in parallel to your tool, you can even hide it in a box if you don't want to look at it.

And please please please don't do like some of our customers, one used a dimmer that comes with an infra red remote to turn his machine shop tools on and off. Another used the same infra red controller to control a plug in breast pump to remotely reduce output to a more comfortable level.