What You Should Know About Speed Controls

[Mikey98118]

What you should know about speed controls

 

(1)  The first rule for speed control, is do not attempt to adjust speed while a tool is engaged; move it away from the work surface first.

(2)  It is a popular marketing ploy to include inbuilt speed control circuits on compact equipment, such as rotary tools; avoid their use like the plague. If you end up with a variable speed tool, just leave its inbuilt controls on high (in effect, not engaged), and employ a separate router or fan speed controller (for brushed motors), or a brushless speed controller (for motors without carbon brushes). Plug the right type of speed controller onto your tool’s power cord; it will do the job, without any risk of overheating the equipment’s delicate speed circuit, and shutting down the tool, until the burned-out circuit can be bypassed. Speed control circuits on the very cheapest power tools just use potentiometers (variable resistors); as in heavily power robbing. A modern variable frequency drive slows the number of pulses per second in the right kind of DC motor (which is found on most hand-held equipment these days), but not the amount of power in each pulse, so it’s drop in torque is comparatively minor, as motor speed slows. Many of the cheaper inbuilt speed controls are erratic, and give poor control in their bottom range (or none in some settings), long before they fry.

  Dremel mounted the first speed controls on rotary tools; they were cross-slide types. This was and still is a hardier design then the tiny inbuilt circuits on other tools. Dremel has kept these controls low priced, easily available, and plug-in on their oldest rotary tool models; none of which is true of the speed control circuits on their new models. The Dremel #100 has a sliding on/off switch. The #200 model uses a sliding combination on/off switch and speed control; these switches are interchangeable. You can use an external speed control on a #100 because it’s sliding switch only turns the tool on and off. But don’t use a separate speed control on the #200, because they warn that it will mess up that model’s own speed control circuit.

(3)  Even with external speed controllers, do not run motors below half-speed for very long, to avoid overheating their windings (ex. just long enough to drill a single micro hole in pipe or tube).

(4)  You can still overheat the motor by bogging it down under a heavy load; even on full speed (and faster on reduced speeds), but it happens slowly enough for the heating motor housing to give warning, in time to let the armature windings cool off. But, overheating an inbuilt control circuit happens suddenly. Your first warning is usually a dead tool, and then you may notice a little smoke…or not.

(5)  The faster you race a gasoline engine the hotter it gets; it’s natural to expect that about electric motors too; but the opposite is true. As you slow an electric motor down, it heats up.

(6)  Flex-drives complicate motor heating problems, because the faster you run them, the hotter they get. Foredom Tool’s top of the line KTXH440 is set up to run between 500 and 15,000 RPM; these are, by definition, the best of the best. How can we expect a Chinese import freebie to last at 35,000 RPM? So, you have competing needs with a flex-drive mounted on a rotary tool. What to do? Run the tool at half speed, in short bursts to let both drive and motor cool down; use it no more than you must. Does this sound inconvenient? What part of flex-drives don’t belong on rotary tools didn’t you get?

If you blow a control circuit, do you have to throw away your tool and buy another? That depends; if you are into electronics, it is simple to de-solder the circuit and replace it with a short length of wire. If not, it is still simple, but you will have to buy a soldering tool, some rosin core solder, and some electric wire of the same gauge (size) or larger than what was used in the circuit; the cost will be about equal to replacing the tool, but you’ll end up with a repair tool out of the deal. Your “fixed” tool will only run at full speed, unless you use a separate speed controller, but that is what you should have been doing in the first place. The separate speed controller can also be used on many other tools.

Speed controllers for brushless motors: Most speed controllers are designed for motors with carbon brushes (brushed motors). Brushless motors (BLDC) need brushless speed controllers; they aren’t hard to come by, or expensive. But there are no plug and play versions available for hand tools; at present they’re only available as kits. The easiest kits to deal with have all the electronics contained in a perforated metal control box, to which you must add electric cords and/or wires; one set incoming from your power source, another outgoing to the tool; wire a receptacle to it (if you want to plug in two different brushless tools at once), or use the last few inches of the extension cord you probably just cut off to make a lead to the power source.

The RioRand 7-70V PWM DC Motor Speed Controller Switch 30A is available through Amazon.com; it has four terminals for wires to mount on; negative and positive “input” terminals from the power source, along with negative and positive terminals “to motor”; they are all plainly marked; there is a speed control dial on the side of its perforated metal body. Why metal, and why perforated? For heat dissipation. Black wires go to negative and red wires go to positive on this brushless motor speed controller.

When the armature fries: If you see smoke and electrical sparks coming from a tool’s air vents, you just overheated its armature for the last time. If replacement parts are available for your tool, you will easily find them online. Just input the product name and add “parts list.” You will need another armature assembly, and a new set of brushes. These parts are available for the Dremel #100 & #200, along with sites showing installation instructions; not that that any hand-holding is needed, since all you require is a little screwdriver.