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10A Universal Motor Speed Controller, Mk.2

This latest speed controller can be used with power tools and appliances rated up to 10A. It is suitable for use with brush-type universal motors such as those in small lathes, electric drills, grass trimmers, circular saws, routers, nibblers and jigsaws.

by John Clarke

Although there are countless battery-powered and 230VAC power tools with inbuilt speed controls these days, there is still a need for a stand-alone speed controller. Apart from power tools, many appliances need to have their motor speed reduced. 

In fact, we are constantly being surprised by the range of uses that readers have for this type of speed control.

Apart from drills and circular saws, a speed control is particularly useful for controlling routers and jigsaws when cutting materials such as plastics that will melt when cutting at high speed. 

Other items that can benefit from speed control are hobby lathes that use sewing machine motors, food mixers when the in-built speed control has failed and grass trimmers that constantly break the Nylon line when used at full speed.

The circuit is a revised version of the popular 5A Speed Controller published in October 2002. It is housed a larger diecast case, not only making it easier to build but also providing for the increased heat dissipation which comes from its uprating to suit 10A universal motors.

Speed control range

The speed controller will enable you to set the motor speed over a wide range, from about 80% of full speed at low loads, down to a very slow rate, depending on the motor and its gearing.

Because this speed controller does not apply full power to the motor at any of its settings, it cannot provide speed control up to full speed. That is why we have incorporated a 10A bypass switch, to enable full speed without unplugging the appliance from the speed control.

Some power tools and appliances don’t run smoothly at very low speeds when run from this type of phase control circuit. They sometimes display a behaviour known as “cogging” whereby they run in short bursts. 

So the practical minimum speed for any appliance motor depends on its freedom from cogging. This will depend on the design of the particular motor but in general we can state that the cheaper the appliance, the less likely it will run smoothly at very low speeds.

Another factor that limits the minimum speed at which an appliance can be run is that most universal motors have an inbuilt fan for cooling. 

Below certain speeds that fan is largely ineffective, so there is no cooling at all. This should be considered if you want to use an electric drill as a power screwdriver with this control. 

By all means, use it as a screwdriver but only for short periods – or run the risk of overheating and burning out the motor.

At mid settings of the speed control, the circuit gives good speed regulation. This means that the circuit slightly increases the applied voltage to compensate if the motor is loaded down.

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