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Soft Starter For Power Tools

Stop that dangerous kick-back...

By Nicholas Vinen

Our Soft Starter project from April 2012, which tames switch-on current surges primarily in equipment with switch-mode supplies, has been very popular. But readers started asking “what about something similar for power tools?”

Many of the smaller mains power tools these days have speed controllers built into the trigger, so they are very controllable when you turn them on.

But larger power tools such as circular saws, plunge routers, angle grinders and worst of all, large electric drills for concrete core drilling, have a simple trigger or thumb switch which applies full power to the motor. Core drilling is particularly dangerous, as you have to brace the core drill bit hard against the wall or floor and then press the trigger. The resulting torque kick can easily jerk the whole tool out of your hands! And you can be injured in the process!

Click for larger image
Shown here with two of the hand tools most likely to be used with the Soft Stater, an electric hand saw and plunge router. The unit is housed in the Jiffy Box in front. If used on a building site or other "rough" environments, it could be housed in an aluminium diecast box.

Features & Specifications

Inrush current limiting:             <20A

Minimum load power:             ~100W

Maximum load current:             10A

Minimum tool restart interval: 60s recommended

Why does it kick?

The reason for that enormous initial torque is the very high surge current pulled by a universal (series wound, brush) motor when power is first applied. Because the motor is not rotating, it is not generating any back-EMF to oppose the applied mains voltage and the resulting surge current can easily be ten times the rated current of the motor with full load.

Elsewhere in this article we show some scope grabs depicting these massive currents which luckily die away to much lower values within less than half a second. It is those massive currents which cause the lights to flicker when you switch on a big power tool; the mains voltage sags noticeably.

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