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230VAC 10A Full-Wave Motor Speed Controller

This full-range Motor Speed Controller will give smooth control from near zero to full speed on electric drills, routers, circular saws, lawn edgers, food mixers - in fact, any appliances with universal (brush-type) motors.

By John Clarke

Features:

• Full control of motor speed from near zero to maximum
• Speed regulation under load
• Smooth low-speed motor operation
• Rated for universal motors rated up to 2300W
• Over-current protection and limiting
• Fuse protection
• Rugged earthed diecast case
• Interference suppression filter

Our last Motor Speed Controller, published in February 2009, utilised a simple phase-control circuit which works reasonably well with most universal motors. However, there are some applications where a wider and smoother control range is required.

One shortcoming of the February 2009 design is that the maximum speed from the motor when under speed control is significantly reduced. So for an electric drill that normally runs at say 3000 rpm, the maximum speed might be reduced to around 2200 rpm. This is inevitable with a controller circuit that effectively half-wave-rectifies the 230VAC mains waveform to give a maximum output voltage of around 160V RMS.

The second drawback of the February 2009 design has to do with low speed control. While the circuit does allow your drill or other appliance to run at quite low speeds, the result leaves much to be desired. There isn’t much torque available and the speed regulation is poor. This means that if you’re operating the drill at a low speed and you put a reasonable load on it, its speed will drop right away or it may stall completely.

Worse still, the motor will tend to “cog”, caused by erratic firing of the Triac within the Drill Speed Controller, so that the motor receives intermittent bursts of power. An electric motor that is cogging badly is virtually useless and the only cure is to increase the speed setting – and this rather defeats the purpose if you want to operate at low speed.

This new SILICON CHIP Motor Speed Controller overcomes these drawbacks. The design does not use phase-control circuitry but uses switch-mode power supply techniques to produce an outstanding controller for universal brush-type motors.

By the way, before we go further we should point out that virtually all mains-powered power tools and appliances use universal motors. These are series wound motors with brushes.

And most power tools will do a better job if they have a speed control. For example, electric drills should be slowed down when using larger drill bits as they make a cleaner cut.

Similarly, it is useful to be able to slow down routers, jigsaws and even circular saws when cutting some materials, particularly plastics. The same applies to sanding and polishing tools and even electric whipper snipers are less likely to snap their lines when slowed down.

Click for larger image
Click for larger image

These waveforms illustrate the operation of a typical phase-controlled SCR. In Fig.1 (left) the SCR is triggered fairly late in the positive half-cycle, so the motor voltage is just 143V RMS and it runs at a relatively low speed. Compare this with Fig.2, right, where SCR is triggered earlier in the half-cycle and the RMS value rises to 163V. Hence the motor runs faster.

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