There is often the need to speed control single phase fans particularly in commercial hood or kitchen exhaust systems.
Traditionally, as far as single phase motors are concerned, this has been achieved by lowering the supply voltage to the motor by means an electronic or triac based device.
Lowering the voltage to the main winding of single phase motor reduces the magnetic flux density within, enabling the rotor to “slip” against fan or impellor load.
Simple enough in principal but there are traps in this method.
Fan motors over 60 watts generally have two windings, a main and an auxiliary.
A capacitor in series with the auxiliary winding provides a “phase shift” creating the “rotating” “magnetic field as is necessary in an electric motor.
Triac type controllers “chop” the sine wave to reduce the voltage. This switching can produce harmful voltage spikes that may effect the insulation of the motor but is more likely to kill any capacitor in the circuit.
Capacitor failures are often the cause of motor burn outs.
It is therefore imperative that the two windings of the motor be isolated and the variable voltage applied to the main winding only and a constant pure voltage to the auxiliary.
To achieve this wiring configuration the cable from the variable speed controller MUST be four core, ie, a core for the variable voltage, one for the pure voltage, a neutral and an earth. It must be assured that all voltages are isolated from the motor on switch off.
Care must also be taken that the motor is not left in a stall condition and that sufficient air passes over the motor to cool it. This is achieved by setting the “lower limit” on the controller and only controllers with this function incorporated in them should be used.
Motors with triac type devices should be “hard started”. That means the motors should be started at full speed, to overcome friction, then reduced to the required speed.

Another method of reducing the voltage to the motor is by the transformer type of controller. These are bulkier, heavier and generally more expensive than the triac type. The transformer may have up to five secondary voltage tappings which are switched to achieve the desired speed.
Whilst four wires to the motor is not as critical, as the wave form is regular, better control is achieved when four wires are used.

Generally, both of the above controllers are only available for motor loads up to 5 amps which is somewhat restrictive.
Avoid the use of two pole motors (2800 rpm) with these controllers, as control is often erratic below 75% of full load speed..

By far the best method of speed controlling motors where single phase only is present is by varying the frequency of the supply to the motor.
Control is more precise and the motor speed is not affected by mains voltage fluctuations.

Commonly known as inverters, VSDs or variable speed drives, these devices are less likely to burn out motors.

Magically , with the used of modern semi conductor technology, one type of VSD converts 230v single phase to a 3 phase variable frequency output which is connected to the more reliable three phase type of motor.
Standard three phase motors up to 3kw are usually able to be connected to accept this type of drive. These drives have many features.
• Variable speed control by trimpot on drive
• Remote control on /off and speed
• Controlled ramp up or down with speed
• Control by external transducers 0-10V 4-20ma (Temperature or flow)
• Up to 4 pre set speeds as option
• Minimum speed preset
• Over speed motors by 20% ie. 60htz
• Controlling a more reliable motor (no capacitors!)
• Excellent control of 2 pole motors
• Control of motors up to 3kw
• ENERGY saving mode.

A basic VSD on the market today is comparable in price of the less favoured “transformer” type and is more cost effective in the long term.

In all cases choose a device with “C” tick Compliance with the EMC standards.