Personal Electronic Equipment: Hairdryers

Nearly every household possesses a hairdryer. Designs vary from hand-held models to large hooded ones supported on a stand. They all work on the same principle of blowing air over a heating element by means of a fan. The fan is either a simple bladed type or a cylindrical version. As the air passes over the hot element, it is heated and is then vented from the appliance. The element is usually a wound exposed wire on a heat-resistant mount. A more solid version is mounted in a zig-zag fashion on a mica former. The element is protected by a thermostat to control the safe working temperature of both heater and appliance, and is usually self-resetting.

The motor may be one of four types: (AC) brush, induction motor (capacitor start), induction motor (shaded-pole) and DC permanent-magnet motor.

Problems

The commonest faults are damage to the flex through wear and tear or misuse, overheating as a result of clogging or a blocked air intake and damage to the fan either from rough handling or from fluff or loose hairjamming and slowing it.

Flex damage and failure often occur because of the continuous movement when the hairdryer is in use. Take care when storing the flex after use, do not pull or stretch it when using the dryer and inspect it regularly for faults, such as outer sheath cracks or splits and heat damage. Pay particular attention to the entry point of the lead and to the plug cord grip. Renew any faulty or suspect flex immediately. Do not use the hairdryer until this has beendone.

Check the hairdryer regularly to ensure the air intake to the fan is clean and clear of fluff and loose hair. A blockage results in the reduction of the airflow which leads to the heater thermostat or TOC tripping repeatedly, eventually resulting in failure of the stat or heater or both.

The DC motor

The DC motor runs only on direct current rather than on normal household alternating current. Its operation is similar to that of the brush gear motor.

DC power is supplied to a wound armature via acommutator and carbon brush system. A field coil system may also be used, but more often a permanent magnet field, requiring no electrical supply, is used. Power in the form of direct current is supplied only to the brushes.

This induces rotation of the armature within the permanent magnet poles. Such motors are easily reversed simply by reversing the supply voltage to the brushes. It is important therefore, to take care when dismantling the appliance because if the terminals are reversed the fan action of the dryer will be reduced or even rendered nonexistent. As always, make notes when stripping down for servicing.

This type of motor is used because it is smaller, has a lower power consumption, high reliability, smooth running and low cost. Failure of the motor will require complete renewal; make sure only the correct replacement is obtained.

There is a bridge rectifier mounted within the appliance. This is recognizable as a small printed circuit board or a small rectifier unit with four wires leading to it. When the hairdryer is switched on, power flows from the live terminal, through the element to the neutral terminal; heating then takes place. A connection (tap) is made part way through the element to supply power to the bridge rectifier. The element acts as a resistor and so reduces the voltage, usually to 1/10, to the rectifier. The reduced AC voltage supplied to the rectifier is then converted by it to DC ready for use in the motor.

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