Hair Dryer Construction
A hairdryer is made up of two major components. A fan generates a flow of air, and a heating element provides a source of heat. Both these components have physical characteristics which can cause power line problems when they are used. When these devices are turned on, their initial demand for electricity can be quite high compared to their normal operational requirements.
Most hair dryers are equipped with a universal motor. This type of motor has many properties that are desired in a hair dryer, including high speed rotation, low cost and ability to operate at different speeds with minimal control circuitry. Universal motors are also characterized by high start-up torque, the ability to go from stop to full speed very quickly.
This rapid start-up requires a large amount of electricity. A universal motor may require ten times more energy at start-up than it does running at full speed. This requirement only lasts for the time it takes to reach its operating speed, typically less than a second.
The process is analogous to an automobile engine working harder, and using more fuel, to accelerate a vehicle than to maintain a constant speed.
The heating element of a hair dryer is commonly made of a nickel-chromium alloy wire. A characteristic of this material is that electrical resistance increases as the material heat up. The result of this property is that the material draws the most electricity when it is cold. Thus, the heating element of a hair dryer also draws the most electricity when it is first switched on. While this effect is not as pronounced as the motor start-up current, it contributes to the heavy demand for electricity when the hairdryer is switched on.
Power Circuit Considerations
A hairdryer will be stamped with a wattage rating by its manufacturer. As an example, consider a 1500 watt hairdryer. Household electricity is 110 volts, so a 1500 watt hairdryer will draw 12.5 amps (1500/110) of current. Modern bathrooms are required to have 20 amp circuit breakers, so the hairdryer should not overload the circuit.
However, for a brief period of time at start-up, the hairdryer may draw twice its rated current or more. The 20 amp circuit is now called upon to supply 25 amps of current. Most circuit breakers will allow a temporary surge of current without tripping. However, if other devices are drawing power from the same circuit, this may be enough to trip the breaker.
Even if the breaker does not trip, there will be a momentary voltage drop on the circuit as the current surges. This can be seen in lights dimming or flickering.
Occasionally this surge may be sufficient to trip a Ground Fault Interrupter (GFI) device, required in all bathroom circuits. Older houses may have a 15 amp circuit breaker installed, worsening the problem.
References and Resources"The Electrical Engineering Handbook;" Wai Kai Chen; 2004
"Introduction to Electricity and Electronics: Electron-flow Version;" Allen Mottershead; 1982