Most clothes dryers operate from a 240- V rms outlet and have several temperature settings, such as low, medium, and high. Let’s examine the manner in which the dryer creates heat for drying and the way in which it regulates the temperature.

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First we will consider the heat itself. A simple model for this situation is shown in Fig. 9.37a, where a resistive heating element is connected across the 240-V rms supply. For a particular dryer, the resistance of the element is roughly 11 Ω. The current through this element is

[latex]I_{he} = \frac{240}{11}[/latex] = 21.81 A rms 9.39

Since the element is resistive, the voltage and current are in phase. The power dissipation is

[latex]P_{he} = I^{2}_{he} R_{he} = (21.81)^{2}[/latex](11) = 5236 W

This value, more than 5 kW, is a lot of power! Note, however, that this is the power dissipated under the assumption that the element is connected to the 240-V rms supply 100% of the time. If we manipulate the percentage of the time the element is energized, we can control the average power and thus the average temperature.
A fairly standard method for temperature control is shown schematically in Fig. 9.37b, where a standard residential single-phase, three-wire service powers the heating element at 240 V rms and the control circuit at 120 V rms. The temperature switch is connected to three resistors. Each temperature setting produces a different current through the thermostat heater, which is just another resistor. Each switch setting will alter the temperature of the themostat heater that controls the temperature set point. We can calculate the power dissipated in the thermostat heater for each temperature setting. If we let [latex]R_{S}[/latex] be the resistance that corresponds to the switch setting, then

[latex]P_{Th} = I^{2}_{Th} R_{Th} = \Big( \frac{120}{R_{Th} + R_{S}} \Big )^{2} R_{Th} \left\{\begin{matrix}= 1.3 \ W \ ext{for the low setting} \=1.79 \ W \ ext{for the medium setting} \=4.1 \ W ext {for the high setting} \end{matrix} ight.[/latex] 9.40

We see that the thermostat heater power dissipation is lowest at the low setting and increasesas the switch is moved to the high setting.
Now let us consider the critical issue in the temperature control system. The thermostat heater is located physically adjacent to the control thermostat (very similar to the ones used to control heat and cooling in your homes where you set the temperature manually). In the dryer, the thermostat heater acts as the desired setting. If the temperature at the thermostat exceeds the setting, then the thermostat switch opens, deenergizing the heating element and allowing it to cool off. When the thermostat determines the temperature is too low, the thermostat switch will close, energizing the element and increasing the temperature. In this way, the thermostat switch opens and closes throughout the drying cycle, maintaining the correct temperature as selected by the temperature switch.

Note that all of our calculations have been made for power levels, not temperatures. The exact temperatures of the heating element, the thermostat heater, and the thermostat itself depend on how heat moves about within the dryer—a thermodynamics issue that cannot be addressed with a simple circuit diagram.
Finally, note the high limit thermostat and its associated switch. This thermostat is mounted very close to the heating element. If the control thermostat fails, there is no temperature control and we can expect trouble. The high limit thermostat will detect these excessive temperatures and deenergize the heating element. Once the temperature drops, normal operation can resume. Thus, the high limit thermostat is used to protect the dryer and by extension your home.

Question 9.26: Most clothes dryers operate from a 240- V rms outlet and hav...

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