Electric stove heating element and switches

Question

I am having difficulty in understanding how heating elements and switches work to control how much heat is dissipated in the element of electric stoves or other similar settings.

From one perspective I have learned the more resistance in say a conductor the hotter the conductor itself gets. The smaller wires get hotter as thicker wire with respect to the same voltage applied to both.

I am certain that the heating element hardly has any resistance in that sense. Or if the lower resistance heating element won't get as hot as the higher resistance element.

With respect to the control switches, if they control voltage by means of having their own some sort of metallic variable resistors which in turn must get hot and get old. And if this varying voltage determines the amount of current to the heating element and not the heating element itself.

I am also wondering if a heating element with more resistance will get hotter than the one with less and what happens to resistance as the heating element gets old?

It is the type of stuff that bugs me.

Answer 1

From one perspective I have learned the more resistance in say a conductor the hotter the conductor itself gets. The smaller wires get hotter as thicker wire with respect to the same voltage applied to both.

1. Don't confuse power and temperature. Temperature will stabilise when heat last to surroundings = power in.
2. Power, \$ P = \frac {V^2} R\$ so as resistance decreases power increases. Thicker wires will dissipate more power.

I am certain that the heating element hardly has any resistance in that sense.

Rearranging the formula we can calculate the resistance. For example, a 1000 W heater element on a 230 V supply, \$ R =\frac {V^2} P = \frac {230^2} {1000} = 53\ \Omega \$. Whether you think that is "hardly any" I can't say.

Or if the lower resistance heating element won't get as hot as the higher resistance element.

Try putting different values into the equation.

With respect to the control switches, if they control voltage by means of having their own some sort of metallic variable resistors which in turn must get hot and get old. And if this varying voltage determines the amount of current to the heating element and not the heating element itself.

Figure 1. A typical bimetallic strip cooker power controller. Read about its operation in another answer. Image source unknown.

Most heaters use on-off control. If you look at a ceramic hob you can see the heater turn on and off with the duty cycle varying from about 5% to 100% as the control knob is adjusted from min to max. Note that these control power, not temperature. The temperature will depend on what is put on the hob. No pan -> very hot. Pan full of water, not so hot.

I am also wondering ... what happens to resistance as the heater gets old.

Hot wire is more fragile than cold and can get damaged. Any narrow spots in the wire have to carry the same current as the rest of the wire and due to their higher resistance per unit length in this area will run hotter and tend to fail at that point.

Answer 2

Heating element wattage P = V²/R where V is the rated voltage and R the element resistance.

The common methods of electric stove control are:

1. Three-heat switch control

In this method two identical heater elements are switched 'two-in-series', 'single' or 'two-in-parallel' using a three-heat switch.

The three-heat switch is a four position rotary switch (which includes an 'off' position) with the switching scheme as shown below.

Thus the wattage selected would be 25%, 50% and 100% of the total wattage.

2. On-Off control, using a bimetal 'energy regulator'

In a bimetal 'energy regulator', the contact carrier is a bimetal strip which, on getting indirectly heated by the element current, warps and trips the circuit. Upon the bimetal strip cooling, the circuit is made on once again. The point at which the tripping occurs is adjustable.

In other words, duty cycle control is effected using a bimetal 'energy regulator'.

3. Duty cycle control, using a triac

In this method, duty cycle control is effected by changing the firing angle of a triac which is in series with the heating element.