Today we will discuss about a very basic and silly looking thing used in every home, that is regulator of fan which control the speed of fan(or is it so.....!).to get the full picture we have to first understand basic speed control methods and their feasibility.
Fan is an electric machine, basically a single phase squirrel cage induction motor with externally connected capacitor (to provide starting torque) and blades. Having a fan with good and smooth speed control is always a challenge for engineers. So first let us have some basic idea of some parameters which affect the speed of fan. so by controlling one of the parameter we will be able to achieve most practical speed control.
Parameters which can be used to control speed of induction motor are mainly,
- Frequency of supply:- Speed of fan is directly proportional to frequency. But frequency of our grid is constant. India has national grid and frequency of 50 Hz. (why is it necessary to have voltage and current of same frequency? answer in next post...) Changing the frequency is quit a messy thing as it require many power electronic circuits and components (which brings harmonics for free...!) like Inverter, rectifier, filters and size of that unit, with efficient and quality design it could be as big as your television and so no one would like to employ that....
- Pole changing:- When AC supply is given to fan, 'electromagnetic poles' are induced in the winding, which depends upon winding configuration. Speed of fan can be controlled by changing poles but to do so, winding configuration of motor has to be changed. It is not practical to change winding configuration when your fan is rotating.
- Voltage control:- Torque in an induction motor is proportional to square of applied voltage. By changing the voltage, torque can be controlled and that changes the speed too (as here the load on the fan is only windage losses, so torque will be directly proportional to square of speed so we can say V ∝ N). It is widely used method for fan speed control. but voltage can also be controlled by different techniques, let's check them one by one.
- Voltage control by Resistance/Rheostat - That old school rectangular big box in which 5 step are given to control speed of fan. It is basically a resistive speed controller in which 5 resistors are connected in series and tapping is provided after every resistor as shown in the figure below...
If you want to control speed in 'n' steps then resistance required are 'n-1'. When your knob is at position 5 than full voltage is applied to stator so fan will rotate at max speed. By moving the knob to lower taps it will connect resistances and voltage applied to the stator of fan will reduce accordingly.
Main disadvantage of this type of regulator is that when current flow through resistors power will dissipate in form of heat. So additional power will be consumed which will definitely raise your electricity bill amount. At 5th position no resistor is connected so power loss in regulator is zero but at position 1 all resistor are in series with stator, so maximum heat is generated at position 1. this means that when fan is at lowest speed it will consume more power........so if you are having one of this regulators the you batter run it on full speed. For dissipation of heat, heat sink is provided on top of regulator.
2. Voltage control by capacitor - It is similar to resistive control but instead of resistors, capacitors are used. This method is very popular now-a-days because it is less bulky, energy efficient, have long life and cheaper than electronic regulators . Here capacitor is connected in series with stator of fan.
Capacitors are connected in series with the stator and tapping provided are same as resistive voltage regulator. As you know in an ideal capacitor current leads the voltage by 90°. Equation of real power (power responsible for electricity bill) is P = VI cos(Φ) so power consumption is ideally zero in capacitor type regulator. In practical some resistance is present (due to dielectric loss and contact losses) but it is negligible. A fan rated 230V, 50W consumes Approx. 25-30 Watts at low speed. So, no power is lost in regulator and in addition there is no heating problems and the bumper advantage is that capacitor also improves power factor of circuit.
3. Power electronics regulator - Above two regulator control speed in steps but this type of regulator control speed smoothly. It consist of power electronic devices such as diac, triac. They chop AC waveform and can increase or decrease RMS value of voltage. These are energy efficient and lossless regulators but more costly. The speed can be reduced by changing how much time waveform is turned on and off (for how much time voltage is applied and removed). In this type of controller at lower speed fan consume lesser energy due to low voltage applied without loss of power (a small amount of power is require to operate this power electronic devices and they also have some heat loss- but they are negligible compare to resistive regulators).
The problem with this regulators are the harmonics. When switching of power electronic device occur, harmonics are introduced in the supply voltage, which have long term deteriorating, derating and damaging effects in electrical M/c and devices. Above figure is the example of such a chopped waveform. Here you can see that the output voltage is not pure sinusoidal.
Above two regulator reduce RMS voltage directly but this regulator reduce voltage by chopping the waveform, so harmonics are not a problem in case of resistive and capacitive regulators.
Their disadvantage is that due to rough usage and sensitive nature they are very much prone to internal damage.
Hope you get a basic idea about the speed control methods and different type of regulators. So next time wherever you see a regulator you will be able to identify and educate your friends also....
👌..good explanation
ReplyDeleteThanks for reading.
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