Last Updated on May 26, 2020 by Amit Abhishek
One of the most important questions of all time. What is the role of the capacitor in a celling fan is the most repeated question in any competitive environment. It is being asked numerous times during company placement, interview, or during a class lecture.
Why do fans have capacitors or what’s the function of capacitors in single phase motor; question maybe manifested in various forms, but the intent is to know one simple answer most of us don’t know. But here, in “shipfever.com” you’ll understand it once and for all.
In simple words a capacitor acts as a different phase for the single-phase motor including that in your fan. It split the current in different phases so there can be sufficient phase difference to generate magnetic torque.
It is required to have one capacitor installed on each single phase motor ( Example: Fan ); to generate net positive magnetic flux.
In the absence of a capacitor there is a complete shift in magnetic flux for each current cycle; that leads to net-zero magnetic fields. Now to keep things simple and easy to understand let’s start from scratch.
Capacitor – Construction And Its Working
Before digging up, it’s vital to understand what is the function of capacitors? A capacitor is a device capable of storing energy in the form of electrical charge; which then discharges that energy when the supply of current is turned off.
Compared to the same size battery; a capacitor can store much smaller amount of energy, usually 10,000 times smaller but useful enough for so many circuit designs.
A capacitor consists of two terminals connected to the two metal plates separated by an insulating material called Dielectric; represented by the Yellow portion of the diagram.
These conductive metal plates are made up of aluminum or other metal alloys; while dielectric is the non-conductive part of capacitors.
A capacitor dielectric is made of insulating materials, such as paper, Mica, Teflon, Air, ceramic or anything that restrict the flow of current. The capacitance of a capacitor is given by:
Where C is capacitance, and “ε” is absolute permittivity of the dielectric. A is overlapping surface area, F is Farad (unit) and d is the distance between the two plates. A capacitor with one-farad value can store a charge worth of one coulomb with 6.25 millions electrons.
Now, let us see how the capacitor works! We know a metal has an equal amount of positive and negatively charged particles; which means it is electrically neutral. Now if we connect the terminals of the capacitor with battery; these electrons on the plates will try to flow towards the negative terminal connected to the battery.
This results in a positive charge being accumulated on the capacitor plate connected to the positive terminal of the battery; while the negative charge on the opposite Plate connected to the negative terminal of the battery.
However because of the dielectric; the electron won’t be able to pass through the capacitor and will start accumulating on the plate.
After the plate is sufficiently energized by these electrons; the battery will be unable to make the flow of current (flow of electron).
This happed because the accumulated electron will repel the new electron coming in (Potential difference will be the same as capacitor and battery). At this point, capacitor is fully charged.
The first plate has developed a net negative charge and another plate has developed a net positive charge; creating an electric field with attractive force between them which holds the charge of the capacitor.
How dielectric can increase the capacitance of a capacitor?
Dielectric contains the molecules that are polar; which means they can change their orientation based on the charge on the plate.
So molecules align themselves with the electric field such that; it enables the electron to be attracted toward the positive plate while repelling more electron out of the positive plate.
A capacitor with high dielectric constant ( The ratio of the permittivity of a substance to that of vacuum ) tends to show more charge stored then in vacuum by reducing the effective electric field.
This reduction in the effective electric field allowing for a reduction in the potential difference between the two plates. With a reduction in potential difference more charges can be added to the capacitor of the same value.
So a dielectric made of glass with relative permittivity of 7; can store seven times more charge that it would have stored under vacuum.
In practice Atom in the dielectric align themselves such that; positive side of the atom is facing negative plate and vice-versa. This lower the voltage, which increases the capacitance.
This can be further represented by:-
Where Q is coulombs, V is voltage and Q/V= Farad (Unit)
So, once the capacitor is fully charged we remove the battery; it will hold the charge for a long time, acting as an energy reservoir.
Now if we short the two plate, the electron will flow until plate become electrically neutral.
This was the basic working of the capacitor, easy to understand but boring and impractical to test yourself. So let’s see a practical example of capacitor working; something you can do and see for yourself.
Practical Example Of Capacitor Working
Take a 100-watt bulb, a capacitor of 2.25 mfd which is generally used in fans and some wire. If you wish you can replace it with a 9v bulb, a simple capacitor of 200 uF, 9v battery, and some wire.
Now connect the bulb, capacitor, and electricity source (battery/power); as per the below diagram. You will find that the bulb will grow bright at the start and start to go dimmer and dimmer as time goes by. The capacitor is at full charge when the bulb stops glowing.
Now as you remove the power source or battery depending on your model and connect the two terminals of capacitors to the bulb; you will find the bulb starts to glow again and go dimmer and dimmer as the capacitor discharge.
From this experiment you understand how a capacitor stores charge; and then release it when disconnected to the power source.
Why Do Fan Have A Capacitor?
Now since you have understood the basics of a capacitor and how does it work; It won’t be hard for you to understand why we need it to run a motor ( As in Fan ).
Any single phase motor as used in fan consists of a stator and main winding with a rotor shaft. Now with alternating current supply to the stator winding; alternating flux is produced which then induce a current in the primary winding. Let’s learn about this through the bellow diagram.
The above diagram represents the circuital arrangement of the fan. According to the diagram it has one running and one starting winding; also known as “stator” connected to the power supply.
One capacitor is serially connected with the starting winding and one rotor. Here the starting winding has low inductive reactance but high resistance; while the main windings have high inductive reactance but low resistance.
Now let consider an electrical fan circuit without any capacitor as shown bellow:
Running Fan Without A Capacitor
Now with the switch closed the circuit is complete; A 230-volt alternating current flows through the starting and running winding.
This produces a non-rotating but pulsating magnetic field from 0 to 180-degree intensity.
When we have the same phase for two winding, no magnetic rotation will be created; hence the rotor will not be rotated for this type of arrangement of the fan without a capacitor.
To create magnetic rotation we have to create two different phases for two different winding.
Now since we have a single-phase connection at home we cannot make two phases to give. To overcome this problem capacitor is used.
Now let’s consider the circuital arrangement with the capacitor; which is serially connected to the starting winding. Now when the switch is closed; the same 230V AC is entering the running winding and capacitor.
The phase that gets enter into the capacitor will get shifted 90 degrees in phase inside capacitors and comes out, and now this shifted phase is entering into the starting winding while the applied phase is in the running winding.
Due to two different phases in each winding, the magnetic rotation will be created and thus cause the rotor to rotate.
When the motor speed reaches 70% of its synchronous speed; the capacitor along with stator winding is disconnected using a centrifugal switch. The capacity of the capacitor is chosen based on the initial load torque requirement and are generally 2.25 mfd.
Note: To learn More about Single phase Induction Motor You can Refer “A Textbook of Electrical Technology: Volume 2” by B.L Theraja; Click Here to check its Current Price On Amazon (India). ( For more information on our links to third party / Amazon Read our terms and condition and Disclaimer. )
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