(Permanent split capacitor motor

Or

Capacitor start capacitor run motor)

Introduction

An induction motor is a rotating machine which converts electrical energy into mechanical energy and operates on principle of mutual induction. A single phase power system is more economical than a three phase system and the power requirement in most of the houses, shops, offices are comparatively small, which can be easily fulfilled by a single phase system. So, the single-phase power system is more widely used than three phase system for both domestic and commercial purposes. Single phase induction motors are available in market with various ratings and sizes. These are simple in construction, cheap in cost, reliable and easy to repair and maintain. These are found in vacuum cleaners, fans, washing machines, centrifugal pumps, blowers, washing machines, etc. By respecting all the scientific facts we will try here to make a single phase induction motor using scraps available in our surroundings. It will be a completely handmade motor which is different from commercial structure. It will run on a comparatively lower voltage say 12V AC.

Components required.

1. Machine bolt
2. Transformer (12 V)
3. Push pin
4. Plastic pipe
5. Aluminium disc
6. Insulated copper wire
7. Capacitor 

Components description

Machine bolt

Four galvanised iron machine bolts are required with same quantity of nuts and 8 washers two washers for each bolt. The function of the washer is to provide proper brace to winding. The size of the bolt is 1/8 inch having length 1.5 inch each. The machine bolt works as core for the machine.

Transformer

It consists of two coils of insulated copper wire linked by an iron core. Transformers are used to step up (increase) or step down (decrease) AC voltages. Energy is transferred between the coils by the magnetic field in the core. There is   no electrical connection between the coils. Here we have used a shell type step down transformer which step downs 230 volt AC to 12 volt AC. It can supply maximum 500 mA current. This is sufficient for the motor for rotation. If you find a 12 volt transformer with more current rating then that’s also good thing.

Push pin

It is just a paper push pin found in notice boards. It is used here as an axis for the aluminium rotor.

Plastic pipe

It is just a small piece of straw used in soft drinks. It provides a better height to the rotor disc to face the magnetic field of stator windings.

Aluminium disc

It is a very thin piece of aluminium sheet easily found in hardware shops. Using scissors we have cut it into a circular disc having a diameter of 3 cm. Make a very small hole at its centre for inserting the push pin.

Insulated copper wire

The wire which is made of copper and can insulate electric current is called insulated copper wire. Copper wire used in an electric motor winding is insulated with a coating of non-conductive insulation like plastic or enamel which prevents the current from passing between the wire turns. The windings of the wire multiply the effect of the circulating current due to which magnetic effect increases. We have used the thinnest gauge of wire available in our laboratory. The important precaution is that 1500 turns of insulated copper wire should be placed properly in that small bolt.

Capacitor

Generally a capacitor sometimes called condenser stores electric charge in it, but it has so many applications in electrical and electronic circuits. It can be used as a filter , voltage booster or phase modifier etc. We have used a 47µF capacitor of electrolytic type. It has been observed that if input voltage changes with winding turns, the capacitor value also changes.

Circuit Diagram

Construction

The rotor is neither squirrel cage type nor wound type. It is just an aluminium disk and it contains no windings. The stator has a main winding of 3000 turns of insulated copper wire and an auxiliary winding of 3000 turns wound on machine bolt. The windings are set in quadrature on the base by tying cotton threads to avoid any displacement. The auxiliary winding contains a capacitor in series. The capacitor is permanently connected both in starting and running conditions. The rotor is placed in such a way that it should not touch any of the windings. Special care should be taken to minimise the gap between stator winding and rotor to minimise reluctance.

Working

The single phase induction motor is not self starting because the produced stator flux is alternating in nature and particularly during the starting period, the two components of this flux cancel each other, and the net torque produced is zero. Here the motor operates just like a balanced two-phase motor. By using capacitor the angle between current in main winding and current in auxiliary winding is made 90°. That means current in main winding lags the current in auxiliary winding by 90°. Thus a single phase supply is intentionally split into two phase supply to apply the stator windings. The motor operation can be explained easily with the help of double field revolving theory.

A rotating magnetic field is produced in stator when supply is given. This magnetic field is induced in rotor and according to Faraday’s laws of electromagnetic induction, an electro motive force is produced in rotor. This induced emf produces current in rotor as it is a conducting disc. Thus the induced current produces another magnetic field in rotor. The rotor magnetic field always tries to catch the revolving magnetic field of stator and thus the rotor rotates. The motor always rotates at a speed which is slightly less than the synchronous speed. Synchronous speed is the ratio between product of frequency with a number 120 and number of magnetic poles.

Mathematically,

Ns = 120f/P

Where,

Ns = Synchronous speed

f = supply voltage frequency,

P = Number of poles of the motor.

Faraday’s law of electromagnetic induction.

Faraday’s law of electromagnetic induction states that whenever a conductor cuts magnetic flux, an  emf ( electromotive force ) is induced in that conductor. Magnitude of the induced emf is equal to the rate of change of flux linkages.

Mathematically ,

e = -N(dΦ/dt)

N – Number of turns of coil

Φ – flux linkage

t – time

dΦ/dt – rate of change of flux with respect to time.

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