SINGLE PHASE INDUCTION MOTOR

electrical theory and electrical fundementals for all electrical related people . students , engineers, electrician #electricaltheorems,electrical,

The single phase ac motors are further classified as:
• Single phase induction motor s or   asynchronous motor s
• Single phase synchronous motors
• Commutator motors
This article will provide fundamentals, description and   working principle of single phase induction motor

Construction of Single Phase Induction Motor

Like any other electrical motor asynchronous motor also have two main parts namely rotor and stator.

Stator: As its name indicates stator is a stationary part of induction motor. A single phase ac supply is given to the stator of Single phase induction motor.

Rotor: The rotor is a rotating part of induction motor. The rotor is connected to the mechanical load through the shaft. The rotor in single phase induction motor is of squirrel cage rotor type.

Stator of Single Phase Induction Motor

The stator of the single phase induction motor has laminated stamping to reduce eddy current losses on its periphery. The slots are provided on its stampings to carry stator or main winding. In order to reduce the hysteresis losses, stamping are made up of silicon steel. When the stator winding is given a single phase ac supply, the magnetic field is produced and the motor rotates at a speed slightly less than the synchronous speed N s   which is given by

  except there are two dissimilarity in the winding part of the Single phase induction motor.

Rotor of Single Phase Induction Motor

The construction of the rotor of the single phase induction motor is similar to the
Squirrel cage three phase induction motor. The rotor is cylindrical in shape and has slots all over its periphery. The slots are not made parallel to each other but are bit skewed as the skewing prevents magnetic locking of stator and rotor teeth and makes the working of motor more smooth and quieter. The squirrel cage rotor consists of aluminium, brass or copper bars. These aluminium or copper bars are called rotor conductors and are placed in the slots on the periphery of the rotor. The rotor conductors are permanently shorted by the copper or aluminium rings called the end rings. In order to provide mechanical strength these rotor conductor are braced to the end ring and hence form a complete closed circuit resembling like a cage and hence got its name as ‘squirrel cage induction motor”. As the bars are permanently shorted by end rings, the rotor resistance is very small and it is not possible to add external resistance as the bars are permanently shorted. The absence of slip ring and brushes make the construction of single phase induction motor very simple and robust.

Working Principle of Single Phase Induction Motor

When single phase ac supply is given to the stator winding of single phase induction motor, the alternating current starts flowing through the stator or main winding. This alternating current produces an alternating flux called main flux. This main flux also links with the rotor conductors and hence cut the rotor conductors. According to the Faraday’s law of electromagnetic induction, emf gets induced in the rotor. As the rotor circuit is closed one so, the current starts flowing in the rotor. This current is called the rotor current. This rotor current produces its own flux called rotor flux. Since this flux is produced due to induction principle so, the motor working on this principle got its name as induction motor. Now there are two fluxes one is main flux and another is called rotor flux. These two fluxes produce the desired torque which is required by the motor to rotate.

Why Single Phase Induction Motor is not self starting?

According to double field revolving theory, any alternating quantity can be resolved into two components, each component have magnitude equal to the half of the maximum magnitude of the alternating quantity and both these component rotates in opposite direction to each other. For example - a flux, φ can be resolved into two components

Each of these components rotates in opposite direction i. e if one φ m   / 2 is rotating in clockwise direction then the other φ m   / 2 rotates in anticlockwise direction.

When a single phase ac supply is given to the stator winding of single phase induction motor, it produces its flux of magnitude, φ m . According to the double field revolving theory, this alternating flux, φ m   is divided into two components of magnitude φ m   /2. Each of these components will rotate in opposite direction, with the synchronous speed, N s . Let us call these two components of flux as forward component of flux, φ f   and backward component of flux, φ b . The resultant of these two component of flux at any instant of time, gives the value of instantaneous stator flux at that particular instant.

Now at starting, both the forward and backward components of flux are exactly opposite to each other. Also both of these components of flux are equal in magnitude. So, they cancel each other and hence the net torque experienced by the rotor at starting is zero. So, the single phase induction motors are not self starting motors.

SINGLE PHASE TRANSFORMER

electrical theory and electrical fundementals for all electrical related people . students , engineers, electrician #electricaltheorems,electrical,

A transformer is a static piece of apparatus by mean of which electric power in on e circuit is transformed into electric power of the same frequency in another circuit .it can raise or lower the voltage in a circuit but with a corresponding decrease or increase in current. the physical basis of a transformer is mutual induction between two circuit linked by a common magnetic flux .in its simplest form it consist of two inductive coils which are electrically separated but magnetically linked through a path of low reluctance .the two coils posses high mutual inductance. If one coil is connected to a source of alternating voltage ,an alternating flux is setup in the laminated core, most of which is linked with the other c oil in which it produces mutual inducted emf according to faraday’s laws of electromagnetic induction E =MdI/dt .If the second coil circuit is closed ,a current t flows in it and so electric energy is transferred from the first coil to the second coil. The first coil in which electric energy is fed from the ac supply mains called primary winding and other from which energy is drawn out is called secondary winding

The rate of change of flux linkage depends upon the amount of linked flux, with the second winding. So it desired to be linked almost all flux of primary winding, to the secondary winding. This is effectively and efficiently done by placing one low reluctance path common to both the winding. This low reluctance path is core of transformer, through which maximum number of flux produced by the primary is passed through and linked with the secondary winding. This is most basic   theory of transformer .

constructional parts of transformer

• Primary Winding of transformer - which produces magnetic flux when it is connected to electrical source

• Magnetic Core of transformer - the magnetic flux produced by the primary winding, will pass through this low reluctance path linked with secondary winding and creates a closed magnetic circuit

• Secondary Winding of transformer - the flux, produced by primary winding, passes through the core, will link with the secondary winding. This winding is also wound on the same core and gives the desired output of the transformer

                     

• The  transformer  which increases the   a.c. voltage   is called   step up   transformer .

• The  transformer  which decreases  the   a.c. voltage   is called   step down transformer .

E s is the voltage generated in the secondary coil.

E p  is the voltage given to primary coil.

N s  is the number of the turns in the secondary coil.

N p  is the number of the turns in the primary coil. 

where   K  is called as transformation ratio.

• If K>1 then the transformer as step up transformer.

• if K<1 then the transformer as  step down transformer.

FLEMINGS LEFT HAND RULE

electrical theory and electrical fundementals for all electrical related people . students , engineers, electrician #electricaltheorems,electrical,

Fleming's left-hand rule   (for motors), and  Fleming's  right-hand rule  (for generators) are a pair of visual  mnemonics. They were originated by  John Ambrose Fleming, in the late 19th century, as a simple way of working out the direction of motion in an  electric motor, or the direction of electric current in an  electric generator.

When current flows in a wire, and an external magnetic field is applied across that flow, the wire experiences a force perpendicular both to that field and to the direction of the current flow. A left hand can be held, as shown in the illustration, so as to represent three mutually orthogonal axes on the thumb, first finger and middle finger. Each finger is then assigned to a quantity (mechanical force, magnetic field and electric current). The right and left hand are used for generators and motors respectively.

To work out the  direction  of force experienced we use  Fleming's Left Hand Rule.

• Your  first  finger points in the  direction of the magnetic field  (North to South).

• Your  second  finger points in the  direction of conventional current  (positive to negative).

• Your  thumb  points in the direction of the  thrust or force  on the conductor.

LEN'Z LAW

electrical theory and electrical fundementals for all electrical related people . students , engineers, electrician #electricaltheorems,electrical,

Lenz's law is named after the German scientist H. F. E. Lenz in 1834.   Lenz's law  obeys Newton's third law of motion (i.e to every action there is always an equal and opposite reaction) and the conservation of energy (i.e energy may neither be created nor destroyed and therefore the sum of all the energies in the system is a constant)
Lenz law is based on   Faraday's law   of induction so before understanding Lenz's law one should know what   Faraday’s law of induction   is. When a changing magnetic field is linked with a coil, an emf is induced in it. This change in magnetic field may be caused by changing the magnetic field strength by moving a magnet toward or away from the coil or moving the coil into or out of the magnetic field as desired. Or in simple words we can say that the magnitude of the emf induced in the circuit is proportional to the rate of change of flux

Lenz law states that when an emf is generated by a change in magnetic flux according to Faraday's Law, the polarity of the induced emf is such that it produces a current whose magnetic field opposes the change which produces it.

The negative sign is used in   Faraday's law of electromagnetic induction, indicates that the induced emf ( ε ) and the change in magnetic flux ( δΦ B   ) have opposite signs.

Where
ε = Induced emf
δΦ B   = change in magnetic flux
N = No of turns in coil