The basic construction of a single-phase induction motor is similar to the three-phase induction motor.

1.       Except that, its stator winding consists of a single-phase winding.

2.       A centrifugal switch used in some types of motors, in order to cut out a winding, used only for starting purpose.

3.       It has a distributed winding and a squirrel cage rotor.

4.       When fed from supply its stator winding produce an alternating flux not revolving.

5.       Hence, in these types of motor an auxiliary winding is used to the main winding for starring purpose.

Phase difference between two fluxes is 120o

For starting purpose, extra winding is not used.

Centrifugal switch is not use.

When fed from Three-phase supply its stator windings produce a flux of constant magnitude, which is rotating at synchronously.

It is self-starting.

Phase difference between two fluxes is 180o

Extra winding is essential for starting purpose.

A Centrifugal switch used in some types of motors.

When fed from Single-phase supply its stator winding produce a flux, which is alternating
The capacitor designed for short duty service and generated not more then 20 periods of operation per hour.
When the motor reaches 75% of full speed, the centrifugal switch S is cuts out both the starting winding and the capacitor from the suapacitor used for making necessary phase difference between [IS & IM].
Current [IM] drawn by the main winding lags the supply voltage V by a large angle.
Current [IS] drawn by the starting winding lead the supply voltage V by a certain angle.
The two currents are out of phase with each other by about 80o as compared to nearly 30o for a split-phase motor.

The resultant current [I] is small and almost in phase with V.

Both torques and the resultant torque for sleeps between zero and +2. At standstill, s=1 and (2-s) =1.
numerically equal but, being oppositely directed produces no resultant torque. So there is no starting torque in a single-phase induction motor


In this motor, four leads are brought outside its housing; two from the main winding and two from the starting-winding circuit. These four leads are necessary for external reversing. As usual, internally, the starting winding is connected in series with the electrolytic capacitor and a centrifugal switch. The direction of rotation of the motor can be easily reversed externally by reversing the starting winding leads with respect to the running winding leads

It is so arranged that the phase difference between the currents in the two stator windings is very large. Hence the motor behaves like a two-phase motor. These two currents produce a revolving flux and hence make the motor self –starting.

ALTERNATORS, A.C GENERATORS, OR SYNCHRONOUS GENERATOR.

ALTERNATORS:
 A.C generators or Alternators is a special machine, which converts mechanical power to three-phase [3-Φ] A.C supply.

CONSTRUCTION:
ȀIn a synchronous generator, a DC current is applied to the rotor winding, which produce a rotor magnetic field

ȀThe prime mover, producing a rotating magnetic field within the machine, then turns the rotor of the generator

ȀThe rotor of a synchronous generator is essentially a large electromagnet. The magnetic poles on the rotor can be of either salient or non-salient construction.

ȀA salient pole is a magnetic pole that sticks out from the surface of the rotor. On the other hand a non-salient pole is a magnetic pole constructed flush with the surface of the rotor.
This generator Consists of an armature winding and magnetic fiel

Voltage produce by this generator is hig Current is taken from armatuIn this generator armature is stationery but field is rotating

This generator Consists of an armature winding and magnetic field
Voltage produce by this generator is lowent is taken from brushes.
In this generator armature is rotating but field is stationary.rent is not applied to the rotor winding for produce the rotor magnetic field.
 
A DC current must be supplied to the field circuit on the rotor. Since the rotor is rotating, a special arrangement is required to get the DC power to its field windings. There are two common approaches to supplying this DC power

1.       Supply the DC power from an external DC source to the rotor by means of slip rings and brushes.

2.       Supply the DC power from a special DC power source mounted directly on the shaft of the synchronous generator.

BACK E.M.F:

When the motor armature is rotate, hence the conductor s also rotate and cut fluxes. According the law of electromagnetic induction an e.m.f is produced, is in opposition of the supply voltage. Because of its opposite direction it is referred to as counter e.m.f  or Back  e.m.f. [ EB]
b = ΦZN * [P / A] volt Where N is r.p.s
Back e.m.f depends on armature speed. If speed is high Eb is high. If speed is low Eb is low.
VOLTAGE EQUATION OF A MOTOR:

The voltage V applied across the motor armature has to

1.      Overcame the back e.m.f Eb

2.      Supply the armature ohomic  drop IaRa

[ V = Eb + IaRa ] Voltage equation of a motor.

Now, mollifying both side by Ia

[ Via = EbIa + Ia2Ra ]

Via     = Electrical input to the armature.

EbIa   = Mechanical power developed in the armature.

Ia2Ra = Cu loss in the armature.

Out of the armature input, some is wasted in I2R loss and the rest is converted into mechanical power within the armature.

Motor efficiency is given by the ratio of power developed by the armature to its input

[ EbIa / VIa ] = [ Eb / V ]

High the value of Eb as compared to V, higher the motor efficiency.
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There is no basic difference between DC motor and generator. The same DC motor can be used interchangeably as a generator.
COMPARISON BETWEEN MOTOR AND GENERATOR:
MOTOR
Rotor is rotate.
Converting Electrical Energy into mechanical energy.
No prime mover needed.
Current supplied in armature.
Convert electrical energy into mechanical energy.
Motor Force direction is given by Fleming’s Left-Hand rule.
Supply is converting to mechanical energy and End by the generator.
Armature is rotate
Converting of Mechanical Energy into electrical energy
Prime mover is used by generator.
Current taken from Armature.
Convert mechanical energy into electrical energy.
Generator E.M.F direction is given by Fleming’s Right-Hand rule.
Mechanical energy is converting to electrical energy and End by the User or loads.
  1. A brushless exciter is a small AC generator with its field circuit mounted on the stator and its armature circuit mounted on the rotor shaft.

2. The three-phase output of the exciter generator is rectified to direct current by a three-phase rectifier circuit also mounted on the shaft of the generator and is then fed into the main DC field circuit.

By controlling the small DC field current of the exciter generator, it is possible to adjust the field current on the main machine without slip rings and brushes.

Construction of a DC generator:

A DC generator consists of the following essential parts.

1.      Magnetic frame or Yoke.

2.      Pole-Core and Pole-Shoes.

3.      Pole Coils or field Coils.

4.      Armature Core.

5.      Armature Windings or Conductors.

6.      Commutator.

7.      Brushes and Bearings.

YOKE:

The outer frame or yoke serves double purpose.

1.      It provides mechanical support for the poles and acts as a protecting cover for the whole machine.

2.        It carries the magnetic flux produced by the poles.

POLE CORES AND POLE SHOES:

The field magnets consist of pole core and pole shoes.

1.      The spread out the flux in the air gap.

2.      They support the exciting coils.

POLE COILS:

When current is passed through these coils, they electromagnetise the poles which produce the necessary flux that is cut by revolving armature conducts.

ARMATURE CORE
It houses the armature conductors or coils and causes them to rotate and hence cut the magnetic flux of the field magnets.

ARMATURE WINDING:
The armature windings are usually former-wound. It cuts the flux

he brushes whose function is to collect current from commutator.
 
Q:-Write down the functionality of the essential parts of a practical Generator?

The curve showing relation between terminal voltage V and field current If when the generator is loaded is known as load saturation curve.

Ob = OCC curve at no load.  Eo = Open circuit voltage.

Oc = Open circuit field current at no load.

E = Load voltage at load.

The curve Ld is practically parallel with the curve Ob. The terminal voltage V will be less then this generated voltage E by an amount of IaRa.

At load condition the terminal voltage will decrease due to demagnetize effect

∆ bdc = called drop reaction for full load.

∆ mnt = called drop reaction triangle for half load.

FACTORS EFFECTIVE VOLTAGE BUILDING OF A DC GENERATOR:

Its represent the normal operation the prime mover rotation is clockwise and both the residual flux ΦR and the field flux ΦF are directed to the left.

It represent the reversed connection of the field circuit which cases ΦF to oppose ΦR consequently the generator voltage builds down from its original residual value.

Its represent revered residual magnetism. Armature voltage is reversed w hich farther reversed the field current.

So both ΦF ΦRare reversed. So the voltage buildup is in the reversed direction.


It represents the reversed armature rotation causes the reversal of the voltage produced by the residual magnetism.

The reversed field current flow causes ΦF to oppose ΦR so that the voltage builds down from its original residual value.

1.       open circuit its shunt field resistance should be less then the critical resistance.

2.       If excited on load circuits its shunt field resistance should more then a certain minimum value of resistive which is given by internal

This curve is a OCC curve. Rc is critical resistance. Rsh is shunt field resistance.

• Before loading a shunt generator it is allowed to build up its voltage.
• Usually there is always present Residual magnetism in the poles; hence a small E.M.F. is produced initially. This E.M.F. circulates a small current in the field circuit which increases the pole flux. When flux is increased, generated E.M.F is increased which farther increases the flux.
• OA is the induced E.M.F. due to residual magnetism & Oa flowing current.
• This current aids residual flux and hence provides a large induced E.M.F. Oa. In turn, this increased E.M.F Ob causes an even large current Oc which creates more flux for a still large E.M.F.
• Now the generated e.m.f in the armature has

1.                 Supply the ohomic drop IfRsh

2.     To oversees the opposing self induced e.m.f in the field coil.

If Rsh grater then Rc the generator will failed to excite.

CONDITIONS FOR BUILD-UP OF A SHUNT GENERATOR:

1.       There must be some residual magnetism in the generator poles.

2.       For the given direction of rotation the shunt field coils should be correctly connected to the armature.

3.       If excited on characteristics.

VOLTAGE REGULATION:

The voltage regulation of a DC generator is the change in voltage. When the load is reduced from rated value to zero expressed as a percentage of voltage regulation.

% VR = [VNL – VFL] / VFL

% VR = [VNL – VRL] / VRL

If , Voltage at no load                VNL = 240 V

Q: Why you refer less voltage regulation?

ANS
·    We prefer terminal voltage constant
·   want VFL high so %VR less.

ARMATURE REACTION:

By armature reaction is meant the effect of magnetic field set up by armature current on the distribution of flux under main pole of a generator.

Armature magnetic field has two effect;

demagnetizes or weakens the main flux. So the induced e.m.f in the armature is decreased.

It cross magnetize or distorts it.

Critical resistance is that resistance which is higher then the shunt field resistance. If the critical resistance is not higher then the shunt field resistance the generator is not possible to build up voltage.

After drawing OCC curve, Then tangent is drawn to its initial portion. The slope of the curve gives the critical resistance.

The value of the resistance represented by the tangent to the curve is known as critical resistance for a given speed.
 

USES OF DC GENERATOR:

1.      Shunt generator with field regulator are used for ordinary lightening and power supply.

2.      Series generators are not used for power supply because of the rising characteristics. Used in distribution system particularly in Railway scurvies.

3.       Compound wound generator is the most widely used DC generator. Because its external characteristics used in motor driven widely.

 

Q: Justify your logic a series generator is not widely used in power supply?

ANS:

Because there is too much losses i.e. armature loss. So that voltage loss is high.

If the series field ampere turn are such as to produce the same voltage at rated load as at no load then the generator is flat compound wound generator.


OVER COMPOUND WOUND:

If the series field ampere turn are such as to produce the voltage at rated load as grater then the no load then the generator is over compound wound generator.

UNDER COMPOUND WOUND:

If the series field ampere turn are such as to produce the voltage at rated load as laser then the no load then the generator is under compound wound generator
The field winding of the generator is disconnected from the machine and connected to an external DC power supply. The field or exciting current If is varied and value read on Ammeter A. The machine is derived at constant speed by the prime mover. Ifisincreased by suitable steps & the corresponding value of Eo are measured. On plotting the relation between If & Eo a curve is obtained and that is the O.C.C curve.
IMPORTANCE OF O.C.C CURVE:

1.       The O.C.C or No-Load saturated curves for Self-Excited generator whether shunt or series connected, are obtained in a similar way.

2.       O-A is called Residual voltage.

3.       Due to Residual magnetism in the poles, some e.m.f [=OA] is generated even when [If = 0]

4.       After point B saturation of poles starts.

5.       O.C.C for a higher speed would lie above this curve and vice versa.