CONCEPT OF GENERAL TERMS PERTAINING TO ROTATING

MACHINES

Physical concept of force and torque production.

Concept of general terms pertaining to rotating machines.

Generated emf in full pitched and short pitched winding.

FLOW OF PRESENTATION

The force on a moving particle of electric q in a magnetic field is given by the Lorentz’s force law

F = q ( V × B ) Newtons Where, v = velocity motion of conductor, m/s

B = flux density, Tesla The force acting on a current carrying conductor

can be directly form the equation

PHYSICAL CONCEPT OF FORCE AND TORQUE PRODUCTION

l = length of conductor C = contour of the

conductor For a homogeneous conductor of length l carrying

current I in a uniform magnetic field, the above equation can be written as

In a rotating magnetic field, the torque about an axis is given by

Where, r = radius vector from the axis towards conductor

1) Electrical and mechanical degreeThere is a definite relationship between and mechanical rotation of a conductor in machine.

Electrical degrees It is the degree or the cycle of emf induced in the

conductor of a machine.

CONCEPT OF GENERAL TERMS PERTAINING TO ROTATING MACHINES.

TWO POLE MACHINE

VARIATION OF EMF WITH TIME ANGLE (ωt) IN 2-POLE MACHINE

VARIATION OF EMF INDUCED IN A CONDUCTOR WITH TIME ANGLE IN 4-POLE MACHINE

ARMATURE COIL

2) Conductor :The active length of wire in which emf is induced and embedded in the slot is called a conductor. In shows fig. two active conductors ‘ab’ and ‘cd’.

3) Overhang :The portion of coil which just provides a connection between the two active conductors and does not take part in energy conversion process is called overhang.

4) Coil :The two active conductors ‘ab’ and ‘cd’ along with overhang constitute one coil.

5) Pole pitch :It is the peripheral distance between the two adjacent poles.The pole pitch is always expressed in electrical degrees and it is always equal to 180° electrical.

6) Coil span :The distance between the two sides of a coil span.

7) Full pitchedThe coils having span which is equal to one pole pitch are called full pitched.

Short pitched coilIf the coils span is less than the pole pitch, the coils are called short pitched coils.

ADVANTAGES OF USING SHORT PITCHED COILS

1. Overhang will reduce i.e. save copper of end connections.

2. Distorting harmonics can be reduced or totally eliminated.

3. Due to reduction of high frequency harmonics, eddy current and hysteresis losses are reduced.

4. Iron losses decrease & efficiency will increase.

Disadvantage

1. total voltage around the coils is somewhat reduced.

2. because the voltages induced in the two sides of the short pitched coil are slightly out of phase, their resultant vectorial sum is less than their arithmetic sum.

Time for one revolution, dt =

Now, frequency of generated emf,

Speed of rotor , N =

Average generated emf/phase = 4f Ø Tph

1.11 Ep = 4.44 kw f Ø Tph ( kw = kc kp )

GENERATED EMF IN FULL PITCHED AND SHORT PITCHED WINDING

Let, Z = number of conductors or coil sides in series per phaseTph = no. of turns or coil per phaseØ = flux per pole in Weber P = number of rotor polesN = speed of rotor, rpm f = frequency of generated emf kc = coil span kp = pitch factor kd = distribution factor of stator winding kw = winding factor

REFERENCES :

DC MACHINES AND TRANSFORMERS BOOK ( DR. J.G. JAMNANI)

E DRAW MAX ( FOR DRAWING )

Thank You……