Basic principle of electric machines
Electric machines convert mechanical energy to electrical energy and vice versa.
Generator:
Function- Converts mechanical energy to electrical energy.
Principle- Generation of e.m.f. in a moving conductor in magnetic field.
Motor:
Function- Converts electrical energy to mechanical energy.
Principle- Force created due to interaction between magnetic field.
Machines require varying flux for their operation
Example: E.m.f. in a conductor is not induced if there is no change of flux associated
with it.
From Faraday’s laws of electromagnetic induction, induced e.m.f. in a conductor is
proportional to the rate of change of flux associated with it.
Categorizing machines with respect to variation of flux
Flux is only time varying- Transformer
Flux is only space varying- DC Machine
Flux is both time and space varying- Induction Motor
Construction of a DC Machine
-1-
Figure 1. Generalized construction of a DC Machine.
Functions of different parts of the DC Machine
Yoke-
-Stationary part of the machine.
-Made up of cast steel.
-Placeholder for main field poles and interpoles.
-Gives an easy path for magnetic field lines to complete.
-Protects all the inner parts of the machine.
Main Pole-
-Stationary part of the machine.
-Remains attached to the yoke.
-Produces the main magnetic field.
Armature-
-Rotating part of the machine.
-Made of laminated sheets.
-Holds the armature winding.
-Produces electrical energy from mechanical energy in case of a generator.
In this case, the armature is rotated by external source and the armature
winding generates e.m.f.
-Produces mechanical energy from electrical energy in the case of a motor.
In this case, armature winding is given a current supply from outside. The
armature rotates and produces mechanical energy.
Commutator- -Rotating part of the machine.
-Made from separated copper bars insulated from each other.
-Attached with an armature shaft.
-Carries current to the brush from armature windings in case of generator.
-Carries current to the armature windings from brush in case of motor.
Brush-
-Stationary part of the machine.
-Made up carbon.
-2-
-Remains pressed with rotating commutator to establish electrical
connection between rotating part.
Interpole-
-Stationary part of the machine.
-Remains attached to the yoke.
-Produces a magnetic field that opposes the armature field.
-Reduces sparking at commutator surface.
Armature winding
Armature winding is the method to wind the conductors over the armature core.
Windings are connected in parallel to increase current rating. Windings are connected in
series to increase voltage rating. Two types of windings are mainly encountered:
Lap winding- Here the armature winding is such that part of each winding overlaps a part
of another winding.
For lap winding: Number of parallel paths (a) = Number of poles (P) in a machine
Wave winding- Here the armature winding is like a wave pattern.
For wave winding: No of parallel path (a) = 2
Generator e.m.f. equation
Let a P pole machine has φ webers as its flux per pole
The total flux cut by an armature conductor in one revolution is Pφ webers.
Let the machine rotate at n revolutions per second.
So, e.m.f. generated in one conductor = Pφ n volts
Let the total number of conductors be Z
So e.m.f. generated = Pφ n Z volts
Let the number of parallel paths be a
=
So, e.m.f. generated per parallel path
Z
Pφ n
a
volts
-3-
The total e.m.f. generated E a equals e.m.f. generated per parallel path
Z
= Pφ n
volts
Therefore, E a a
Again, let the armature rotates at a speed of N rpm.
N
So n = 60
Z
N
= Pφ
E
60 volts. This is the expression of the generator e.m.f.
Then a a
Concept of voltage build up in a generator
-Generators require strong field flux for generation of sufficient electrical power
-Strong field flux is created by electromagnets, i.e. sending current through field winding
of field poles.
-Current to the field is obtained by the generator itself, i.e. from e.m.f. generated in the
armature
-But to generate e.m.f. in the armature, field flux must exist
-This problem is overcome by what is called voltage build up
-A residual magnetism exists in the field poles which is small in magnitude
-This weak field flux creates small e.m.f. in armature when generator is started
-The small e.m.f. causes a relatively larger field current to flow causing a larger field flux
-This larger field flux creates a larger e.m.f. in armature
-The loop repeats and voltage gradually builds up to maximum value limited by the
construction, resistances of coils and the speed of operation
Points to remember in the above concept
-Voltage can only build up if residual field can generate armature e.m.f. of proper
polarity to increase the field
-The loading in case of shunt generator must not be above a critical value
-4-
Field connections in generator
Case 1- Separately excited field
A separately excited generator supplying a load.
Here the field coil is excited from a separate source
The field flux does not depend on the armature e.m.f. generated.
Voltage build up always takes place and not dependent on loading
Here, let the
e.m.f. generated in the armature = E g volts
Resistance of armature winding = R a ohms
Armature current
= I a amps (This is the current through the circuit)
Resistance of load
Voltage across the load
= R L ohms
= V volts ( This is the voltage we get from generator)
-5-
Voltage supplied to the field
Field current
= V f volts
= I f amperes
Now, voltage drop due to armature resistance = I a R a volts
Then, E g - I a R a = V (1)
And I a = I L as load current is the same as armature current.
Thus the equation (1) can be modified as
V= - R a I L + E g
This equation is called the external or the load characteristics of the generator.
So, a curve of I L vs V will be a straight line with a negative slope.
Figure showing the load characteristics of a DC separately excited generator
Case 2- Shunt field
A shunt generator supplying a load.
Here the shunt field coil is excited from the armature
The field flux depends on the armature e.m.f. generated.
-6-
Voltage build up takes place only when load resistance is above a critical value
Here, let the
e.m.f. generated in the armature = E g volts
Resistance of armature winding = R a ohms
Armature current
= I a amps (This is the current through the circuit)
Resistance of load
Load current
Voltage across the load
= R L ohms
= I L ohms
= V volts (This is the voltage we get from generator)
Resistance of the field
Field current
= R f ohms
= I f amps
Now, voltage drop due to armature resistance = I a R a volts
Voltage across field = I f R f volts
Voltage across load = I L R L volts
Now, E g - I a R a = I f R f = V= I L R L (1)
Ia = If + IL
(2)
From the above equations, load characteristics of the shunt generator can be found as
follows.
V = Eg – ( If + IL) Ra
=– R a I L +( E g - I f R a )
Now,
E g ∝ φ, φ∝ If, therefore , E g ∝ If
Always remember that field current generates the field flux.
Then, V =– R a I L +( k - R a ) I f
k is a constant depending on the characteristics of the generator.
Thus, the characteristic curve under load can be divided into two parts.
The first part is under light load where field current is constant. This represents a straight
line with a negative slope.
In the later part, the load increases and the field current is diverted to the load, so that I f
decreases. Then, the curve deviates from the linear part as shown.
-7-
Figure showing the load characteristics of a DC shunt generator
Case 3- Series field
A series field generator supplying a load.
Here the series field coil is excited from the armature itself
The field flux depends on the armature e.m.f. generated.
Voltage build up takes place only when load is connected
Here, let the
e.m.f. generated in the armature = E g volts
Resistance of armature winding = R a ohms
-8-
Armature current
= I a amps (This is the current through the circuit)
Resistance of load
Voltage across the load
= R L ohms
= V volts (This is the voltage we get from generator)
Resistance of the field = R f ohms
Now, voltage drop due to armature resistance = I a R a volts
Voltage across field = I a R f volts
Then, from the circuit of the series generator, V =E g - I a R a - I a R f
(1)
Now, (i) E g ∝ φ, φ∝ Ia, therefore , E g ∝ Ia Always remember that field current generates
the field flux. Here in series circuit armature current flows through the field.
(ii) I a = I L
Putting these values of (i) and (ii) in equation (1), we get
K a is constant depending on a particular generator.
V = K a I L –( R a + R f ) I L
This equation is called the load characteristics of the series generator.
Initially, K a and R a +R f is constant, so V∝ IL . This represents the first part of the curve.
In the next part, φ becomes constant due to saturation, so that I L = constant. This
represents the peak part of the curve with a small flat top.
Next, I L = constant and ( R a + R f ) increases due to heating effects.
Thus
V = K 2 – ( R a + R f ) K 3 where K n are constants. This represents the later drooping part
of the curve.
Figure showing the load characteristics of a DC series generator
Compound field generator
-9-
Here, both the series and shunt field exist in a generator. If the series field and shunt field
opposes each other, it is called differential compound field generator. If the series and
shunt field supports each other, it is called cumulative compound field generator.
- 10 -