1
Ex.No. DATE:
OPEN CIRCUIT CHARACTERISTICS OF SELF EXCITED
DC SHUNT GENERATOR
AIM:
To obtain open circuit characteristics of self excited DC shunt generator and to find
its critical resistance.
APPARATUS REQUIRED:
S.No. Apparatus Range Type Quantity
1 Ammeter (0-2)A MC 1
2 Voltmeter (0-300)V MC 1
3 Rheostats 220, 1.2A Wire Wound 2
4 Tachometer - Digital 1
5 Connecting Wires 2.5sq.mm. Copper Few
PRECAUTIONS:
1. The field rheostat of motor should be in minimum resistance position at the time of
starting and stopping the machine.
2. The field rheostat of generator should be in maximum resistance position at the
time of starting and stopping the machine.
3. SPST switch is kept open during starting and stopping.
THEORY:
The O.C.C is a curve showing the relationship between the no load emf generated
and the shunt field current (Eo and If ). Even when the field current is zero there is some
residual magnetism present in the poles. Hence there is a small voltage generated even at
zero field current, which is called the residual voltage. As the field current is increased, Eo
also increases and the curve traced is almost a straight line. As If is further increased the
poles start getting saturated, the straight line relation no longer holds good and the curve
bends and becomes almost horizontal.
, 2
In a D.C. generator, for any given speed, the induced emf in the armature is directly
proportional to the flux per pole.
Eg =(ΦZNP)/60A Volts
Where Ф is the flux per pole in webers,
Z is the no. of conductors in the armature,
N is the speed of the shaft in rpm,
P is the no. of poles and
A is the no. of parallel paths.
A = 2 (wave) A = P (lap)
CRITICAL RESISTANCE:
It is that value of resistance in the field circuit at which the generator will just excite
(or voltage build up begins). If the resistance is higher, the machine will fail to build up
voltage. It is given by the slope of the tangent drawn to the linear portion of the
magnetization curve from the origin.
Conditions for voltage build up in a d.c shunt generator
1. There should be some residual magnetism in the poles.
2. For the given direction of rotation, the shunt field coils should be properly connected.
That is, The coils should be so connected that the flux generated by the field current aids
the residual flux.
3. When excited at no load, the shunt field circuit resistance should be less than the critical
resistance.
CRITICAL SPEED:
It is that value of speed at which the given shunt field resistance represents the
critical resistance. It is determined as follows. For the same value of If determine E1 and
E2 from the field resistance lines. Then
E1/E2 =N1/Nc ; Nc = (E2/E1)N1
Where, Nc is the Critical speed
, 3
PROCEDURE:
1. Connections are made as per the circuit diagram.
2. After checking minimum position of motor field rheostat, maximum position of
generator field rheostat, DPST switch is closed and starting resistance is gradually
removed.
3. By adjusting the field rheostat, the motor is brought to rated speed.
4. Voltmeter and ammeter readings are taken when the SPST switch is kept open.
5. After closing the SPST switch, by varying the generator field rheostat, voltmeter
and ammeter readings are taken.
6. After bringing the generator rheostat to maximum position, field rheostat of motor
to minimum position, SPST switch is opened and DPST switch is opened.
, 4
TABULAR COLUMN:
Field Current Generated Voltage
S.No.
If (Amps) Eg (Volts)
RESULT:
Thus open circuit characteristics of self excited DC shunt generator are obtained
and its critical resistance is determined.
Ex.No. DATE:
OPEN CIRCUIT CHARACTERISTICS OF SELF EXCITED
DC SHUNT GENERATOR
AIM:
To obtain open circuit characteristics of self excited DC shunt generator and to find
its critical resistance.
APPARATUS REQUIRED:
S.No. Apparatus Range Type Quantity
1 Ammeter (0-2)A MC 1
2 Voltmeter (0-300)V MC 1
3 Rheostats 220, 1.2A Wire Wound 2
4 Tachometer - Digital 1
5 Connecting Wires 2.5sq.mm. Copper Few
PRECAUTIONS:
1. The field rheostat of motor should be in minimum resistance position at the time of
starting and stopping the machine.
2. The field rheostat of generator should be in maximum resistance position at the
time of starting and stopping the machine.
3. SPST switch is kept open during starting and stopping.
THEORY:
The O.C.C is a curve showing the relationship between the no load emf generated
and the shunt field current (Eo and If ). Even when the field current is zero there is some
residual magnetism present in the poles. Hence there is a small voltage generated even at
zero field current, which is called the residual voltage. As the field current is increased, Eo
also increases and the curve traced is almost a straight line. As If is further increased the
poles start getting saturated, the straight line relation no longer holds good and the curve
bends and becomes almost horizontal.
, 2
In a D.C. generator, for any given speed, the induced emf in the armature is directly
proportional to the flux per pole.
Eg =(ΦZNP)/60A Volts
Where Ф is the flux per pole in webers,
Z is the no. of conductors in the armature,
N is the speed of the shaft in rpm,
P is the no. of poles and
A is the no. of parallel paths.
A = 2 (wave) A = P (lap)
CRITICAL RESISTANCE:
It is that value of resistance in the field circuit at which the generator will just excite
(or voltage build up begins). If the resistance is higher, the machine will fail to build up
voltage. It is given by the slope of the tangent drawn to the linear portion of the
magnetization curve from the origin.
Conditions for voltage build up in a d.c shunt generator
1. There should be some residual magnetism in the poles.
2. For the given direction of rotation, the shunt field coils should be properly connected.
That is, The coils should be so connected that the flux generated by the field current aids
the residual flux.
3. When excited at no load, the shunt field circuit resistance should be less than the critical
resistance.
CRITICAL SPEED:
It is that value of speed at which the given shunt field resistance represents the
critical resistance. It is determined as follows. For the same value of If determine E1 and
E2 from the field resistance lines. Then
E1/E2 =N1/Nc ; Nc = (E2/E1)N1
Where, Nc is the Critical speed
, 3
PROCEDURE:
1. Connections are made as per the circuit diagram.
2. After checking minimum position of motor field rheostat, maximum position of
generator field rheostat, DPST switch is closed and starting resistance is gradually
removed.
3. By adjusting the field rheostat, the motor is brought to rated speed.
4. Voltmeter and ammeter readings are taken when the SPST switch is kept open.
5. After closing the SPST switch, by varying the generator field rheostat, voltmeter
and ammeter readings are taken.
6. After bringing the generator rheostat to maximum position, field rheostat of motor
to minimum position, SPST switch is opened and DPST switch is opened.
, 4
TABULAR COLUMN:
Field Current Generated Voltage
S.No.
If (Amps) Eg (Volts)
RESULT:
Thus open circuit characteristics of self excited DC shunt generator are obtained
and its critical resistance is determined.
