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EXPERIMENT:12
AIM: Speed control of DC Shunt Motor using
a) Armature control and
b) Field control methods.
APPARATUS REQUIRRED:
Sr.no
1
2
3
4
5
Apparatus
Voltmeter
Ammeter
Ammeter
Rheostat
Tachometer
Range
(0-300)V
(0-5)A
(0-2)A
250 ohm/1.5A
Quantity
1
1
1
2
1
Fuse Rating:
40% of rated current
40*17/100=6.8≈10 A
THEORY:
(A)Flux Control method
The speed of the DC motor is inversely propositional to the flux per pole, when the
armature voltage is kept constant. By decreasing the flux the speed can be increased and vice –
versa. Hence the main flux of field control method the flux of a DC motor can be changed by
changing field current with help of a shunt field rheostat. Since shunt field current is respectively
small shunt field rheostat has to carry only a small amount of current which means I2R losses is
small so that rheostat is small in size .This method is very efficient.
(B)Armature Control method
This method is used when speed below the no load speed are required.As the supply
voltage is normally constsnt the voltage across the armature is varied by inserting a variable
rheostat in series with the armature circuit. As conductor resistance is increased potential
difference across the armature is decreased, herby decreasing the armature speed. F or a load of
constant torque speed is approximately propositional to the potential difference across the
armature.
PRECAUTIONS:
1. Armature rheostat must be kept at maximum resistance position.
2. Field rheostat should be kept at minimum resistance minimum position.
PROCEDURE:
(A)ARMATURE CONTROL METHOD:
1. Connect as per the circuit diagram
2.Close the DPST switch
3. Start the motor using three point starter
4.By keeping the field current(If) as constant value, adjust the armature rheostat and note
down the corresponding armature voltage and motor speed.
5.Repeat the step four till the motor reaches the rated speed.
(B)FLUX CONTROL METHOD:
1. Connect as per the circuit diagram
2.Close the DPST switch
3. Start the motor using three point starter
4.By keeping the armature voltage as constant value, adjust the field rheostat and note
down the corresponding field current and motor speed.
GRAPHS:
1.Field current Vs speed
2. Armature voltage Vs speed
CIRCUIT DIAGRAM:
OBSERVATION TABLE:
(A)ARMATURE CONTROL METHOD
SL.NO.
Field Current If =
Armature
Voltage
V
A
Speed N
RPM
1.
2.
3
4
5
6
(B)FIELD CONTROL METHOD
SL.NO.
1.
2.
3
4
5
6
CONCLUSION:
Armature Voltage Va=
V
Field Current
Speed N
If
RPM
A
2 graph pages