Kathmandu University
Department of Electrical and Electronics Engineering
BASIC ELECTRICAL LABORATORY EXPERIMENTS (ENGG 103)
EXPERIMENT 6:- Speed control of separately excited DC motor using
armature voltage control
Objectives: 1. Study of dc motor speed control of separately excited DC motor using
armature control method.
Materials and Equipments Required:
i)
Voltmeter (0-300 V) DC
ii)
Tachometer
iii)
Ammeter (1-5 A) DC
iv)
DC motor
Theory:
In a DC motor the armature is fed by DC current. The flux produced by field (either
permanent magnet or field winding supplied by excitation dc) interacts with this armature
current to produce a repulsive torque which rotates the armature of DC motor. As
armature rotates the flux changes its magnitude and hence e.m.f. is induced on it. This
e.m.f. opposes the supplied voltage. This e.m.f. is called back e.m.f.
Back emf Eb= Ka Φ ω
In case of dc motor back e.m.f Eb is zero initially i.e. during starting. (Eb= Ka Φ ω i.e.
Eb α ω at starting ω= 0). Hence armature draws high current from supply during starting
if initially armature voltage is set high. This high current may cause sparking and damage
to motor. Due to high current the torque driving the armature is also high (T= KaΦIa
i.e.T α Ia) which may cause damage to the mechanical load connected to the motor. So
armature voltage is gradually increased to the rated value.
Control of speed can be understood with the motor equation
= Vs - IaRa
There are usually three methods commonly used to secure speed control are
i)
Armature voltage control
ii)
Armature resistance control and
iii)
Field Control
Experimental Setup:
Procedure:
i)
Measure the armature and field resistance using multimeter.
ii)
Connect the circuit as shown in figure 1.
iii)
Connect the voltmeter in parallel with supply and armature terminal of motor
and ammeter in series with armature of motor.
iv)
Input fixed 190 V DC supply to field winding of motor and variable DC supply
to the armature of motor.
v)
Note the armature current and speed for different armature voltage (DC) input.
Armature voltage control
i)
In this method the voltage applied to the armature terminal is controlled by the
variable voltage supply itself.
ii)
The voltage supplied is directly proportional to the speed of the motor
iii)
Thus the change in the supply voltage can adjust the speed of motor.
Observations:
Armature Voltage Control
Armature Resistance Ra= ……………………………………..
Field Resistance Rf= ………………………………………….
Field Current If= ………………………………………………
S.N.
Input DC voltage (V)
1
220
2
210
3
200
4
190
5
180
Conclusions and Inference:
Armature Current Back emf
(Ia)
Eb = V-IaRa
Speed (RPM)
Discussion
i)
ii)
iii)
iv)
v)
Comment in the observation with qualitative reasons for the speed control of
motor. Draw the graph of Speed vs. armature voltage and back emf vs.
armature voltage.
What happens if the additional resistance is added in series with armature?
Why at initial case motor is not supplied with direct rated voltage?
What will happen if the field circuit of the DC motor is open circuited while it
is running at rated speed?
Explain theory of separately excited DC motor.
Instructor’s Signature
Date
Performance Date
Submission