1) A separately excited DC motor has separate DC voltage supplies for the field coil and armature coil, allowing independent control of each. 2) Reversing the direction of rotation can be achieved by changing the polarity of either the field voltage or armature voltage, but not both. Reversing the field voltage is generally preferable due to lower field currents. 3) Experiments and MATLAB simulations were conducted to verify the direction of rotation under different polarity combinations of the field and armature voltages. The results supported that changing one or the other voltage reverses rotation, while changing both returns it to the original direction.

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S
Electrical Machines
Assignment #02
(Separately Excited DC Motor)
(Rotation Control)
RO
Engr : Dr. Shahid Atiq
Student: Hasnain Yaseen
Reg. No: EE171021

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SEPARATELY EXCITED DC MOTOR
Separately excited DC motors also have both stator and rotor. Stator is the static part of motor,
which consists of the field windings. And the rotor is the moving armature which contains
armature windings or coils. Separately excited dc motor has field coils similar to that of shunt
wound dc motor. The name suggests construction of this type of motor. Usually, for other DC
motors, the field coil and the armature coil both are energized from a single source. The field
of them does not need any separate excitation. But, in separately excited DC motor, separate
supply Provided for excitation of both field coil and armature coil. Figure below shows the
separately excited dc motor.
Here, the field coil & armature coil are energized from two separate DC voltage. Armature
voltage source may be variable but, constant DC voltage is used for energizing the field coil.
So, those coils are electrically isolated from each other in this motor, and this connection is the
specialty of this type of DC motor.
Task-1
For motor operation Observe the direction of rotation under following conditions
➢ change Polarity of IF and keep polarity of VA unchanged
➢ Change the polarity of VA and keep polarity of IF unchanged
➢ Change both polarity of IF and polarity of VA
THORETICAL PART
For a “standard” brushed DC motor, it is easiest to control the rotation of the
motor on a separately excited motor, where the field winding has separate
terminals to the (main) armature winding. This is typically done on a motor
designed for speed control. To reverse the motor, you need to change the polarity
of the supply voltage to either the field winding or the armature winding, but
not both. Generally it is better to reverse the field voltage because the field
current is less than the armature current, so your reversing switchgear is more
lightweight. If you cannot separate the field terminals and armature terminals,
then reversing cannot be done. As below in figure we can check that

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when we change the polarity of Field supply then the rotation of motor become
reverse(CASE :02)because when we change the polarity of Field then the force
that rotate the rotor of the motor acts in opposite direction. When we change the
polarity of of armature voltage then also the rotation of the rotor become
reverse(CASE :03).But when we change the both polarities then rotation of the
motor is the same as in (case :01) because when we change the polarity of both
field and armature then the force that rotate the armature acts in the same
direction as in conventional way(CASE :01).So by comparing CASE:01&CASE:04)
we can say that by change the polarity of both field and armature voltage the
direction of the rotation of motor remain same. Now lets check this theory
practically and by simulation at MATLAB.
Fig .(2.1)
EXPERIMENTAL PART
We check the rotation control of separately excited Dc Motor Experimentally and our final
results are recorded in the table (2.1).
Case :01
When polarity is in conventional way. When we connect the positive terminal of
Armature and IF to the positive of the DC supply and connect the negative end with the
negative end of the supply.
Then we observe that
➢ Field Current (IF) =0.28 with positive sign
➢ Armature Voltage (Va) =101V with positive sign
➢ The direction of the rotation of the rotor of motor is Clockwise.
Case :02
When we change Polarity of IF and keep polarity of VA unchanged.
Then we observe that
➢ Field Current (IF) = -0.27 with negative sign
➢ Armature Voltage (Va) =101V with positive sign

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➢ The direction of the rotation of the rotor of motor is Counter Clockwise.
Case :03
➢ When we Change the polarity of VA and keep polarity of IF unchanged.
Then we observe that
➢ Field Current (IF) = 0.27 with postive sign
➢ Armature Voltage (Va) = -101V with negtive sign
➢ The direction of the rotation of the rotor of motor is Counter Clockwise.
Case :04
When we Change both polarity of IF and polarity of VA.
Then we observe that
➢ Field Current (IF) = -0.26 with negative sign
➢ Armature Voltage (Va) = -101V with negative sign
➢ The direction of the rotation of the rotor of motor is Clockwise.
Table (2.1)
Sr NO:
Field Current (IF)
(A)
Armature Voltage (VA)
(V)
Direction of Rotation
Of Rotor
CASE :01 0.28 101 Clockwise
CASE: 02 -0.27 101 Counter Clockwise
CASE:03 0.27 - 101 Counter Clockwise
CASE:04 -0.26 -101 Clockwise

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MATLAB SIMULATION
CASE:01
When polarity is in conventional way. Then we can observe from graphs that
speed(Clockwise), Field Current & Armature Current all positive.

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CASE:02
When we change Polarity of IF and keep polarity of VA unchanged. Then we observe from
graphs that the speed(Counter Clockwise)& Field current are negative but Armature current
is still positive.

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CASE:03
When we Change the polarity of VA and keep polarity of IF unchanged.Then we can
observe from graphs that speed (Counter Clockwise)&Armature current are negative
but now field current is positive.

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CASE:04
When we Change both polarity of IF and polarity of VA. Then we can observe from
the graphs that Field current & Armature current are negative but now
speed(Clockwise) is positive .

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CONCLUSION:
So we can conclude from our experimental and MATLAB simulation results that
When polarity is in conventional way. Then speed (Clockwise), Field Current &
Armature Current all are positive. When we change Polarity of IF and keep polarity of
VA unchanged. Then the speed (Counter Clockwise)& Field current are negative but
Armature current is still positive. When we Change the polarity of VA and keep polarity
of IF unchanged. Then the speed (Counter Clockwise) &Armature current are negative
but now field current is positive. When we Change both polarity of IF and polarity of
VA. Then the Field current & Armature current are negative but now speed(Clockwise)
is positive .