Motor:-

• An electric motor is a device which converts an electric energy into a mechanical energy. This mechanical energy can be supplied to load. The motors can operate on d.c are known as d.c. motors. D.C. motors are classified into 3 types depending upon the connection of field winding with the armature winding. They are

1. D.C. Shunt motor
2. D.C. Series motor
3. D.C. Compound motor

D.C. Shunt motor:-

• In D.C. Shunt motor the field winding is connected across the armature winding and the combination is connected across the input supply voltage V. The digaram of D.C. Shunt motor is given below.

• Let Rsh be the resistance of shunt field winding and Ra be the resistance of armature winding. The resistance of armature winding i.e Ra is very small compared to the resistance of shunt field winding i.e. Rsh.

• As the field winding is connected across the armature winding and the combination is connected across the input supply voltage V, so the voltage across armature winding and field winding is equal to supply voltage V.
• From the above diagram the line current IL is the sum of shunt current Ish and armature current Ia and is given by

$I_L=I_{sh}+I_a$

• The shunt current Ish is given by

$I_{sh}=\dfrac{V}{R_{sh}}$

• The flux produced in the field winding is proportional to the shunt current Ish and is given by

$\phi \propto I_{sh}$

• In d.c. shunt motor as the supply voltage is constant , the shunt current Ish is constant which produces a constant flux. Hence d.c. shunt motor is sometimes called as constant flux motor.

Characteristics Of DC Shunt Motors

• Three characteristic curves are considered for DC Shunt Motors. They are

1. Torque vs. armature current (Ta - Ia)
2. Speed vs. armature current (N- Ia)and
3. Speed vs. torque(N-Ta).

• The characteristic curves for DC Shunt Motors are shown below

Torque - Armature Current characteristics:-

• For d.c. motors we have

$T \propto \phi I_a$

• In d.c. shunt motor as the supply voltage is constant , the shunt current Ish is constant which produces a constant flux. Thus the above equation is modified as follows

$T_a \propto I_a$

• That is armature torque is proportional to armature current. Hence the Ta-Ia characteristic for a dc shunt motor will be a straight line through origin. That is armature current increases linearly with armature current. But armature current depends on load. So as the load increases, armature current increases, increasing the torque linearly. Heavy starting load needs heavy starting current, shunt motor should never be started on a heavy load.
• Shaft torque is also plotted against armature current Ia. It is shown in dotted line. At no load Tsh=0 and increases linearly with armature current Ia.
• At any point shaft torque is always less than armature torque. The difference between armature torque and shunt torque for a given armature current is known as loss torque and is denoted as Tf.

Speed - Armature Current characteristics:-

• For d.c. motors we have

$N \propto \dfrac{V-IaRa}{\phi}$

• In d.c. shunt motor as the supply voltage is constant , the shunt current Ish is constant which produces a constant flux. Thus the above equation is modified as follows

$N \propto V-IaRa$

• As load increases the armature current increases and hence drop IaRa also increases. In d.c. shunt motor as the supply voltage is constant , V-IaRa decreases and hence speed reduces. As the resistance of armature winding i.e Ra is very small hence drop IaRa is also small as Ia varies from no load to full load. So the speed changes are also small as Ia varies from no load to full load.

Speed - Torque characteristics:-

• The Speed - Torque characteri