Fig. shows the connections of the DC shunt motor.
Since the motor is fed by the constant supply voltage, hence flux is constant, therefore-
NOTE: The flux in a DC shunt motor is approximately constant because the field current is constant since the field winding is connected directly to the constant supply voltage.
1. Torque and Armature current characteristics of DC Shunt Motor (Ta/Ia):
We know that in a d.c. motor,
Since the motor is fed by the constant supply voltage, hence flux is constant, therefore-
- Hence, Torque and Armature current characteristics (Ta/Ia) is a straight line and it is passing through the origin as shown in Fig below.
- The shaft torque is less than the armature torque which is shown by a dotted line. From the curve,
- It is clear that a very large current is required to start a heavy load. Therefore, a shunt motor should not be started on a heavy load.
2. Speed and Armature Current Characteristics of DC Shunt Motor (N/Ia)-
The speed N of a. d.c. motor is given by;- Under normal conditions, the flux and back emf of the DC shunt motor are constant. Therefore, the speed of a shunt motor remains constant after that by varying armature current.
- When the load is increased, Eb (= V - IaRa) and flux decrease due to the armature resistance drop and armature reaction respectively. However, back emf decreases slightly more than flux, therefore, the speed of the motor decreases slightly with the load.
3. Speed and torque characteristics of DC Shunt Motor (N/Ta)-
- The curve is obtained by plotting the values of the speed of a DC motor and Armature torque for different armature currents. It observed that speed falls as the load torque increases.
- From the characteristics, it is clear that the Speed and torque characteristic is approximately similar to the Torque and Armature current characteristics.
Conclusions from DC Shunt Motor Characteristics:
- There is a small change in the speed of a shunt motor from no-load to full-load. Hence, it is essentially a constant-speed motor.
- The starting torque is low because of torque proportional to the armature current.
FAQs on DC Shunt Motor Characteristics:
1. Why is the flux in a DC shunt motor approximately constant?
The flux in a DC shunt motor is constant because the field winding is connected directly to the constant supply voltage, ensuring a constant field current.
2. What is the Torque and Armature current characteristic (Ta/Ia) of a DC shunt motor?
The Torque and Armature current characteristic (Ta/Ia) of a DC shunt motor is a straight line passing through the origin. Since the motor is fed by a constant supply voltage, the flux is constant, and torque is proportional to the armature current.
3. Why should a DC shunt motor not be started on a heavy load?
A very large current is required to start a heavy load in a DC shunt motor. Since a high armature current can cause damage, it is not advisable to start the motor under a heavy load.
4. What is the Speed and Armature current characteristic (N/Ia) of a DC shunt motor?
The Speed and Armature current characteristic (N/Ia) shows that the speed of the motor decreases slightly with increased armature current, especially under load. This is due to a drop in back emf and slight reduction in flux.
5. How does load affect the speed of a DC shunt motor?
When the load on a DC shunt motor increases, the armature current increases, causing a slight decrease in speed due to the drop in back emf and armature reaction.
6. What is the Speed and Torque characteristic (N/Ta) of a DC shunt motor?
The Speed and Torque characteristic (N/Ta) demonstrates that speed decreases as torque (or load) increases. This relationship is similar to the Torque and Armature current characteristic.
7. Why is a DC shunt motor considered a constant-speed motor?
A DC shunt motor experiences only a small change in speed from no-load to full-load, making it essentially a constant-speed motor.
8. Why does a DC shunt motor have a low starting torque?
The starting torque of a DC shunt motor is low because torque is proportional to armature current, and at start-up, the current needs to increase substantially to generate torque.
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