Question

Mention general characteristics of DC shunt motor and its applications.

Hint:

When an application demands highly predictable, constant-speed operation without requiring extreme starting torque, the DC Shunt Motor is the standard choice.

Answer 1

Step 1: Analyzing DC Shunt Motor Excitation:
In a DC shunt motor, the field winding is connected in parallel (shunt) with the armature winding across a constant DC voltage supply. As a result, the shunt field current ($I_{sh} = V/R_{sh}$) remains constant, producing a practically constant magnetic flux ($\Phi \approx \text{constant}$).

Step 2: General Operating Characteristics:
The performance of a DC shunt motor is characterized by:

• Nearly Constant Speed: The speed equation is $N \propto \frac{V - I_a R_a}{\Phi}$. Because $\Phi$ is constant and the resistive drop $I_a R_a$ is tiny, the motor maintains a nearly constant speed from no-load to full-load conditions (dropping by only $2\%\text{--}5\%$).
• Linear Torque-Current Relationship: Torque is directly proportional to armature current ($T \propto \Phi I_a \propto I_a$). This creates a linear, predictable torque output.
• Medium Starting Torque: It provides moderate starting torque, making it unsuitable for extremely heavy starting loads.
• High Control Flexibility: Speed can be easily adjusted above base speed by adding resistance in the field circuit, or below base speed by adding resistance in the armature circuit.

Step 3: Industrial Applications:
Because of its constant speed and excellent speed-control characteristics, the motor is widely used in:

• Machine tools such as lathes, industrial drill presses, and milling machines.
• Centrifugal water pumps, blowers, and air compressors.
• Constant-speed fans and conveyor belts.