Why does a DC shunt motor have low starting torque?

A DC shunt motor has low starting torque because torque is proportional to armature current, and the armature current at start is limited to protect the motor from damage.

Characteristics of DC Shunt Motors

Q: What is the Torque and Armature Current characteristic (Ta/Ia) of a DC shunt motor?

The Torque and Armature Current characteristic of a DC shunt motor is a straight line passing through the origin. Since the flux is constant, torque is directly proportional to armature current (T ∝ Ia).

Q: Why should a DC shunt motor not be started on a heavy load?

A DC shunt motor requires a very large armature current to start under heavy load. This excessive current can damage the motor, so it should not be started with heavy load connected.

Q: What is the Speed and Armature Current characteristic (N/Ia) of a DC shunt motor?

The Speed and Armature Current characteristic shows that the speed of a DC shunt motor decreases slightly as armature current increases. This happens due to a small drop in back EMF and flux under load conditions.

Q: How does load affect the speed of a DC shunt motor?

When load increases, the armature current increases, causing a slight reduction in back EMF and flux due to armature reaction. As a result, the speed of a DC shunt motor decreases slightly.

Q: What is the Speed and Torque characteristic (N/Ta) of a DC shunt motor?

The Speed and Torque characteristic of a DC shunt motor shows that speed decreases gradually as torque or load increases. This characteristic is similar in nature to the Torque and Armature Current characteristic.

Q: Why is a DC shunt motor considered a constant-speed motor?

A DC shunt motor is considered a constant-speed motor because its speed varies only slightly from no-load to full-load due to nearly constant flux.

In DC Shunt motor, the field winding is connected in parallel (shunt) with the armature winding. Because the field winding is connected directly across the supply, it receives almost constant voltage, resulting in nearly constant speed under varying load conditions.
This property makes the DC shunt motor very useful where speed regulation is important.

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.

Speed Equation of DC Shunt Motor

The speed of a DC motor is given by:

N \propto \frac{E_{b}}{Φ​​}

Where:

N = Speed of motor

Eb = Back EMF

Φ = Flux per pole

In a shunt motor the magnetic flux is practically constant because the field winding is connected in parallel (shunt) directly across a constant voltage supply.

👉 Speed remains nearly constant even when load changes.

Torque Equation

Torque in a DC motor is given by:

T \propto Φ I_{a}

Since Flux is constant in DC Shunt Motor:

T \propto I_{a​}

👉 Torque is directly proportional to armature current.

1. Torque and Armature current characteristics of DC Shunt Motor (Ta/Ia)

Torque is Proportional to Armature Current. Since the motor is fed by a constant supply voltage, flux is constant, therefore,

T \propto I_{a}

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:

N \propto \frac{E_{b}}{Φ}

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 the armature current.

dc-shunt-motor-speed-armature-current-characteristics

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 observes that speed falls as the load torque increases.

dc-shunt-motor-speed-torque-characteristics

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.
It used where constant speed is required, irrespective of load variations. Due to its good speed regulation and smooth operation, it is preferred in many industrial and commercial applications.

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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