(SEM 7) THEORY EXAMINATION 2017-18 ELECTRIC DRIVES

SECTION A

(Short answers – explained clearly)

1. What is meant by dynamic torque?

Dynamic torque is the torque required to accelerate or decelerate a motor. It comes into action when the motor speed is changing. Dynamic torque is equal to the product of moment of inertia and angular acceleration. When a motor starts, stops, or changes speed, dynamic torque is required in addition to load torque. If the speed is constant, dynamic torque becomes zero.

2. What is meant by classes of duty of motor?

Classes of duty of a motor describe how the motor is loaded over time during its operation. Different applications require motors to operate continuously, intermittently, or for short periods. These duty classes help in selecting a suitable motor so that overheating and damage are avoided. Examples include continuous duty, short-time duty, and intermittent periodic duty.

SECTION B

(Medium answers – conceptual explanation)

3. Explain dynamic braking for a separately excited DC motor.

Dynamic braking is a method of stopping a DC motor by disconnecting it from the supply and connecting the armature to a braking resistor. The field supply remains unchanged. When the motor continues to rotate, it acts as a generator and produces electrical energy. This energy is dissipated as heat in the braking resistor, creating a braking torque that slows down the motor. Dynamic braking is simple and commonly used, but the energy is wasted as heat.

4. Explain regenerative braking for a separately excited DC motor.

In regenerative braking, the DC motor operates as a generator and feeds power back to the supply. This happens when the motor speed is higher than the no-load speed or when the load drives the motor. The direction of current reverses, but the torque remains opposite to the direction of rotation. Regenerative braking improves efficiency because energy is not wasted but returned to the power source. It is widely used in electric trains and elevators.

SECTION C

(Long answers – detailed explanation)

5. Explain different types of braking of an induction motor.

There are three main types of braking in induction motors: regenerative braking, dynamic braking, and plugging. In regenerative braking, the motor feeds energy back to the supply when operating above synchronous speed. In dynamic braking, the supply is disconnected and a DC current is injected into the stator, producing a stationary magnetic field that creates braking torque. In plugging, the phase sequence of the supply is reversed, producing a strong braking torque but causing high current and losses. Each method is used based on application requirements.

6. What are the components of load torque? Explain each.

Load torque consists of various components that oppose motor motion. Friction torque arises due to bearings and mechanical contact. Windage torque is caused by air resistance and increases with speed. Useful load torque is the torque required to perform the actual work, such as lifting or driving a machine. In some applications, torque may also depend on speed or position. Understanding load torque components helps in proper motor selection.