(SEM VII) THEORY EXAMINATION 2018-19 ELECTRIC DRIVES

SECTION – A

(Short answer questions – brief explanations)

1. What is an Electric Drive?

An electric drive is a system that converts electrical energy into mechanical energy to control the motion of a machine. It consists of an electric motor, power converter, controller, and load. Electric drives are widely used because they offer precise speed control and high efficiency.

2. Advantages of Electric Drives

Electric drives provide smooth speed control, high efficiency, reliability, and easy automation. They require less maintenance compared to mechanical drives and are suitable for a wide range of industrial applications.

3. What is Load Torque?

Load torque is the torque required by the driven machine to perform useful work. The motor must supply sufficient torque to overcome load torque and losses for smooth operation.

4. Define Duty Cycle

Duty cycle represents the time sequence of operation of a motor under load and no-load conditions. It helps in selecting a motor with the correct power rating.

5. What is Starting Torque?

Starting torque is the torque developed by a motor at zero speed. It must be high enough to overcome inertia and static friction of the load.

6. What is Braking in Electric Drives?

Braking is the process of stopping or reducing the speed of a motor. It can be mechanical or electrical. Electrical braking methods include regenerative, dynamic, and plugging.

7. What is Speed Control?

Speed control is the process of maintaining or changing the speed of a motor according to load and operating conditions. It improves performance and energy efficiency.

8. What is Moment of Inertia?

Moment of inertia represents the resistance offered by a rotating body to change in speed. Higher inertia requires more torque for acceleration or deceleration.

9. Define Regenerative Braking

Regenerative braking occurs when a motor acts as a generator and feeds energy back to the supply during deceleration, improving system efficiency.

10. Applications of Electric Drives

Electric drives are used in fans, pumps, conveyors, cranes, elevators, rolling mills, electric vehicles, and machine tools.

SECTION – B

(Medium answer questions – explain clearly)

1. Types of Electric Drives

Electric drives are classified as group drive, individual drive, and multi-motor drive.
Group drive uses one motor to run several machines. Individual drive uses one motor for one machine, offering better control. Multi-motor drive uses multiple motors for different parts of the same machine, common in rolling mills and cranes.

2. Electric Braking Methods

Electrical braking improves safety and control.
In regenerative braking, energy is returned to the supply.
In dynamic braking, energy is dissipated as heat in resistors.
In plugging, motor connections are reversed to produce braking torque.
Each method is selected based on efficiency and application needs.

3. Characteristics of Load Torque

Load torque may be constant, proportional to speed, or proportional to square of speed. For example, conveyors have constant torque, fans have torque proportional to speed squared, and machine tools have variable torque depending on operation.

4. Thermal Effects in Electric Drives

Heat is produced due to losses in motor windings and mechanical friction. Excessive temperature reduces insulation life. Proper cooling and duty cycle selection are important to ensure safe operation.

SECTION – C

(Long answer questions – detailed explanation)

Q3(a) Components of Load Torque

Load torque is made up of different components that act on the motor shaft.
Friction torque is required to overcome bearing and mechanical friction and is always present.
Windage torque occurs due to air resistance and increases with speed.
Useful torque is required to perform actual work such as lifting, pumping, or cutting.
 

Inertia torque is needed to accelerate or decelerate rotating parts and depends on moment of inertia and rate of change of speed.

The total load torque is the sum of all these components and determines motor selection and control strategy.

Q3(b) Induction Motor with Flywheel (Numerical Concept)

In this problem, a 6-pole, 50 Hz, 3-phase wound rotor induction motor is connected to a flywheel. The motor speed is calculated using synchronous speed formula.
 

The stored energy in the flywheel depends on moment of inertia and angular speed.
During load fluctuation, the flywheel supplies or absorbs energy, reducing speed variation.
Such systems are used where load varies suddenly, like rolling mills and presses.