(SEM VII) THEORY EXAMINATION 2022-23 ELECTRIC DRIVES

SECTION A – Short Answers (2 Marks Each)

(a) Classification of mechanical loads based on speed–torque characteristics

Mechanical loads are classified as constant torque, torque proportional to speed, torque proportional to square of speed, and constant power loads.

(b) Constant power operation

In constant power operation, output power remains constant while speed varies, so torque changes inversely with speed.

(c) Steady-state stability

A drive is steady-state stable if a small disturbance in speed or load produces restoring torque that brings the system back to its operating point.

(d) Heating time constant of motor

It is the time required for motor temperature to reach about 63.2% of its final steady-state temperature after load application.

(e) Methods of reducing energy loss during starting

Reduced voltage starting, star-delta starting, auto-transformer starting, rotor resistance control, and electronic soft starters.

(f) Advantages and disadvantages of electrical braking

Advantages include smooth braking and energy recovery. Disadvantages include additional equipment cost and complexity.

(g) Power electronic control methods of AC motors

AC voltage control, variable frequency control (V/f control), cycloconverter control, and PWM inverter control.

(h) Thyristor control vs Ward Leonard system

Thyristor control is preferred due to higher efficiency, lower maintenance, compact size, and better dynamic response.

(i) Cycloconverter

A cycloconverter directly converts AC power at one frequency to AC power at a lower frequency without DC link.

(j) Variable frequency control giving high torque/current ratio

Because flux is maintained constant by keeping V/f constant, high starting torque is achieved with lower current.

SECTION B – Long Answers (10 Marks Each)

(a) Motors employed in electric drives

Electric drives use DC motors (series, shunt, compound), induction motors (squirrel cage and slip ring), synchronous motors, brushless DC motors, and switched reluctance motors. Selection depends on speed control, torque requirement, efficiency, and application.

(b) Loading of electric motor and duty cycle

Motor loading refers to variation of load torque with time. Duty cycles include continuous, short-time, intermittent, periodic, and variable load duty. Proper selection ensures thermal safety.

(c) Dynamic braking of DC series motor

In dynamic braking, motor is disconnected from supply and connected to braking resistance. Speed-torque characteristic shows torque decreasing with speed and becoming zero at finite speed due to residual flux reduction.

(d) DC separately excited motor with 3-phase full converter

In continuous conduction mode, average armature voltage is controlled by firing angle. Speed is controlled by varying armature voltage while maintaining constant field current.

(e) Static Scherbius drive

It recovers slip power from rotor circuit of slip-ring induction motor using rectifier-inverter system, improving efficiency and enabling speed control below synchronous speed.

SECTION C – Long Answers (10 Marks Each)

(3a) Multi-quadrant operation of electric drive

Electric drives operate in four quadrants representing forward motoring, forward braking, reverse motoring, and reverse braking. This enables bidirectional speed and torque control.

(3b) Advantages and applications of drive systems

Drive systems offer precise speed control, high efficiency, automation, and energy saving. Applications include electric traction, elevators, rolling mills, CNC machines, and robotics.

(4a) Regenerative braking of separately excited DC motor

In regenerative braking, motor acts as generator, converting mechanical energy to electrical energy and feeding it back to the supply when back EMF exceeds supply voltage.

(4b) Energy loss during starting of induction motor

Energy loss during starting equals energy supplied minus kinetic energy gained. Loss is reduced using reduced voltage starters, rotor resistance control, and soft starters.