Electrical Drives (EX-801) - Important Questions

Draw the block diagram of electrical drives and explain the function of each block, including the converters.

Explain constant power and constant torque drives.

Explain steady-state / mechanical stability of an electric drive and why it depends on the relative motor and load torque-speed characteristics.

Compare AC and DC drives and state the advantages of electrical drives.

A motor equipped with a flywheel is to supply a load torque of 1000 N-m for 10 sec. followed by a light load period of 300 N-m long enough for the flywheel to regain its steady state speed. It is desired to limit the motor torque to 700 N-m. What should be the moment of inertia of flywheel? Motor has an inertia of 10 Kg-m$^2$. Its no load speed is 500 rpm and the slip at a torque of 500 N-m is 5%. Assume speed torque characteristic of motor to be a straight line in the region of interest.

Find equilibrium points from given motor and load torque equations and determine their steady-state stability.

For a DC shunt motor driving a constant-torque load, calculate the new speed when the supply voltage drops, given rated voltage, current, speed, and armature and field resistances.

Explain why field control is used for speeds above rated speed and armature voltage control is used for speeds below rated speed in DC motors.

For a wound rotor induction motor, find number of poles, slip, slip with doubled rotor resistance, and new rotor copper loss.

Explain why DC field application leads to pull-in to synchronism near synchronous speed but not at considerably lower speed.

Write short notes on any two of the followings:

Explain about the different speed torque characteristics of different machines and give their utility in section for industrial loads.

Explain with circuit diagram the principle and working of chopper control of a separately excited DC motor.

Explain the different types of braking employed in DC motors.

Find the firing angle of a single-phase fully controlled converter feeding a separately excited DC motor for given speed and torque condition.

Draw and explain the torque-speed characteristics for dynamic braking of a DC series motor and explain why torque becomes zero at final speed.

Define a DC chopper and describe the various types of chopper configurations with diagrams.

Sketch the voltage and current waveforms of a single-phase semi-converter fed DC series motor for a given firing angle.

Discuss phase-controlled and chopper-controlled DC drives.

Discuss about Energy Recovery System.

Explain V/f (variable frequency) control of three-phase induction motor with diagram, including why V/f is kept constant below base speed and voltage is kept constant above base speed, and range of speed control.

Explain the conventional / static Scherbius system/drive for speed control of the induction motor.

Write short notes (any two):

Numerical on stator voltage control of induction motor driving fan load to find terminal voltage, current and torque at reduced speed.

Describe the static Kramer drive for speed control of the induction motor.

For a 3-phase induction motor given direct-on-line starting current and torque ratios, determine the reduced applied voltage required to obtain full-load torque at starting and the corresponding starting current.

Explain with suitable block diagrams the various types of VSI controlled induction motor drives.

Numerical on induction motor operated from variable frequency supply (e.g., 80 Hz) to find motor speed at rated torque and slip at maximum torque.

Write short notes on any two of the followings.

Discuss conventional methods of starting, braking and speed control.

Discuss about current sources fed 1M drives.

Explain the working of VSI-fed synchronous motor drive with block diagram (closed-loop operation).

Speed control of self-controlled synchronous motor drives

Explain with the help of the construction, operation and switching scheme of Brushless D.C. motor.

Self-controlled and load commutated inverter (LCI) fed synchronous motor drives and their comparison

Numerical on variable-frequency V/f control of wound-field synchronous motor to calculate torque, power, armature and field currents and constant-power range.

Explain VSI-fed and CSI-fed synchronous motor drives.

Self-controlled brushless and commutatorless DC and AC motor drives

Explain the construction, principle of working and fundamentals of switched reluctance motor with neat diagram.

Explain digital control of electric drives with a block diagram.

Explain the working principle of PMSM. (Permanent Magnet Synchronous Motor) motors.

Describe the various electric drive schemes used in the cement industry.

Describe the various electric drive schemes used in the steel industry.

Numerical on synchronous reluctance motor to calculate load angle, line current and input power factor from $X_d$, $X_q$ and load torque.

Briefly describe how to control load angle in Stepper Motor.