ei2301-ie_unit 4_part a and part b_with answers.pdf

7/27/2019 EI2301-IE_Unit 4_Part A and Part B_With answers.pdf

1/25

1

Unit IVDC and AC DRIVES1. What are drives and electrical drives?

Motion control is required in large number of industrial and domestic applications like

transportation systems, rolling mills, paper machines, and textile mills. Machine tools, fans,

pumps, robots washing machines etc. Systems employed for motion control are called drives

and the prime movers such as diesel or petrol engines, gas or steam turbines, steam engines,hydraulic motors and electric motors, for supplying mechanical energy for motion control

and drives employing electric motors are known as electrical drives.

2. What are the advantage and disadvantages of D.C. drives?The advantages of D.C. drives are,a. Adjustable speedb. Good speed regulationc. Frequent starting, braking and reversing.

The disadvantage of D.C. drives is the presence of a mechanical commutator, which

limits the maximum power rating and the speed.

3. Give some applications of D.C. drives.The applications of D.C. drives are,a. Rolling mills b. Paper mills

c. Mine winders d. Hoists

e. Machine tools f. Tractiong. Printing presses h. Excavators

i. Textile mils j. Cranes.

4. What is braking? Mention its types.The motor works as a generator developing a negative torque, which opposes the motion,

is called barking.

It is of three types. They are,

a. Regenerative braking.b. Dynamic or rheostat braking.c. Plugging or reverse voltage braking.

5. What are the three types of speed control?The three types of speed control as,a. Armature voltage control

b. Field flux controlc. Armature resistance control.

6. What is called continuous and discontinuous conduction?A D.C. motor is fed from a phase-controlled converter the current in the armature may

flow in discrete pulses is called discontinuous conduction.A D.C. motor is fed from a phase controlled converter the current in the armature may

flow continuously with an average value superimposed on by a ripple is called continuous

conduction.

7. Give the applications of induction motors drives.Although variable speed induction motor drives are generally expensive than D.C. drives,

they are used in a number of applications such as fans, blowers, mill run-out tables, cranes,

conveyors, traction etc., because of the advantages of induction motors. Other applicationsinvolved are underground and underwater installations, and explosive and dirty environments.

8. Mention the advantages of a converter fed indication motor over a line fed motor.A converter fed induction motor has the following advantages over line fed motor.a. Smooth start up is guaranteed by variable frequency starting from a low value.b. Soft starting and acceleration at constant current and torque are possible.


2/25

2

c. The network is no longer subjected to a high switching surge current as with thedirect switch On of cage induction motor, and as such, special starting equipment canbe omitted even at high ratings.

d. High moments of inertia can be accelerated without need to over dimension themotor.

e. The converter acts as a decoupling device. Therefore feedback from the motor to thepoint of short circuit does not take place, when line short circuits occur. The short

circuit rating on the basis of which the switchgear has to be over dimensioned is

therefore low, permitting a saving to be made.

9. How is the speed control by variation of slip frequency obtained?Speed control by variation of slip frequency is obtained by the following ways.

a. Stator voltage control using a three-phase voltage controller.b. Rotor resistance control using a chopper controlled resistance in the rotor circuit.c. Using a converter cascade in the rotor circuit to recover slip energy.d. Using a cycloconverter in the rotor circuit.

10. What is meant by V/f control?

When the frequency is reduced, the input voltage must be reduced proportionally so as tomaintain constant flux. Otherwise the core will get saturated resulting in excessive iron loss

and magnetizing current. This type of induction motor behavior is similar to the working

of dc series motors.

11.What is slip controlled drive?When the slip is used as a controlled quantity to maintain the flux constant in the motorthe drive is called slip conrolled drive. By making the slip negative (i.e., decreasing theoutput frequency of the inverter) The machine may be made to operate as a generator and the

energy of the rotating parts fed back to the mains by an additional line side converter or

dissipated in a resistance for dynamic barking. By keeping the slip frequency constant,braking at constant torque and current can be achieved. Thus braking is also fast.

12. How is the D.C. dynamic braking is obtained?D.C. dynamic barking is obtained when the stator of an induction motor running at a

speed is connected to a D.C. supply. D.C. current flowing though the stator produces astationary magnetic field. Motion of rotor in this field induces voltage in the rotor winding.

Machine, therefore, works as a generator. Generated energy is dissipated in the rotor circuitresistance, thus giving dynamic barking.

13. What is meant by regenerative braking?Regenerative braking occurs when the motor speed exceeds the synchronous speed.

In this case,the induction motor would run as the induction generator is converting the

mechanical power into electrical power, which is delivered back to the electrical system. Thismethod of braking is known as regenerative braking.

14.Why the drive motor must be slightly over dimensioned in slip energy recoveryscheme?

The losses in the circuits cause a slight reduction in efficiency. This affiance is furtheraffected by additional losses due to the non-sinusoidal nature of the rotor current. Therefore

the drive motor must be slightly over dimensioned. The motor used must have a rating 20%higher than the required power.

15.How is super synchronous speed achieved?Super synchronous speed can be achieved if the power is fed to the rotor from A.C.

mains. This can be made possible by replacing the converter cascade by a cycloconverter. Acycloconverter allows power flow in either direction making the static sherbiuss drive

operate at both sub and super synchronous speeds.


3/25

3

Part B

1. Explain the operation of a single phase fully controlled converter fed separately excited DCmotor with neat waveforms and derive the speed torque characteristics.


4/25

4


5/25

5


6/25

6


7/25

7


8/25

8


9/25

9


10/25

10


11/25

11

2. Deduce an expression relating speed and torque of a single phase full converter fedseparately excited DC motor drive operating in the continuous current mode and

discontinuous modes

.same answer of the previous question

3. Explain the motoring and braking operation of three phase fully controlled rectifier control ofDC separately excited motor with aid of diagrams and waveforms. Also obtain the expression

for motor terminal voltage and speed.


12/25

12


13/25

13


14/25

14

4. Explain in detail the working of a multi quadrant control of chopper fed DC series motor


15/25

15


16/25

16


17/25

17

5. With necessary diagram, explain the theoretical principles of stator voltage control


18/25

18


19/25

19


20/25

20


21/25

21

6. Derive an expression for the torque of an inverter fed three phase induction motor when it isoperated with V/F control. Show that the maximum torque remains unaltered in this scheme.


22/25

22

Low speed.


23/25

23

7. Explain static rotor resistance control in closed loop speed control


24/25

24


25/25

ei2301-ie_unit 4_part a and part b_with answers.pdf

Documents