# WBUT Question Papers EE

## Electric Drives B Tech 7^{th} Sem Dec 2008

Time : 3 Hours ]

[ Full Marks : 70

GROUP – A (Multiple Choice Type Questions)

Choose the correct alternatives for any ten of the following : 10 x 1 * 10

1) To get speed higher than the base speed of a D.C. shunt motor

**a) **armature voltage control Is used

**b) **field control is used

**c) **armature resistance control Is used

**d) **frequency control is used.

0) A typical passive load is

a) Holst b) Friction

**c) **Blower ■ d) Pump.

Hi) Intermittent duty rating of an electric motor

**a) **is equal to name plate rating

**b) **is less than name plate rating

**c) **is greater than name plate rating ‘

**d) **has no bearing to its name plate rating.

Iv) In constant torque drive

**a) **power is proportional to the speed

**b) **power is proportional to the square pf speed

**c) **power is inversely proportional to the speed

**d) **power is independent of speed.

«^nC»(B)/nEIC-7/BB-701/06/«)9) 4

**v) **A drive l|a9 following parameters :

. J« 10 kg-m * T_{m}– 100-01 N. N^n. ,

T _{L} ( passive ) = 0 05 N, N-m.wheretrt* speed in rpm.

. Then the steady state speed Is

a} 700 rpm b) 800 rpm

c) 667 rpm , d) 680 rpm. |

**vi) **Regenerative braking is a v ‘ s ‘ a) first quadrant (T-w) operation

; b) second quadrant operation

**c) **multiquadrant operation >

**d) **third quadrant operation. ____

vfi) The slip of an induction motor during d.c. rheostatic braking is

a) s b) 2 – s

c) 1-s d) none of these.

viii) A three phase induction motor having a combination of diode rectifier & line commutated inverter in rotor circuit, can give

**a) **speed below synchronous speed only

**b) **speed above synchronous speed only

**c) **both sub-& super-synchonous speed

**d) **no change in speed. •

ix} Which operation is not possible for semi-converter fed D.C. drive system ?

**a) **Ilnd quadrant (V-I) b) III quadrant

c) IVth quadrant d) All of these. |

x) The value of co-efficient of adhesion will be high when rails are

**a) **greased b) wet

**c) **sprayed with oil d) none of these.

xl) The speed time curve for a local train Is shown In figure below.
^ <srrS. |

In this AB represents

**a) **coasting

**c) **braking

xil) The common method of speed control used In 25 kV, 50 Hz, 1-phase traction system Is

**a) **tap changing control b) reducing current method

**d) **none of these.

**c) **series parallel method

GROUP -B ( Short Answer Type guestions )

Answer any three of the following.

What do you mean by group. Individual & multimotor drives ?

Deduce the condition for steady state stability of a motor load combination. Can this condition be applied for synchronous motor ? 4+1

A drive has the following parameters :

T = 150-0-1 N, N-m, where N Is the speed in rpm.

Load torque T _{L} = 100 N-m.

Initially the drive is operating at steady state. The characteristics of the load torque are changed to T _{L} = – 100 N-m. Calculate the initial & final equilibrium speeds.

Deduce the expression of loss of energy during stating of a separately excited D.C. motor ’

GROUP – C (Umg Answer Type Questions)

Answer any three questions.

**7.**a) With the help of relevant torque-speed characteristics, discuss different methods

of braking of D.C. shunt motor.

**b) **A 500 V D.C. shunt motor taking an armature current of 240 A, while running at 800 rpm, is braked by disconnecting the armature from the supply & closing it on a resistance of 2-02 SI, the field excitation remaining constant. The armature has a resistance of 0-5 £2. Calculate the initial braking current. 12 + 3

**8.**a) Deduce the relation necessary to obtain the heating & cooling curve of an

electric motor.

**b) **A motor has a thermal heating time constant of 50 minutes. When the motor runs continuously on full load, Its final temperature rise is 80’C.

. D What would be the temperature rise after 1 hour, if the motor runs continuously on full load ?

ii) If the temperature rise on 1 hour rating is 80’C. find die maximum steady state temperature at this rating. ,

iU) How long will the motor take for its temperature to rise from 50*C to 80*C,’ if it is working at its 1 hour rating ? 6 + 9

**9.**a) State the.advantages & disadvantages oHVord-Leonard drive system.

**b) **Discuss with relevant diagrams, the principle, of speed control of induction motor, above & below synchronous speed by feeding energy to the source.

**c) **The rotor of an 8-pole, 50 Hz, 3-phase induction motor has a resistance of 0*2 (2 per phase & runs at 730 rpm. If the load torque remains unchanged, calculate the additional rotor resistance that will reduce Its speed by 10%. Neglect stator impedance. 3 + 7 + 5

10 £0 Explain the principle of Variable voltage variables frequency ( VWF ) control of |

Induction motor.

**b) **A 400 kW, 3-phase, 33 kV, 50 Hz, unity power factor, 4-pole, star connected synchronous motor has the following parameters :

JO _{a} ■ 0, X _{s} ■ 13 O, rated field current * 10 A. The machine is controlled by variable frequency control at a constant j ratio.

Calculate the torque & field current for rated armature current, 900 rpm & 0-8 leading power factor. 8 + 7

11. Write short notes cm any three of the following : 3×5=15

£0 Series parallel control of D.C. motor

**b) **Self-controlled synchronous motor drive

**c) **3-phase fully controlled rectifier fed D.C. motor

**d) **EMU.