ELECTRICAL MACHINE-1

Aryabhatta Knowledge University

Sem. iv   2013                               Time 3hr                            Full marks: 70

ELECTRICAL MACHINE-1                 

Attempt any five questions in which question no 1 is compulsory

1.       Choose the correct option (any seven) :

(a)    Three –phase alternators are invariably connected in star so as to

(i)                  Reduce the size of stator conductors used

(ii)                Increase the terminal voltage

(iii)               Reduce magnetic losses

(iv)              All of the above

(b)   The phase sequence of a three-phase alternator will reverse ,if

(i)                  The field current is reversed keeping the direction of rotation same

(ii)                The field current remains the same but direction of rotation is reversed

(iii)               The field current is reversed and the number of poles is doubled

(iv)              The number of poles is doubled without reversing the fields current

(c)    The regulation of an alternator of an alternator is given as

(i)                  100(V-E˳)/V

(ii)                100(E˳-V)/V

(iii)               100V/E˳

(iv)              100(E-V)/E˳

(d)   In the measurement of X_d ,X_q (in ohms), following data are obtained by the slip test on salient pole machine:

I_d max=10A       ,     I_d min=6.5A      ,V_d max=30 V   ,V_d min=25 V

Which one of the following is correct?

(i)                  X_d=3Ω      ,        X_q =3.86Ω

(ii)                X_d=4.165Ω ,     X_q=2.5Ω

(iii)               X_d=3  Ω   ,         X_q=2.5Ω

(iv)              X_q=4.16Ω   ,     X_q=3.86Ω

(e)   In which of the following is reluctance power is developed?

(i)                  Salient pole alternator

(ii)                Non salient pole alternator

(iii)               Squirrel cage induction motor

(iv)              Transformer

(f)     Which motor can conveniently operate at lagging as well as leading power factor?

(i)                  Squirrel-cage induction motor

(ii)                Wound-rotor induction motor

(iii)               Synchronous motor

(iv)              Dc shunt motor

(g)    An inverted V curve of synchronous motor shows the variation of

(i)      Power factor and DC excitation at constant load

(ii)    Supply voltage and field current at constant excitation

(iii)   Power factor and supply voltage during hunting supply voltage and excitation current at constant load

(h)   A single-phase induction motor is running at N r.p.m. Its synchronous speed is N_s .if its slip with respect to speed is s, what is the slip with respect to the backward field?

(i)      S

(ii)    –s

(iii)   (1-s)

(iv)  (2-s)

(i)      The torque-speed characteristic of two- phase induction motor is largely affected by

(i)      Voltage

(ii)    Speed

(iii)   X/R ratio

(iv)  Supply frequency

(j)     In a.c. series motors, armature coils are usually connected to commutator

(i)                  Through resistor

(ii)                Through inductors

(iii)               through capacitors

(iv)              solidly

2.       (a) draw the following phasor diagram of a 3 phase synchronous machine:

(i)                  Type: non salient pole , mode generator , pf-lagging

(ii)                Type : salient pole ,mode motor ,pf –leading

(iii)               Type : non –salient pole, mode motor ,pf- leading

(b)    a 3 phase ,10 kVA ,400 V ,star connected alternator supplies the rated load at 0.8 p-f  lagging. If the armature resistance is 0.5 ohm and synchronous reactance is 10 ohms , find the torque angle and voltage regulation.

3.       (a) from the phasor diagram of a salient –pole alternator working at a leading pf , but with pf angle θ less than load angle δ ,obtain the following relation:

(i)                  Tan(δ-θ)=  [i_a X_q –V_t sinθ]/[V_tcosθ+I_a r_a ]

(ii)                E_f =V_t cosδ+I_a r_a cos(δ-θ)+I_d X_d  

(b)   Find excitation voltage of an alternator when delivering rated output at 0.8 pf lagging and at a rated voltage. For this alternator, X_d =1discuss the effects.25p.u. and X_q=1.00 p.u.

4.       Show that for alternators running in parallel ,the division load between them is governed mainly by the speed- load characteristics of their prime movers

5.       (a) discus the effects of field current on synchronous motor  power factor with the help of            

phasor diagram.

(b) prove that power develop in three-phase synchronous motor  is given by

P=3[E_fV_t sinδ/X_d +(1/2)V_t² (1/X_q-1/X_d)sin2δ]

6.       (a) Derive the equivalent circuit of a single-phase  induction motor with the help of double revolving field theory .

(b) A –pole ,50 Hz ,ingle –phase induction motor ,when running at normal speed, has forward field and backward field resistance of 60 ohms and 1.4 ohms respectively . The stator current at a full -load slip of 5% is 3A. Determine the full load torque in N-m.

[Assume rotational losses to be 4 % of the internal power developed]

7.       A 2-phase ,4-pole servomotor has the following parameters at 50Hz :

R₁=360 ohms, r₂=250 ohms, x₁=x₂ =50 ohms, x_m=900 ohms

This motor is operated with 115 V across the control winding and 75 V across the control winding, the latter leading the former by 90°.

Calculate the stator currents ,output power and motor torque at a slip of 0.5 .the rotational losses under condition given above are 0.7 watt .both the windings have equal no of turns.

8.       (a) Give the constructional features, working and application of single –phase reluctance motors.

(b) Describe the working of single sub –synchronous motor. Why should its rotor posses considerable flywheel effect?

9.       Write short notes on any two of the following :

(a)    Permanent magnet Dc motor

(b)   Hysteresis motor

(c)    Starting of synchronous motor

(d)   Two- reaction theory in salient pole synchronous generator