Objective Questions on Alternator | Page – 2

  1. Which of the following is not determined by circle diagram?

    Frequency is not found by circle diagram.

  2. To determine the losses an alternator is driven by

    Efficiency of an alternator is calculated by ‘measurement of losses’ method. A quite accurate and simple method of determining the losses is to use a calibrated D.C. motor to drive the alternator. It is used because the motor is calibrated in the sense that all it losses have been determined for varying condition of operations so that its output is then readily be obtained.

  3. The distribution factor for a 36 slots, 4 poles, single layer three phase winding alternator will be.

  4. Cooling of synchronous generator is done by

    Cooling of synchronous generators is done by all of these methods ie air cooling ,direct water cooling & hydrogen cooling.

  5. Calculate the highest speed at which 50 hz alternator can operate

    Synchronous speed = (120f) / P WHERE P = 2 since it is not possible to have less than 2 poles. Ns = 3000 rpm.

  6. If two mechanically coupled alternator deliver power at 50 KHz & 60 KHz respectively. Then highest speed of the alternator is-

    The two coupled alternators have to run at the same speed. Ns = 120f ⁄ P or P = 120f ⁄ N or P1 ⁄ P2 = f1 ⁄ N.N ⁄ f2 = f1 ⁄ f2 = 50 ⁄ 60. So, the lowest number of poles (corresponding to higher speed) can be, if two mechanically coupled so, P1 = 10 and P2 = 12. Then the highest speed of motor is (120X50) &frals; 12 = 600 rpm.

  7. A synchronous generator is feeding a zero power factor(lagging) load at rated current,the armature reaction is

    The distribution of electric current & fluxes with phase angle φ = +π ⁄ 2 form maximum emf which coincide with centre of pole. The field created by the armature reaction mmf will be in opposite to main field flux & will result a demagnetizing effect.

  8. Distributed winding & short chording employed in AC machines will result in

    Due to distribution winding & short chording both, emf & harmonics are reduced.

  9. A stand-alone engine driven synchronous generator is feeding a partly inductive load. A capacitor is now connected across the load to completely nullify the inductive current. For this operation condition

    When capacitor is connected across load, the power factor improves. Then burden on generator decreases because of capacitor. Hence field electric current of the synchronous generator reduces.

  10. In diesel engine, gas engine type of machine contains __________pole type of alternator.

    We know that cylindrical rotors will most likely to be located on alternators where steam power is readily available. Salient pole types rotors will most likely to be located on alternator where medium speed obtained and such a machine which have large diameter and small axial length. As Diesel engine, gas engine, and gas turbine prime movers are considered as medium speed machine, and their diameter also very large compared to other machine hence this machine have salient poles.

  11. An alternator has 18 slots per pole and a coil span of 15 slots the pitch factor for the 3rd harmonic is

    Pitch factor of the third harmonic = cos 3α / 2 = cos 3 X 30 / 2 = cos 450 = 0.707
    α = (18 − 15 / 18) X 180 = 300.

  12. The voltage regulation of an alternator having 0.75 pf leading with a no load induced emf of 2400 V and rated terminal voltage of 3000 V is ___ .

    % regulation = ( no load - full load / full load) X 100
    = (2400 - 3000 / 3000) X 100 = -20%.

  13. For the same maximum voltage between conductors the ratio of copper required for a two wire dc system to a three phase 3 wire system of the same length and transmitting the same power at power factor cos φ is

    1 : (cos2Φ/1.5 )= cos2Φ/1.5 = 2/3 cos2Φ.

  14. A full pitch coil in an alternator has a span 18 slots. Coil span for a chording angle of 600 electrical will in slot be

    18 slot - 1800i.e. 1 slot - 100
    Chording angle is 600
    Coil span is (1800 - 600) = 1200
    12 slot are used i.e. 1 to 13.

  15. Turbo generator is a how many pole machine

    Turbogenerator is a high speed machine of speed 3000rpm. Therefore to match with 50Hz (frequency)
    Number of poles=2.

  16. In synchronous machine the damper winding is used to

    Damper windings are provided to damp out oscillation. They also help in starting as they form a cage winding.

  17. What are the methods for determining regulation of an alternator?

    There are 3 methods for determining voltage regulation of an alternator i.e synchronous impedance method, M.M.F method and zero p.f / potier triangle method.

  18. Methods of synchronizing of alternator

    There 4 methods of synchronizing an alternator - 1. All dark lamp method ,
    2. all bright lamp method , 3. One dark two bright lamp method & 4. Synchroscope method.

  19. In a synchronous alternator rigidity factor or stability factor(Psyn) expressed as a

    In a synchronous alternator the rate at which the synchronous power P varies with load angle δ is known as rigidity factor or stability factor denoted by Psyn.
    Psyn = dP/ dδ
    Power output per phase of cylindrical rotor generator; P = V / Zs[Efcos(θz-δ) - V cosθz]
    ∴ Psyn = dP / dδ = (VEf / Zs)sin(θz-δ)
    In many synchronous machine Xs>>R. Therefore for a cylindrical rotor machine neglecting saturation and stator resistance Psyn will be; Psyn = (VEf/Xs)cosδ.

  20. In synchronous generator a synchronous dead load (lamp, furnaces etc) has no ____________ hence it has no natural frequency of oscillation.

    When a mechanical rotatory system having inertia and restoring torque that tends to restore its position when displaced,the system has a natural frequency of oscillation.
    A synchronous machine operating in parallel with other machine or infinite bus bars forms such a system. Here the restoring torque is due to the synchronizing torque which depends on displacement. But a synchronous dead load has no restoring torque and by this it has no natural frequency of free oscillation.