Alternator Objective Questions With Explanation Part-1

1. The speed of a 4-pole 60 Hz synchronous machine will be
A. 1800 r.p.m.
B. 2400 r.p.m.
C. 3000 r.p.m.
D. 3600 r.p.m.

Explanation:
In a synchronous machine,
Frequency, fs = 60 Hz
Number of poles = 4

Now, N = 120f/p = 120 × 60/4
=7200/4

⇒ N = 1800 rpm

2. The rotor preferred for alternators applied to hydraulic turbines are
A. salient pole type
B. cylindrical rotor type
C. solid rotor type
D. any of the above

Explanation:
In synchronous generators or alternator, generally salient-pole type of rotors is driven by hydro-turbines and cylindrical type rotors are driven by steam turbines

 Salient pole rotor Cylindrical rotor Large diameter and short axial length Small diameter and long axial length Used for low-speed alternators Used for high-speed turbo alternators Has projecting poles Has no projecting poles Needs damper windings Do not need damper windings Windage loss is more Windage loss is less
• In an alternator, the rotor is usually made to rotate by giving DC supply, as stator windings(Output) are stationary and field is rotating to get a 3 phase induced emf at the output terminals of 3phase alternator.
• Hence, other than DC excitation to the rotor, the output of 3 phase alternator doesn't generate 3 phase induced emf but generates a random induced voltage which is unwanted.
• The main purpose of a DC exciter in a generator(alternator) is to provide a rotating magnetic field continuously.
• Excitor is that Which is used to induce the e.m.f in the armature coil.
• So, DC power is given to the exciter and the exciter is nothing but a coil, and therefore exciter creates a magnetic field.
• Under excited alternator works at the leading power factor
• Normal excited alternator works at the unity power factor
• Overexcited alternator works at lagging power factor
3. What is the largest size of alternator being manufactured in India ?
A. 500 MW
B. 250 MW
C. 210 MW
D. 110 MW

Explanation:
500 MW is the largest size of alternator being manufactured in India

Important Notes:
• An overexcited alternator always supplies lagging current to the connected load, which means that load is of lagging nature. Lagging load take active and reactive power from the supply or alternator. Therefore, reactive power flows outwards from an over-excited alternator.
• Under excited alternator works at the leading power factor
• The normal excited alternator works at the unity power factor
• The overexcited alternator works at lagging power factor
For Synchronous motor its opposite of alternator,
• Under excited synchronous motor works at lagging power factor
• The normal excited synchronous motor works at the unity power factor
• The overexcited synchronous motor works at a leading power factor

4. The speed of a p-pole synchronous machine in r.p.m. is given by
A. 120f/p
B. 120p/f
C. 1/120pf
D. 120fp

Explanation:
The speed of a synchronous speed is given by,
N = 120f/p

Where, f is frequency
P is number of poles

5. The frequency of voltage generated in large alternators is
A. 50 Hz
B. 60 Hz
C. in kilo cycles
D. in mega cycles

Explanation:
The frequency of voltage generated in large alternators is 50 Hz

The output frequency of an alternator is decided by two factors, rotor speed and number of poles.

N = 120f/P

Where,
N = Speed of rotor in rpm
f = Frequency of generated emf
P = Number of Poles

6. Hydrogen is used in large alternators mainly to
A. reduce distortion of waveform
B. cool the machine
C. strengthen the magnetic field
D. reduce eddy current losses

Explanation:
• Hydrogen is the least expensive, has less weight, has high thermal conductivity, less density and less viscosity. Less weight, less density & less viscosity attributes to its flow rate. High thermal conductivity helps in better heat exchange. The least expensive help in balance sheets, more power in fewer investments.
• In order to reduce the high temperature of the alternator hydrogen gas is used as a coolant. The coolant, Hydrogen gas is allowed to flow in a closed cyclic path around the rotor. Heat exchange takes place and the temperature of hydrogen gas increases, for better cooling of the rotor in the next cycle it has to be cooled. Cooling of hydrogen gas is done by passing it through heat exchangers generally constituted with water. Now Hydrogen gas after cooling is allowed to pass through driers ( mainly silica gel which absorbs moisture) and allowed to pass again through the rotor.
Important Notes:
Coolant: It is a substance which is used to reduce or regulate the temperature of a system.

An ideal coolant has:
• Low viscosity
• Low cost
• Non- toxic in nature
• Chemically inert
• Neither causes nor promotes corrosion of the cooling system
• Hydrogen is used as a high-performance gaseous coolant. Its thermal conductivity is higher than all other gases; it has high specific heat capacity, low density and therefore low viscosity, which is an advantage for rotary machines susceptible to windage losses. Hydrogen-cooled turbo generators are currently the most common electrical generators in large power plants.
7. An alternator coupled to which prime mover will usually have the highest rotating speed ?
A. Steam engine
B. Reciprocating diesel engine
C. Francis turbine
D. Steam turbine

Explanation:
Hydraulic Turbine:
• A hydraulic turbine uses the potential energy and kinetic energy of water and converts it into usable mechanical energy
• The mechanical energy made available at the turbine shaft is used to run an electric power generator which is directly coupled to the turbine shaft
• The electric power which is obtained from hydraulic energy is known as hydroelectric energy.
• It is employed to run the salient pole alternator used in low-speed from 100 rpm to 1500 rpm.
Types of Hydraulic Turbine:
Impulse turbine
• If the energy available at the inlet of the turbine is only kinetic energy, the turbine is so-known as an impulse turbine.
• The available energy at the inlet is only kinetic energy if the inlet pressure and outlet pressure are the same equal to atmospheric pressure.
• Example: Pelton Turbine
Reaction Turbine:
• At the inlet of the turbine, the water possesses kinetic energy as well as pressure energy
• Example: Francis Turbine, Kaplan Turbine
Diesel engine, Steam turbine, Gas turbine employed for cylindrical pole alternator at high speed.

8. In an alternator the voltage generated per phase is proportional to
A. number of turns in coil
B. flux per pole
C. frequency of waveform
D. all of the above

Answer: D. all of the above

Explanation:
The output frequency of an alternator is decided by two factors, rotor speed and number of poles.

N = 120f/P

Where,
N = Speed of rotor in rpm
f = Frequency of generated emf
P = Number of Poles

In an alternator the voltage generated per phase is proportional to number of turns in coil, flux per pole, frequency of waveform.

9. Salient pole type alternators are generally used on
A. low voltage alternators
B. hydrogen cooled prime movers
C. high speed prime movers
D. low and medium speed prime-movers

Answer: D. low and medium speed prime-movers

Explanation:
• Cylindrical rotors are used in high-speed alternators having speeds between 1500 rpm to 3000 rpm.
• Salient pole rotors are used in low-speed devices, from 100 rpm to 1500 rpm.
Important Notes:
Salient pole rotor:
• A salient pole rotor has a large diameter and short axial length, length to diameter is ratio is very small.
• The salient pole rotor contain projected poles and the field winding is a concentrated winding on the pole core.
• The rotors are not mechanically stable and does not offer high operating speeds because of the projected nature.
• Any mechanical instability of the rotor i.e. vibration or hunting should be suppressed. This is achieved by providing damper bars in the poles/rotors.
• So that salient pole rotor type:- causes speed fluctuation and suitable for- low and medium-speed operation.
Cylindrical rotor:
• A non-salient pole type rotor has a small diameter and long axial length, length to diameter is ratio is very high.
• As cylindrical rotor has no projections and there is a closed portion to contain field winding which is distributed.
• Therefore it acts as a smooth rotor and it does not require any damper winding as the smooth rotor it self acts as good damper.
• So that cylindrical rotor type: causes no speed fluctuation and suitable for high-speed operation.
• ex. Thermal power plant

Note:

 Salient pole rotor Cylindrical rotor Large diameter and short axial length Small diameter and long axial length Used for low-speed alternators Used for high-speed turbo alternators Has projecting poles Has no projecting poles Needs damper windings Do not need damper windings Windage loss is more Windage loss is less

10. Turbo-alternators are generally used to run at
A. 1500 r.p.m.
B. 3000 r.p.m.
C. 5000 r.p.m.
D. 15000 r.p.m.