Alternator

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Q1: Which formula is used to calculate EMF/phase in a ideal alternator?

• $E = \frac{\phi F T}{2.22}$
• $E = \frac{\phi F T}{4.44}$
• $E = 2.22 \phi F T$
• $E = 4.44 \phi F T$

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Q2: Which rule is used to find the direction of induced emf in an alternator?

• Cork screw rule
• Right hand palm rule
• Fleming's left hand rule
• Fleming's right hand rule

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Q3: What is the name of the part of alternator?

• Stator
• Exciter
• Salient pole rotor
• Smooth cylindrical rotor

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Q4: What is the formula to calculate emf equation of an alternator?

• $E = 4.44 K_d K_c T \phi$
• $E = 2.22 K_d K_c F \phi$
• $E = 4.44 K_d K_c F T \phi$
• $E = 1.11 K_d K_c F \phi$

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Q5: How alternators are rated?

• KVA
• KW
• MW
• KV

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Q6: Which formula is used to calculate the percentage voltage regulation in alternator?

• $\frac{V_{NL} - V_{FL}}{V_{NL}} \times 100$
• $\frac{V_{NL} - V_{FL}}{V_{FL}} \times 100$
• $\frac{V_{NL} - V_{FL}}{V_{NL}} \times 100$
• $\frac{V_{FL} - V_{NL}}{V_{FL}} \times 100$

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Q7: What is the supply frequency of an alternator having 6 poles runs at 1000 rpm?

• 25 Hz
• 40 Hz
• 50 Hz
• 60 Hz

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Q8: Calculate the speed of an alternator having 2 poles at a frequency of 50 Hz.

• 1500 rpm
• 2500 rpm
• 3000 rpm
• 6000 rpm

Show Calculation

**Formula:** $N_s = \frac{120 \times f}{P}$ Where: $N_s$ = Synchronous speed in Revolutions Per Minute (RPM) $f$ = Frequency in Hertz (Hz) = 50 Hz $P$ = Number of poles = 2 $N_s = \frac{120 \times 50}{2}$ $N_s = \frac{6000}{2}$ $N_s = 3000$

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Q9: What condition the lamps become dark in dark lamp method of parallel operation of two alternators?

• Terminal voltages are equal
• Voltage and frequency are equal
• Voltage and power rating are equal
• Frequency are same in both alternator

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Q10: How to compensate de-magnetizing effect due to armature reaction in an alternator?

• Reducing the speed of alternator
• Reducing field excitation current
• Increasing field excitation current
• Increasing the speed of alternator

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Q11: What is the use of synchroscope?

• Adjust the output voltage
• Adjust the phase sequence
• Adjust the supply frequency
• Indicate the correct instant for paralleling

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Q12: What is the name of the equipment that provides D.C to the rotor of alternator?

• Exciter
• Inverter
• Converter
• Synchroniser

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Q13: What is the purpose of damper winding in alternator?

• Reduces the copper loss
• Reduces windage losses
• Reduces the hunting effect
• Improves the voltage regulation

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Q14: Which condition is to be satisfied before parallel operation of alternators?

• Rating must be same
• Phase sequence must be same
• Rotor impedance must be same
• Stator impedance must be same

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Q15: What is the speed of an alternator connected with a supply frequency of 50 Hz at rated voltage having 4 poles?

• 1000 rpm
• 1500 rpm
• 3000 rpm
• 4500 rpm

Show Calculation

**Formula:** $N_s = \frac{120 \times f}{P}$ Where: $N_s$ = Synchronous speed in Revolutions Per Minute (RPM) $f$ = Frequency in Hertz (Hz) = 50 Hz $P$ = Number of poles = 4 $N_s = \frac{120 \times 50}{4}$ $N_s = \frac{6000}{4}$ $N_s = 1500$

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Q16: What condition the two lamps become bright and one lamp dark during paralleling of two alternators?

• Terminal voltages are equal
• Voltages and frequencies are equal
• Voltages and phase sequence are equal
• Both the alternators receive same frequency

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Q17: What causes the terminal voltage of an alternator reduces, if the load increases?

• Field resistance
• Armature reaction
• Inductive reactance
• Armature resistance

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Q18: What is the purpose of using damper winding in AC generator?

• Prevents heating
• Reduces copper loss
• Reduces windage loss
• Prevents the hunting effect

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Q19: What is the type of alternator?

• Brushless alternator
• Three phase alternator
• Single phase alternator
• Salient pole type alternator

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Q20: Calculate the speed in r.p.s of the 2 pole, 50Hz alternator?

• 50 rps
• 100 rps
• 1500 rps
• 3000 rps

Show Calculation

Number of poles ($P$) = $2$ Frequency ($f$) = $50 \text{ Hz}$ **Formula:** $\text{Speed (r.p.s)} = \frac{2 \times f}{P}$ $\text{Speed (r.p.s)} = \frac{2 \times 50}{2} = \frac{100}{2} = 50 \text{ r.p.s}$

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Q21: What is the advantage of using rotating field type alternator?

• Easy to locate the faults in the field
• Easy to connect the load with alternator
• Easy to dissipate the heat during running
• Two slip rings only required irrespective of No. of phases

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Q22: What is the effect in increasing the field excitation current in alternator?

• Prevents demagnetizing
• Over voltage protection
• Dead short circuit protection
• Alternator will be over loaded

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Q23: Calculate the pitch factor ($K_p$) for a winding having 36 stator slots 4 pole with angle ($\alpha$) is 30° in alternator.

• 0.942
• 0.965
• 0.978
• 0.985

Show Calculation

$K_p = \cos\left(\frac{\alpha}{2}\right)$ Where: * $\alpha$ = Short pitch angle in electrical degrees (given as $30^\circ$) $K_p = \cos\left(\frac{30^\circ}{2}\right)$ $K_p = \cos(15^\circ)$ $K_p = 0.9659$

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Q24: What is the cause for hunting effect in alternators?

• Due to over load
• Running without load
• Running with fluctuation of speed
• Due to continuous fluctuation in load

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Q25: Calculate the voltage regulation in percentage if the load is removed from an alternator, the voltage rises from 480V to 660V

• 27.2%
• 32.5%
• 37.5%
• 38.5%

Show Calculation

$\%VR = \frac{V_{NL} - V_{FL}}{V_{FL}} \times 100$ $\%VR = \frac{660 - 480}{480} \times 100$ $\%VR = \frac{180}{480} \times 100$ $\%VR = 0.375 \times 100$ $\%VR = 37.5\%$

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