How can a common three-phase, squirrel cage motor be made to generate?

It must be connected in a three-phase circuit and be driven by a prime mover at a faster-than-synchronous speed. It won’t generate unless it is connected in a circuit that is already supplying current, or is given a short burst of current to the windings.

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How DC Generator be converted into Alternator?

By providing two collector rings on one end of the armature and connecting these two rings to two points in the armature winding 180 degrees apart.

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Why is it preferable to generate alternating current rather than direct current?

Alternating current can be changed in voltage by means of transformers. This is necessary because to transmit power over any distance it must be at high voltage. Too much power is lost when transmitting at low voltage. Direct current cannot be changed in voltage without first changing it to alternating current and then raising the voltage; the operation must then be reversed at the receiving end.

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On an alternator, does the dc part (the poles) or the ac part (the conductors) rotate?

It is immaterial which part rotates. However, the dc field is usually made the rotating part, and the stator is usually the ac part of the device. This is because the dc field excitation can be of relatively low voltage, and it is easier to insulate a rotating part for low voltage than for high voltage. The ac output is usually a much higher voltage, and it is much more practical to insulate the stator for the high voltage. Also, with this arrangement there are

no brushes required on the output side.

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How do synchronous motors differ from alternators?

Figure: Diagram of a simple alternator with a rotating magnetic core (rotor) and stationary wire (stator) 

They may be just like alternators; however, if they are, they won’t be self-starting and will have to be started by some means until they approach synchronous speed, at which time they can be connected to the line and pull into speed. Most synchronous motors have a squirrel cage winding in addition to the dc field. They start as a squirrel cage motor, and when they are about up to the speed of the alternator, the dc field is energized. The poles then lock in position with the revolving field of the armature, and the rotor revolves in synchronization with the supply circuit.

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21 Common Interview Questions & Answers on AC Generators - Part-2

Figure: Three Phase Generator Output

Q1. Magnetic induction occurs when there is relative motion between what two elements?

A1. A conductor and a magnetic field.

Q2. What is the part of an alternator in which the output voltage is generated?

A2. Armature.

Q3. What are the two basic types of alternators?

A3. Rotating armature and rotating field.

Q4. What is the main advantage of the rotating field alternator?

A4. Output voltage is taken directly from the armature (not through brushes or slip rings).

Q5. Most large alternators have a small dc generator built into them. What is its purpose?

A5. To provide dc current for the rotating field.

Q6. How are alternators usually rated?

A6. Kilovolt-amperes (volt amperes).

Q7. What type of prime mover requires a specially designed high-speed alternator?

A7. Steam turbine.

Q8. Salient-pole rotors may be used in alternators driven by what types of prime movers?

A8. Internal combustion engines, water force and electric motors.

Q9. What does the term single phase indicate?

A9. One voltage (one output).

Q10. In single-phase alternators, in order for the voltages induced in all the armature windings to add together for a single output, how must the windings be connected?

A10. In series.

Q11. What determines the phase relationship between the voltages in a two-phase ac generator?

A11. Placement of armature coils.

Q12. How many voltage outputs are available from a two-phase three-wire alternator?

A12. Three.

Q13. What is the relationship of the voltage at C in figure 3-7 to the voltages at A and B?

A13. C is 1.414 times greater than A or B.

Q14. In a three-phase alternator, what is the phase relationship between the individual output voltages?

A14. Each phase is displaced 120º from the other two.

Q15. What are the two methods of connecting the outputs from a three-phase alternator to the load?

A15. Wye and Delta.

Q16. Ships’ generators produce 450-volt, three-phase, ac power; however, most equipment uses 117volt, single-phase power What transformers and connections are used to convert 450-volt, three-phase power to 117-volt, single-phase power?

A16. Three single-phase, delta-delta, step-down transformers.

Q17. What two factors determine the frequency of the output voltage of an alternator?

A17. Speed of rotation and number of poles.

Q18. What is the frequency of the output voltage of an alternator with four poles that is rotated at 3600 rpm?

A18. 120 Hz.

Q19. The variation in output voltage as the load changes is referred to as what? How is it expressed?

A19. Voltage regulation. As a percentage.

Q20. How is output voltage controlled in practical alternators?

A20. By varying the voltage applied to the field windings.

Q21. What generator characteristics must be considered when alternators are synchronized for parallel operation?

A21. Output voltage, frequency, and phase relationships.

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Related Post: 

1. 16 Common Interview Questions and Answers in Generators - Part-1
2. 37 Interview Questions and answers on DC Generators - Part-3

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Some Useful Electrical Equations.

Useful Electrical Equations:

· For Sinusoidal Current: Form Factor = RMS Value/Average Value = 1.11

· For Sinusoidal Current: Peak Factor = Max Value/RMS Value = 1.414

· Average Value of Sinusoidal Current (Iav) = 0.637 x Im (Im = Max.Value)

· RMS Value of Sinusoidal Current (Irms) = 0.707 x Im (Im = Max.Value)

· A.C Current = D.C Current/0.636.

· Phase Difference between Phase = 360/ No of Phase (1 Phase=230/1=360°, 2 Phase=360/2=180°)

· Short Circuit Level of Cable in KA (Isc) = (0.094 x Cable Dia in Sq.mm) /√ Short Circuit Time (Sec)

· Max.Cross Section Area of Earthing Strip (mm2) = √(Fault Current x Fault Current x Operating Time of Disconnected Device ) / K
K = Material Factor, K for Cu = 159, K for Al = 105, K for steel = 58 , K for GI = 80

· Most Economical Voltage at given Distance = 5.5 x √ ((km/1.6) + (kw/100))

· Cable Voltage Drop (%) =
(1.732 x current x (RcosǾ+jsinǾ) x 1.732 x Length (km) x 100) / (Volt(L-L) x Cable Run.

· Spacing of Conductor in Transmission Line (mm) = 500 + 18 x (P – P Volt) + (2 x (Span in Length)/50).

· Protection radius of Lighting Arrestor = √h x (2D-h) + (2D+L).
Where h= height of L.A, D-distance of equipment (20, 40, 60 Meter), L=V x t (V=1m/ms, t=Discharge Time).

· Size of Lighting Arrestor = 1.5x Phase to Earth Voltage or 1.5 x (System Voltage/1.732).

· Maximum Voltage of the System = 1.1xRated Voltage (Ex. 66KV = 1.1 × 66 = 72.6KV)

· Load Factor = Average Power/Peak Power

· If Load Factor is 1 or 100% = This is best situation for System and Consumer both.

· If Load Factor is Low (0 or 25%) = you are paying maximum amount of KWH consumption. Load Factor may be increased by switching or use of your Electrical Application.

· Demand Factor = Maximum Demand / Total Connected Load (Demand Factor <1)

· Demand factor should be applied for Group Load

· Diversity Factor =
Sum of Maximum Power Demand / Maximum Demand (Demand Factor >1)
Diversity factor should be consider for individual Load

· Plant Factor (Plant Capacity) = Average Load / Capacity of Plant

· Fusing Factor = Minimum Fusing Current / Current Rating (Fusing Factor>1).

· Voltage Variation (1 to 1.5%) = ((Average Voltage – Min Voltage) x 100)/Average Voltage
Ex: 462V, 463V, 455V, Voltage Variation= ((460 – 455) x 100)/455 = 1.1%.

· Current Variation (10%) = ((Average Current – Min Current) x 100)/Average Current
Ex: 30A,35A,30A, Current Variation = ((35-31.7) x 100)/31.7 = 10.4%

· Fault Level at TC Secondary
= TC (VA) x 100 / Transformer Secondary (V) x Impedance (%)

Motor Full Load Current = Kw /1.732 x KV x P.F x Efficiency

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Why Power Plant Capacity Rated in MW and not in MVA?

For the following reasons, a Power plant capacity rating may be expressed in MW instead of MVA:

In a Generating station, the prime mover (Turbine) generates only and only Active Power. That’s why we rated a power plant capacity in MW instead of MVA. It's mean no matter how large your generator is, but it depends on the capacity of the  engine (Prime mover/Turbine) I.e. a 50MW turbine connected to a 90MVA alternator in a power plant will generate only 50MW at full load. In short, a power plant rating is specified in terms of prime mover /Turbine (Turbine rating may be seen by nameplate rating which is in MW or Horsepower (HP) not in MVA) and not by the alternator set coupled to it.

Another thing is that, electric power company charges their consumer for kVA while they generate kW (or MW) at the power station (Power plant).They penalize their consumer for low Power factor because they are not responsible for low power factor and kVA but you. Moreover, in power plant, power factor is 1 therefore MW is equal to MVA …… (MW = MVA x P.f).

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Mention The major features of a good protective gear for alternator and transformer

Major features of a good protective gear

ü  High speedy

ü  Selectivity

ü  High sensitivity

ü  Stability

ü  Simplicity

ü  Reliability

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Write down the Protective System of Alternator and Transformer

Alternator protection

ü  Differential protection system

ü  Balanced Earth fault protection

ü  Stator-inter run protection

Transformer protection

ü  Using Buchholz relay

ü  Combined leakage and overload protection

ü  Circulating-current scheme for transformer protection

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Write down some major Faults of Alternator and Transformer.

Major faults in Alternator

• ü  Failure of prime-mover
• ü  Failure of field
• ü  Over Current
• ü  Over Voltage
• ü  Over Speed
• ü  Stator Winding faults
• ü  Unbalanced loading

Major faults in Transformer

ü  Open circuits

ü  Over heating

ü  Earth Fault

ü  Phase-to-Phase fault

ü  Over Load

ü  Phase to ground fault

ü  Incipient fault

ü  High voltage surge fault

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What are the causes of over speed and how alternators are protected from it?

Sudden loss of all or major part of the load causes over-speeding in alternators. Modern alternators are provided with mechanical centrifugal devices mounted on their driving shafts to trip the main valve of the prime mover when a dangerous over-speed occurs.

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What is pitch-factor of an Alternator?

The pitch or coil span factor is defined as the ratio of actual coil voltage to the coil voltage for a full-pitch coil.

Hence, pitch factor, K_P = Vector sum of induced emf per coil/ Arithmetic sum of induced emf per coil.

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