Electrical Machines Flashcards
Explain why the armature of most AC generators is on the stator instead of the rotor
Centrifugal force is no longer a problem
Much lighter so higher speeds and less wear on bearings
Brush gear not required
Explain 3 ways to increase the emf of an AC generator
Increase field strength
Increase speed of armature
Add more loops to armature
What are 2 ways of increasing the frequency output of an AC generator?
Increase number of poles
Increase speed
What is the frequency equation?
F=Np/60
What is a bus bar made of?
Copper, brass or aluminium
What does a bus bar do?
Conducts electricity within a switchboard/ distribution board
Describe 2 types of 3 phase connections
Star connected- the end of each phase are connected at one central point
Delta connected- 3 phases connected together forming a loop
Draw a graph of the output of a three phase generator
docket…
What are the advantages of 3-phase systems?
Produce more power
Greater economy, (3-4 conductors rather than 6) reduces cost and complexity of wiring
Provides a rotating magnetic field
What is the relationship between phase and line voltages in a star connected 3 phase system?
Line voltage = √3 phase voltage
Explain the one wattmeter and two wattmeter methods of measuring 3-phase power.
A single wattmeter can be used to measure power dissipated in a balanced 3 phase load.
Two wattmeters show the total power dissipated in a balanced or unbalanced 3 phase load
Measures current and voltage in reference to the third
Explain the purpose of using an unearthed electrical system
Improves equipment reliability not personal safety
Why is it leathal to touch a bare conductor in an unearthed system?
Because of the cabling to hull capacitance and low resistance of human skin, enough current can flow to kill a person
What is the purpose of the DC exciter in an AC generator?
Provided the DC excitation current to the generator rotor via the exciter commutator and a set of slip rings
What is the advantage of the brushless generator over the conventional generator?
Reduces maintenance requirements and improves overall efficiency.
Describe the two types of generator rotors and their applications in the RN
Salient pole rotor- all HM ships
Field coils wound around the laminated pole pieces which are fixed to the rotor shaft ( 6 pole machines in RN) (short and fat)
Cylindrical rotor- subs gas turbine T45 and QEC
Circular cross section with filed windings sited in slots on surface of rotor ( two pole rotor) (long and thin)
Explain how the field current is applied to the rotor of the conventional and brushless generators
conventional- dc exciter directly to the generator rotor using slip rings
brushless- 3 phase ac generator converts to dc using a diode pack
Describe the construction of the stator of an AC generator
Steel plate (yoke) with a soft iron laminated core
Windings are placed in slots in the stator core
Space between stator core and yoke for ventilation
What is the voltage regulation equation?
(VNL-VFL)/VFL x 100%
Why is a voltage regulator necessary?
To ensure the generator is supplied with a constant voltage, voltage regulator reduces droop
How is hand control of a generator achieved?
increases and decreases the resistance to alter the field current of the DC exciter?
Explain the purpose of each main component of AVR
TRU –> senses the output voltage of the generator and transforms and rectifies it
Reference voltage unit –> produces a DC voltage proportional to the desired output voltage
High frequency generator –> Provides a stable high frequency power supply
Comparator unit –> Compares the voltage signal from the TRU with the reference voltage unit
Thyristor bridge rectifier –> controls the amount of current to the exciter field.
Exciter generator –> output is designed to supply the rotor of the main AC generator
What are the two main operating parameters required of an AVR by the RN?
no load to load–> maximum terminal voltage change less than 1%
suddenly applied load –> not change more than 15% and must be back within 1% within 1 second
What is the purpose of the thyristor bridge in an AVR?
Controls the amount of current to the exciter field
Explain 2 reasons for paralleling AC generators
Transfer load from one generator to another or to and from shore supply
Enable 2 or more generators to share the ships electrical load
List and explain 4 conditions that must be met before generators are paralleled
same terminal velocity
frequency must be lightly higher than the running generators frequency
same phase rotation
Both generators must be in phase when the circuit breaker is shut
Explain the procedure for paralleling two AC generators using a sychroscope including the reason for each action
Adjust frequency so sychroscope is slowly rotating clockwise (slightly higher frequency)
Oncoming generator breaker is closed at 11:55 position as slight delay (breaker shuts when generators in phase)
Oncoming generator picks up some load due to frequency difference (prevent motoring)
Describe operation of a sychroscope
It indicates the frequency and phase difference between 2 AC supplies
Explain the relationship between True, Reactive and Apparent power.
True- in phase with busbar voltage (power that can be used to do work by the machine)
Reactive- at 90° to the true power (power that creates and maintains magnetic fields in the machine)
Apparent- calculated from true and reactive using Pythagoras theorem (Power that must be generated to supply the machine)
How does the speed of the prime mover and setting of the AVR affect the load sharing of an AC generator set?
prime mover- controls frequency -true power
AVR - voltage- reactive power
Explain why a generator requires a droop characteristic in order to operate in parallel with another generator
Stable operating point for true and reactive power
What is the normal voltage droop of the shore side supply?
No droop
What is the most common power factor on board HM ships?
0.8 lagging
Explain why the AC generators designed in chapter 7&8 will not work for a high frequency generator
When rotor speed is increased, causes problems with centrifugal force, prime mover cant run reliably at high speeds.
Explain how the 1600 Hz generator produces such a high frequency output at a relatively low prime mover speed.
Inductor generator, rapidly varies the magnetic flux interacting with the stator winding as the rotor turns, induces a rapidly varying EMF
What is an application of a high frequency generator?
1600 Hz Permanent magnet generator
If a transformer is designed to a step-up voltage, how will the current change?
decrease
Describe two types of transformer core.
Core- cheaper construction, for low voltage power transformers, and very large power transformers to save weight.
Shell- less flux leakage, used in light current and higher voltage applications
Describe an application of a single phase transformer with equal number of primary and secondary windings.
Isolation transformer, separate electrical networks into isolated zones, and to protect sensitive electronic equipment from external earths.
used in radio and audio circuitry
Explain the main difference between an autotransformer and a conventional transformer
Auto transformer as a single winding common to both primary and secondary circuits
What are the advantages and disadvantages of an autotransformer?
adv - cheaper and lighter as less copper and only one metal core
- can be configured as variable voltage transformer
disadv - provide no electrical isolation between primary and secondary side
State an application of the autotransformer
Explain how a current transformer works.
The core of the transformer surround the bus-bar, the busbar forms a one turn primary coil. an ammeter is placed in series with the secondary windings, the ammeter is calibrated to measure secondary current and display actual busbar current.
State 2 applications for a current transformer
Portable clamp on ammeter
?
Why can a current transformer become dangerous if the ammeter or load is removed from the secondary side?
It will produce a large EMF in the secondary if left as an open circuit
