Electrical Sciences AC Generators & Transformers
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The part of an AC generator in which voltage is produced
Generally used in low power applications where a variable voltage is required.
Generally used in electronic circuits that require constant voltage or constant current with a low power or volt-amp rating
The ratio of primary current to secondary current
Generally used in electrical power distribution and transmission systems
The ratio of the useful power output to the total power input
In an AC generator consists of coils of conductors within the generator that receive a voltage from a source (called excitation) and produce a magnetic flux
Occur when iron cores in an AC generator are subject to effects from a magnetic field
Steps down the current of a circuit to a lower value and is used in the same types of equipment as a potential transformer
Instrument current transformer (CT)
Steps down voltage of a circuit to a low vale that can be effectively and safely used
Instrument potential transformer (PT)
Normally low power transformers used to isolate noise from or to ground electronic circuits
Can be caused by bearing friction, brush friction on the commutator, and air friction
The inducing of an EMF in a coil by magnetic flux lines generated in another coil.
Used in electronic circuits and come in many different types and applications.
The coil of a transformer that is energized from an AC source.
The component that is used to drive the AC generator.
The rotating component of an AC generator. Depending on the type of generator, this component may be the armature or the field.
The coil that delivers from the AC source to the load.
Electrical connections that are used to transfer power to and from the rotor of an AC generator.
The stationary part of an AC generator. This component may be the armature or the field, depending on the type of generator.
The ratio of turns of wire in the primary winding to the number of turns of wire in the secondary winding.
The ratio of primary voltage to secondary voltage.
Air friction losses in a generator caused by air turbulence from the rotation of the rotor.
- State the purpose of the following components of an AC generator:
- Slip rings
The field in an AC generator consists of coils of conductors within the generator that receive a voltage from a source (called excitation) and produce a magnetic flux.
The armature is the part of an AC generator in which output voltage is produced.
The prime mover is the component that is used to drive the AC generator.
The rotor of an AC generator is the part that is driven by the prime mover and that rotates.
The stator of an AC generator is the part that is stationary.
Slip rings are electrical connections that are used to transfer power to and from the rotor of an AC generator.
Given the speed of rotation, 3600RPM and number of poles, 2, calculate the frequency output of an AC generator
- 7200/120 = 60Hz
List the three losses found in an AC generator
- Hysteresis losses
- Mechanical losses
- Internal Resistance
STATE the purpose for voltage regulation equipment
To maintain the output voltage of a generator at a desired value
Given the prime mover input and generator output, determine the efficiency of an AC generator
Efficiency of an AC generator can be calculated by dividing the output by the input and multiplying by 100.
Describe the basis behind the KW and current rating of an AC generator
Power (kW) ratings of an AC generator are based on the ability of the prime mover to overcome generation losses and the ability of the machine to dissipate the heat generated internally. The current rating of an AC generator is based on the insulation rating of the machine.
Describe the conditions that must be met prior to paralleling two AC generators, including consequences of not meeting these conditions
- Their terminal voltages must be equal. A mismatch may cause high currents and generator or distribution system damage.
- Their frequencies must be equal. A mismatch in frequencies can cause one generator to “motor,” causing an overload in the generators and the distribution system.
- Their output voltages must be in phase. A mismatch in the phases will cause large opposing voltages to be developed, resulting in damage to the generators and distribution system due to high currents.
Describe the difference between stationary field, rotating armature AC generator and a rotating field, stationary armature AC generator
The disadvantage of a stationary field, rotating armature is that the slip-ring and brush assembly is in series with the load circuits and, because of worn or dirty components, may interrupt the flow of current.
- A stationary armature, rotating field generator has several advantages: (1) a load can be connected to the armature without moving contacts in the circuit; (2) it is much easier to insulate stator fields than rotating fields; and (3) much higher
- voltages and currents can be generated.
Explain the differences between a wye-connected and delta-connected AC generator, including advantages of each type
The advantage of the delta-connected AC generator is that, if one phase becomes damaged or open, the remaining two phases can still deliver three-phase power at a reduced capacity of 57.7%.
The advantage of a wye-connected AC generator is that each phase only has to carry 57.7% of line voltage and, therefore, can be used for high voltage generation.
Given the type of transformer, DESCRIBE the operating characteristics and applications for that transformer type
Distribution transformers are generally used in power distribution and transmission systems.
Power transformers are used in electronic circuits and come in many different types and applications.
Control transformers are generally used in circuits that require constant voltage or constant current with a low-power or volt-amp rating.
Autotransformers are generally used in low-power applications where a variable voltage is required.
Isolation transformers are normally low-power transformers used to isolate noise from, or to ground, electronic circuits.
Instrument potential and instrument current transformers are used for operation of instruments such as ammeters, voltmeters, wattmeters, and relays used for various protective purposes.
Define the following terms as they pertain to transformers
- Mutual induction
- Turns ratio
- Impedance ratio
The induction of an EMF in a coil by magnetic flux lines generated in another coil is called mutual induction.
The turns ratio is defined as the ratio of turns of wire in the primary winding to the number of turns of wire in the secondary winding.
The ratio between the primary and secondary impedances is referred to as the impedance ratio.
Efficiency of a transformer is the ratio of the power output to the power input.
Describe the differences between a wye-connected and delta-connected transformer
In a delta connection, all three phases are connected in series to form a closed loop.
In a wye connection, three common ends of each phase are connected together at a common terminal, and the other three ends are connected to a three-phase line.
Given a block diagram of a typical voltage regulator, DESCRIBE the function of each of the following components
- Sensing circuit
- Reference circuit
- Comparison circuit
- Amplification circuit(s)
- Signal output circuit
- Feedback circuit
Stationary Field, Rotating Armature AC Generator
Simple AC Generator – Rotating Field, Stationary Armature
Current lags voltage in a(n) ________ circuit.
Voltage lags current in a(n) _________ circuit.
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