Unit 32 - Three Phase Alternators

Revolving Armature Type

- 3 separate windings connected in delta or wye - Windings rotated inside a stationary magnetic field (similar to DC generator) - Generated AC is brought to the load unchanged by means of slip-rings - Found only in alternators of low power rating

Synchronization of Alternator

- Generators can be added to removed depending on load - Before connecting two generators together, it must be synchronized - Usually done automatically

How do we typically Control Output Voltage

Increasing or decreasing the strength of the magnetic field of the rotor

Increasing the number of turns of wire has the same effect as _______

Increasing the length of the conductor

The revolving armature type alternator is capable of _____ power output and is, therefore, _____ often used than the revolving field type.

Less; Less

North America is powered by AC generators connected together in______ powered by _____

Parallel Steam Turbines

_______ pole pieces become electromagnets

Rotator

A(n) _____ is used to measure the difference in voltage and frequency between two alternators.

Synchronscope

What regulates the main exciter output (Ix)?

The DC control current Ic from the pilot exciter

Using Indicator Lights

Two alternators connected together through a synchronizing switch Set of lamps acts as resistive load when switch contacts are open Voltage developed across the lamps is proportional to the difference in voltage between the two alternators

Revolving Armature Type has a _______

Very Limited Output Power (limited in amount of output voltage and kVA capacity it can develop)

To synchronize an alternator, we must:

1. Adjust speed of regulator of the turbine so the frequency of alternator is close to frequency of the system 2. Adjust excitation so that voltages are equal 3. Observe the phase angle between voltages by means of a synchroscope, when the pointer is at the 0 mark, proceed 4. The line circuit breaker is closed, connecting the generator to the system

What are mounted on the same shaft and turn together with the alternator?

Armature of the AC exciter and the rectifiers

What does the main exciter do?

Feeds the exciting current to the field of the synchronous generator through brushes and slip-rings

Using a(n) _____ is one method of preventing induced voltage from becoming excessive.

Field discharge resistor

When are rotating armature types used?

For low power ratings and not used to supply electric power in large quantities Used for output power less than 5kVA

In order to increase the efficiency of the cooling, some alternators operate in a(n) _____ atmosphere.

Hydrogen

Excitation is changed automatically as it ________

responds to the load changes of the system in order to maintain a constant line voltage output from the alternator

What are the 3 factors that determine the output voltage of an alternator?

1. Conductor length of the armature or stator winding 2. Strength of the rotator magnetic field 3. Speed of rotor

Rotor

- The rotating member - Provides Magnetism needed to induce voltage into stator windings - Magnets of the rotor are electromagnets and require some source of external DC to excite the alternator - DC is known as excitation current

Alternators of small kilovolt-ampere rating are generally _____-cooled.

Air

The synchronizing switch should never be ____ when the lamps are glowing

Closed

What is a key part of a large alternators overall design?

DC field excitation

Two basic types of Alternators:

1. Revolving armature type (least often used) 2. Revolving field type

A three-phase alternator containing six poles is turning at 1000 RPM. What is the output frequency?

50 Hz

An alternator containing eight poles per phase is turning at 900 RPM. What is the output frequency?

60 Hz

What must an exciter ensure?

A stable terminal voltage and must be able to respond to sudden load changes in order to maintain stability

What can be varied in order to change the strength of the magnetic field?

DC Excitation current

Power is carried to the outside circuit of a revolving armature type alternator via brushes riding against the _____.

Slip-rings

Rotating armature requires ______ and ______ to conduct the current from the armature to the load

Slip-rings Brushes

Short-Circuit Test

- Excitation is reduced to zero and the 3 stator terminals are short-circuited together - With generator running at rated speed, the exciting current is gradually raised to its original Ixn - Resulting short circuit current Isc in the stator windings is measured

Single phase alternators exist and are used as _____

portable power units for emergency home use or to operator power tools in remote locations

Open-Circuit Test

- Generator is driven at rated speed and the exciting current is increased until the rated line-to-neutral voltage is obtained - Corresponding exciting current Ixn and line-to-neutral voltage En are recorded

Stator

- Identical to that of a 3-phase induction motor - Composed of a cylindrical laminated core containing a set of slots that carry a 3-phase lap winding (always connected in wye and neutral connected to ground) - In general: the greater the power rating, the higher the voltage - Nominal line-to-line voltage seldom exceeds 25 kV because the increased slot insulation takes up valuable space at the expense of the copper conductors

What is the difference between the brush and brushless types of excitation?

3-phase rectifiers replace the commutator, slip-rings and brushes - Basically, have traded the commutator (a mechanical rectifier) for an electronic rectifier

9. Before two alternators can be connected in parallel, what 4 things must be true between the two machines?

1. Output voltages need to be equal 2. Phase rotations should be equal 3. Output voltages must be in phase 4. Frequency should be the same

What are the 2 main reasons a wye connection is preferred over delta for stators? (Reason 1)

1. Phase voltage is only 58% that of the line voltage - can reduce the amount of insulation in the slots which means you can increase the cross section of the conductors (increase current and power output)

What can result at high voltages due to difficulty of insulating brushes, armature, and slip-rings?

Arc-overs and Short Circuits

Since the voltage applied to the rotating field is low voltage DC, what problems is eliminated?

The problem of high voltage arc-over at the slip-rings does not exist

What is important when it comes to maintaining stability when there is a sudden change in load?

The reaction time of the exciter

What is a distortion due to in delta connections?

The third harmonic voltages which has a frequency 3x that of the fundamental frequency (180 Hz)

Why employ a brushless excitation system?

To eliminate the problem of brush wear and maintenance

A brushless exciter uses _________

Stationary electronmagnets

The non-rotating part of a revolving-field-type alternator is called the _____.

Stator

___________ alternators have a fan attached to one end of the shaft.

Air-cooled

What machines operate on the same principle of electromagnetic induction as DC generators, but they have no commutator?

Alternators

When a brushless exciter is used, what converts the AC produced in the armature windings into DC before it is supplied to the field winding?

Bridge Rectifiers

An alternator that has a separate small alternator mounted on the shaft of its rotor is equipped with a(n) _____.

Brushless Exciter

How is the revolving field type constructed?

By placing 3 sets of windings 120 degrees apart

The power rating of the main exciter depends on ______

The capacity of the alternator

What is one factor that determines the amount of induced voltage? What is this often stated as?

The length of the conductor The number of turns of wires in the stator because the voltage induced in each turn adds

The lamps are used to indicate what 2 conditions? (descriptions of each on next 2 cue cards)

1. Indicate when the phase rotation of one machine is matched to the phase rotation of the other 2. When the phase of one machine is synchronized with the phase of the other machine

What are the advantages of using hydrogen?

- Hydrogen is less dense than air - Lower density reduces windage loss of the spinning rotor - Hydrogen has the ability to absorb and remove heat much faster than air

The frequency of the main exciter is generally _______ times the alternator frequency (60Hz)

2-3 times

Reason 2:

2. The distorting line-to-neutral harmonics do not appear between the lines because they effectively cancel each other out. Therefore the line voltages remain sinusoidal under all load conditions

Typically for a 1000 kW alternator, a _______ is required. Whereas a 500 MW alternator requires a ______

25 kW exciter is required (2.5% of its rating) 2500 kW exciter (0.5% of its rating)

What does a brushless system consist of?

3-phase stationary field generator (revolving armature type) whose AC output is rectified by a group of rectifiers

At a pressure of 1 atm, hydrogen has a specific heat of approx. ____ The specific heat of air at a pressure of 1 atm is approx. ____ This means hydrogen has the ability to absorb approx. _____ times more heat than air

3.42 0.238 14.37

4. Direct current must be supplied to the field before the alternator can produce an _______ 5. The direct current supplied to the field is called ______ 6. The output frequency of an alternator is determined by ______

4. output voltage 5. Excitation current 6. The number of stator poles and the speed of rotation

7. The output voltage is controlled by ______ 8. Three factors that determine the output voltage of an alternator are:

7. The amount of DC excitation current 8. a) Conductor length of the armature or stator winding b) Strength of the rotator magnetic field c) the speed of the rotor

When a large load is lost (major disturbance) on the utility system, the alternator can experience __________. What must the exciter do?

A large drop in the voltage across the terminals of the alternator Exciter must react quickly to keep the AC voltage from falling

Synchroscope

An instrument which has a pointer which continually indicates phase angle Zero mark indicates the voltages are in phase If generator freq is higher than system, pointer rotates clockwise If generator freq is lower than system, pointer rotates CCW (generator is lagging) Turbine speed is fine-tuned so that the pointer barely creeps across the dial Final check is made to see that the alternator voltage is still equal to the system voltage

How does the Revolving Armature Type Operate?

As the rotor spins, a voltage is induced and its value depends on the speed of rotation and the DC exciting current in the stationary poles The frequency of the voltage depends on the speed and the number of poles on the field

How is the increase in frequency obtained?

By using more poles on the exciter than on the alternator

How can the magnetic field strength can be controlled by _______

Controlling the DC excitation current to the rotor

Alternators use the same principle as _________. Difference is they ______

DC Generators Don't have a commutator to change AC produced in armature to DC

The direct current that is an input to the alternator's rotor is the _____ current.

Excitation

The higher power output alternators use a revolving _____.

Field

A grid, such as our power utility grid is said to be an ________ because it has so many generators essentially connected in parallel that neither the voltage nor the frequency of the grid can be altered

Infinite Bus

Reactance Xs is an _______ and is typically ______ to _____ times greater than R (we can typically neglect resistance unless we are interested in efficiency or heating effects)

Internal impedance, 10-100x greater than Resistance

What does the nominal line voltage of an alternator depend on?

Its kVA rating

What is one factor that controls the output frequency of an alternator?

Number of stator poles and rotor speed

By increasing or decreasing the strength of the magnetic field of the rotor, the _________of an alternator is controlled.

Output voltage

If one alternator does not produce sufficient power to supply a load, another alternator can be connected to the circuit. The second alternator is connected in _____.

Parallel

The direction of magnetic field rotation is known as _____ rotation.

Phase

RPMs needed to Produce 60 Hz

RPM/#Stator Poles 3600/2 1800/4 1200/6 900/8

The rotating part of a revolving-armature-type alternator is called the _____.

Rotor

Some alternators use _____ and _____ to provide the excitation current to the rotor

Slip-rings and brushes

How is the reaction time of the exciter accomplished?

Using two DC generators as the exciters 1. A main exciter 2. A pilot exciter

12. Two devices used to prevent a high voltage being induced in the rotor when the DC excitation current is stopped are: 13. Many large alternators use a brushless exciter to supply ______

12. Field-discharge Resistor; Diode 13. Direct current to the rotor winding

What is the exciter voltage between under normal conditions? How is it regulated?

125V and 600V - Either manually or automatically by control signals that are sent to control Ic which is produced by the pilot exciter

1.

- If all 3 lamps blink on and off at the same time, or in unison, the phase rotation of alternator 1 is correctly matched to the phase rotation of alternator 2 - If they blink on and off but not in unison, phase rotation between the two machines is not correctly matched, and two lines of alternator 2 should be switched

2.

- If the positive peak of Alt 1 does not occur at the same time as the positive peak of Alt 2, there is a potential between the two machines. This permits lamps to glow. Brightness indicates how far out of synch they are - When peak voltages of the two machines occur at the same time, there is no potential different between them. The lamps should be off

How can you check phase alignment using a voltmeter

- Two alternators are parallel using 3 lamps - AC Voltmeter connected across the same phase of each machine indicates when the potential different between the two machines is 0 (synchronizing switch should eb closed at this point)

Revolving Field Type

- Used for larger output powers - Uses a stationary armature called a stator and a rotating magnetic field - Permits higher voltage and kVA ratings because the outside circuit is connected directly to the stator and is not routed through slip-rings and brushes

Field Discharge Protection

- When DC excitation current is disconnected, the collapsing field can cause contacts to arc and damages to rotor windings

Output Frequency of an alternator is determined by what two factors?

1. Number of stator poles (constant) 2. Speed of rotation of the rotor

What two devices can be used to prevent a high voltage being induced in the rotor when DC excitation current is stopped?

1. Field-discharge Resistor 2. Diode

An alternator is said to be synchronized when it meets to following conditions:

1. Freq. Alter = Freq. System 2. Voltage Alt = Voltage System 3. Voltage alt. is in phase with Voltage of system 4. The phase sequence of the alternator is the same as that of the system

What are the two methods for determining when the phase rotation (direction of magnetic field rotation) of one alternator is matched to the phase rotation of the other?

1. Indicator Lights (3 lamps connected between 2 alternators) 2. Synchroscope (can also be used to determine difference of frequency between 2 alternators)

REVIEW: 1. Two basic types of 3-phase alternators are: 2. Rotating-armature type is the least used because of its ____ 3. The rotating-field-type alternator contains ______

1. Rotating-armature, rotating-field 2. Limited voltage and power rating 3. Electromagnets

What are the 2 main methods of cooling alternators?

1. Small alternators are generally air-cooled (have a fan attached to one end of the shaft that circulates air through the entire assembly) 2. Large capacity - hydrogen atmosphere

10. Three lamps connected between the two alternators can be used to test for _____ 11. A synchroscope can be used to ________

10. Phase Rotation 11. Determine phase rotation and difference of frequency between two alternators

An alternator containing eight poles per phase is turning at 1500 RPM. What is the output frequency?

100 Hz