AC Machines: Three-Phase Alternators -- part 2

The Rotor

The rotor is the rotating member of the machine. It provides the magnetism needed to induce voltage into the stator windings. The magnets of the rotor are electromagnets and require some source of external DC to excite the alternator. This DC is known as excitation current. The alternator cannot produce an output voltage until the rotor has been excited. Some alternators use slip rings and brushes to provide the excitation current to the rotor. A good example of this type of rotor can be found in the alternator of most automobiles. The DC excitation current can be varied in order to change the strength of the magnetic field. A rotor with salient (projecting) poles is shown.

++++9 Basic brushless exciter circuit. Rectifier Field Armature; +, -

++++10 Brushless exciter assembly.

The Brushless Exciter

Most large alternators use an exciter that contains no brushes. This is accomplished by adding a separate small alternator of the armature type on the same shaft of the rotor of the larger alternator. The armature rotates between wound electromagnets. The DC excitation current is connected to the wound stationary magnets. The amount of voltage induced in the rotor can be varied by changing the amount of excitation current supplied to the electromagnets. The output voltage of the armature is connected to a three-phase bridge rectifier mounted on the rotor shaft. The bridge rectifier converts the three-phase AC voltage produced in the armature into DC voltage before it’s applied to the main rotor windings. Because the armature, rectifier, and rotor winding are connected to the main rotor shaft, they all rotate together and no brushes or sliprings are needed to provide excitation current for the large alternator. A photograph of the brushless exciter assembly. The field winding is placed in slots cut in the core material of the rotor.

++++11 Two-pole rotor slotting.

Alternator Cooling

There are two main methods of cooling alternators. Alternators of small kilovolt ampere rating are generally air-cooled. Open spaces are left in the stator windings, and slots are often provided in the core material for the passage of air.

Air-cooled alternators have a fan attached to one end of the shaft that circulates air through the entire assembly.

Large-capacity alternators are often enclosed and operate in a hydrogen atmosphere. There are several advantages in using hydrogen. Hydrogen is less dense than air at the same pressure. The lower density reduces the wind age loss of the spinning rotor. A second advantage in operating an alternator in a hydrogen atmosphere is that hydrogen has the ability to absorb and re move heat much faster than air. At a pressure of one atmosphere, hydrogen has a specific heat of approximately 3.42. The specific heat of air at a pressure of 1 atmosphere is approximately 0.238. I.e., hydrogen has the ability to absorb approximately 14.37 times more heat than air. A cutaway drawing of an alternator intended to operate in a hydrogen atmosphere.

++++12 Two-pole, turbine-driven, hydrogen-cooled alternator.

Frequency

The output frequency of an alternator is determined by two factors:

1. the number of stator poles

2. the speed of rotation of the rotor Because the number of stator poles is constant for a particular machine, the out put frequency is controlled by adjusting the speed of the rotor. The following chart shows the speed of rotation needed to produce 60 hertz for alternators with different numbers of poles.

rpm : Stator Poles

3600 : 2

1800 : 4

1200 : 6

900 : 8

The following formula can also be used to determine the frequency when the poles and revolutions per minute (rpm) are known:

f = PS /120

f = frequency in hertz

P = number of poles per phase

S = speed in rpm

120 = a constant

=== Exc.===

What is the output frequency of an alternator that contains six poles per phase and is turning at a speed of 1000 rpm?

Solution:

f =6 3 1000 rpm

/ 120

f =50 Hz

===

Output Voltage

Three factors determine the amount of output voltage of an alternator:

1. the length of the armature or stator conductors (number of turns)

2. the strength of the magnetic field of the rotor

3. the speed of rotation of the rotor The following formula can be used to calculate the amount of voltage

induced in the stator winding:

E =BLV / 10^8

where 10^8 = flux lines equal to 1 weber

E =induced voltage (in volts)

B =flux density in gauss

L =length of the conductor (in cm)

v =velocity (in cm/s)

One of the factors that determines the amount of induced voltage is the length of the conductor. This factor is often stated as number of turns of wire in the stator because the voltage induced in each turn adds. Increasing the number of turns of wire has the same effect as increasing the length of one conductor.

Controlling Output Voltage:

The number of turns of wire in the stator cannot be changed in a particular machine without rewinding the stator, and the speed of rotation is generally maintained at a certain level to provide a constant output frequency. Therefore, the output voltage is controlled by increasing or decreasing the strength of the magnetic field of the rotor. The magnetic field strength can be controlled by controlling the DC excitation current to the rotor.

Paralleling Alternators

Because one alternator cannot produce all the power that is required, it often becomes necessary to use more than one machine. When more than one alternator is to be used, they are connected in parallel with each other.

Several conditions must be met before parallel alternators can be used:

1. The phases must be connected in such a manner that the phase rotation of all the machines is the same.

2. Phases A, B, and C of one machine must be in sequence with Phases A, B, and C of the other machine. For example, Phase A of Alternator 1 must reach its positive peak value of voltage at the same time Phase A of Alternator 2 does.

3. The output voltage of the two alternators should be the same.

4. The frequency should be the same.

++++13 The voltages of both alternators must be in phase with each other. Alternator 1; Alternator 2

Determining Phase Rotation:

The most common method of detecting when the phase rotation (the direction of magnetic field rotation) of one alternator is matched to the phase rotation of the other is with the use of three lights. In ++++14, the two alternators that are to be paralleled are connected together through a synchronizing switch. A set of lamps acts as a resistive load between the two machines when the switch contacts are in the open position. The voltage developed across the lamps is proportional to the difference in voltage between the two alternators. The lamps are used to indicate two conditions:

1. The lamps indicate when the phase rotation of one machine is matched to the phase rotation of the other. When both alternators are operating, both are producing a voltage. The lamps blink on and off when the phase rotation of one machine is not synchronized to the phase rotation of the other machine. If all three 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 the lamps blink on and off but not in unison, the phase rotation between the two machines is not correctly matched, and two lines of Alternator 2 should be switched.

++++14 Determining phase rotation using indicator lights. Alternator 1 -- Alternator -- 2 Load; Synchronizing; Indicator lights 2

The lamps also indicate when the phase of one machine is synchronized with the phase of the other machine. If the positive peak of Alternator 1 does not occur at the same time as the positive peak of Alternator 2, there is a potential between the two machines. This permits the lamps to glow. The brightness of the lamps indicates how far out of synchronism the two machines are. When the peak voltages of the two alternators occur at the same time, there is no potential difference between them.

The lamps should be off at this time. The synchronizing switch should never be closed when the lamps are glowing.

++++15 Synchroscope.

++++16 AC voltmeter indicates when the two alternators are in phase. AC voltmeter Load; Synchronizing; Alternator

The Synchroscope:

Another instrument often used for paralleling two alternators is the synchro scope. The synchroscope measures the difference in voltage and frequency of the two alternators. The pointer of the synchroscope is free to rotate in a 360 degree arc. The alternator already connected to the load is considered to be the base machine. The synchroscope indicates whether the frequency of the alternator to be parallel to the base machine is fast or slow. When the voltages of the two alternators are in phase, the pointer covers the shaded area on the face of the meter. When the two alternators are synchronized, the synchronizing switch is closed.

If a synchroscope is not available, the two alternators can be paralleled using three lamps, as described earlier. If the three-lamp method is used, an AC voltmeter connected across the same phase of each machine indicates when the potential difference between the two machines is zero. That is the point at which the synchronizing switch should be closed.