Question

Describe a brush less alternator with a.c. exciter and static-A. V.R.

State the output voltage characteristics for this type of machine.

In this machine slip-rings and brushes are eliminated and excitation is provided not by conventional direct current exciter but by a small alternator.

The a.c. exciter (Figure) has the unusual arrangement of three-phase output windings on the rotor and magnetic poles fixed in the casing.

The casing pole coils are supplied with direct current from an automatic voltage regulator of the type described in the previous section.

Three-phase current generated in the windings on the exciter rotor passes through a rectifier assembly on the shaft and then to the main alternator poles.

No slip-rings are needed.

The silicon rectifiers fitted in a housing at the end of the shaft are accessible for replacement and their rotation assists cooling. The six rectifiers give full-wave rectification of the three-phase supply.

Brushless alternator

Self-Excited Error Operated A.C. Generator

switchboard

Static excitation system principle

Excitation of self excited brushless a.c generator

This type of generator is the most common type used on modern ships.

1. The main rotor has residual magnetism which produces a weak magnetic field.

2. So when the rotor turns, this weak field flux cuts the main stator winding.

3. A low voltage is generated in the main stator.

4. This output is fed back to the AVR which rectifies the a.c. power to d.c.

5. This d.c. is fed to the exciter stator.

6. A stationary magnetic field is created in the exciter stator by the d.c. fed to it.

7. The exciter rotor, when it rotates, cuts this stationery field.

8. Since the exciter rotor windings are wound to produce 3 phase a.c. power, 3 phase a.c. is generated in it.

9. All of this 3 phase a.c. is led to a bank of rotating diodes mounted on the same shaft.

10. The diodes convert all of this a.c. to d.c.

11. This d.c is the “excitation” of the main generator.

12. This d.c. in the main rotor add to the weak magnetic field already there due to residual magnetism of the main rotor.

13. So the total field flux produced by the generator rotor is now increased: field flux due to current feedback from AVR & field flux due to residual magnetism.

14. Since more flux now cuts the generator stator windings, a higher voltage is generated.

15. This process of voltage build-up continues until the generator rated terminal voltage (usually 440V) is reached. The AVR regulates the voltage to this value.

b)

Both the 'conventional' alternator with d.c. exciter/carbon pile regulator combination and the brushless machine described have error-operated AVR and excitation systems (Figure).

The voltage has to change for the AVR to register the deviation from normal and to then adjust the excitation for correction.

The suddenness of the initial volt dip (blamed specifically on transient reactance) is such that the response from the error-operated system cannot come until the dip is in the second slower stage.

Thus neither machine can prevent the rapid and vertical volt dip due to transient reactance, but the faster acting voltage regulator of the brushless machine will arrest the voltage drop sooner on the slower secondary part of its descent.

The carbon pile regulator is slow compared with the static type but better recovery by the brushless alternator is also achieved by field forcing, i.e. boosting the excitation to give a quicker build-up.

AVR will control generator voltage to ± 2.5% [or better] of it’s set value over the load range.

This is the steady state voltage regulation.

Transient voltage dip is usually limited to 15% for a specified sudden load change with recovery to rated voltage with in 1.5 seconds.

Typical volt dip/recovery pattern for an alternator

Voltage dip recovery for brushless alternator [error operated]