Brushless Alternator Working, Paralleling & AVR Explained

1. Excitation Without Touching Parts

A brushless alternator is two alternators built end-to-end on one shaft. The smaller one is the exciter; the larger one is the main machine — and they are wired back-to-front on purpose.

Exciter stage

Stationary exciter field feeds a rotating exciter armature (power coils).

Shaft rectifier

Three-phase exciter output rectifies on the rotor (rotating rectifier assembly) straight into the main rotating field.

Main armature

The stationary main armature delivers load power. Varying the small exciter current steers the entire output.

Bootstrapping: residual magnetism seeds the whole loop — leftover field makes a little EMF, which feeds field current, which grows flux, until saturation caps it at equilibrium. If field resistance exceeds the critical value, the loop never builds and the set stays dead.

Maintenance is sea-duty realism: isolate, lock out and post notices before touching anything. Clean ventilation passages and filters, prove stator and rotor insulation, hold the air gap at 1.5–2 mm, check terminal-box tightness and cable glands, keep forced ventilation and heaters working — then run a no-load test and log noise, temperature and volts against PMS.

2. Paralleling Ritual — Synchroscope and Lamps

Four conditions, no exceptions: same voltage, same frequency, same phase, same phase sequence. Miss any one and closing the breaker parallels two disagreeing sources through the bus.

1. Meters proven, set started correctly, pressures, temperatures and alarms sane, prime mover stable for a full minute.

2. Match voltages bus-to-incoming, selector to the incoming set, and read the synchroscope: clockwise means fast, counter-clockwise slow.

3. Trim the governor to a slow clockwise drift (~4 s/rev) — incoming slightly fast so it takes load on closing instead of motorising and dragging the bus.

4. Close the breaker just before 12 (mechanism delay lands the contacts on phase), selector off, share load across governors.

When the scope dies, lamps take over. Three bulbs wire bus-to-incoming: as speeds differ they breathe bright and dark at the slip rate. Dark-lamp wiring goes dark at matched phase; bright-lamp wiring goes bright. The sequence method is preferred — lamp brightness appears to rotate, clockwise for fast, anti-clockwise for slow, and correct sync reads the top (key) lamp dark with the two bottom lamps equally bright.

Oral closer: name the four conditions, the slow-clockwise drift, close-just-before-12 for mechanism delay, and 2-bright-1-dark as the lamp backup.

3. AVR Loop & the Three Protections

The sensing unit steps output down, rectifies and smooths it to a DC signal, compares it against a zener-diode reference, and amplifies the error into the field-circuit thyristors — voltage restored against load, speed, temperature and power-factor drift.

Droop CTs

Let paralleled sets share reactive load instead of fighting over it.

V/Hz slope

Sheds excitation under speed so turbocharged recovery is not strangled.

Soft-start + latch

Ramps volts up sanely; latched shutdown cages runaway excitation until standstill.

Protection functions: overcurrent and short-circuit protection respond to excessive current; reverse-power protection detects real-power flow from the bus into a generator whose prime mover is no longer supplying sufficient torque; undervoltage protection responds to loss of voltage. On AC systems the relay detects the direction of real power, rather than reverse current as used in a simple DC explanation. Pickup values and time delays depend on the installation, prime mover and coordination study.

Testing: prove relay operation by the maker-approved test method, such as secondary injection or a relay test set, with the circuit isolated and protected as required. Preferential-load shedding and breaker trips must be verified using the vessel’s approved test procedure; do not intentionally motor a paralleled generator or alter live protection settings as a routine test.

4. Parallel Load Sharing — Governor kW and AVR kVAr

When two alternators are connected to the same bus, they run at the bus frequency and their output must be coordinated. The prime-mover governor controls the mechanical input and therefore the generator’s share of real power (kW). The AVR changes field excitation and therefore mainly controls terminal voltage and the share of reactive power (kVAr).

For a load increase, the governor increases prime-mover torque so the set contributes more kW; the AVR responds to voltage change by adjusting field current. These controls interact through the network, so use the vessel’s load-sharing mode and observe the meters rather than treating the two adjustments as interchangeable. A mismatch in excitation can make one set carry excessive reactive current even when its kW share looks normal. Unequal governor response can overload one prime mover while total bus load appears acceptable.

For manual operation, compare the individual kW and kVAr (or power-factor) indications after closing, then make small adjustments using the correct control and allow the readings to settle. Automatic controllers may perform this balance through droop, cross-current compensation or digital sharing. The control method and allowed operating range are specific to the switchboard, alternator and prime mover.

Warning — Do not chase the wrong meter.

Governor adjustment mainly changes kW sharing; excitation adjustment mainly changes kVAr sharing and voltage. Follow the approved operating procedure and do not exceed engine, alternator or excitation limits.