The slides, written out
AI-drawn illustration. Simplified to explain the idea, not to scale.
Slide 1 · SYNCHRONISING ALTERNATORS
SYNCHRONISING ALTERNATORS
Conditions, Correct Practice & Consequences of Incorrect Synchronising
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Slide 2 · a) CONDITIONS FOR
a) CONDITIONS FOR
SYNCHRONISING ALTERNATORS
Before you close the incomer's breaker onto the busbars,
you must satisfy four conditions.
1
SAME VOLTAGE
The incoming machine's RMS voltage must equal
the busbar voltage. You set this with the automatic
voltage regulator / field excitation; check it on the
voltmeter with the selector switch.
2
SAME FREQUENCY
The incoming machine's frequency must match the
running machine (or busbar). You adjust this with
the prime mover's speed governor.
3
SAME PHASE SEQUENCE
The incoming machine's phase rotation (R-Y-B or
A-B-C) must match the busbars, so that like phases
are connected when the breaker closes.
Phase sequence is normally verified using an approved
synchronising lamp arrangement or a synchroscope.
If the rotation is wrong, synchronising cannot be
achieved correctly — the remedy is to interchange
any two phase connections.
4
SAME PHASE ANGLE (IN PHASE)
The incoming voltage phasors must align with the busbar
phasors, i.e. corresponding line-to-line voltages reach their
positive maximums at the same instant. You judge this
with the synchroscope: the pointer should rotate slowly in
the FAST direction and you close the breaker slightly
before the 12 o'clock position so that the contacts
close when the machines are in phase.
Slide 3 · b) CUMULATIVE DAMAGE FROM INCORRECT SYNCHRONISING
b) CUMULATIVE DAMAGE FROM INCORRECT SYNCHRONISING
If you close the breaker when the machines are not in synchronism — typically because the phase angle between the incoming and running voltages is too large at closure — this is the picture:
At the instant the breaker closes, the voltage phase difference drives a large circulating current between the machines.
That current produces a powerful magnetic force that tries to "pull" the generators into synchronism — forcing rapid acceleration of one rotor and deceleration of the other.
The consequence is severe mechanical and electrical stress, which over repeated or severe mis-synchronisation events becomes cumulative damage.
The large circulating current may trip each generator's breaker — ultimately a blackout.
CUMULATIVE DAMAGE INCLUDES:
MECHANICAL
• Deformation of stator windings
• Movement between the stator core and frame
• Twisted rotor shafts
• Localised crushing of shaft-end keyways
• Broken couplings
• Severe vibration at the loss-of-synchronism condition
ELECTRICAL
• Failure of the rotor diodes in brushless machines
• Flashover at the slip rings in the case of alternators with a rotating armature
OPERATIONAL
• The large circulating current may trip each generator's breaker — ultimately a blackout.
So the "cumulative" angle the examiner wants: a single sloppy close might not break anything outright, but the repeated mechanical shock and electrical stress progressively degrade windings, shafting, and couplings until a catastrophic failure or a blackout shows up later, often at the worst moment — like during cargo discharge or manoeuvring.
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Slide 4 · KEY TAKEAWAYS
KEY TAKEAWAYS
Synchronising alternators is essential for safe and reliable power system operation.
Always ensure the four synchronising conditions are met.
Use instruments (voltmeter, frequency meter, synchroscope) and approved lamp methods correctly.
Close the breaker only when the synchroscope pointer is rotating slowly in the FAST direction and near the 12 o'clock position.
Incorrect synchronising causes severe mechanical and electrical stress, leading to cumulative damage and possible blackout.
REFERENCE
This content is based on accepted maritime engineering principles and the following sources:
IMO – SOLAS Regulations (Electrical Installations)
IMarEST – The Marine Engineer's Handbook
DNV – Rules for Ships: Electrical Installations
ABB – Synchronizing of Alternators (Technical Guide)
Drafted by SailR from our daily lessons. AI-drawn illustration. Simplified to explain the idea, not to scale. Last updated 19 Oct 2026.
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