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Generator Maintenance, Generator Rewinding

Synchronisation Of Generators: Procedure, Conditions & More

Without proper synchronisation of generators, they are not able to operate at its maximum efficiency and eventually, this can potentially cause damage to the healthy power system.

Though such a situation is not desirable, a simple generator overhauling by a professional can be done to have a quick assessment of the equipment’s state of health – afterwards, they will determine if it’s necessary to either repair, redesign or completely replace the machinery.

But instead of immediately going into that stage, you should first understand what synchronisation between generators is about to ensure that they are in good working condition.

Synchronisation between generators

Typically, this process of connecting generators in parallel is done before it is being reconnected to the power system.

A standard alternating current (AC) generator will have to match the parameters of another generator at the same exact frequency and speed in order for it to supply power to an electrical grid. On the other hand, a direct current (DC) generator will simply have to be connected to a power network by making adjustments to its field excitation or speed – without having to match them exactly.


Before synchronisation can happen – there are certain characteristics that the engineer has to look out for:

  • Frequency: Frequencies of both generators have to be the same number.
  • Line voltage/voltage magnitude: Both generators should supply or be programmed to give out the same voltage.
  • Phase sequence: There should be the same number of phases, single or three phases for both systems.
  • Phase angle: For both the generator voltage and voltage of the bus bar – the phase angle should be at zero.
  • Waveform: The waveforms have to be in sync with one another for the system to successfully operate.


First and foremost, you will have a generator which supplies power to the bus bars at rated frequency and voltage, such as by opening the valves on a steam turbine.

Afterwards, another generator will come in to be synchronised in parallel with the previous generator. At this point, the speed of the alternator of the second generator will be increased to have a varied frequency, adjusting the speed until it is the same exact frequency as the first generator.

Following up to this step would the adjusting of the voltages. Once the three voltages are in phase with one another – it would mean that the two generators are synchronised.


Though you have understood the procedure of how it works – the techniques involved have not been discussed. Here are the common techniques which are used to check on the previous conditions as mentioned and achieve total synchronisation between them.

Three Dark Lamps Technique

Three lamps will be connected across the switches that are to be synchronised.

Correct Phase Sequences

The three lamps will switch on and off concurrently – and will darken or brighten together in sequence.

Wrong Phase Sequence

The lamps will switch on and off after one another and not be in harmony.

How to fix it

After turning off the machine, change the connections of any two leads of one generator. Ultimately, the aim is to reduce the rate of flickering to match the frequency – which can be done by adjusting the alternator’s speed with its prime mover control.

Two bright and one dark lamp technique

One lamp will be connected in between corresponding phases whereas the remaining two are cross-connected between two other phases.

Correct phase sequences

Unlike the three dark lamps method where they have to be simultaneously lit up or dim down together, this method requires the set up to have flicker in the exact order to have the right frequency.

Wrong phase sequences

The sequence of the lamps will be in the wrong order. Either indicating that there has to be a reduced or increased alternator speed.

How to fix it

Just like the three dark lamps method, the engineer will have to adjust the primer mover control accordingly until the desired flickering of light is attained.

Synchroscope technique

Using an instrument known as Synchroscope – the engineer will have to observe the pointer until the exact frequency is achieved.

Correct phase sequences

With the same goal to achieve the same frequency, in this case, the pointer will remain stationary once there is the same number.

Wrong phase sequences

The pointer will rotate to either face the fast or slow side to state whether the incoming alternator is lower or higher than the other generator’s frequency.

How to fix it

The speed of the alternator has to be controlled to either adjust the frequency to become higher or lower to arrive at the same number of frequency.

Keeping track of your generator’s condition is something you have to do regularly. But at times, you may not be able to spot every single and error and that’s when a professional who does generator maintenance in Singapore has to come in.

So, do what you must to maximise the efficiency of your generator. And when necessary – have a generator rewinding done to get your equipment back in shape.

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