In the video below, somebody synchronizes two 3-phase generators using sync lamps. When the three bulbs blink at very low frequency, and just before going dark all of them (which means that the frequencies of the two gensets are nearly equal and the voltage difference between them is just about to become zero), the two power sources are put in parallel and, from what I understand, they remained locked in frequency, phase shift and amplitude to each other. Why does this autosynk happen? The one who run the demonstration offered a short explanation but it is not clear enough.

(Evidently he does not refer to something like putting in parallel two batteries of 1.2 and 1.5 V, situations in which the resulted voltage would be somewhere in the interval 1.2 - 1.5 V but there will be voltage drops on the internal rezistors of the batteries, which means a waste of power.)

Synchronizing AC generators using sync lamps

Source: Synchronizing AC generators using sync lamps

  • \$\begingroup\$ If one is more heavily loaded it slows down, generates less voltage, which transfers the excess load to the other one. \$\endgroup\$
    – user16324
    Aug 1, 2021 at 16:48

5 Answers 5


I referenced that video (or another in the series) in my answer to Electrical phase in a power grid and power transmission which may help.

In the video below, somebody synchronizes two 3-phase generators ...

In electrical engineering we often refer to these as "machines" rather than specifically as a motor or a generator because they can operate as either. If connected to the grid and driven they will act as a generator and export power to the grid. If a mechanical load or brake is applied to the shaft they will act as motors and import energy from the grid to drive the load.

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Figure 1. A generator and load on three-phase system. Note here how both machines are in synchronisation due to their direct connections. Image source: Instrumentation Tools.

Why does this autosynk happen?

I've explained how synchronisation is achieved in my other answer and you've summed it up at the top of your question. A better follow up question might be, "What would it take to break synchronisation?" The answer is that you would need to exceed the machine's capability to stay in synchronisation.

  • In the case of the motor you just have to apply a large enough load or stall the motor. The problem is that the motor will draw more current to the point that the windings will reach extreme temperatures, the insulation burn off and internal short-circuits occur followed by smoke and fire. To prevent this overload trips are installed which allow short overloads during startup but trip out on a prolonged fault to protect the motor.
  • In the case of the generator being over-driven a similar situation will occur when the generator tries to export enough energy to speed up the whole grid. Again, an overload will trip out to protect the machine from overload.

Have a look at my other answer and see if the combination answers your question.


The output frequency of a synchronous generator is entirely determined by speed. The output voltage is determined by speed and excitation. The speed of a generator is controlled by the speed of the prime mover that is driving it. In the video, the prime mover is an induction motor with an electronic speed control.

If one generator starts to lag behind the other in phase or voltage, it and any electrical load will take power from the other generator to counteract that change. That will unload the prime mover of the generator that is falling behind allowing it to catch up.

In practical systems, the individual generator excitation and the prime mover "throttle" are actively controlled to keep the generator synchronized with the grid and to regulate the power delivered to the grid by the individual generator. In the video, the operator adjusted to induction motor drive "throttle" to achieve initial synchronization.


After being synchronized things stay in sync if their frequencies do not change. Things not perfectly in sync (nothing is perfect) would slowly drift apart if something is not done to force their synchronization.

The connection in parallel is sufficient to force the synchronization. Power from one reduces load on an other, allowing that other to speed up a bit and catch up to the one. You can also think of it as other imposing a load on one and slowing it down.

Generators and motors are fairly similar. The big difference is usually that while motors are designed to continuously rotate by passing the rotor from one step to another, generators will only behave like motors within a small portion of the full turn - when stationary and rotating parts are at certain alignments. The exchange of energy at these specific pionts is sufficient to cause that auto-sync you are talking about.

If they were randomly connected in parallel, they would not align though since those certain alignments would not be achieved simultaneously, and worse - could be misaligned to the point where in the example scenario one tries to drive the other backwards imposing more load rather than reducing it!


To parallel three-phase ac generators, three things must be identical: phase sequence; voltage; and frequency.

Phase sequence must be the same, abcabca... In the video, all lights are dark or bright together. If the phase sequence was incorrect, one would be bright, one dark and one dim.

Voltage is a little tricky for induction motors used as generators. Synchronous machines allow adjustment of excitation to adjust the voltage produced. The voltage you get with induction motors is not adjustable.

Generators are driven by prime movers. A diesel engine, steam turbine, water turbine (water dam). The prime mover determines the frequency.

The video shows synchronizing induction motor generators using sychronizing lamps. Normally (on ships), you would have a synchroscope with synchronizing lamps as backup. Actually, if you were just doing it with lamps, you want to have frequency meters. You want the oncoming generator to be running a little faster than generator on the bus. You close breaker at 11:55 or as lights go dark. In the video, the second time he does it, he misses it slightly.

You want it dark because that means the voltages on either side of the lamps are the same. If they are not, the online generator(s) force the oncoming generator to be identical to bus. The will be driven as a motor to be in step with bus. The brighter the lights when the breaker is switched on and the more current that will flow, possibly tripping breakers.

You want the oncoming generator to be running a little fast as it comes online, so that it will take up it's share of the real load, the kW, from the prime mover. Excitation of the generator will control the reactive power balance, the kVAR.

If you have multiple generators online driving a load, the action of the generators will keep themselves balanced.

If one generator slow downs, it delivers less power to the load and starts to speed up, then the other will deliver more power to the load and start to slow down. This continues until both are producing the same voltage, power factor and frequency.

If there is a difference in voltage between the generators, current will flow between the generators. The generator delivering more of the load drives the other as a motor forcing it to stay sychronized.

Think of it like two clydesdale horses yoked together pulling a load. They both have to go at the same speed to move the load.


It seems you are thinking of the electrical signal but not the mechanical force. The conversion of mechanical energy into electrical energy is TWO-WAY. The generator can also be a motor. And the voltage at the output of the generator and the mechanical force on the shaft are linked together in a very direct physical way. If the electrical frequency is changed, a large torque will be applied to the shaft which will make the mechanical speed match the new frequency.

So once the outputs are paralleled, the shafts are literally forced to rotate at the same speed because any small deviation in electrical voltage or phase will result in a large mechanical force on both shafts causing their speeds to remain matched.

If you connect two brushless DC motors together electrically, spinning the shaft of one them (mechanically) will cause the other shaft to spin also.

Large generators on the grid are, in effect, mechanically synced up by way of their common connection to the grid, and none of them can speed up or slow down without causing all of them to speed up or slow down.


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