# How do power generators generate AC at a constant frequency?

As far as I know, alternating current is generated at power plants by converting rotational energy to electricity. The frequency of the AC depends on the rotational frequency of the rotor. In USA, the frequency must be 60 Hz, correct me if I'm wrong.

However, I don't know how power plants can maintain such constant frequency. This might be easier in nuclear or thermal plants, but how about in case of wind turbines, where the wind speed isn't constant, and hence, the turbines can't spin at the constant rate?

I tried looking around, and I found this Reddit discussion. I however do not trust Reddit as a reliable source, so I wish to get a reliable answer here.

So, in short, how does a generator generate constant frequency despite inconsistencies in the energy source (such as irregular wind flows)?

First you need to understand that because the grid is alternating current, and the generator and motor are wound in such a way, that the magnetic field inside the stator (the fixed housing) will rotate around at a speed determined by the frequency and the machine type. This rotating field can be made by the grid, or the rotor, a rotating (electro)magnet.

Wind turbines are an exception to this. Most generators are synchronous machines. Meaning that the magnetic field on the rotor, matches that present at the stator.
This applies to permanent magnet generators and most generators found on ships and in emergency power systems. Also rental diesels and most on site power plants.

Wind turbines use asynchronous generators. This means that the stator field rotation does not have to match the rotor rotation. This is similar to induction motors with slip.

When the wind is pushing, it will try to push the rotor beyond the speed of the alternating magnetic field on the stator. This generates energy.
When the wind is not pushing, the rotating stator field will try to take the rotor with it, making a fan. This obviously isn't the ideal case, hence some electronics prevent this.

The major disadvantage is that these cannot start easily from blackout, but they are cheap. Compared to DC-AC converting wind turbines.

I have skipped the process of synchronizing the generator to an active grid. But this basically means that you match the voltage, speed and phase so that there is 0 volt over the circuit breaker, thus no current will flow when turned on. After this, more mechanical power can be given to convert to electricity.

This is a danish wind company with some extra info.

Those wind turbines not only need to operate at the same frequency they also need to operate at the same phase meaning the all need to reach the peak of the sinewave voltage at the same time.

Clearly that's nigh impossible just by doing mechanical adjustments.

So how is it done ? Well, electronics of course.

The energy from the wind turbine's rotor could be converted to a DC voltage. I'm assuming wind turbines generate AC as that would be most efficient. This AC is then rectified into a DC voltage which is then converted to "mains-style" AC by means of fast switching transistors.

The electronics take care of the frequency and phase. It can just monitor the existing voltage and synchronize to that.

The same needs to be done for solar panels as these generate DC directly.

Note that the mains frequency although quite constant, is only constant on average. You might notice that a mains synchronized clock might run fast at some time but might be running on time a day later.

In the power plant they also keep a clock running on the mains frequency and if that one is running fast they know that they need to slow down for a while.

• Ahh, I get it. So basically, it's just being converted to DC, and then back to desired frequency AC right? – Pritt Balagopal Sep 12 '17 at 7:27
• That is what I think is most likely how it is done. Also read Jeroen3's answer for a non-electronic method. This method might only be used in very old/cheap wind turbines. – Bimpelrekkie Sep 12 '17 at 7:33
• This answer would be much better if it was backed by sources instead of speculation. As it stands, it's an answer of how it could be done, not how it is done :) – marcelm Sep 12 '17 at 12:04
• Re, "Clearly that's nigh impossible just by doing mechanical adjustments." I've read the instructions for connecting mobile generators in parallel. (E.g., the big trailer-mounted generators that travelling carnivals use). Basically, fire them both up, disconnected from each other, and bring both up to approximately the right speed. Then, they've got this panel with light bulbs connected to both circuits. When the light bulbs go out, the machines are in-phase. So, adjust the throttle of one machine until the bulbs go out, and then throw the switch that connects them together. – Solomon Slow Sep 12 '17 at 14:35
• ...Once they're connected, they'll stay in lock-step. There's just a small matter of keeping the throttles adjusted so that they're both working equally hard. I forget the details of how that bit works. – Solomon Slow Sep 12 '17 at 14:38

A hydro generator has movable blades that can change the properties of the generator.

Wikipedia have a in-depth description of how it is done: https://en.wikipedia.org/wiki/Water_turbine