# Car Alternator

## Main Question or Discussion Point

I am wondering how can an alternator charge the battery if the rpm change constantly? Can someone please explain me how it manages to produce a constant voltage?

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OmCheeto
Gold Member
I am wondering how can an alternator charge the battery if the rpm change constantly? Can someone please explain me how it manages to produce a constant voltage?
It has a voltage regulator which changes the field strength to maintain a constant output voltage.

So an alternator as a whole is like a smart generator...now why then they don't use this system for windturbines instead of a brake system

OmCheeto
Gold Member
So an alternator as a whole is like a smart generator...now why then they don't use this system for windturbines instead of a brake system
I've worked on lots of alternators, but I've never built, worked on, nor even seen an actual wind turbine in operation before. But I believe they use generators which I had never heard of before. Since I've never heard of them, I don't know how they work exactly.

Wiki has this comment about their use:

Induction generators are mechanically and electrically simpler than other generator types. They are also more rugged, requiring no brushes or commutators.
I would imagine they use braking systems to keep the turbines from spinning too fast, and destroying themselves. Automotive alternators are hooked to the engine and are probably geared so that their safe operating speed is never exceeded based on the safe operating limit of the driving motor.

Wind turbines typically use synchronous generators, which output variable frequency because the wind speed is variable. The output is then rectified to DC and connected to a line common to all the other wind turbines. Then the DC current is converted to AC at constant frequency with an inverter.

OmCheeto
Gold Member
Wind turbines typically use synchronous generators, which output variable frequency because the wind speed is variable. The output is then rectified to DC and connected to a line common to all the other wind turbines. Then the DC current is converted to AC at constant frequency with an inverter.
bolding mine

This is untrue according to my memory.

This is backed up by a wiki.windpower.org(and therefore unreliable) comment:

Most wind turbines in the world use a so-called three phase asynchronous (cage wound) generator, also called an induction generator to generate alternating current. This type of generator is not widely used outside the wind turbine industry, and in small hydropower units, but the world has a lot of experience in dealing with it anyway
[ref]bolding again mine

Asynchronous generators are rarely used because their speed has to be almost twice the synchronous speed for maximum efficiency, and they have to be magnetized externally to be able to start. However, it's possible that they found some use for them since they are cheaper and could in principle run at variable speeds (although not at maximum efficiency).

Apparently both systems are used.

OmCheeto
Gold Member
Asynchronous generators are rarely used because their speed has to be almost twice the synchronous speed for maximum efficiency, and they have to be magnetized externally to be able to start. However, it's possible that they found some use for them since they are cheaper and could in principle run at variable speeds (although not at maximum efficiency).

Apparently both systems are used.
Rarely used?

GE 1.5 MW Series
Most widely deployed wind turbine - 16,500+ turbines installed globally

GE 1.5 MW Series Generator
* Doubly-fed three-phase asynchronous generator
google vestas Vxx gnerator type said:
Vestas V80, V82, V90 - asynchronous generator
Or are you talking about home style wind generators?

Bandit127
Gold Member
Car alternators produce 3 phase AC. Voltage depends on engine speed, typically 13 V at tick over to around 30 V at full speed.

The AC gets converted to DC by the rectifier. The voltage regulator then lets about 13.8 V through and wastes the rest as heat.

A blown regulator is a common problem on some Honda motorcycles. The battery then gets over 20 V and "boils" dry.

The rectifier and regulator are inside the cover on common car alternators.