Varying Speed AC Generator to Steady DC

Hi all.

I have a 240 volt, 10 Amp AC generator being powered by excess compressed air that would otherwise be wasted. I want to use it to charge a bank of 12 V DC batteries wired to be 24 V. My issue is that the compressed air pressure varies slightly to the point that the generator run between 40 ad 59 Hz. The voltage varies along with the speed.

Is there off the shelf equipment that can take the varying AC voltage and turn it into something useful to charge the batteries?

I initially started with a PULS CPS20 Power supply, but found the lower end of the generator caused the power supply to trip when it gave the generator load. Under no load the generator is spinning around 63 Hz. When the power supply gave it load it would drop down to 40 Hz and the CPS20 would automatically shut it off.

Any help is much appriciated!

A rectifier and smoothing system to convert to reasonably smooth DC, then a solar MPPT charge controller?
They control the output current to get the most power from the source, rather than overloading it.


There again, with rectification & smoothing, the power supply you have may work, if it's not slowing the generator too much?
Most switched-mode PSUs will run fine from DC, and if it's only the frequency that is tripping it out, that would be a way around that.

For 30A at 12V it should not be taking much above 3A at 240V (up to 6A if the voltage falls to 120V) so the smoothing system would not have to be all that massive.

It would need a safety enclosure as the charged capacitors are very dangerous if touched!

Thanks for the reply.

When you say a rectifier and smoothing system, I generally understand what you mean. However, I am hoping to find something off the shelf (no idea how to build any of this). Is there a smoothing system that I can purchase, preferably with a rectifier already built in.

What I struggle to understand is the effect of the input voltage being variable. Do I need to put something inline to get the voltage to remain somewhat consistent or is it really not that important?

This is just you trying to get some new answers to your old thread, isn't it? What applied to the other thread still applies to this thread.

The old thread - https://www.electro-tech-online.com...d-control-of-a-generator.164762/#post-1436183

To my way of thinking you should not count on something that is this variable. Take the power when you can get it and be thankful. I may be wrong but it seems like your trying to get power out of something that isn't yours to get the power from. And you don't want to get caught doing it. Sorry if that is a rude statement but you don't/haven't given a real answer to why your wanting this.

Shortbus - Rude, yes. Incorrect, yes. Disappointed to see you are still not helping in any capacity, but thanks for the bump.

Still hoping to see if anyone is able to recommend an off the shelf smoothing system. I have recently been exploring invertor generators. Their back end system seems to be very similar to what I am looking for.

Also if any could help me understand if and why I need to put something inline to get voltage to be consistent to the power supply/ batteries I would much appreciate it.

The PSU you mentioned can work from under 100V AC to over 240V AC; it has a more than 2:1 input voltage range.

The problem with it shutting off appears to be due to the AC frequency dropping too low.

I do not know offhand of any off-the-shelf rectifier+smoothing systems.

I was under the impression it does not work under 100V AC, given its range. Is there a lower limit to the volts input?

Why would it shut off due to frequency if it is turning it into DC anyway?

Ill keep looking on my end. Thanks for the help!

Soul21 said:

I was under the impression it does not work under 100V AC, given its range. Is there a lower limit to the volts input?

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According to the makers data, the guaranteed operating range is 85V to 264V (110V - 15% to 240V + 10%).

Either its PFC stage cannot handle low frequency AC, or the smoothing caps cannot maintain enough voltage between half cycles, so it's low voltage cutout circuit is being continuously tripped, at a guess.

That makes sense. Thanks!

Sounds like I could use a transformer to see if its voltage

What is the generators loaded output voltage at low RPM?

The prime mover is pressurized air (150 psi - 190 psi). The generator is a 240 v 60hz 10 amp motor (wanco AB20L) The load below is a test load 120v 500 watt heater.

The real load will be a bank of 12 volt batteries wired to be 24 volt. I am struggling to understand why the frequency/rpm remains the exact same even though the prime mover is increasing.

With load:
150 psi / 55 volts / 0.9 amps / 33 Hz
170 psi / 78 volts / 2.1 amps / 33 Hz
190 psi / 112 volts / 4.1 amps / 33 Hz

Without load:
150 psi / 85 volts / 37 Hz
170 psi / 168 volts / 47 Hz
190 psi / 235 volts / 59 Hz

What are you using for the turbine? What is the cfm for each pressure value

The turbine to generator (motor) ratio is just wrong. It needs a 2:1 increase so the output is at around 60Hz on load, with proportionally higher voltage.

The low voltage and low frequency are just not suitable for running an off-the-shelf PSU.

That may overspeed the generator when off load though.

ps. The only place I can find mention of a "wanco AB20L" motor is a cross post of yours on another site?


Again, a far simpler solution is get rid of the motor, which is not designed for that type of use, and replace it with a 24V alternator running at a suitable ratio via V Belts.

That gives you a 24V charging system, using something internally regulated and designed to operate from a variable speed input.

Just look for a 24V one wire alternator (one that's not intended to be externally controlled):

ChrisP58 said:

What are you using for the turbine? What is the cfm for each pressure value

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Currently unknown. Working to get a meter on it. Rough estimate is somewhere around 5,000 cfd

rjenkinsgb said:

The turbine to generator (motor) ratio is just wrong. It needs a 2:1 increase so the output is at around 60Hz on load, with proportionally higher voltage.

The low voltage and low frequency are just not suitable for running an off-the-shelf PSU.

That may overspeed the generator when off load though.

ps. The only place I can find mention of a "wanco AB20L" motor is a cross post of yours on another site?


Again, a far simpler solution is get rid of the motor, which is not designed for that type of use, and replace it with a 24V alternator running at a suitable ratio via V Belts.

That gives you a 24V charging system, using something internally regulated and designed to operate from a variable speed input.

Just look for a 24V one wire alternator (one that's not intended to be externally controlled):

https://www.walmart.com/ip/New-50-A...-10Si-1-Wire-Self-Energizing-Hookup/130660714

https://www.walmart.com/ip/Powermas...ysler-One-Wire-Single-Pulley-75-Amp/147787639

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Totally understand the rational behind the alternator and might be the best solution (that or axial flux machine). My question to that is why do modern wind turbines use the traditional AC generator rather than a large alternator?

Soul21 said:

Totally understand the rational behind the alternator and might be the best solution (that or axial flux machine). My question to that is why do modern wind turbines use the traditional AC generator rather than a large alternator?

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An alternator IS a traditional AC generator.

Nigel Goodwin said:

An alternator IS a traditional AC generator.

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It is a type. I am using an AC generator, but rjen is suggesting to use an alternator. Modern wind turbines are not using an alternator machine.

Soul21 said:

It is a type. I am using an AC generator, but rjen is suggesting to use an alternator. Modern wind turbines are not using an alternator machine.

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What else do you think they are using? - they obviously aren't using a car alternator, but they use an alternator just the same.

Soul21 said:

It is a type. I am using an AC generator, but rjen is suggesting to use an alternator. Modern wind turbines are not using an alternator machine.

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Both a vehicle alternator and (often) commercial large-scale wind turbine have regulation built in to the generation system itself.

If you watch a wind farm, quite often the blades all turn at the same rate, as they commonly use synchronous alternators that directly produce 50Hz or 60Hz, as appropriate.

The blade pitch is controlled to match the load requirement. In some types, the alternators they drive are likely regulated as well, to match the available power and load.
(Other non-synchronous types use either DC generators or frequency converters).


Likewise, a vehicle alternator has active internal control of its rotor field current, to hold the output voltage constant regardless of drive speed, within a wide range - plus internal three-phase rectification, which . The maximum current drops off as the speed drops, but the voltage is constant.

They are explicitly designed to handle a wide input speed range.

Your motor used as a generator has no internal regulation so the voltage and frequency vary with speed, and the voltage varies with load. That makes it very difficult to adapt a load to.

At low speed its too low voltage and frequency - but if geared up to give a reasonable minimum frequency & voltage, it would overspeed and likely fail either electrically or mechanically (rotor burst) with maximum air and light load..

ps. To clarify terminology re. "Alternators" - Generator is a generic term, including both electrical dynamos and alternators (and many non-electrical power devices).

An alternator is an AC-producing electrical generator.

eg.