Varying Speed AC Generator to Steady DC 3

[Dawsonh4]

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 appreciated!

 

[waross]

Look for a switch-mode charger that will accept 100 Volts to 250 Volts, or similar.
The generator will have an UFRO feature that will drop the voltage as the frequency drops to avoid low frequency saturation of iron cored devices, such as motors and transformers.
If the generator is tripping you may need a circuit that monitors the output current and reduces the charging rate when the current approaches the tripping set point.
(Or set the charger to a lower charging rate.)

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Dawsonh4]

Thanks Waross.
I will look for the switch-mode charger.

As for the UFRO feature. The generator (wanco AB30L) does drop voltage as frequency drops, but I thought that was simply a product of lower RPM. Am I mistaken and it is actually a feature that is built in?

 

[waross]

Both.
With a constant excitation, the voltage will drop with a dropping frequency.
With an increased load, the voltage will drop.
The AVR sees the drop in voltage and increases the excitation to hold the voltage up.
The AVR also monitors the frequency and intentionally lowers the voltage when the frequency drops.
Not all generators have a conventional AVR.
The Wanco AB30L uses capacitor excitation and it is normal for the voltage to drop when the speed drops.
That is a good thing (voltage dropping with frequency) if you are powering transformers or motors.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[TugboatEng]

Have you done any calculations to estimate how much power is available? You said your load dropped it to 40hz at which point your AVR tripped. If the AVR didn't trip would the speed have continued to drop until it stalled?

You said the generator spins at 63hz under no load. That means you have enough power to spin the generator and do nothing else.


If you do find you have power available , I would recommend using a proportional controller with PWM output and solid state relay to regulate the power output while maintaining nominal speed of the generator.

 

[Dawsonh4]

I know I am deviating from the original intention of this thread, but curious as to why the need for capacitor excitation on a generator. What would happen if the capacitors were smaller (or bigger) or not there at all?

I appreciate the lesson

 

[Dawsonh4]

TugBoat - I have recorded data with the generator under load, using a 500 watt heater. The generator is being spun using pressurized air as the prime mover. The 63 Hz I mentioned is at an inlet of 200 psi (current max). The data I have recorded is with varying prime mover pressure (150 - 200 psi). 150 psi give 37 Hz and 80 volts, 170 psi gives 37 Hz and 110 V, 200 psi gives 37 Hz and 198 V.

I assumed these capacitors have something to do with Hz remaining constant even as prime mover and volts increase. What would happen with smaller capacitors? Not planning on replacing them, just curious.

 

[Dawsonh4]

Accidently deleted your post.

Apologize I had data from a different test in the previous post. Here is the correct data:

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​

 

[TugboatEng]

Is this a 50hz or 60hz generator? It looks like 60 hz. Any kind of load appears to drop your speed to 30hz. It appears that your turbine is mismatched to your generator. You may consider using a pulley to double the generator speed.

Then, use a proportional controller that senses hz to vary the duty cycle of a SSR on the load side. 0% duty at 60hz and 100% at 63+ hz (a standard range of speeds that will satisfy off-the-shelf voltage regulators).

 

[Dawsonh4]

This is a 240 v 60hz 10 amp motor. The wanco AB20L powering a test load 120v 500 watt heater.

The real load will be a bank of 12 volt batteries wired to be 24 volt.

I would like to avoid gears/ pulley. I’m not sure why hertz stays the same with varying input to prime mover. Would the pulley ratio even help this?

 

[3DDave]

see thread237-510959

Don't see the AB20L model, but the AB series are belt driven per

https://www.wanco.com/wp-content/uploads/2017/06/Manual_BeltDriveGenerators_204230.pdf

Operate AB- and AR-Series generators at a constant speed of 3600
RPM for 60 Hz models or 3000 RPM for 50 Hz models.
❑ All AB25 and AB30 generators are running at the correct speed
when their voltmeters read 120V.
❑ All other AB- and AR-Series generators are operating at the
correct speed when their voltmeters read 240V.

All AB- and AR-Series generators are inherently regulated. By
maintaining the appropriate speed as described above, the proper
voltage of 120 or 240 volts will be delivered by the generator.
Lowering the generator shaft speed decreases the voltage;
increasing the shaft speed increases the voltage.

Is this what the manual that came with the generator says?

 

[TugboatEng]

Your turbine likely has peak efficiency around 30hz and is very inefficient at 60hz. As it slows the efficiency increases which causes the power to increase until it matches the load.

 

[Dawsonh4]

It seems like it must be something different than efficiency. Its exactly 33 Hz in each case not matter the power of the prime mover.

 

[TugboatEng]

Googling turbine efficiency vs RPM yields a few charts like this:

You can see that efficiency drops off rapidly after a nominal rpm. If you were to look at it in terms of torque vs RPM you would have a very steep slope near the nominal speed. This means that changes in load produce small changes in speed near the nominal speed. Everywhere else, small changes in load make for large changes in speed.

Otherwise it's your voltage regulator rolling off due to underspeed at 33hz.

 

[3DDave]

So, no manual? Wrong maker, wrong model?

There must be some source of information for the device you have.

 

[waross]

At 60 Hz, the capacitors set the voltage.
More capacity gives a greater voltage output.
There is not a conventional AVR on a capacitor excited generator.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Compositepro]

Charging a battery while supplying variable loads when operating and variable speed as is what every alternator on any car does, and you in your power range. It seems you want to rewrite conservation of energy, somehow. The basic principles of physics here are simple and invariable. This sounds like a hobbyist question.

 

[waross]

Further to Compositepro's post;
Find a surplus motorcycle or automotive alternator complete with regulator.
Install it and forget it.
Move on to the next challenge.
That simple.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[3DDave]

Previously discussed in the thread I linked to - this item has been purchased and commitments made, no change apparently can be allowed.

 

[wayne440]

Sometimes it is prudent to say "this isn't practical with the equipment you have" and let the decision maker(s) decide whether to live with that or allow the needed changes.
Perhaps this is one of those times.