How to limit voltage output from 3-phase permanent magnet alternator.

Thread Starter

izrunas

Joined Mar 3, 2020
1
So here's the issue... My new 2000w wind turbine is designed to produce 3-phase 96VAC, but my grid-tied controller is only designed for 3-phase 40-90VAC. I realize that voltage and rpm's are proportional, but due to the high winds we regularly experience, I'd expect the 90+ condition to happen too often. I had considered a voltage-controlled relay to switch from feeding the controller to shunting to a braking system (to keep the turbine from flying apart at excessive rpm's).

How can I build a 3-phase AC voltage regulator to prevent the voltage from exceeding 90VAC? If it was single phase, I'd just use a handy motor speed controller to trim the voltage and put a BIG fan on it. But, I don't even know where to start when it comes to 3-phase AC voltage regulation.

I have also considered a 3-phase step-down transformer (96VAC to 48VAC), but good luck finding such a unicorn. :)
Any advice would be greatly appreciated!
 

MisterBill2

Joined Jan 23, 2018
13,709
One alternative will be to copy what the power companies do, and use an automated transformer tap changer. With just one tap per phase on the primary and one per phase on the secondary you can have a total of four possible ratios, either stepping down or stepping up.
OR, in place of that 96volt to 48 volt transformer, use a 240 to 120 volt transformer. THOSE are fairly common. Or, try a 480 volt to 240 volt transformer. And keep in mind that there is also a WYE/Delta change that produces significant voltage differences.
So there you have a number of suggestions that do not involve any electronics and are quite efficient. And rather reliable as well.
Also, if the individual phases are accessible on your alternator, just switching to the "WYE" connection scheme will give a lower output voltage.
 
Did you manage to figure the circuit?
I have a 5kW turbine that generates in excess of 100V and I'd like to limit it to 72V as that is the maximum input to my DC-DC converter. Any help would be appreciated.

As far as I understand it you need:
- two braking resistors (power rating dimensioned according to turbine)
- two triacs

the triacs need to connect one resistor between L1 and L2 and the other one between L2 and L3 whenever voltage is too high, this will brake the turbine causing it to rotate slower which will keep the voltage bellow the maximum desired.
Unfortunatelly I'm not good enough at analog electronics to understand how to make the triacs connect phases to resistors only when voltage is too high.
 

MisterBill2

Joined Jan 23, 2018
13,709
To burn the excess 30+ volts in resistors is to waste almost 30% of the turbine output as heat. That is quite a waste. Given that the turbine generator produces 3 phase AC power and that the DC to DC converter requires a DC input, transformers make the best sense.
To use standard transformers the simple approach will be to use standard step-down transformers for 240 to 120 volts, but to wire the primary in series with the secondary so that the step down ratio will be 3:2. This will drop 100 volts to 66 volts, fairly close to the max. With rectification the output might be a bit more. There are also transformers with less standard ratios available, and even a number of three phase transformers, from transformer manufacturers. So really a trio of transformers will be the most efficient wayto match the turbine generator output to the converter input requirements. It may also be possible to change the turbine generator 3-phase connection from WYE to Delta, and get a more convenient output.
 

ThePanMan

Joined Mar 13, 2020
519
Are the turbine blades the self adjusting angle of attack type? If you set them up right then regardless of wind speed the turbine won't over-rotate. Also, you can trim it to produce the voltage you want.

Or change the alternator to a self-exciting type. That way once producing you can regulate the voltage going to the rotor (or armature, always get those mixed up). All you need is just a little excitation source and the alternator should produce sufficient output. You're only looking for about 70 amps. The drawback is you'd need to brake the blades in excessive wind speeds. Perhaps you can tell I know very little about wind turbines.
 
With the transformer energy will also be wasted, just in the lower range, because 40V will be transformed into a voltage that is below the lower limit of operation.

Small wind turbines do not rotate the blades out of wind and the turbine needs to be braked in high winds to avoid mechanical damage.
The braking resistor is needed.
Energy is burned in the resistor only when there is excess energy that is not being consumed, this true in high winds or if there is no load (during the night while everyone is sleeping and wind blows just regularly)
 

MisterBill2

Joined Jan 23, 2018
13,709
@ Pan Man: Changing an alternator type generally means replacing the alternator with a different one. The alternator is usually the most expensive part of the whole system. Transformers would be much simpler and cheaper, and often within the capability of a lot of folks to make. And transformers are often available as surplus .
Not using all of the possible output is much less wasteful than burning power in resistors. and my one example was based on a lot of information that we did not get.
 
Are the turbine blades the self adjusting angle of attack type? If you set them up right then regardless of wind speed the turbine won't over-rotate. Also, you can trim it to produce the voltage you want.

Or change the alternator to a self-exciting type. That way once producing you can regulate the voltage going to the rotor (or armature, always get those mixed up). All you need is just a little excitation source and the alternator should produce sufficient output. You're only looking for about 70 amps. The drawback is you'd need to brake the blades in excessive wind speeds. Perhaps you can tell I know very little about wind turbines.
No. Turbine blades are fixed.
I can't change the generator, it is 'the turbine'.
 

dendad

Joined Feb 20, 2016
4,251
For the transformer idea, I found this example to save me drawing it. If you use a 110V to 24V transformer for each phase, connected as shown so the 24V secondary bucks the input voltage.
1660935294119.png
Chose the transformers to have a 24V current rating a bit higher that the max alternator current.
But it will be interesting to see if the transformers work over the frequency range.
The other way is to get a 110V to 48V switch mode power supply to feed your existing charger. You will need a 3 phase rectifier in front of the new power supply if the rectifier is in the charger.
Adding the transformers will make the wind gen charging drop out at a higher minimum wind speed.
Transformers will be pretty expensive I think. A new charge controller may be cheaper.
 

ThePanMan

Joined Mar 13, 2020
519
it will be interesting to see if the transformers work over the frequency range.
Ah! Yes. Transformers like certain frequencies and alternators that spin at varying rates don't live by that rule. I've seen transformers designed for 50/60Hz & 400Hz (aircraft). Even SMPS's use transformers that are designed for very high frequency. But as long as you're operating in the proper freq. the transformer would be happy.
 

shortbus

Joined Sep 30, 2009
9,754
All of this "over volting" is why a PM(permanent magnet) alternator is a bad idea. Not easy to regulate the out put. Cheaper to start out but much harder to make it work reliably.
 

MisterBill2

Joined Jan 23, 2018
13,709
Probably it would cost much less to replace the DC/DC converter with a version able to accept the higher input voltage.
But transformers able to handle a wide range of frequencies are certainly understood, and so such a device should be available, and far more effective.
And now the previously unasked question: What is the normal output of the three phase 5KW alternator???
Once again we do not have enough information to offer a best suggestion.

Since the normal output range of the turbine should be known, that will determine the normal frequency range that the transformer must handle best. So it is not an unknowable quantity. Except to us.
 
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