# Using a step up transformer with a generator.

#### fabieville

Joined May 25, 2009
33
I have a windblue DC-500 Motor Driven Permanent Magnet Alternator, it produces both AC and DC output. I was wondering if i could use it as a wind generator. It produces 24VDC at approx. 2500RPM and the current at that reading is about 125amps. I know the volts-rpm ratio is not suited for a wind turbine application because when you work out the calculation it produces 1VDC at approx. 104RPM, but i was just looking at the amount of amps it generated and i was thinking to myself saying what if you connected it to a step up transformer and step up the voltage very high so that when it reach approx. 180-200 RPM you get at least 18VDC going to a 12VDC battery would it really work? Is this possible? I know you would have a decrease in amps but would it be a fairly good amount of amps generated at 200RPM going into the battery after stepping up the voltage? giving that the current rating for this motor is very high.

I know i would have to use the AC output on the generator to step up the voltage then run it through the rectifier to get back dc voltage going to the battery.

#### rvh002@gmail.com

Joined May 15, 2009
119
What exactly is your question ?

#### Kermit2

Joined Feb 5, 2010
4,162
The power output will not be a linear function vs the RPM. It will not start to generate large currents until it is well above the 50% mark on RPMs. Even though the voltage might reach 50% of the volts at half speed, the current available will be around 20-30% of the rated output.

If you have a large distance to send power to a battery. A step up transformer can be a blessing. You would need two of them. First step up the voltage from 24 to say 480 for the travel over copper wire to the battery house, then step it back down to 24 volts and rectify for application to the batteries.

Transmitting large currents over long distances causes immense voltage drop and loss of valuable power due to the i2r loss. Current squared times resistance loss.

#### Wendy

Joined Mar 24, 2008
22,647
A gear box, especially something like an automatic transmission, should help with the RPM issues. Whether it is economical I can't say.

I hadn't thought about the step up/step down idea, but it makes sense. It is how the commercial power company does it with their power grid.

#### timrobbins

Joined Aug 29, 2009
318
Fabieville,

Firstly, are you saying you have checked that you can bypass the rectifier in the alternator and get to the AC, because that model has just a rectified DC output?

If you can access the AC terminals, then you will need to somehow find out (test) how many phases are available (the type of rectifier will identify that), and what the frequency versus rpm is.

No point conjecturing too much until that info is known.

However, with such a large power output available, are you somehow mechanically limiting the input rpm from the turbine, or is it bounded in some way? You need to understand the max levels that could exist at the terminals - otherwise one day you may find all your equipment failing due to overvoltage or overcurrent.

Ciao, Tim

#### timrobbins

Joined Aug 29, 2009
318
PS. are you trying to direct couple the turbine, or do you have control over the transmission ratio?

#### fabieville

Joined May 25, 2009
33
Yes you can bypass the rectifier and get to the AC output.

If using the step up transformer would pose an issue or not worthwhile what about using a dc-dc converter on the dc output phase and step up the voltage so that when it reaches about 180-200 RPM you would get approx. 18VDC at a fairly good amount of current????

#### fabieville

Joined May 25, 2009
33
PS. are you trying to direct couple the turbine, or do you have control over the transmission ratio?
The answer is no to the questions above in the quote.

#### timrobbins

Joined Aug 29, 2009
318
You will need some ingenuity and electrical awareness to proceed, as you could easily damage parts.

A good technique to harvest lower power levels is to use a dc-dc converter with a wide input range, and to use a voltage sensing relay to disconnect the converter when alternator voltage gets too high. You can do this with a number of converters, each one aligned with the voltage/power capability as the alternator output increases.

The problem with AC, even if it is single phase, is the frequency variation - which may be quite low. No harm in certainly trying, but you would need to be aware of the current ratings of the windings, and determine what type of transformer would be appropriate. You could start with say 6V to 115V transformer, rectified into a 24V battery.

Ciao, Tim

#### fabieville

Joined May 25, 2009
33
A good technique to harvest lower power levels is to use a dc-dc converter with a wide input range, and to use a voltage sensing relay to disconnect the converter when alternator voltage gets too high.
I know that i could step up the voltage but how would i maintain a good amount of amps at the output after I step up for eg. 2VDC to 18VDC?

The generator is rated for high current and when at 2VDC the short circuit current at that time is very high according to the chart on the site.

I just want to know that when it reaches 2V at approx. 200RPM the circuit converts that 2V to at least 16V or higher at a reasonable amount of current at the output (5 amps or greater), that i can use to charge a 12V battery.
That's all I am seeking.
I was looking at the
LM2621 Low Input Voltage, Step-Up DC-DC IC but it is only rated for 1A load current so I dont think that this could work or how would you modify it so that can handle and deliver more current ?

#### timrobbins

Joined Aug 29, 2009
318
Firstly, you need to properly understand the output power you can get from the alternator - as a function of wind speed, and rpm (they are different, and depend on your turbine and transmission.

You may well have only 5W of available power when the rectified DC is 2V - and if you could transform the 5W with 100% efficiency, then you would only have <0.5A charging current at 12V.

Prepare a table for us with wind speed, rpm of alternator, and power output of alternator (obtained from manufacturer's curves) - that would test your understanding!

Ciao, Tim

#### Kermit2

Joined Feb 5, 2010
4,162
With a 10 ft diameter turbine blade and a VERY good generator you would harvest something around 150 Watts of power.

Remember I'm talking about a perfectly matched system-rotor blade and generator. You want to use this generator WAY out of it specified operational range. SO you will get even less effecient harvesting of power.

75 to 100 Watts with a 10 ft dia. Blade.

Read This and Figure out that you need to rethink this project if you want useable power from the wind

#### timrobbins

Joined Aug 29, 2009
318
Kermit - are you normalising to some wind speed and turbine efficiency with that power level?

Fabieville - the crux of your thinking is incorrect - you can't normally achieve high power at very low rpm when you have a system that is designed for high rpm. Simplisitically you get hgh power at high rpm, and you get very low power at low rpm. So your request for a converter to transform high currents at low voltage has no sense. Is that understandable?

Ciao, Tim

#### t06afre

Joined May 11, 2009
5,934
Kermit - are you normalising to some wind speed and turbine efficiency with that power level?

Fabieville - the crux of your thinking is incorrect - you can't normally achieve high power at very low rpm when you have a system that is designed for high rpm. Simplisitically you get hgh power at high rpm, and you get very low power at low rpm. So your request for a converter to transform high currents at low voltage has no sense. Is that understandable?
Ciao, Tim
That is core of the problem. A high RPM generator will not work on low RPM. But you may end up destroying it. On internet you will find many DIY wind generator designs. Use Google and terms like "homemade windmill generator", "wind turbine", "wind generator" This may be a good place to start. http://www.otherpower.com/otherpower_wind_alternators.html We can not help you if your design approach is wrong from the start. Please understand that is why you do not get much response on your request

#### Kermit2

Joined Feb 5, 2010
4,162
Kermit - are you normalizing to some wind speed and turbine efficiency with that power level?
....Ciao, Tim
I assumed a constant 10 mph wind, and a wind power capture efficiency of 35%, with a 70% efficient generator, with 59% being the maximum possible power you could capture from wind, I think that would be an excellent result for a homemade unit. Most homemade small wind gennys will not reach that 35% harvest level, but will get around 10-20%.

The power on AVERAGE is overall very low, but on a windy day you can get some considerable amperage out of one. Power in wind goes up by the square of wind speed, or something like that.

#### marshallf3

Joined Jul 26, 2010
2,358
Don't we already have a long thread on this exact same subject?

#### fabieville

Joined May 25, 2009
33
ok i just understand the whole concept now. Thanks for all of the info i really appreciate your help in getting me to understand what I really thought could be possible but somehow my idea was way off.

#### billybob

Joined Sep 26, 2010
1
ok i just understand the whole concept now. Thanks for all of the info i really appreciate your help in getting me to understand what I really thought could be possible but somehow my idea was way off.
Not to prolong an already "dead" subject. But fabieville. no one can know everything. That and from what I have seen here, most if not everyone was way off with you. This is to be expected though. Wind power is often a misunderstood concept. Having been off-grid myself for several years, I do not know everything either. Even after doing a few years worth of research / experimentation on this exact subject.

First off, there are 10 foot radius machines ( PMA Windmills ) that produce well over a kW at peak output. And, if I understand which kind of alternator you're using ( AC Delco 10/12 si case ? ). Machines similar to this have/can *potentially* produce 800W + output. I have one neighbor that has used these in a very windy area, and one that still does to power off-grid power systems ( charging batteries ). With a much smaller than 10' diameter set of blades.

However, with the above said. These types of alternators are less than ideal for the task for several reasons, and the given alternator you're speaking of does not sound like it would work well anyhow. Wind turbines typically work best at a much lower RPM. Typically, they start charging at a couple hundred RPMs, possibly less. Now, it is possible to use gearing, or pullies to increase the speed of the alternator. But if you understand how wind power works, you would very likely begrudge the losses you would incur from doing so. Do a little googling on the topic "Betz limit", and perhaps you will start to see what I speak of here.

Also, there are several web sites dedicated to this specific subject. The best would probably be;

http://www.otherpower.com/ and their forums page is: http://www.fieldlines.com/board/

Just be prepared to be disappointed. Unless, you're very motivated, and are willing to read/learn what the real limitations are concerning wind power.