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Old 11-29-2012, 05:07 PM   #1
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Help: 3-phase to single phase power conversion!


I'm an engineering student working on a project to build a small hydroelectric energy storage system. We've recently hit some hiccups and are looking for help/advice, and I thought this may be the right place to post. A little background first:

1) The system we're working on consists of a pelton wheel attached to an alternator which leads to an inverter/charge controller and, finally, a 12V battery. Our current setup has an expected alternator output of about 15V.

2) This project is non-profit and entirely student run so we rely heavily on donations and scrap parts. The work we're doing now is to build a prototype for eventual use in rural Thailand to power a UV water filter (i.e. clean the water with its own kinetic energy!)



The problem: the alternator that was donated to us has a 3-phase output (i.e. 3 wires, 1 is neutral), and we are a little stumped on how to use the power. We've looked into cheap rectifiers that can make the output transmittable to the charge controller and the rest of the system but we have little experience with 3-phase power.

The alternator is by far the most expensive part of our system and we've already done an extensive amount of work building watertight housing and adapters for it to attach to the pelton wheel. It's probably out of the question to get a new one- the one we have has the perfect torque for our system as well.

Any suggestions on what to do? I know this is a DIY forum but I figure there may be some experienced people out there willing to help! Let me know if you need any additional info and I'll be happy to provide it. Thank you.

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Old 11-29-2012, 05:30 PM   #2
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Help: 3-phase to single phase power conversion!


As you described it alternator sounds like 1 phase. (3 wires,1 is neutral).

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Old 11-29-2012, 05:35 PM   #3
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Help: 3-phase to single phase power conversion!


Quote:
Originally Posted by bobelectric View Post
As you described it alternator sounds like 1 phase. (3 wires,1 is neutral).
3 phase in a Y configuration also uses 4 conductors ( 3 phase delta connected uses just 3.)

To the OP: You can obtain single phase output from a 3-phase source by connecting between any 2 "hot" conductors. You'll need to measure this phase-to-phase voltage and see if it's sufficient to drive your rectifier/filter/charge controller circuit.

Oops. Need a clarification. Does the alternator have 3 wires plus a neutral or 3 wires total?
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Old 11-29-2012, 05:40 PM   #4
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Help: 3-phase to single phase power conversion!


Read his post.
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Old 11-29-2012, 06:51 PM   #5
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Help: 3-phase to single phase power conversion!


Quote:
Originally Posted by Dave632 View Post
3 phase in a Y configuration also uses 4 conductors ( 3 phase delta connected uses just 3.)

To the OP: You can obtain single phase output from a 3-phase source by connecting between any 2 "hot" conductors. You'll need to measure this phase-to-phase voltage and see if it's sufficient to drive your rectifier/filter/charge controller circuit.

Oops. Need a clarification. Does the alternator have 3 wires plus a neutral or 3 wires total?
You can't just use two phases of the alternator and ignore the third. The load must be balanced or it will be extremely inefficient and cause problems.

You need a 3-phase rectifier to obtain DC from your three phase AC source. Six diodes. That's it. Do you have an electrical engineering person involved in this project? If not, you need to get one. If so, you need to get a different one. This is an extremely simple problem - much simpler than all of the other electrical system design challenges you have to solve to make this all work properly. There are many much more subtle and difficult issues involved here beyond just making 3-phase AC into DC.
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Old 11-29-2012, 07:34 PM   #6
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Help: 3-phase to single phase power conversion!


It's easy to make this alternator produce DC power. You'll need 6 basic diodes, each one needs to be rated for the full amperage of the alternator, and at least twice the output voltage with no load attached.

Assuming the alternator is connected in a wye configuration, connect the diodes in 3 groups of two each, + to -, and one of the 3 output wires also connects to this junction. Ignore the 4th wire, it is the center of the wye, and will not be used here.

Just about every automotive alternator produces 3 phase AC which is converted to DC by integral diodes. Most are wye connected with the center of the wye spliced and isolated in the windings.

Depending on the amount of current involved, the easiest way to convert the 3 phase AC into DC would be to go to your local auto junkyard, pick up a used alternator, take it apart, and use its diode bridge. It'll have 3 terminals that the windings connect to, and often the heatsink (the finned aluminum frame) will be the DC-, and there'll be some sort of a bolt-like piece that's the DC+.

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Old 11-29-2012, 08:13 PM   #7
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Old 11-29-2012, 09:30 PM   #8
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Quote:
Originally Posted by TTW View Post
This is a very good drawing of a 3 phase alternator with DC conversion. The 4th wire (neutral) in the OPs case is the point where all 3 windings connect.

This is how most automotive alternators are connected. The diode bridge is built into the alternator.
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Old 12-01-2012, 12:57 PM   #9
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Help: 3-phase to single phase power conversion!


Hi everybody- thanks for the prompt responses, we truly appreciate it.

Our team of 4 is currently only mechanical and chemical engineers- we've focused more on the pelton wheel, water flow characteristics, and alternator itself. We definitely need to get an electrical engineering person on board, as this is a simple problem. However until we do find somebody, we're trying to be as proactive and productive as possible.


Here's a link to the actual alternator we have:

http://www.ginlong.com/wind-turbine-...L-PMG-500A.htm


The alternator has a TOTAL of 3 wires, and I believe that the configuration you showed will work. On the spec sheet it mentions "rectified DC output"- not sure if this means we have to supply our own rectifier or there's a way to do it with the alternator itself. We will be calling the company to find out more info, but that may or may not be helpful.

Again, thank you for the help. This community is fantastic!
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Old 12-01-2012, 01:42 PM   #10
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Help: 3-phase to single phase power conversion!


Quote:
3 Phase star connected AC output
The drawing supplied by TTW is what you are looking for. Note that a full speed this generator will be putting out nearly 55 volts, much more than your original project was designed to accept. It should put out about 15 volts at 150 RPM. Of course rectifying the output will increase the voltage further. Sounds like you need to get a student electrical engineer as part of your team.
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Old 12-01-2012, 01:59 PM   #11
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Help: 3-phase to single phase power conversion!


If you look at the specification sheet for the altinator....it's 3ph "Star" (wye)....according to the OP, there are only 3 wires....so I'm assuming there is no connection to the common of the wye setup....but you don't really need it....

Just use the diodes as noted above......

However, I see bigger issues....at full RPM, the output can be up to 50v.....any type of standard 12Vdc battery charger is going to get fried.

Basically, I think the OP will need an inverter.....there are commercial units available....but I doubt they can afford it.

This would be a good project for a couple of undergrad EE students......
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Old 12-01-2012, 02:28 PM   #12
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Help: 3-phase to single phase power conversion!


More than likely the generator case is the ground. But stranger things... The manufacture's website is nearly useless.
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Old 12-01-2012, 02:30 PM   #13
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Help: 3-phase to single phase power conversion!


Quote:
Note that a full speed this generator will be putting out nearly 55 volts, much more than your original project was designed to accept. It should put out about 15 volts at 150 RPM.
That's about what we are going for. The alternator is attached to a pelton wheel which turns from water flow redirected through pipes from an nearby water source. The flow rate of the water should limit our alternator to reasonably low RPM's even during max flow conditions. The reason we got this alternator was the low starting torque- though hindsight is 20/20 and we probably should have just geared down a much smaller alternator. Luckily we got this one on a huge discount.

Quote:
However, I see bigger issues....at full RPM, the output can be up to 50v.....any type of standard 12Vdc battery charger is going to get fried.
You're right- but we're working with the pipes to limit the flow rate of the water so that high RPM's are impossible. We've already looked into different inverters and charge controllers but we know there are going to be significant electrical challenges past this one and we're going to need some EE students on board.
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Old 12-01-2012, 02:32 PM   #14
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Help: 3-phase to single phase power conversion!


The alternator in the link will indeed produce 50 volts DC, but this rating is open circuit.

In real life, the output voltage will depend on the current.

Provided a charge controller is used, the two design considerations here are as follows;

1) The output voltage of the alternator is higher than the battery voltage.

2) The output voltage of the alternator is lower than the maximum input voltage of the charge controller.

The actual voltage of the alternator will depend on current. The charge controller will determine current based on load and the charge of the battery. Therefore the operating voltage of the alternator is of no consequence.

Rob
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Old 12-01-2012, 04:35 PM   #15
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Help: 3-phase to single phase power conversion!


On a microhydro, you typically set it up for a given output and run it at that level 24/7. The charge controller is set up in dump mode so that it dumps excess current, usually to heating elements.

Here is a link that has resources you can study and has experts on the subject.
http://tech.groups.yahoo.com/group/microhydro/

Be ready to start talking head and available flow for at least one test site, before you start asking questions.

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