3 phase rectifier efficiency

[(unknown)]

Greetings all,

I've finally succeeded in converting my hydroplant over to DC by using a 230V 3 phase induction motor running as generator, feeding the output into a 300D to 60D 3 phase transformer (originally a 600D to 208Y120 transformer which I modified to 120D output by opening up the neutral), and thence to a 3 phase bridge rectifier to get 65VDC more or less.

It's working very well and seems alot more efficient than my old FuKing ST synchronous generator.

My only concern is the amount of heat produced by the rectifier. I've got a large (6" x 8") aluminum finned heat sink on it, and it's uncomfortable to touch, with my hydroplant running only at half power (1.25kw).  I plan on adding some fan cooling to the ventilated enclosure, but I'm wondering if there's anything I can do to increase the efficiency?  Perhaps adding a capacitor?

--Doug

[drdongle]

The only way a cap will reduce the heat is buy introducing capacitive reactance in series with each of the 3 phases. That is the cap will present a series resistance at what ever frequency your motor/alternator is providing. I can't recommend it it isn't a very efficent or reliable method, which introduces more possiable points of failure. Either use a larger heatsink and/or add a fan or use individual heat sinks for the rectifiers.

[SamoaPower]

Generally, the more over-rated the diodes are for current, the lower the voltage drop, hence better efficiency.

Use schottky diodes.

Use synchronous rectifiers. Much better efficiency but complex.

[Opera House]

All that water for cooling and you want to use a fan!!!!!

[(unknown)]

The electronics shack is 250' from the hydro shack, so water cooling really isn't practical, although it's an interesting idea.

[Nando]

You are not giving enough information.

Please be accurate

Can you tell us the head and the water volume you have and where are you located.

Can you describe your motor arrangement ( including capacitor values), turbine type, RPM, and ELC controller type and brand ?>

Can you measure the current, what is the size of the fins and how many per linear inch and the thickness of the base plate.

What is the carrying current of the diodes.

The initial size given by you is small for the 27 amps which may be dissipating around 75 to 100 watts and the heat sink too hot to touch then it seems that the heat sink needs to be at least 2 times the present size you have if the diodes are the correct one, dissipating not more than about 1 volt for a total of 54 watts average.

Get a 48 volts fan blower it will do wonders to the heat sink, may be two if necessary to cover the full length of the Heat sink.

Nando

[Nando]

Doug:

I forgot to respond to your basic question:

In this case since you have a 3 phase charging scheme is impractical to use

capacitors because you have a lot of current and it is 3 phase and in a

sense the diodes conduct all the time, the capacitor may reduce the ripple

and may increase a bit the current by it is not worthy in your case.

Since you are at half power it is clear that you heat sink area seems to need

increasing at least 4 times to allow full power and a good pair of blowers to

keep the heat sink cool -- also since you have water why don't you the water

to cool the heat sink -- this is the procedure we have been using with all

our hydro plants, water cooling, in your case if you let me know more about

your site I can suggest the cooling arrangement for you.

Nando

[(unknown)]

Here is some of the information you requested:

Total current is about 24 amps at 50 volts.  I don't think I can measure the diode current since I'm using bridge rectifier.

Heat sink is 6" x 6", 1" tall fins, 4 fins/inch, 3/8" thick baseplate.

I have enough room to move up to a 7" x 10" heat sink with 2" tall fins, also room enough to add qty 2 x 48VDC fans.

[Nando]

Use both heat sinks if you can place half of the diodes in one and half in the other,

Are you using the quad diodes blocks and 3 of them ?.

If so, then one in the 6X6 and two in the 7X10 and place the fans close to the fins for the stream of air to go along the fins, never across the fins.

You really need both heat sinks and the area still a bit under size and with the two fans the temperature will go down a lot.

If you have the whole thing in a box, make sure that the is circulated out often with another third fan. if the fans can not be ducted with external air and an exiting hole with a grill.

Let me know

Nando

[(unknown)]

It's a monolithic bridge rectifier. One big IC.

[Flux]

You don't say what you are doing with the 65v dc, I assume you are not using it direct, are you charging a battery or something.

If you are charging batteries then capacitors will do nothing, if you are running a direct load then capacitors will mess up your voltage.

Decent press fit or stud mounted diodes will produce less heat than a isolated base dedicated bridge. You will do better with 3 individual diodes on each heat sink than a potted bridge on a bigger sink.

With decent stud diodes you can run the heatsinks up to about 60 deg C ( too hot to hold) With potted bridges you may have to hold the sink temperature down to keep the junctions within their thermal limits.

Sink fins need to be vertical for air cooling, good size fins are better than a larger area with small fins.

Fans are fine short term but usually end in disaster long term, they always fail, in this case I certainly wouldn't consider one.

Your voltage is too high for schottky to gain you much,I haven't seen any that I would run at 65v (150v piv with battery). The volt drop on the higher volt ones tends to be higher anyway.

For air cooling to work you need the sinks at a reasonable temperature, you will need monster sinks if you want them cold. I hate those potted bridges.

Flux

[(unknown)]

I'm charging a battery bank.

[(unknown)]

How do these look?

http://www.alliedelec.com/Images/Products/Datasheets/BM/SEMIKRON_INC/Semikron-Inc_Actives-and-Passiv
es_5500069.pdf

[Nando]

Why don't tell us the brand name and the part number, stop hiding information.

It seems then that in this case you need to move it to the 7 X 10 size with a good fan in/out as I said it before.

Nando

[(unknown)]

Sorry, but I'm at a loss to understand your accusation that I'm hiding information.

[Nando]

The need to give the type of rectifier you are using to be able to give you an accurate assistance.

Knowing the part number then it is easy to determine how many watts are dissipated in your case, then it is easy to calculate what heat sink area is needed to protect the rectifiers from burning up, which seems to be getting close in your case and save you from a disaster.

It is the lack of giving information to get proper assistance, which is quite common in this group and others, as well, and the requester thinks that with simple question like with the one you presented any one can give you a clear solution to the problem.

Go back to the initial message asking for assistance, then read the replies and see how many gave you a logical solution, also observe who was the only one that said:

You are not giving enough information

Then observe who continue trying to get the proper information from you and what was the initial response ( the use of 2 heat sinks) then you changed everything when you said that you have one block rectifier -- this information should have been given from the beginning -- AND THIS GOES TO any one that asks for assistance --BE ACCURATE and informative to arrive a practical and logical solution and NOT WITH some responses that do not have any room or part for a practical solution.

I am not trying to bug you or anyone, it is the need to be accurate to get accurate responses instead of getting useless responses.

Nando

[Nando]

Do you have the Semikron diodes or a block rectifier ?.

Can you exactly say what you have ?.

Also since you are reporting half power charging are you planning to into full power charging and if so how much power.

You can measure the current if you place the current detector at the input to the rectifier,

Nando