Capacitance Calculations

[Windpower]

Morning All! I have a home made generator unit based around a 5 1/2 HP briggs engine and 63 amp delco alternator. This system is both setup to supply 12 Volts for charging batteries and is setup to supply 100 Volts DC. I have been experiencing problems with engine stalling with heavy loads and have been looking into the possibility or not of using a cap of suitable size effectively with the alternator? In my quest to find the answers to my question I came across the website known as the "Backshed" the site is dedicated to alternative energy and windpower. I started this topic where I caught the attention of Len who is a electronics engineer by trade. In the process of coming up with the calculations for a suitable cap Len asked that I post the below chart so someone can confirm his calculations as being accurate regarding the size of the cap needed? This is the original discussion on the backshed https://www.thebackshed.com/forum/forum_posts.asp?TID=4448&PN=1

Robert

 

[ericgibbs]

Test Reply.E

 

[Windpower]

Drawing

Hey all! I thought I had uploaded the follow attachment? Did not take for some reason or another.

Robert

 

[ericgibbs]

hi Robert and Welcome.

We have a minor Permission to Post/Access problem to your Thread. I am sure Electromaster will fix it as soon as possible.

Moderation:E

EDIT:
4th Jan 2012 Thread moved.E

 

[tcmtech]

Well now that this thread is someplace that everyone can get into....

If you are running a common Delco alternator in a modified setup to get a higher than stock voltage out of it you can also expect it will take a higher than stock input power to drive it at peak loads as well.

Assuming your 5.5 Hp engine is up to snuff its should be able to support around 4100 watts mechanical output peak before it starts dropping off. At 100 volts that would be around 41 amps but most automotive alternators are around 50 - 80% efficient in their normal working range so realistically I would be hesitant to expect much over 20 - 25 amps at 100 volts before you simply run out of engine power.

As far as capacitors go at those power levels they are completely impractical or outragiously expensive for anything more than multi millisecond pulses so a good old fashioned battery bank is the only practical way you will be able get the necessary reserve power for any extended overload period.

 

[BrownOut]

Can you explain how you're getting 100V from your alternator? Did you just bypass ther regulator? If so, how do you get 12V for battery charging? Thanks!

 

[Windpower]

Anyone seen any 1 farad 415 Volt Aluminum caps for sale? I am looking for one similar to a motor run cap in shape.


Brownout, Do to the extreme danger of DC at high voltages I elect not to disclose how I did it. DC is lethal at these voltages unlike AC which is most likely survivable if you were to get zapped. I would look at this if you need a small amount of power? https://www.qsl.net/ns8o/Induction_Generator.html Or simply obtain a portable generator which sometimes shows up on the cheap from Harbor Freight.

Robert

 

[tcmtech]

Yep they are about $25,000 and the size of a 30 gallon drum!

 

[BrownOut]

Brownout, Do to the extreme danger of DC at high voltages I elect not to disclose how I did it.

It's not possible to propose a solution if you don't disclose the details of your design. I don't need a generator, I have all the backup power I require. I find it laughable you think you need to protect us. Anyway, good luck with that.

 

[tcmtech]

As far as getting 100+ volts out of a common alternator its not that hard. Usually just feeding the rotor with a 12 - 15 volt DC source and spinning it fast enough is all it takes. The other modifications may be switching the windings from a delta to Wye configuration and or stacking the windings in series if its a multi winding alternator.

I have several Leese Neville units that have a stock field configuration of double delta three phase. By reconfiguring them to a single Wye the original 12 volt 150 amp output is now capable of running as a 42 volt 42 amp output with the rotor still being driven at 12 volts. By increasing the speed and lowering the amp draw they can easily run 300 volts at 5 - 10 amps on a load bank.

You can go even higher if you find a multi winding alternator like some of the big 24 volt 140+ amp commercial units are set up with. Many of them will have 3 - 5 parallel windings for each phase so if you stack them all together and you can get some serious voltage and power out of them.

 

[BrownOut]

I was only interested in that the OP had configured. Some of the drawings I saw on the other forum looked unworkable, and the article suggested using a battery as a voltage regulator, which I found reckless. Oh well, darwin may take care of it all in the end.

 

[Windpower]

I was only interested in that the OP had configured. Some of the drawings I saw on the other forum looked unworkable, and the article suggested using a battery as a voltage regulator, which I found reckless. Oh well, darwin may take care of it all in the end.

Brownout, My warnings were simply suggested by a fellow on the backshed site. This is the link where I first mentioned the modifications. https://www.thebackshed.com/forum/forum_posts.asp?TID=4236&PN=1 I have a hand made drawing of how it all comes together if you want to send me a PM and I will share my drawings.

I came up with the idea after seeing Lindsey Publications book "Alternator Secrets" that was given to me by a family member who knew my interest in generators and wind power. The book discusses how to get higher output voltages from automotive alternators and how to modify them for wind power.

Robert

 

[BrownOut]

Send me a PM and I will share my drawings.

No thanks.

 

[RCinFLA]

Without changing the rectifier diodes you run the risk of hitting their breakdown voltage.

An automotive alternator will produce over 100 vdc if field winding current is kept high and the alternator rpm is fast enought. Problem is at some point you will blow out the rectifier diodes which are typically rated at 100v breakdown. The rectifiers will see a reverse voltage of twice the output DC voltage.

A capacitor will do nothing. Three phase full wave rectifier output is used to minimize ripple.

 

[Windpower]

The diodes in the alternator have withstood running at no load 105 volts so far. I suspect this bit of information from the backshed might shine some light on the matter?

A bit off topic, but a delta connection would very likely produce some nasty circulating harmonic currents, because the waveforms are not pure sine waves.

A star connection with a floating mid point solves that particular problem very neatly, which is why they do it that way.

Most of the alternator diodes I have tested will work to at least 200 volts and probably much more, even though they may be rated to only 100 volts max.

What happens is, a manufacturer builds a whole bunch of diodes and they are individually automatically non destructively avalanche tested (for breakdown voltage).

The really good ones might be rated at 600 volts, and will probably withstand much more than 600 volts. These are stamped xxx/600 (or something similar)

The next batch might avalanche between 500v and 600v, these are stamped xxx/500, and so on down to the crappiest which become xxx/100 devices.

But superb quality control in your diode factory has resulted in every diode coming off the production line testing better than say 580 volts.
But you still need to supply bulk orders of very cheap xxx/100 diodes to the alternator manufacturers.
So they just stamp as many of the good diodes as being rated at only 100 volts.

So don't be too surprised if your alternator diodes easily survive voltages way above the rated 100 volts.



__________________
Cheers, Tony.


The alternator I have is from the 1970's. Back then things were not as cheap as they are now so the unit was built to last. I am not saying something won't happen? But I have not had any problems with the diodes yet.

Robert

 

[Windpower]

I have seen a method of tapping the three phases to get AC out of the alternator before. The method is to tap two of the phases to a pair of outlets. The third phase is used as a common ground. Anyone have any experience regarding this type of connection? My stator is y connected so I hope that helps some.

Would adding any 10 UF caps to the AC tap filter any of the high frequency ripple?

Robert

 

[tcmtech]

I dont see where the concern about a delta connection is valid being I have worked on countless alternators from many different manufactures and models and I have seen delta connection used in factory designs many many times.

In fact I have seen that with many aftermarket high amp conversion stators they simply switched the stock Wye connection over to a Delta connection and traded off low RPM voltage output capacity for higher RPM current output capacity.

 

[Windpower]

I believe the issue with delta has to do with getting 100 volts output and not melting the windings in the process. The alternator is not designed to deliver a large amperage output much above the nameplate rating. So as the voltage goes up the amps go down, however alot of heat is being developed in the process since a automotive alternator really does not have enough windings for 100 volts output. Which stresses the stator to the point the temperatue would keep rising until the copper melted.

Robert

 

[tcmtech]

I dont see how you figure voltage has to do with melting windings. Current flow is what heats windings up.

If an alternators windings were properly laid out and installed it will not matter what connection configuration it uses. If wound properly in a delta configuration the phasng at the ends of each winding will ballance out to zero potential diference between them. If not its going to be an unballanced short circuit at any voltage and the short circuit current is only going to be limited by the winding resistances themselves.

So you are getting 100 volts out of common automotive alternator, whould you like a cookie or something?

 

[RCinFLA]

Alternator wire size on stator is based on current. They are also a 'Y' 3 phase configuration, not delta. Many alternator regulators, which most now have regulator within case back, use the center of 'Y'.

What has been said about grading diodes for avalanche is somewhat true but high voltage designed diodes take physically more silicon for same current rating. For a target voltage design the breakdown under worse case temp is not generally more then 150% to 200% of rating. A manufacturer may want to reduce their fab variety for economy of scale and may decide to package higher voltage fabbed parts within a lower voltage spec'd part number. But you run the risk. An alternator supplier might change vendors to save a couple of cents and you are back to their original spec rating.

Beside blowing diodes, the other risk is the mechanical stress of driving them at high rpm with high torque load over extended time. This puts more stress on shaft bearing then normal automotive use.