Help with voltage regulator for 28V DC starter-generator

[Badad]

I need a solution for a voltage regulator for a 28V DC starter generator.

I have a vehicle gas turbine from mid 60:ies that is equipped with a starter generator EEMCO 72121. I intend to put it in a boat and of course there are many problems to solve. This is one of them.
The voltage regulator has always been missing and now I need to find or build one. I have the starter relay.

The starter generator has 4 connection points and the start cables are connected to 2 of them.

I have a function description and scheme (attached) but the components are not specified. If you could help me by specifying the components I can build one myself but there might be other solutions out there that will do the job just as well.

Any help on the parts needed or hints to resources that will help me are very appreciated
Thank you

 

[duke37]

The regulator is of the old style with special relays to set the voltage. These often overcharged the batteries so shortening their life. Modern regulators use more accurate voltage generation.

Cars use 12V but some larger vehicles use 24V and regulators should be available. You will need to find the current required by the field and get a regulator which can manage this.

 

[Badad]

Thank you so much for answering my post. Most vehicles today use AC alternators I believe. Can I use a regulator for alternators? How can I find out the current needed by the field?

 

[duke37]

Alternators are similar to DC generators (dynamos) except that they use diodes to rectify the AC instead of a commutator to do the same job and they are inside out.

The current needed will depend on the resistance of the field winding. This may be difficult to measure with a digital meter because of the inductance. I do not think the current would be very different.

I think that most of the newer alternators have built in regulators but you may also find external ones.

 

[duke37]

There is a CAV regulator on UK e-bay for £30 and the firm autoelectric007 say they manufacture dynamos and alternators and will answer any questions.

 

[duke37]

I seem to have lost a reply so here we go again.

Generators (dynamos) are similar to alternators except for the way AC is rectified to DC and that an alternator is inside out !

The field current is determined by the field resistance which can be measured. There may be problems using a digital meter due to the inductance.

 

[Anoxynym]

To find out how much field current the generator will draw, measure the resistance of the field. The usual full force field is with the output voltage applied to the field winding.

A starting point for estimating field current is 28V divided by the field winding resistance. The regulator will reduce this current when the generator is up to speed to control the generated voltage.

Field_current = 28V / Field_resistance

This is sort of worst case. In actual operation the current will be less than this to regulate the generator output. However, you will see this much current every time you run the generator, so you should take this as a design requirement.

Actually, I always pad my worst case estimates by about 20% or more to make the system more robust and provide for unforeseen circumstances.

Don't forget to allow a lot of margin for component ratings. Automotive service such as this is an extremely severe temperature and vibration environment. If you are thinking of a marine application, that adds another level of difficulty due to the humidity and corrosive salty environment.

When you do find a solution that works, make sure you buy spares. You probably do not want to lose your generator a thousand miles from shore.

 

[Badad]

Hi both and others,
Thanks for helping me out. I measured the field core with analog meter and got 15 Ohm and with a digital and got 22 Ohm. Say in worst case that it will take 2A.
I have tried to find out what a CAV regulator is but didn't get far. Would such a regulator do the job provided it is right spec?
I basically need three functions or?
1. Keep the voltage from the commutator at or below 28V.
2. Limit the current to the field core at 2A
3. Prevent the battery from driving the starter generator when the voltage falls below the battery voltage.
Can I put together a CDL and a Zehner diod somehow. Excuse my naive questions.

 

[duke37]

I do not know what a CDL is.

CAV is a company which I think is part of Lucas. They seem to split their sales, Lucas to 12V and CAV to commercial 24V. CAV do diesel injection equipment. Is Motorcraft a US company?

I would think that a 24V alternator regulator would work and should regulate the output to between 27.6V and 28.8V ( double 13.8 to 14.4).

There is no need to limit the field current, that is done by the regulator.
An alternator will not pass reverse current because it has built in rectifying diodes. You will need a diode in the generator output to stop reverse current, this will be a big device (100A?) which will need a heat sink.

You will may need a switch to cut off the field current when the engine is not running. I suggest that you get a circuit diagram of a truck or bus to see how the regulator is connected.

The regulator will adjust output voltage as temperature changes. A Zener diode will not do this and will need extra circuitry. A commercial regulator will be boxed and will have some protection from the wet.

 

[duke37]

I have had a look on the net and some of the regulators use SCRs to control the current from the three alternator phases, this is not what you want.

I found a description of a 12V dynamo regulator which would need some modification for 24V. You will see that it is a lot more than a zener diode, in fact a zener is not good enough as a reference.

ludens.cl/Electron/dynareg/dynareg.htm

 

[Badad]

That Ludens design is exactly what I'm looking for. Thanks for pointing to it. I'll ask him how to convert it to 24v -28v.
Problem no 2 is to get all the parts.

 

[Badad]

I went through the Ludens design but the owner of the site says that extensive redesign is needed and he is not prepared to do it. Unfortunately I'm not able to do it so I'm stuck again.