6 Volt Alternator Conversion - How to Determine Alternator Cut In Speed?

[A Woolf]

I am working on an alternator conversion for one of my cars.  I have acquired a small Denso style one wire alternator that is setup to charge a six-volt battery.  The challenge I am having with this alternator is I need to know at what speed (rpms) does the alternator cut in and start charging.  This information is needed to determine what pulley sizes are needed to complete the installation.

 

The obvious solution is to ask the alternator shop where I purchased the alternator.  I took it back to them and asked the question about cut in speeds.  They could not tell me.  Their test setup runs at fixed RPM and they didn’t know what speed the motor on the test stand turned.  They seemed to think it should start charging about 1600 rpm but weren’t interested in knowing more than it was working or not. 

 

My next thought was to setup the alternator in my lathe and try to spin it up and determine the cut in speed.  This is not a perfect solution because the lathe is not variable speed but I can bracket the speeds and get a reasonable idea of where the alternator starts charging.  I made a mount for the alternator and mounted a pulley in the lathe headstock.  A V-belt completes the mechanical setup.  The positive post of a six-volt battery is connected to the alternator and the negative side is connected to the case of the alternator.  Testing the output is done with a digital VOM. With the setup as shown I can turn the alternator as maximum of 1970 rpm.  I am using a machinist’s tachometer to check the alternator rpm at the alternator pulley.  So far, I have had no success in getting the alternator to start charging.  As best as I can figure the next step is to gear it up and spin the alternator faster. 

 

I do have some questions that I hope this group can answer:

1.      How much voltage is required to excite the alternator windings? 

2.      What does the battery voltage need to be for alternator to work?

3.      Does 2000+ rpm seem to be a speed for reasonable for the cut in speed for an alternator?

4.      Do I have the alternator wired correctly to the battery?  It is setup with a positive wire to the alternator output post and the negative side tied to the case of the alternator.  It is a one wire style alternator with a built-in regulator. 

 

Alan 

 

[plymouthcranbrook]

Well although not anywhere near an electrical genius I did google the question and several items came up. One said that about 2000 rpm was a number for an alternator to begin to work. Another mentioned that the alternator pulley should be about half the crank pulley size to create best output which peaked at about 6,000 rpm.  I don’t know if that helps or if I am just repeating what you already know. Some of the reports went into great detail which left me at the head of the page.

Edited July 18, 2023 by plymouthcranbrook (see edit history)

[A Woolf]

Your information is interesting because most of the literature I have seen recommends a 3 to 1 ratio for the alternator.  Generally speaking most modern cars idle at 500-600 rpm.  A three to one ratio would mean the alternator is turning at 1500-1800 rpm.  And every car I am aware of will charge the battery at idle.  

 

Thanks for the input.  Alan 

 

 

[Bloo]

There is no cut-in. If it behaves as if there is a cut-in, it must be a function of the "one wire" regulator, and in that case I can't answer.

 

On an Alternator, there is no need for any function like a cutout because the rectifier diodes are already there to rectify the alternating current. They prevent the reverse current when shut down or turning too slow that a cutout is meant to prevent.

 

There is no need for current regulation because the stator regulates itself somehow by the way it is wound. I am a little fuzzy on why that works, but overcurrent damage as seen on generators is not really possible. Once the alternator gets enough RPM to charge at full current, it just charges the same as you spin it faster.

 

Voltage is regulated by varying the magnetic strength of the field, just like on a generator. The only real difference is that the field is located on the rotor instead of the frame. Also some alternators use pulse width modulation to vary the field strength. Others vary a resistance like a generator system does.

 

8 hours ago, A Woolf said:

1.      How much voltage is required to excite the alternator windings? 

Almost none? I'm not sure how to answer that except to say in might be the wrong question. Alternators can sometimes bootstrap themselves on residual magnetism, but it is not 100% reliable. On a GM alternator with an internal regulator for instance, there is always a tiny current bled from the battery through the regulator that provides a little bit more magnetism. Also, if there is an idiot light, turning the ignition switch on bleeds a little current through the light to provide a little magnetism. Some brands of alternator rely on this and may not start charging reliably if the bulb is not there. My guess is if this unit is truly meant to be one wire (for a tractor or something) it probably has a tiny (microamps) internal bleed current, and is never completely "off".

 

8 hours ago, A Woolf said:

2.      What does the battery voltage need to be for alternator to work?

I doubt it needs anything more than the tiniest bit of current to start, and if there's any residual magnetism left it might not even need that.

 

8 hours ago, A Woolf said:

3.      Does 2000+ rpm seem to be a speed for reasonable for the cut in speed for an alternator?

I doubt you could quantify it because I don't believe it cuts in at all. The RPM where it first achieves full output would be a much more interesting thing to know.

 

8 hours ago, A Woolf said:

4.      Do I have the alternator wired correctly to the battery?  It is setup with a positive wire to the alternator output post and the negative side tied to the case of the alternator.  It is a one wire style alternator with a built-in regulator. 

Yes.

 

Most cars I believe are capable of about half of their output current (or maybe a little less) at idle, and when you are underway, equivalent to a little more than fast idle, they are capable of full charge. I'm not sure if this applies to late models, but from the early 60s through the 90s it was pretty much true. If you set it up to do this, nobody could really call it wrong.

 

There were exceptions in that period though, mostly high performance cars who turned the alternator slower either to save horsepower, or to extend the upper RPM range of the alternator without damage, or both. Those cars dim down pretty bad at idle. That probably doesn't matter, but it brings us to the subject of RPM range. With generators you pretty much had to get it right because the acceptable generator RPM range was narrow, and if you spun them too fast you could easily damage them with centripetal force. Alternators are similarly limited, but the range is much wider. The field is would like a spool around the rotor shaft, and that is much harder to damage than an armature.

 

I would put an ammeter in series with either your positive or negative lead. You'll want better connections than clipcords and probably bigger wire. Then put a load tester on the battery. Crank the load tester up higher than the rated output of the alternator. and vary the speed until you find the speed at which full output is achieved. Maybe find half output as well. You must make these tests very quickly and then shut down the load tester or the load tester will overheat. The load tester is necessary because the alternator will only put out as many amps as you ask of it. There is a hard limit set by the stator. The lowest possible RPM at that point is what you are looking for. The current will just stop rising there and refuse to go any higher as RPM increases, unlike a generator which would burn up without current regulation.

 

You didn't say what the car is. If it is a really old slow-spinning antique engine, you could probably set it up to have full charge current available at idle without overspeeding the alternator if you want. On the other hand, you could run it a little slower to save horsepower if you are low on that. You would have to run the arithmetic using the RPM range of your engine. As I mentioned earlier in the post, half current (or a little less) available at idle is a common default setup.

 

Edited July 19, 2023 by Bloo (see edit history)

[plymouthcranbrook]

I looked at this:

 

chrome-extension://efaidnbmnnnibpcajpcglclefindmkaj/https://www.matec-conferences.org/articles/matecconf/pdf/2017/04/matecconf_aigev2017_01076.pdf

 

and this:

 

https://www.quora.com/How-fast-in-RPMs-does-the-car-s-alternator-have-to-spin-in-order-to-produce-50-watts-and-how-fast-to-produce-100-watts

 

and I glanced at this:

 

https://www.motortrend.com/how-to/0206sr-understanding-your-alternator/

 

 I had always been told the main improvement of an alternator over a generator was the fact that the alternator would always charge at idle as opposed to a generator which often did not.

Edited July 19, 2023 by plymouthcranbrook (see edit history)

[Frank DuVal]

2 hours ago, Bloo said:

Also, if there is an idiot light, turning the ignition switch on bleeds a little current through the light to provide a little magnetism. Some brands of alternator rely on this and may not start charging reliably if the bulb is not there.

On the GM systems, there is a resistor in parallel to the lamp so the field circuit still works if the lamp burns out. They thought of that.

 

I've had the internally regulated GM alternators not start charging until I brought the engine above idle. This was evidenced by the idiot light being on when the engine was started and went out when the RPM was raised. Why that happened, I never bothered to find out.😁 This was about 1000 engine RPM. This action mimics the action of the "cut out" of a generator in there is no charging until the RPM crossed a line. Interesting that once that line was crossed, it would continue charging at the normal idle speed. Most all the other GM cars I've owned or worked on seem to charge when started, as evidenced by the idiot light not turning on at idle on start up (light turns off when engine started).

 

I too would find max RPM for alternator and figure pulley ratios so I would not exceed that with normal full engine RPM.

 

Is it a true one-wire alternator, as in only one wire from the output terminal to the battery? (They do not count the alternator frame being connected to chassis of vehicle)

[Bloo]

31 minutes ago, Frank DuVal said:

On the GM systems, there is a resistor in parallel to the lamp so the field circuit still works if the lamp burns out. They thought of that.

 

I've had the internally regulated GM alternators not start charging until I brought the engine above idle. This was evidenced by the idiot light being on when the engine was started and went out when the RPM was raised. Why that happened, I never bothered to find out.😁 This was about 1000 engine RPM. This action mimics the action of the "cut out" of a generator in there is no charging until the RPM crossed a line. Interesting that once that line was crossed, it would continue charging at the normal idle speed. Most all the other GM cars I've owned or worked on seem to charge when started, as evidenced by the idiot light not turning on at idle on start up (light turns off when engine started).

Maybe the wrong pulley? But they ALWAYS start, resistor or no resistor, even if the bulb circuit is completely missing. Some others alternators will not. I was led to believe back in school that those 10SI/12SI alternators were just better at retaining their residual magnetism. Nope. Joe Padavano posted a schematic of the internal regulator in some other thread. There's a small current path. Suddenly it all makes sense.

 

Now the old external regulated ones had a kick-in point when the field relay turned the system on....  Not that it really mattered, but they did.

 

Edited July 19, 2023 by Bloo (see edit history)

[A Woolf]

11 hours ago, Bloo said:

There is no cut-in. If it behaves as if there is a cut-in, it must be a function of the "one wire" regulator, and in that case I can't answer.

 

On an Alternator, there is no need for any function like a cutout because the rectifier diodes are already there to rectify the alternating current. They prevent the reverse current when shut down or turning too slow that a cutout is meant to prevent.

 

There is no need for current regulation because the stator regulates itself somehow by the way it is wound. I am a little fuzzy on why that works, but overcurrent damage as seen on generators is not really possible. Once the alternator gets enough RPM to charge at full current, it just charges the same as you spin it faster.

 

Voltage is regulated by varying the magnetic strength of the field, just like on a generator. The only real difference is that the field is located on the rotor instead of the frame. Also some alternators use pulse width modulation to vary the field strength. Others vary a resistance like a generator system does.

 

Almost none? I'm not sure how to answer that except to say in might be the wrong question. Alternators can sometimes bootstrap themselves on residual magnetism, but it is not 100% reliable. On a GM alternator with an internal regulator for instance, there is always a tiny current bled from the battery through the regulator that provides a little bit more magnetism. Also, if there is an idiot light, turning the ignition switch on bleeds a little current through the light to provide a little magnetism. Some brands of alternator rely on this and may not start charging reliably if the bulb is not there. My guess is if this unit is truly meant to be one wire (for a tractor or something) it probably has a tiny (microamps) internal bleed current, and is never completely "off".

 

I doubt it needs anything more than the tiniest bit of current to start, and if there's any residual magnetism left it might not even need that.

 

I doubt you could quantify it because I don't believe it cuts in at all. The RPM where it first achieves full output would be a much more interesting thing to know.

 

Yes.

 

Most cars I believe are capable of about half of their output current (or maybe a little less) at idle, and when you are underway, equivalent to a little more than fast idle, they are capable of full charge. I'm not sure if this applies to late models, but from the early 60s through the 90s it was pretty much true. If you set it up to do this, nobody could really call it wrong.

 

There were exceptions in that period though, mostly high performance cars who turned the alternator slower either to save horsepower, or to extend the upper RPM range of the alternator without damage, or both. Those cars dim down pretty bad at idle. That probably doesn't matter, but it brings us to the subject of RPM range. With generators you pretty much had to get it right because the acceptable generator RPM range was narrow, and if you spun them too fast you could easily damage them with centripetal force. Alternators are similarly limited, but the range is much wider. The field is would like a spool around the rotor shaft, and that is much harder to damage than an armature.

 

I would put an ammeter in series with either your positive or negative lead. You'll want better connections than clipcords and probably bigger wire. Then put a load tester on the battery. Crank the load tester up higher than the rated output of the alternator. and vary the speed until you find the speed at which full output is achieved. Maybe find half output as well. You must make these tests very quickly and then shut down the load tester or the load tester will overheat. The load tester is necessary because the alternator will only put out as many amps as you ask of it. There is a hard limit set by the stator. The lowest possible RPM at that point is what you are looking for. The current will just stop rising there and refuse to go any higher as RPM increases, unlike a generator which would burn up without current regulation.

 

You didn't say what the car is. If it is a really old slow-spinning antique engine, you could probably set it up to have full charge current available at idle without overspeeding the alternator if you want. On the other hand, you could run it a little slower to save horsepower if you are low on that. You would have to run the arithmetic using the RPM range of your engine. As I mentioned earlier in the post, half current (or a little less) available at idle is a common default setup.

 

Lots to consider and I appreciate the input.  One thing you mentioned that I had not considered is putting a load on system.  I believe that may be necessary to get the alternator to start charging.  I would like to know more about the bench testers that are used to test alternators.  Do they apply a load to the alternator? 

 

Just to clarify this alternator will be going on a prewar car.  The measured idle speed of the engine is just under 500 rpm.  The original generator runs off of the water pump so I will make a custom drive system to drive the alternator at an appropriate speed.  This is not a high speed engine so I am not concerned about spinning the alternator too fast.  My primary concern is to get it to charge at idle.  

 

Alan 

 

[JACK M]

I have installed many one wire alternators and pretty much all of them don't charge at the initial startup. 

Once I feel the oil is all up, I give the engine a goose to get the thing excited. It will then charge at idle.

Edited July 19, 2023 by JACK M (see edit history)

[Bloo]

7 hours ago, A Woolf said:

I would like to know more about the bench testers that are used to test alternators.  Do they apply a load to the alternator? 

Probably, but for the moment I can't remember. Mainly I think the people operating them are looking for "Does it charge?" and "is there a bad diode?".

 

7 hours ago, A Woolf said:

One thing you mentioned that I had not considered is putting a load on system.  I believe that may be necessary to get the alternator to start charging.

 

On a whole car, it is a very convenient way to test. It might be good here too.

 

As I mentioned earlier, the cutout and the current regulator are not there in an alternator system because they are not needed. There is only a voltage regulator. It tries to maintain a specific voltage, more or less. There is often temperature compensation, but lets say about 7.4 volts at room temp (6v system). The natural voltage of the battery is about 6.3 volts.

 

If you put a load tester on the battery, it will drag the voltage down . If you pull more current from the battery than the alternator is capable of, the regulator will drive it to full output trying to maintain the normal unloaded "system voltage" (about 7.4 volts on a 6 volt system).

 

The test method on a whole car is to put an ammeter in series between the battery and alternator. I typically used a dc capable "amp clamp" and a multimeter. Being able to clamp the loop around the main charging wire meant that I did not have to disconnect anything to insert the ammeter. The current must flow through the ammeter on it's way to the battery.

 

Then, connect a load tester to the battery, start the car, hold the engine at a fast idle or a little higher (1500-2000 rpm ought to do it on modern cars), and engage the load tester so it draws more than the alternator is rated for. The regulator will go "full on" trying to keep up and get back to about 7.4 volts, which it will not be able to do because you have exceeded the alternator's output rating. You should see something very close to the amp rating of the alternator displayed on the ammeter you have put in series.

 

You do have to pull current from the battery with something like a load tester to get that current from the alternator. If the regulator is able to push the system right up to about 7.4 volts because the battery is full and nothing is putting on a load, then that is all it will do. It will put out enough current (amps) to hold the system about 7.4 volts and that is all. Not much. An amp or two.

 

The phenomenon @JACK M mentioned, of having to rev up the alternator to get it to start is a completely separate issue. Some other multiple-wire systems do this too, especially ones with a field relay. It's all about getting enough magnetism to get enough current to turn the system on without any signal from the ignition switch to tell it to turn on. Once it is on, it stays on (until you shut the engine down). It shouldn't be going back off at stoplights like a generator with a cutout on a car that is idling too slow to keep the cutout engaged.

 

You would want to spin the alternator up before doing a current test to make sure the system is turned on, and that you are not getting fooled by the behavior @JACK M noted. The most interesting thing to figure out is at what RPM the current hits the maximum rating and plateaus. You can make a good choice about pulleys from there.

 

 

 

 

Edited July 19, 2023 by Bloo (see edit history)

[A Woolf]

16 hours ago, Bloo said:

Probably, but for the moment I can't remember. Mainly I think the people operating them are looking for "Does it charge?" and "is there a bad diode?".

 

 

On a whole car, it is a very convenient way to test. It might be good here too.

 

As I mentioned earlier, the cutout and the current regulator are not there in an alternator system because they are not needed. There is only a voltage regulator. It tries to maintain a specific voltage, more or less. There is often temperature compensation, but lets say about 7.4 volts at room temp (6v system). The natural voltage of the battery is about 6.3 volts.

 

If you put a load tester on the battery, it will drag the voltage down . If you pull more current from the battery than the alternator is capable of, the regulator will drive it to full output trying to maintain the normal unloaded "system voltage" (about 7.4 volts on a 6 volt system).

 

The test method on a whole car is to put an ammeter in series between the battery and alternator. I typically used a dc capable "amp clamp" and a multimeter. Being able to clamp the loop around the main charging wire meant that I did not have to disconnect anything to insert the ammeter. The current must flow through the ammeter on it's way to the battery.

 

Then, connect a load tester to the battery, start the car, hold the engine at a fast idle or a little higher (1500-2000 rpm ought to do it on modern cars), and engage the load tester so it draws more than the alternator is rated for. The regulator will go "full on" trying to keep up and get back to about 7.4 volts, which it will not be able to do because you have exceeded the alternator's output rating. You should see something very close to the amp rating of the alternator displayed on the ammeter you have put in series.

 

You do have to pull current from the battery with something like a load tester to get that current from the alternator. If the regulator is able to push the system right up to about 7.4 volts because the battery is full and nothing is putting on a load, then that is all it will do. It will put out enough current (amps) to hold the system about 7.4 volts and that is all. Not much. An amp or two.

 

The phenomenon @JACK M mentioned, of having to rev up the alternator to get it to start is a completely separate issue. Some other multiple-wire systems do this too, especially ones with a field relay. It's all about getting enough magnetism to get enough current to turn the system on without any signal from the ignition switch to tell it to turn on. Once it is on, it stays on (until you shut the engine down). It shouldn't be going back off at stoplights like a generator with a cutout on a car that is idling too slow to keep the cutout engaged.

 

You would want to spin the alternator up before doing a current test to make sure the system is turned on, and that you are not getting fooled by the behavior @JACK M noted. The most interesting thing to figure out is at what RPM the current hits the maximum rating and plateaus. You can make a good choice about pulleys from there.

 

 

 

 

 On this project I am at the point in time that I need to acquire a couple of items before I can proceed.  I need a load tester and another pulley to increase the speed of the alternator I am attempting to test.  It will be next week before I can get those items.  Then I will test the alternator based on your recommendations.  Thanks for the help and I will report my findings.

 

Alan 

[Frank DuVal]

On 7/19/2023 at 5:01 PM, Bloo said:

The phenomenon @JACK M mentioned, of having to rev up the alternator to get it to start is a completely separate issue. Some other multiple-wire systems do this too, especially ones with a field relay. It's all about getting enough magnetism to get enough current to turn the system on without any signal from the ignition switch to tell it to turn on. Once it is on, it stays on (until you shut the engine down). It shouldn't be going back off at stoplights like a generator with a cutout on a car that is idling too slow to keep the cutout engaged.

 

Hey, I mentioned it first!  Mine was a stock GM internally regulated alternator (100 Amp) system in an '81 Cadillac. 

 

A good load tester is a carbon pile. But, not many around in garages anymore. 

[Bloo]

Yeah I guess you did . Harbor freight has little carbon piles that don't cost a lot. I have one but haven't used it much. It seems to me it needed some modification to be more useful, but at least it isn't spring loaded. That puts it ahead of some of the professional ones in my book. Yes, you have to pay attention not to overheat it.

 

My favorite load tester isn't even a carbon pile, just a fixed resistor. It's branded "Snap-On" but it's really an Associated. It's 117 Amps on a 12 volt system and I guess about half that on 6 volts. I like it because you can use it one handed. That leaves you a hand to operate the throttle. It is great for the kind of testing we have been talking about.

 

 

 

Edited July 21, 2023 by Bloo (see edit history)