Running auto alternator with wire off

[frankiee]

Will an alternator burn up if
1)The field wire is off?
2)The output wire is off?

Thanks

 

[evelrod]

Possibly, depends on the generator and the circumstance. I have heard a lot of second hand anecdotes from 'friends' that indicate failure would be imminent. I have not found this to be true, however.

I have a switch on the field circuit of my ND (from a late model Toyota) on my Mini Cooper race car. I intend to use it only when I need that extra couple hp. So far, I have not used it "on purpose"...Left it off one race, accidentally, though. No harm after ~30 minutes at high revs.
Also, I have had the 'output' wire break on several occasions on various race cars...no harm to the generator.

Rod

 

[frankiee]

I am thinking of a AC Delco one wire alternator but would I am interested in other alternators also.
With my one wire alternator I dont have an option. I would have to pull the output wire. From what I read, the regulator would read no voltage and full field the rotor and depending on the RPMs, 120 volts may be reached.
I read the diodes are either 100 volt or 200 volt.
I am trying to find a site that has specs on GM alternators.
Also I want to find out if it is true with a one wire alt that the regulator will read no voltage and full field the rotor.
I read an article called "Alternator secrets" where the alt can be slightly modified to produce 120 DC and power universal ac dc motors etc. like drills and saws.
Great for the construction site. I could also load test my batteries in the car to see how long the battery takes to go down in voltage if I lose charging

Thanks for the input

 

[MikeHalloran]

With no connection to the battery, the 'logic' in the alternator has no juice with which to reason, and no energy with which to excite the field.

... Okay, there's a little remanance, so maybe you'd see a spike of a few ms before any activity within the alternator depleted the stored energy.

Mike Halloran
Pembroke Pines, FL, USA

 

[evelrod]

I use an AC "one wire" 12v generator on my stock Model A Ford. The biggest drawback with this installation (very low rpm engine) is that I must rev the engine to ~2500 rpm before the thing will generate any current. After the field is excited the first time, no problem...until I shut it off and then, same ole, same ole. Still, it is far better than the OEM DC 6v generator from the 30's.

On another but similar subject---Has anyone knowledge of the amt of power necessary to turn an "alternator"...AC generator as compared to a DC generator? I have a couple of dyno pulls with the ND trying to determine the hp required but, the results were indeterminate.

Rod

 

[MikeHalloran]

On the Model A, you'd probably benefit from a much larger driving pulley, just to get the alternator up into its comfort zone, wrt output and cooling.

Theoretically, for the same output, you should need the same input. Parasitic losses might be marginally less for the alternator.. significantly less if you don't 'gear it up'. The power in any case is limited to ~what you can transmit through a single v-belt, or a couple of HP. Estimated another way: 12Vx100A=1.2kW 1.2kw/0.746=1.6HP, plus a bit to compensate for inefficiencies..

Rod, are you saying that once you get the A started, you can run with no battery? In that case, I stand corrected.



Mike Halloran
Pembroke Pines, FL, USA

 

[frankiee]

So Mike
That makes sence.
One wire disconnected = no input energy and therefore no output and the alternator wont smoke itself. Will just run off the battery. So I can load test my battery by disconnecting the one wire.
For my homemade DC 120 volt I will have to get a 3 wire alt

 

[patprimmer]

Rod

My SWAG is SFA and 2/10ths of SFA.

I agree with Mike, a bigger pulley will help, but I am sure you already know that.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers for professional engineers

 

[evelrod]

Pat, I'm sure your familiar with stock A engines, the bigger drive pulley is pretty much a non starter. I have machined the smallest pulley for the gen that I could get the very large OEM belt to bend around...It's a compromise. At the very low ~450 rpm idle speed the engine is capable of, the output is zero. However, even though the output is zero at idle, the output increases quickly off idle, provided I have 'excited' the damn thing with a flick of the acc pedal when I first start it. No problems with keeping the batt up...in fact, the only problem I have is honking the OEM horn...it's bone stock and has not been touched in 79 years but requires a bit more juice than it once did, I rekon.

Mike:
I think your figures using 100A are, at best, wildly optimistic in my case. The ND on the race Mini Cooper is only 35A with a larger than std pulley---and the AC Delco on the Model A is purportedly a 66A unit (untested) but operates at well below optimal speeds.
I was just curious to know if anyone had actually put a gen to the test. In my dyno runs, switching the "alternator" (I'm sorry, I have never liked that term...goes along with my dislike of the "Americanization" of suspension bits...old fashioned, I suppose) on and off did not alter the output figures enough to determine any loss in wheel hp.

Rod

 

[patprimmer]

Rod

It is probably 45 year since I looked at a model A Ford motor. Even then I did not work on it and took no notice of details.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers for professional engineers

 

[MikeHalloran]

The 100A was arbitrary. 'Merican alternators used to be that big. Now they're typically smaller, in physical size and in maximum current rating.

About that. The alternator's rating is what it can produce at maximum excitation into a load that can absorb the current. I.e., it's a rough measure of how fast it could charge a dead battery, or how many aftermarket lights or amplifiers it could run while not discharging the battery. Once the battery is charged, the regulator is supposed to reduce the excitation enough to just meet the steady state loads. So an alternator running on a dyno without an electrical load only needs enough input to deal with fan and bearing losses. ... and even at maximum current output, it doesn't take a lot of horsepower to drive it, relative to what an engine dyno measures, so the alternator's effect on the engine might get lost in the noise.

I understand the point about the A's thick drive belt. It should be possible to turn an old DC generator into a jackshaft, with say a 2:1 speedup from that to an alternator. It's probably possible to fit a ~35A alternator and some kind of speedup drive fully within the envelope of an old DC generator. Whether it's worth the money/effort is a different question.



Mike Halloran
Pembroke Pines, FL, USA

 

[thruthefence]

Add a relay to the horn circuit & your problems will be over.

 

[evelrod]

Mike---It's not worth the effort...it seems to function with the little rpm 'blip' when I start it...Besides, there are so many other things to worry about driving a 1930 stock automobile on a daily basis (as I do)!

TTF---Re the answer above...There is no relay that can cure the problems of a 1930 Ford. Besides, I rather like adjusting the horn from time to time just as my mom did on her 30 A...."Memories"!!!

Rod

 

[thruthefence]

A guy in the hangar across from me has a '29 sedan, been "in work" for over 20 years; his dad bought it for him to 'keep him out of trouble', he's an architect now, and actually can see the end of the project.

 

[whitehendrix]

frankie..

to answer #2.- no.
to answer #1.- kinda yes and no.
as the other gentlemen have mentioned, your field connection introduces the magnetic field that will ultimately produce power.


it won't burn up if the is no load..

it may pose problems if a load is connected, only due to the fact that theres a reduced voltage output going to the load. dependant on the load's current draw, this may stress any regulatory circuits thru heat and will stress the windings as well.
voltage and current are inversly proportional, therefore, if the voltage goes down, it may be possible to eclipse the ampacity of the unit accidently, and cause you to learn things you may not want to learn.

then again, to err on the safe side of assumptions.. if theres not enough power to run a specific load, then there may not even be enough load in general to load the system to failure.

it could go both ways..

 

[catprofessor]

Disconnecting the field exitiation wire should cause no damage. Alternators with internal regulators need only a brief jolt of current after the engine starts to energize the field coil. Once this happens, the half-wave rectifiers (diode trio) take over and produce current for the field directly from stator output. If the alternator never gets that initial jolt from the ignition switch (in the run position), it cannot produce electricity.

With the output wire disconnected, no current will be produced, therefore no damage will result. With alternators, we need only regulate voltage not amperage as was done with the old DC generators. The current output of the alternator is determined entirely by the load imposed. To see this, monitor current output of an alternator. You will notice that it gradually decreases as the battery charges.

With regard to the amount of energy consumed by an alternator, I estimate it to be about 5 H.P. for 60 amps output. With a direct 1:1 drive ratio using a 1/2 H.P. electric motor to drive the alternator (in my classroom demonstrator) it will only put out about 20-30 amps before stalling the motor. Most alternators are overdriven by the engine which means they will consume several times more hp and as ampere output increases, more energy is required to drive it.

 

[GregLocock]

30A*14V=420W
1/2hp=380W

I should hope so

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[LionelHutz]

I believe a 1-wire alternator will turn-on even if the output wire is disconnected. The regulator is self powered from the alternator. That is why it needs to be rev'd before it will begin charging. There is a weak magnetic field left on the rotor and you have to get the thing spinning pretty fast before it produces enough voltage to power the regulator and turn-on.

I'd like to know what you will use to disconnect it? The switch you use needs to be capable of switching at a minimum 60A and to be safe 100A or more. This is far beyond the capability of a tyical toggle switch.

On the other hand, a 3-wire alternator has the regulator powered by one of the smaller connector wires. The other wire provides some current to the field to assist with the initial turn-on. So, if you disconnect these smaller wires then the thing will stay off and you can use a smaller toggle switch to turn it off.

Overall though, there's not much point in bothering. If the battery is charged then you will not be able to measure a hp difference with it working or not. If you have some high current devices (like a bunch of driving lights or big fans) working then you just might be able to measure the HP loss but at this time the alternator really is needed to keep the voltage up and the battery charged.