# Alternators and how they supply current

Hello,

My question is how do Alternators(Like a cars alternator) supply current? Do they act like a current supply or do they act like a voltage supply that can have current drawn up to its rated value? If I have stepper motors being powered from it would they draw the right amount of current?

Thanks for your time and help!

## Answers and Replies

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phinds
Gold Member
2019 Award
Have you done any research on alternators at all? How far along are you in electronics? What level of study are you at?

Hello,

My question is how do Alternators(Like a cars alternator) supply current? Do they act like a current supply or do they act like a voltage supply that can have current drawn up to its rated value? If I have stepper motors being powered from it would they draw the right amount of current?

Thanks for your time and help!
They are not a very good voltage supply, nor current supply. An alternator can be modeled as a voltage source plus a series inductance and resistance, or as a current source with parallel inductance and resistance. These are the Thevenin & Norton models resp. THe car industry uses a voltage regulator to adjust the field current (in the rotor) so as to maintain a constant terminal voltage. The current is determined by the load resistance.

Constant voltage source is a better and more practical choice than constant current source. The reasons are involved but I can elaborate if needed. So the answer to your question is that car alternators are equipped with a voltage regulator to hold the terminal voltage constant. The current is the voltage divided by the load resistance. It could be set up for constant current source operation, but constant voltage is better. Did I help?

Claude

psparky
Gold Member
An car alternator isn't much different than a standard generator. Although the battery is rated at 12 volts....the voltage regulator keeps the voltage around 14 volts. The current drawn should than be a simple V=IR according the the R...or in other words, the loads. (Starter, lights, windshield wipers, coil for spark plugs, heater fan, etc.) It should also be noted that starting the car....or turning on the starter is almost a dead short. Somewhere in the neighborhood of 600 amps in some cases. Turning the starter has nothing to do with the alternator (other than recharging battery once started)....The battery is in charge of this and only has enough power for roughly 30 seconds or so of constant cranking. Also, 30 seconds of crankin is tough on the starter as the dead short nearly burns/heats it up. That's why you need to rest or take pauses between a car that is not easily starting.

Also keep in mind that the alternator has a full wave rectifier which takes the AC and turns it into 14 volts DC. If you put a voltmeter on a battery when the car is running you will read about 14 volts. As soon as you shut car off, the battery will then read 12 volts.

An car alternator isn't much different than a standard generator. Although the battery is rated at 12 volts....the voltage regulator keeps the voltage around 14 volts. The current drawn should than be a simple V=IR according the the R...or in other words, the loads. (Starter, lights, windshield wipers, coil for spark plugs, heater fan, etc.) It should also be noted that starting the car....or turning on the starter is almost a dead short. Somewhere in the neighborhood of 600 amps in some cases. Turning the starter has nothing to do with the alternator (other than recharging battery once started)....The battery is in charge of this and only has enough power for roughly 30 seconds or so of constant cranking. Also, 30 seconds of crankin is tough on the starter as the dead short nearly burns/heats it up. That's why you need to rest or take pauses between a car that is not easily starting.

Also keep in mind that the alternator has a full wave rectifier which takes the AC and turns it into 14 volts DC. If you put a voltmeter on a battery when the car is running you will read about 14 volts. As soon as you shut car off, the battery will then read 12 volts.
Well that is all true, but you did not answer the OP question at all. OP asked if the alternator outputs constant current or constant voltage. You did not address that at all. But what you posted is generally correct. I think we should stick to the OP question.

Claude

psparky
Gold Member
Well that is all true, but you did not answer the OP question at all. OP asked if the alternator outputs constant current or constant voltage. You did not address that at all. But what you posted is generally correct. I think we should stick to the OP question.

Claude
I thought you answered it pretty well. I threw in the voltage regulator and V=IR thing too. It obvioulsy has a constant voltage.

Does every comment need to be based exactly on the OP's orginial question....or is it reasonable to bring some other things to light?

I learned a lot of that stuff in Autos 1 back in High school.

I thought you answered it pretty well. I threw in the voltage regulator and V=IR thing too. It obvioulsy has a constant voltage.

Does every comment need to be based exactly on the OP's orginial question....or is it reasonable to bring some other things to light?

I learned a lot of that stuff in Autos 1 back in High school.
Not every comment need to be directed at the OP question, but at least a portion of the comments should be, according to forum rules. Anyway, I wasn't complaining, just pointing it out. No offense I assure you. As far as having a constant voltage goes, that is a result of the voltage regulator, NOT the alternator's natural modus operandi. Without feedback and regulation, an alternator is a lousy voltage source, and a lousy current source. That was my point. It is artificially forced to operate in constant voltage mode by means of the regulator. I think we can agree on that. Thanks for your input.

Claude

psparky
Hello,

I am a Sophomore level student in College working towards my degree in Electrical engineering. We briefly talked about generators in class however I am interested in learning more about them. I am trying to find good resources to help and I found this forum during my search and thought why not pop in and ask a question. This is not a homework related question just an I was wondering how this worked question. I am trying to figure out how they work because I have a personal project in which I would like to run an 24VDC, 18 Amp stepper motor off of an alternator rated for a regulated 24VDC and a max of 16 Amps. This is for an electronic system on a vehicle so I need to use the alternator. The stepper will be on for brief moments in time but will mostly remain off. I figured I could probably use the alternator and a 24VDC Sealed lead acid battery but was not sure if I would run out of power or how to calculate if I would. Is it just a simple Power calculation and knowing when the stepper will be on and off? Thanks for all of your help so far, even the off topic answers have been helpful.

psparky
davenn
Gold Member
2019 Award
The stepper will be on for brief moments in time but will mostly remain off.

please clarify that part :)

as in ...how brief and how often ?

The Stepper would be running for ~.375 seconds while it was on with a 1 second or greater time in between cycles. I figured that the alternator and the battery could support the actuator while it was running but was unsure of if the battery would be charged fast enough and if doing higher amp cycles on the battery would ruin it.

Thanks for the article ! It has been helpful.