# DC excitation to car alternator

#### asralan-zia

Joined Dec 22, 2015
2
hello guys!
i have a question here
can i use a 12v supply to excite alternator instead of a battery?
how much amperes supply should be?
should supply be of same ampere rating as of alternator output current ratings?

Joined Jul 18, 2013
28,824
IIRC the current at 12vdc is 2 amps.
Varying the field is going to vary the DC output voltage, rpm will also affect the output, do you intend to regulate it?
Max.

#### MikeML

Joined Oct 2, 2009
5,444
IIRC the current at 12vdc is 2 amps.
Varying the field is going to vary the DC output voltage, rpm will also affect the output, do you intend to regulate it?
Max.
Actually field current controls output current, not output voltage.

In a typical automotive alternator, the field resistance (measured at DC) is about 5 to 8Ω, so the max field current when connected to 12V is ~2A.

At a given RPM, the alternator acts as a current-controlled current-source; the output current is proportional to field current, so if you connect 12Vdc to the field, the field current will be about 2A, the output current will be about 50A, so the current gain is 50/2 = 25. If the output side of the alternator is open circuited with full-on field excitation, the output voltage will shoot up to hundreds of V (hence current source, not voltage source).

The typical automotive voltage regulator senses battery voltage, and controls the field current using PWM, which determines the output current such that it just exactly matches total load current, keeping the battery at a nominal 14.2V. A simulation of this behavior is shown here.

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Joined Jul 18, 2013
28,824
Actually field current controls output current, not output voltage.

.
How come I can vary the voltage output on an unloaded alternator by varying the field?
Just as it does on a DC generator.
Max.

#### MikeML

Joined Oct 2, 2009
5,444
How come I can vary the voltage output on an unloaded alternator by varying the field?
Just as it does on a DC generator.
Max.
An alternator is not like a DC generator. The battery is what keeps the voltage near ~14V. Saying you can control the output voltage of an alternator by controlling the field is like saying you can control the current through an LED by controlling the voltage across it, or like saying you can control the voltage at the collector of a transistor by controlling the base voltage...

What controls the voltage out an alternator is the car's battery being in parallel with the output. Removing the battery while the engine is running almost guarantees the destruction of every item of electronics in the car because without the battery, the voltage can and will shoot up to >100V. Look up "Load Dump".

Read this explanation of what is going on inside the typical automotive charging system.

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Joined Jul 18, 2013
28,824
I don't agree, I have designed and marketed DC generator regulators that were virtually identical to the regulator used in automotive versions after the mechanical one.
The battery does not keep the generator to 14v it is the regulator, by varying the voltage and hence current through the field the output can be Varied.
You can run a Alternator without a battery connected apart from the initial excitation, and use a variable supply to vary the output voltage, empirical bench testing has shown it.
Both Alternator and DC generator both are exited by a DC field.

#### asralan-zia

Joined Dec 22, 2015
2
thanks everyone
so variable DC supply can be used to excite alternator instead of battery in order to get 3-phase output .

Joined Jul 18, 2013
28,824
thanks everyone
so variable DC supply can be used to excite alternator instead of battery in order to get 3-phase output .
That is what I have done, you need automatic regulation for precise control however, if the load is not very constant.
It has been a while but I seem to recall 50-65v out with constant 12v into the field.
Max.

#### bertus

Joined Apr 5, 2008
22,304
Hello,

I moved the thread from the completed projects forum to the projects forum, as it does not comply to the demands for the completed projects forum.

Bertus

#### tcmtech

Joined Nov 4, 2013
2,867
I don't agree, I have designed and marketed DC generator regulators that were virtually identical to the regulator used in automotive versions after the mechanical one.
The battery does not keep the generator to 14v it is the regulator, by varying the voltage and hence current through the field the output can be Varied.
You can run a Alternator without a battery connected apart from the initial excitation, and use a variable supply to vary the output voltage, empirical bench testing has shown it.
Both Alternator and DC generator both are exited by a DC field.
Similar thoughts and feelings based on like experiences here as well.

As I see it the voltage regulator circuit does its best to regulate the output voltage by voltage sensing and controlling the overall power sent to the field coil not current sensing to keep the output current constant regardless of what output voltage it has to make to do so.

I am not sure how anyone could classify a alternator or generator system as being a constant current device when everything about its control systems suggests that they are designed to keep the output voltage as constant and stable as possible regardless of the current draw provided it's within the devices current supplying limit.

As for disconnecting the battery and killing everything electrical on the vehicle I say out rightly BS! I've done it countless times both accidentally and deliberately to every type of vehicle and machine I have ever owned and had to work on for charging system related issues and never once has anyone them ever had a single electrical or electronics device failure from it.

#### shortbus

Joined Sep 30, 2009
10,049
Pretty sure that the field(rotor) starts out at ~5 or 6 V. This is to allow headroom to make the required output voltage. The current pretty much takes care of it's self, to keep up the output voltage.

#### crutschow

Joined Mar 14, 2008
34,835
I agree that the field current directly varies the alternator output voltage. It only indirectly controls the output current.
There is thus no fundamental difference between a DC generator and an alternator generating rectified DC.
The voltage generated in the stator wires is proportional to the rate of change in magnetic flux around the wire (fundamental law of induction).
Thus the alternator output voltage is proportional to the magnetic field intensity (as determined by the rotor field current) and the speed of the rotor.
The load dump spike that occurs if the battery is suddenly disconnected is because the regulator cannot respond rapidly enough to the increased output voltage to reduce the rotor current, not because it's regulating the current.

#### tcmtech

Joined Nov 4, 2013
2,867
Yep what they said.

When a battery is in good condition and fully charged the current going into it is negligible and can be well under an amp per hundred Ah of batteries rated capacity meaning it is by no means working as a load dump keeping a vehicles 100+ amp capable alternators output voltage stable at ~14 volts.

Anyone who has ever had a vehicle or some sort of equipment with a simple old fashioned amp meter that shows whether or not the battery is charging or discharging knows that. Once the battery is charged up it's load and voltage regulation effects on the system are secondary and negligible not primary and critical to normal operating conditions.

#### MikeML

Joined Oct 2, 2009
5,444
...As I see it the voltage regulator circuit does its best to regulate the output voltage by voltage sensing and controlling the overall power sent to the field coil...
Exactly. Since the field (rotor) circuit acts as a resistor (~6Ω), you can consider the average field voltage (I=E/R) , or the average field current (E=IR) as the control input to the alternator.

The job of the Voltage Regulator is to control the average voltage across the Field winding. It is just as accurate to say that the job of the Voltage Regulator is to control the average current through the Field winding. These statements are totally consistent, because the resistive nature of the Field wind makes current proportional to voltage or vice versa.

...not current sensing to keep the output current constant regardless of what output voltage it has to make to do so.
I have never, ever, said that an alternator voltage regulator senses current. The car's alternator voltage regulator senses the battery voltage, which depending on state of charge of the battery, will be somewhere between ~12.6V and 14.5Vdc.

The voltage regulator is nothing but a voltage-controlled switch. It compares the battery voltage to an internal reference voltage and drives a switch. If the battery voltage is less than 14.xxV, the VR applies the full battery voltage to the Field winding by turning on the switch. If the battery voltage is greater than 14.yyV, then the VR turns off the switch. (yy is slightly higher than xx, by a few mV).

I am not sure how anyone could classify a alternator or generator system as being a constant current device when everything about its control systems suggests that they are designed to keep the output voltage as constant and stable as possible regardless of the current draw provided it's within the devices current supplying limit.
I have never said "that a alternator ... charging system as being a constant current". I am saying that an alternator charging system is designed to maintain the battery voltage within +- a few mV of the VR setting, typically 14.2 to 14.5V.

I am saying that the way an alternator charging system works is a bit different than the way a generator charging system.

At a constant RPM, a DC generator's output voltage is proportional to it's applied Field voltage, Vo=k*Vf. The current out of the generator's output terminal is a function of the external load (within its useful range). It is a voltage source..., so much so that generator charging systems need to have a current-cutout circuit to protect the generator from excessive output current in the event that it is charging a badly discharged battery.

A Lundell automotive alternator's output current is proportional to it's field current, Io = k*If. The voltage at the generator's output terminal is a function of the external load (within its useful range). It is a current source..., so much so that its output is determined by magnetic saturation alone and not by the load that it drives.

No over-current protection is needed.Unlike a generator, a Lundell alternator has a nearly flat output current vs RPM curve.

The load-dump jump in output voltage is a consequence of its constant-current nature; remove the load, and the voltage jumps sky-high. A generator does not do that!

As for disconnecting the battery and killing everything electrical on the vehicle I say out rightly BS! I've done it countless times both accidentally and deliberately to every type of vehicle and machine I have ever owned and had to work on for charging system related issues and never once has anyone them ever had a single electrical or electronics device failure from it.
That is because you work on 1930's technology tractors. Load Dump is a well-documented and well-understood problem that surfaced as the electronics in cars became more complex. Just look at the difference in a voltage regulator IC spec'ed for use in automobiles vs one that is not. The suppliers of ICs for the automotive market have gone to great lengths to make their products immune to the effects of Load Dump. It is really bad practice to purposely induce Load Dumps to "test" a charging system.

Joined Jul 18, 2013
28,824
Actually field current controls output current, not output voltage.
Appears to be a somewhat of a contradiction?

An alternator is not like a DC generator.
Actually the method of field control is virtually identical.
Max.

#### MikeML

Joined Oct 2, 2009
5,444
Appears to be a somewhat of a contradiction?
Not at all. Reread what I wrote, more carefully. If you disagree, refute the specific claims I made about how the Lundell alternator behaves in post #14. Don't just keep repeating yourself.

Actually the method of field control is virtually identical.
Max.
Which is exactly what I just said in post #14.

#### MikeML

Joined Oct 2, 2009
5,444
I agree that the field current directly varies the alternator output voltage. It only indirectly controls the output current.
There is thus no fundamental difference between a DC generator and an alternator generating rectified DC...
Then refute the claims I make in post #14. Explain why a Lundell alternator doesn't need current limiting. Explain why an alternator will create a load dump, while a generator does not. Explain why an open-circuited alternator generates hundreds of volts, while a generator will not.

#### crutschow

Joined Mar 14, 2008
34,835
Then refute the claims I make in post #14. Explain why a Lundell alternator doesn't need current limiting. Explain why an alternator will create a load dump, while a generator does not. Explain why an open-circuited alternator generates hundreds of volts, while a generator will not.
The Lundell alternator current is limited by the leakage inductance of the design.
This also means that the open circuit voltage for a given output current has to be much higher than the loaded voltage due to the voltage drop in the leakage inductance. This generates the high load dump voltage. (Here's some discussion on that.)
I assume a generator has lower leakage inductance so does not generate as high a load dump voltage.

None of this negates the fact that primary effect of changing the field current is to change the output winding voltage, as it does for other types of generators. The output current from that also changes, of course, but that's a secondary effect.

#### tcmtech

Joined Nov 4, 2013
2,867
Then refute the claims I make in post #14. Explain why a Lundell alternator doesn't need current limiting. Explain why an alternator will create a load dump, while a generator does not. Explain why an open-circuited alternator generates hundreds of volts, while a generator will not.
They are inherently self limiting due to inductive impedance of the stator windings and its iron cores magnetic properties.

I am not sure what you are referring to as a load dump?

Which alternators will put out hundreds of volts? I've never been able to get one to do that. 80 - 100 maybe just the same as any decent old fashioned DC generator will when they are fed full field excitation power and the armature is left open circuit.

That is because you work on 1930's technology tractors.
I work on vehicles and equipment with modern alternators and electrical systems that have been largely updated to modern designs. All of our farm machinery regardless of age have standard 12 or 24 volt system with 70 - 150+ amp alternators. The oldest vehicle I own and regularly drive now is a 1994 model with all modern EFI and whatnot.

Everything older than that that had generator systems has been converted to common Delco 70 amp or larger alternators including my 1952 international L110 pickup.

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#### tcmtech

Joined Nov 4, 2013
2,867
I am saying that the way an alternator charging system works is a bit different than the way a generator charging system.

At a constant RPM, a DC generator's output voltage is proportional to it's applied Field voltage, Vo=k*Vf. The current out of the generator's output terminal is a function of the external load (within its useful range). It is a voltage source..., so much so that generator charging systems need to have a current-cutout circuit to protect the generator from excessive output current in the event that it is charging a badly discharged battery.
From personal experience and observations I can say that years ago both DC generators and AC alternators used the same type of electromagnetic buzzing solenoid designs to control their voltage output. Also it's not that hard to transplant a modern alternators regulator system into a old DC generators voltage regulator box and have it control things rather than using its old buzzing solenoid design.

As for current limiting to protect things unfortunately the majority of the common automotive alternators will burn themselves up just the same as a DC generator without its current limiting regulation circuit in place will if they are overloaded for too long.

At a constant RPM and constant field winding power both systems behave identically. Output voltage and current limits behave the same. Low current draw results high output voltage. High current draw results in lower output voltage and overloading eventually results in smoke.

At a fixed input RPM's and fixed field excitation levels both behave as voltage sources and neither limits their current until some internal effect like winding resistance or impedance comes into play just like any voltage source works.