# Constant alternate output voltage

Discussion in 'Automotive Electronics' started by elecLear78, Sep 22, 2016.

1. ### elecLear78Member

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I am getting confused that alternator coupled with engine will be revolving from minimum rpm to maximum rpm but still the output alternator voltage is constant. How is it possible? Is there any feedback?

2. ### ericgibbsWell-Known MemberMost Helpful Member

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3. ### MikeMlWell-Known MemberMost Helpful Member

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It is a classic feedback control system...

The Voltage Regulator senses Battery voltage, compares it to a Set Point (nominally ~14.5V) and uses the error to control the duty-cycle of (Pulse Width Modulate) the voltage applied to the Field winding inside the alternator. The field winding of the alternator has a huge inductance, and effectively averages (low-pass filters) the PWMed voltage to make a relatively-smooth average DC current through the field winding.

At any given (useful) engine RPM, the alternator's output current is proportional to the average Field current and the Alternator's shaft RPM. At engine cruise RPM, the Alternator gain is ~25 (2A of Field current produces ~50A of Alternator Output). The Battery integrates the net current into it or out of it (net current in/out of the Battery equals Alternator current minus the car's Load current).

If the Battery voltage is too low, or falling, (compared to the set point), the Voltage Regulator effectively increases its PWM duty-cycle to increase the average Field current. If the Battery voltage is too high, or rising, (compared to the set point), the Voltage Regulator effectively decreases its PWM duty-cycle to decrease the average Field current. This increases or decreases the Alternator's average output current to just match the Load current plus whatever Charging current is required to bring the Battery terminal voltage to the system Set Point voltage.

As the car is first started, a significant amount of charge is removed from the battery due to cranking the starter motor. As the engine speed comes up, the Alternator Charging System is trying to get the Battery voltage from <12V (immediately after the start) to >14.5V (where it is held for the duration of the driving trip).

Initially, the Alternator may deliver ~50A, where most of that is going into the Battery, while the car's normal Loads might consume less than 20A. As the Battery recharges, the Alternator output current drops from the initial high value (50A) to the steady-state value (20A) that just matches the Load current on a sufficiently long trip.

Last edited: Sep 22, 2016
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