Alternator Ripple Voltage

Thread Starter

jethro99

Joined Oct 31, 2020
30
Is it AC or DC?

It is measured usually at the alternator output terminal or at the battery. Following rectification. Which leads me to believe that it is pulsating DC. But I see it referred to as AC ripple on a number of automotive websites.

Thanks
 

MaxHeadRoom

Joined Jul 18, 2013
28,667
Is it AC or DC?

It is measured usually at the alternator output terminal or at the battery. Following rectification. Which leads me to believe that it is pulsating DC. But I see it referred to as AC ripple on a number of automotive websites.

Thanks
Yes AC reverses direction of current flow, each time crossing zero!.

DC, terminology used on sites such as automotive cannot always be trusted. ;)
They still use the word 'condenser'! :rolleyes:
 

crutschow

Joined Mar 14, 2008
34,386
AC reverses direction of current flow, each time crossing zero!.
Had this discussion before.
So you think that it's not AC if it's riding an a DC voltage, but if you the couple it through a DC blocking capacitor to ground then it suddenly becomes AC.

It's common terminology to call it AC in either case, such as in an AC amplifier, even if it's not pedantically correct.
 

MrChips

Joined Oct 2, 2009
30,779
DC and AC have different meanings depending on with whom you are talking.

The original meaning when applied to power sources is:
DC = Direct Current
AC = Alternating Current

In this original meaning, DC is always in one direction whereas AC reverses direction on every half-cycle.

In signal analysis, DC and AC have a different meaning. All signals have DC and AC. The voltage from an alternator has both DC and AC.
DC means the signal at 0Hz.
AC means any signal above 0Hz.

Your automotive alternator, after rectification, could be outputting 14.5VDC with a 1VAC ripple superimposed on the DC. Another way of putting it, the output is 1VAC ripple riding on 14.5VDC. It is not usually possible to measure this with DMM. You need to view the waveform on an oscilloscope.

In summary, the output is DC and AC.
 

MaxHeadRoom

Joined Jul 18, 2013
28,667
It all comes down to the definition word used, 'alternating' which I have always been taught, is that the current alternates in direction, if it refers to DC, it is pulsating.
 

MrChips

Joined Oct 2, 2009
30,779
If we say "alternating" then we mean it reverses direction.
If we say "AC" we mean anything above 0Hz (unless of course we are talking about 50Hz/60Hz power from the power utility company).
 

Thread Starter

jethro99

Joined Oct 31, 2020
30
Your automotive alternator, after rectification, could be outputting 14.5VDC with a 1VAC ripple superimposed on the DC. Another way of putting it, the output is 1VAC ripple riding on 14.5VDC. It is not usually possible to measure this with DMM. You need to view the waveform on an oscilloscope.

In summary, the output is DC and AC.
Are the rectifiers not capable of completely blocking the current flow?
 

MrChips

Joined Oct 2, 2009
30,779
Are the rectifiers not capable of completely blocking the current flow?
Maybe you still don't understand. For the purpose of this discussion, let us stop using the terms "alternating" and "AC".
Instead, I will use "unidirectional" and "bidirectional".

The voltage and current generated by the alternator is "bidirectional".
When fed into the rectifiers, the voltage and current become "unidirectional". This current is used to charge the battery in the automobile.

In power supply systems, the "unidirectional" current is fed to a capacitor ("condenser" for ol' timers). The purpose of the capacitor is to attempt to hold the voltage constant from one power cycle to the next. (On a 60Hz line powered system, this becomes 120Hz after full-wave rectification.)

On an automobile, the battery serves as a very big capacitor. It holds the voltage reasonably steady at about 14V.
 

Thread Starter

jethro99

Joined Oct 31, 2020
30
OK. Unidirectional for flow in one direction and bidirectional for flow in both directions.

You say that the voltage and current out of the rectifiers is "unidirectional". Flowing in one direction in layman's terms I suppose.

Why then does an Oscilloscope show the current flow to the battery as being bidirectional?
 

Thread Starter

jethro99

Joined Oct 31, 2020
30
Let me start over.

If my facts are straight, the portion of a sine wave above a reference line is considered in many a text book to be positive voltage. The portion of a sine wave below a reference line is considered to be negative voltage. Current above the reference line flowing in one direction and current below the reference line flowing in the opposite direction.

If a full wave rectification system is employed, does all of the current now flow in one direction only? Known as DC for Direct Current.

If not, please provide some words to help me understand.

 

sparky 1

Joined Nov 3, 2018
756
For a standard automobile alternator, anything over 0.1 volts VAC is too much, 50mV AC is ok. but over 100mV AC you could have a bad diode.
Always good idea to check the manufacturers specifications because there are some alternators that are just poorly rectified.
 
Last edited:

crutschow

Joined Mar 14, 2008
34,386
the portion of a sine wave above a reference line is considered in many a text book to be positive voltage. The portion of a sine wave below a reference line is considered to be negative voltage.
Current above the reference line flowing in one direction and current below the reference line flowing in the opposite direction.
Yes, but the reference line for this rectified 3-phase signal is well above the high and low point of the ripple voltage/current.
So both the high and low points of the ripple remain positive.
There is no reversal to the opposite direction.
The ripple just causes a small variation in the current from the rectified source into the load.
 

MrChips

Joined Oct 2, 2009
30,779
In reality, there is no such thing as "pure" DC. All real voltages do fluctuate. The sticky point is what is the duration of the sampled record. Is it 1 second, 1 day, 1 year, or 100 million years?
 

Thread Starter

jethro99

Joined Oct 31, 2020
30
In reality, there is no such thing as "pure" DC. All real voltages do fluctuate. The sticky point is what is the duration of the sampled record. Is it 1 second, 1 day, 1 year, or 100 million years?
I thought that voltage from a battery was pure DC. Why would the voltage from a battery fluctuate? Assumption being the sample time is say just a few minutes and the load is moderate. Recognizing that the voltage will decrease over time as the battery looses its capacity.
 

MrChips

Joined Oct 2, 2009
30,779
I thought that voltage from a battery was pure DC. Why would the voltage from a battery fluctuate? Assumption being the sample time is say just a few minutes and the load is moderate. Recognizing that the voltage will decrease over time as the battery looses its capacity.
It is a question of semantics.

Have you ever tried to measure the battery voltage with a DMM? What happens when you turn on your headlights? A voltage change of 0.1V or 0.00000001V is still a voltage change. One needs to define both the percentage change and the time duration over which the change is observed before one can establish "zero Hz" which is what we mean by DC.
 
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