Auto Alternator load test ?

[janmarsh]

Hello all,
I have read the maximum amps from a particular auto alternator can be read by shorting the unit ????

I understand Diodes dont like this at all. Could anyone please explain to me how this is then done ? Many thanks.

 

[patprimmer]

As I understand it, the rating of an alternator is not it's maximum output to a short circuit as this will quickly kill any alternator, but it is the maximum sustainable output it will maintain without damage. The resistance needs to be adjusted to maintain that amperage.

Regards
Pat
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[janmarsh]

Pat,
Many thanks for your help. I am mystified at the use of the term "short". I have come across it on two occasions.

 

[patprimmer]

Short circuit is a VERY common term used by electrician etc to describe a very low resistance path to earth that bypasses the resistance used to obtain work.

Regards
Pat
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[patprimmer]

A short circuit result if you run a wire from the battery positive terminal to the negative terminal.

Regards
Pat
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[evelrod]

The folks gave me a neat little tester for Christmas...It will test generator output voltage and amperage, load test batteries, etc.
All that means is that it's available to the general public fairly cheaply, at that. Just hook it up and run the engine and the little thingy reads output.

A short circuit result if you run a wire from the battery positive terminal to the negative terminal.

Yup...that's a short, all right! Maybe a bit more?

Rod

 

[janmarsh]

Thank you.

 

[TheBlacksmith]

Generally you either "vaporize" the wire or weld it to the terminals and kill the battery depending on the wire size. Current flow (amps) is potential (volts) divided by resistance (ohms). A short is close to zero ohms, and as ohms go to zero for any voltage, current goes towards infinity.

 

[ufe777]

A short is close to zero ohms, and as ohms go to zero for any voltage, current goes towards infinity.

In an ideal world with ideal voltage sources but the source output impedance needs to be taken into account. With a non zero output impedance, as the short resistance tends to zero, the voltage across the short will tend to zero because of the voltage divider effect of the output impedance

 

[TheBlacksmith]

What ufe777 says is true, in a real life short the source impedance or internal resistance becomes dominant and limits the available voltage and thus current, but not before considerable heating of the shorting object has occurred.

 

[thruthefence]

I'm wondering if there are some syntax differences in play here?

Is it possible that when the OP is discussing "shorting" we are really talking about "full fielding" the alternator by "shorting" ( jumpering ) the "output" terminal to the "field" terminal, (regulator isolated from the alternator, of course).

This makes output voltage dependent on RPM

 

[catprofessor]

Regarding testing alternator output: Shorting an alternating current generator (alternator) output to ground will result in serious problems. Generator output can be tested in two ways. A carbon pile resistor or other resistive unit is connected in parallel with the battery with the load applied gradually until the output voltage drops to no less than 12 volts (or whatever the nominal voltage of the generator is). The output can be read using an inductive pickup clamped around the generator output lead. Alternating current generators should also be checked for output ripple to detect bad rectifiers and/or stator windings.

The second method uses an algorithm to analyze voltage output with and without load to determine whether the generator is performing at specifications. This newer type tester checks output ripple and performs battery and starter tests without using a resistive load.

Regarding the term "short." Currently the electrical training material from John Deere makes a distinction between "ground" and "short." Ground is used to denote a circuit in which the positive leg has zero ohms of resistance (touches the ground side of the circuit). They use the term short to denote one circuit inappropriately connected to another (two wires "shorted" together).