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Automotive Alternator Field

Automotive Alternator Field

Automotive Alternator Field

I hope this isn't considered a double post but I'm getting no attention in the automotive group. I'm trying to convert an automotive alternator (slip ring synchronous 3 phase) from 12 volts to 24. I was able to install a 24 volt stator but cannot source a 24v rotor. For industrial applications (100-150kW gen sets) I usually don't have to change the rotor when wiring between high and low voltages so I'm not totally sure what is different in this case. From literature I see that 12 volt alternators have ~4 amps rotor current and 4 ohms resistance. For 24 volt alternators there is ~2.5 amps rotor current and 10 ohms resistance. Would it be reasonable to place a ~6 ohm resistor in series with the field? Should I just run it as is?

RE: Automotive Alternator Field

In a rotating field synch generator, the rotor provides the flux and you can design to stator to produce any voltage (within practicality) for that flux. So, you can run the 24 V stator without doing anything to the rotor.


RE: Automotive Alternator Field

Tugboat Rotor flux is - as usual - proportional to ampere-turns (amps x turns). The 12v alternator requires 4 A into a 4 ohm rotor winding circuit: let's say 100 turns of some wire size. The winding "space" is the area under the pole head that will restrain the wire from moving into the airgap - which means there's a limit on the number of turns/space available for a given diameter and pole count. You indicate your 24v design uses 2.5 A into a 10 ohm circuit resistance. All else being equal, the 24v rotor uses (4/2.5) x (number of 12v turns), with a correspondingly smaller cross-section ... which ends up being a higher resistance circuit.

If you just push 2.5A thru the 12v rotor winding, you'll only develop 2.5/4 x the flux necessary, which means your alternator will only produce about (2.5/4) x 24 volt = 15 v output. Adding an external resistor to the circuit isn't going to help raise that output voltage. On the other hand, you could regulate on voltage and "push" 4 A thru the rotor winding to develop sufficient flux ... since the winding is already designed for that ... but you'd have some more losses somewhere in the system.

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RE: Automotive Alternator Field

It sounds like the best option is to use the 12 volt regulator (4 amp capacity) with a voltage divider to half the sensed voltage.

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