I got my hand on an old car alternator. I can't figure out how it works. What bugs me is that it look like all the phases (and neutral) are connected together right at the output of the stator coils.

When I test the resistance between any of the four yellow wires I get 0.3 ohm or something like that.

When powering the rotor and turning the alternator with a drill there is a DC current at the output.

stator coils and rectifier

Here you can see the 4 wires coming out of the stator, the topmost being neutral.

I must be wrong somewhere but I can't get my head around this.

  • 1
    \$\begingroup\$ Do the maths! What current will that alternator give out? What will the voltage drop across 0.3 ohms be at that current? Then what will the internal voltage have to be to get 12 - 14 V after the rectifier at that current? \$\endgroup\$
    – Transistor
    Jun 15, 2021 at 20:23
  • 4
    \$\begingroup\$ It doesn’t look shorted to me. Hidden under the plastic may be the 6 power diode rectifiers. \$\endgroup\$ Jun 15, 2021 at 20:32
  • \$\begingroup\$ The alternator outputs AC only, there are diodes (probably 6 of them) converting the AC to DC. \$\endgroup\$
    – Gil
    Jun 16, 2021 at 2:32
  • \$\begingroup\$ There are indeed "press fit" diodes but when looking at rectifier wiring the phase are not connected to each other, only through the diode (which should block the current ) and the stator winding. That's where i was wrong, i was expecting a much higher resistance from the stator coils, but the resistance is too low for my cheap multimeter which simply which lead me to conclude that the phases were "soldered" together. \$\endgroup\$
    – Benoit
    Jun 16, 2021 at 4:43

3 Answers 3


They are almost certainly Delta connected - an arrangement where each phase winding is connected to both of the others in a circuit that looks like a triangle (or delta).

The rotor induces AC waveforms in each phase, 120 degrees apart, so there must be a rectifier (6 diodes for 3 phase) between those phase connections and the DC output.

  • 3
    \$\begingroup\$ It's much more likely to be a wye connection. That's why (get it?) there's 4 leads attached to the diode board. \$\endgroup\$ Jun 15, 2021 at 21:43

Why are all the phases connected together on this old alternator

The stator phases would not be 'connected together' but would be as shown in this schematic.

enter image description here

It's a 3-phase alternator with a half-wave rectifier for field excitation, a full wave rectifier for DC output and a voltage regulator for field control.

The rectifiers would be behind the black base plate. The voltage regulator, being a separate assembly, would be connected to the the field winding on the rotor through a pair of brushes and slip rings.


I found my mistake:

I was expecting a much higher resistance in the stator coil, but it is quite the opposite: To be effective, the resistance has to be as low as possible. My cheap multimeter is not precise enough to distinguish between the stator coil low resistance and simply two wires soldered together.

  • \$\begingroup\$ You can accept your answer then so that the question does not remain open. \$\endgroup\$ Jun 16, 2021 at 5:16
  • \$\begingroup\$ I have to wait 23 hours to do that \$\endgroup\$
    – Benoit
    Jun 16, 2021 at 21:10

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