I have been reading about how alternators work and I am wondering where the initial magnetic field comes from when no current is running through the field coils that excite the magnetic field in the rotor. I have found alternators that are using a permanent magnet, where it seems obvious and alternators that apparently do not seem to be using permanent magnets, where it is not clear to me. I can also find on the wikipedia article about excitation that there are such generators:

The magnetic field may be produced by permanent magnets or by field coils.


3 Answers 3


The initial field excitation, in an automotive alternator, is provided by the battery.

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In an automotive alternator, AC output from the star-connected 3-phase winding is rectified by a 3-phase full-wave bridge rectifier, to charge the battery through terminal B+.

The same AC output is also rectified by a 3-phase half-wave rectifier, to supply excitation current to the field winding. The regulator maintains the alternator's output voltage within permissible limits by switching on the field current at the lower limit and switching it off at the upper limit.

The output of the half-wave rectifier is brought out to terminal D+ which is connected to the battery through the ignition light and the ignition switch. On the ignition switch being turned on, the excitation current, fed to the field winding via the D+ terminal, switches on the ignition light.

When the engine starts and the alternator is driven, the external excitation induces armature current. As the speed increases and the B+ and D+ voltages take over the battery voltage, the ignition light goes out.

The ignition light remaining on, indicates that the alternator is faulty.

  • 2
    \$\begingroup\$ Very interesting information about the ignition light in automobiles! \$\endgroup\$ Commented Sep 2, 2020 at 18:36
  • \$\begingroup\$ Thank you, Yanick. \$\endgroup\$
    – vu2nan
    Commented Sep 2, 2020 at 19:07
  • 4
    \$\begingroup\$ @YanickSalzmann One side note, there is usually a resistor in parallel with the charging light. This is in case the light burns out. The alternator can still excite through the resistor. \$\endgroup\$
    – vini_i
    Commented Sep 2, 2020 at 19:33
  • 1
    \$\begingroup\$ "A switching regulator maintains the alternator's output voltage within permissible limits." Really? I was under the impression it's linear. \$\endgroup\$
    – winny
    Commented Sep 3, 2020 at 8:52
  • \$\begingroup\$ Hi Winny, Thanks to your comment, I realize that I have failed to convey what I had intended to by wrongly using the term 'switching regulator'. The regulator is neither linear nor PWM controlled. It regulates by switching on the field current only when D+ voltage is at the lower limit e.g. 13.8 V and switching it off when it touches the upper limit e.g. 14.2 V. I regret the error. I'll amend my answer accordingly. \$\endgroup\$
    – vu2nan
    Commented Sep 3, 2020 at 11:01

In an automobile, obviously the initial field current can come from the battery. But even without that, there is often enough "residual" magnetism in the ferrous parts of the alternator to "bootstrap" the process.

  • 1
    \$\begingroup\$ So in a way even an alternator without permanent magnet has some kind of a permanent magnet for just being ferrous. \$\endgroup\$ Commented Sep 2, 2020 at 14:59
  • \$\begingroup\$ @YanickSalzmann yes. But the residual field is weak. One needs to rev the engine quite high (or use other tricks) in order to get alternator to self-start with a completely flat (or missing) battery. \$\endgroup\$
    – fraxinus
    Commented Sep 3, 2020 at 12:18
  • \$\begingroup\$ Often enough residual magnetism, but not always. I have a diesel genset that includes an alternator that relies on residual magnetism to bootstrap the process. The owner's manual mentions that if the alternator fails to generate due to lack of residual magnetism, you can fix that by momentarily connecting an AC power source to the generator's output in series with a 25-watt light bulb as a current limiter. They don't say where to get the AC power in a blackout . . . . \$\endgroup\$
    – MTA
    Commented Sep 3, 2020 at 12:24
  • \$\begingroup\$ @MTA: I have an old generator with the same problem. You can also bootstrap it by briefly(!) connecting a 12V battery across the output. \$\endgroup\$
    – Dave Tweed
    Commented Sep 3, 2020 at 13:31
  • any AC electrical generator can be called an alternator, but usually the term refers to small rotating machines driven by automotive and other internal combustion engines.

Since the RPM is proportional to the voltage it can create the field current is used to control the current instead by mutual coupling if the stator to the rotor and voltage integrated by the large battery capacitance after the alternator 3 phase rectifier. Since the Alternator must be able to charge the battery at idle near 700 RPM, in theory it could generate 10x the voltage at max RPM but impossible due to load effects of the battery.

Thus you can consider the alternator as being an amplifier of field current regulated by feedback with a 14.2V limiter and have sufficient gain to achieve max power in the kW range at some low RPM such as 1200 RPM.

You might imagine the field flux as a magnetic clutch that affects torque load with field current to meet the load current which has a linear relationship with output torque load and electrical current load.

PM alternators are called Magnetos.

You could run an alternator without a battery but use a ultracap to smoothen the ripple (but RC>>1/f) and thus the field windings have a current source to couple the rotor and amplify the current.

  • The initial voltage comes from the remanence of the transformer steel and cap charge voltage created with sufficient low ESR (≈1 mOhm) and integrated voltage such that current amplification occurs.

Some snowmobiles use this method.

This solves the cold temp battery issues for some snowmobiles.

You might wonder... what current gain does a good alternator need ?

  • \$\begingroup\$ Aside from the informative first part I also find the link to the 'shot' start very interesting! Sounds like an cool concept, I wonder how much power one pull start can generate, i.e how long that minimum magnetic field can be held up by the capacitor \$\endgroup\$ Commented Sep 2, 2020 at 15:34
  • \$\begingroup\$ Leakage time constants are very long compared to pull time and start. If really frozen, torque load might need 2 pulls then start. But reminds me of my youth crack starting an Austin Westminster in the ‘60’s no ultracaps though \$\endgroup\$ Commented Sep 2, 2020 at 15:37
  • \$\begingroup\$ Good point, the super capacitor for the RAID failover also managed to supply charge for days, so leakage is indeed very low. I assume there needs to be a switch to connect the capacitor to the alternator once you are really starting the engine, otherwise it would constantly keep up the field. \$\endgroup\$ Commented Sep 2, 2020 at 15:44
  • \$\begingroup\$ Yes charge and start are separate switches \$\endgroup\$ Commented Sep 2, 2020 at 16:13

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