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I am learning about self-excitation in alternators.

My understanding is that there is residual magnetism in the rotor. As it spins, this generates a current in a stator which is then fed to the (rotating) field coils to increase the strength of the magnetic field, and thus the output voltage.

If the current fed to the field coils were constant, (I think) we would be done. The constant current through the field coils would generate a constant magnetic flux relative to the position/orientation of the coils. Since the field coils are rotating, the magnetic field is rotating relative to the stator. This induces a current in the stator.

However, if the output of the stator due to residual magnetism is fed into the field coils (as during build up), won't the field coils thus be fed an alternating current? Won't this imply that the field coils generate a magnetic field that alternates with respect to the position/orientation of the coils? Won't that imply that imply that the magnetic field "pulses" but does not "rotate" relative to the stator?

If I have correctly identified this "problem," is the solution some form of AC->DC conversion?

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  • \$\begingroup\$ You can use brushes or solid-state. \$\endgroup\$
    – DKNguyen
    Dec 13, 2021 at 23:12
  • \$\begingroup\$ Many use rectifiers. \$\endgroup\$
    – Andy aka
    Dec 13, 2021 at 23:12

3 Answers 3

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All simple alternators with wound rotors (such as on cars). Do rectify the output of the stator to provide the field current.

The exact arrangement may vary but a common arrangement is shown below.

The AC output from the rotor is rectified to feed the field coils through slip-rings. A regulator reduces the current to the field winding when the battery reaches the intended regulation voltage.

At start-up the residual magnetism helps start the process of producing power.

There are a couple of interesting points.

  1. There is a separate high-current rectifier to supply the power to the battery and the vehicle. This avoids power from the battery being needlessly used to drive the field when the alternator is stopped.

  2. The current through dashboard lamp helps in the process of start-up as provides a small amount of current through the field to add to the residual magnetism. If the dashboard lamp fails the engine may need to be revved up much higher than normal to start-up the self-excitation.

enter image description here

Image credit - Pinterest - Carparts.com

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If I have correctly identified this "problem," is the solution some form of AC->DC conversion?

Machines of the type described require either a permanent-magnet rotor or DC supplied to the rotor field coil. Over the 130 years or so that "alternators" or "synchronous generators" have been is use, many excitation schemes have been developed. Since constant output frequency is desirable, the variable DC excitation voltage is generally used to regulate the output voltage.

For many decades, DC was supplied to the rotor through brushes and slip rings. For not quite as many decades, brushless excitation systems have been used. Electro-mechanical, vacuum-tube and semiconductor rectifiers have been used. A DC "exciter" generator mounted on the alternator shaft was one popular source. There are many text books that describe the machine designs.

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Does Alternator Self-Excitation Require Rectification?

Yes.

Automobile alternators, for example, have a single axial field winding, with claw-pole halves at either end providing the alternate North and South poles.

enter image description here Image Credit - ResearchGate

enter image description here Image Credit - Mpoweruk.com

The alternator output is rectified and fed to the field winding through an 'on-off' regulator that switches the excitation current between 2 set points (e.g. 13.8 & 14.2 V for a 12V system).

Initial excitation is provided by the battery through the 'charging' indicator bulb.

enter image description here

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