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I have heard that there are DC generators where resistance can be regulated by using electro-magnetic field. Where permanent magnets are substituted with electro-magnets. Are this kind of generator exists?

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    \$\begingroup\$ Are you looking for self exciting generators? They need some weak magnets too though, you can't do anything without magnetic fields really. \$\endgroup\$
    – PlasmaHH
    Mar 1, 2016 at 15:28
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    \$\begingroup\$ Vehicles alternators are DC generators without permanent magnets, roughly speaking. Underneath there's a 3-phase AC generator with rectifier. Inner rotor is supplied with DC to create rotating field. \$\endgroup\$
    – PDuarte
    Mar 1, 2016 at 16:09
  • \$\begingroup\$ Technically speaking, there are incredibly few actual DC generators that have ever even been invented. A "true" DC generator is most likely one of a Wilmhurst Machine, Van de Graaf Generator, or Homopolar Generator/Faraday Disc. If you're asking about one of these 3, then yes, the faraday disk can be built using an electromagnetic winding for the firld magnet, but at DC, the winding will need to be current-controlled to keep it from shorting. - In any other case, you're not asking about a dc generator, but instead about a rectified AC generator. \$\endgroup\$ Mar 1, 2016 at 17:35

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Look up for "dynamo".

There are DC machines without magnets. The function of the magnets is, as you said, replaced by a field producing coil. They are know as "separately exited" machines, you could take a look in here: http://www.electrical4u.com/types-of-dc-motor-separately-excited-shunt-series-compound-dc-motor/

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