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In a motor where both the field and armature are wound there are at least two possible control scenarios.

The field winding can be excited with constant current and the armature voltage controlled, this is similar to a common permanent magnet DC motor. There is back EMF generated at the armature.

Alternatively, the armature can be excited with constant current and the field winding controlled. Apparently this configuration allows the torque on the motor to be controlled with a small amount of power at the field winding (and high current at the armature) why does this contrast with the original scenario? Finally, is there any back EMF in the field winding?

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  • \$\begingroup\$ do you have the datasheet or link for your motor? \$\endgroup\$ – jsolarski Feb 15 '11 at 5:44
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    \$\begingroup\$ Mmmm, sounds like homework. It's not wrong to ask homework questions usually - provided you tag them and don't ask everyone to just give you the answer. \$\endgroup\$ – AngryEE Feb 15 '11 at 14:26
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    \$\begingroup\$ Mmmmm, doesn't sound like homework to me. \$\endgroup\$ – Jason S May 18 '11 at 0:47
  • \$\begingroup\$ Counter EMF is what limits the current flow in a low resistance armature. If the mechanical load on a motor increases, the CEMF decreases, and the motor draws more current. Automatically keeping in balance. \$\endgroup\$ – Optionparty Jan 16 '13 at 15:38
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If the motor has no permanent magnets and separate rotor and stator windings, then yes you can control it either way. However, such motors are usually intended to be controlled one way or the other. For example, some windings will have significantly higher inductance, which makes controlling them slower or requires higher voltage.

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The second scenario is equivalent to a motor with magnets on the rotor. Yes there is counter EMF from field windings.

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  • \$\begingroup\$ If the motor in question has a split ring commutator, the field on the rotor will actually be AC, at least from the rotor's frame of reference. So, not equivalent to permanent magnets on the rotor. Back emf is going to occur where the time-varying fields pass through coils. If you do have a permanent magnet rotor, you have to commutate the field windings, and you do get back emf in the field windings. \$\endgroup\$ – JustJeff Apr 18 '11 at 0:02

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