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I want to place a DC motor on the driveshaft of a marine steam engine. Since i do not want to loose or add power to the drive but only having the motor there for backup reasons, i want to let the DC motor move without any magnetic resistance.

So the question is simple, how do i remove the magnetic resistance of the motor without adding more power.

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    \$\begingroup\$ Use a Universal motor, which has no permanent magnet and no magnetic resistance (unless you apply voltage to the field coil). You're still left with the mechanical resistance of the brushes, however, which is likely to be a bigger problem. \$\endgroup\$ – Brian Drummond Feb 27 '15 at 14:56
  • \$\begingroup\$ Universal(cause AC and DC) motors have PM magnets, cause the AC in field windings would be switching the rotation polarity all the time. You probably meant parallel excited collector machines. These have field windings on the stator and a standard armature of a brushed DC motor. Both get their power from a single DC source, which in your case will be some kind of battery. The torque-RPM relationship is linear(higher torque->lower RPM, predictably). If you want to control the torque I would suggest separating the excitation and the armature circuits. So that you can have a constant excitation. \$\endgroup\$ – WalyKu Mar 5 '15 at 15:49
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    \$\begingroup\$ Universal motors do not have PM magnets. \$\endgroup\$ – Eric Mar 5 '15 at 19:01
  • \$\begingroup\$ Any clue about involved power? \$\endgroup\$ – Gregory Kornblum Feb 2 '17 at 21:31
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A suggestion, not a complete answer:

Assuming a normal two-winding DC motor, not connected to power, there will be some residual field in both stator and rotor iron. This will generate AC voltage in both coils, and cause eddy current losses in both pole pieces.

The AC voltage is easy to avoid: just ensure both windings are open circuit.

For the eddy currents, it might help to demangetise both rotor and stator iron, so there are no residual fields. This is done with a decaying alternating current.

From a boat DC supply you'd need some sort of H-bridge driver that supplies AC to the motor stator and rotor, starting at say 50 Hz and rising in frequency until the current is small.

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This may be from a perspective of a fledgling electrical novice, but I think mechanically and the problem is in wanting to directly connect the motor to the drive shaft. While practical in application and not losing power I was thinking about a type of gear or belt drive where you could engage or disengage the backup motor when needed. This would eliminate the need for the motor to be turning while moving on steam power and would allow for an easier change of motor if your electric motor were to fail. Perhaps a type of spring system or tensioner pulley like in an automotive application coupled with a free spinning pulley or gear?

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