I'm having a debate with a friend. He claims that if you change the rotation of the shaft of a dynamo the output DC current changes direction. I, however, claim otherwise. The way I see it is that a dynamo rotates a coil inside a magnetic field, this produces an AC sine wave which then gets rectified to DC by the commutator. If you rotate the coil in the other direction it still produces AC and then gets rectified to DC in the same direction as if the shaft were rotating in the original way, right?

So who is correct? If it's my friend then why does the DC current change direction?

Any help is greatly appreciated.

  • \$\begingroup\$ That essentially depends on how that rectification is being done, you can also imagine a brush changing polarity universal motor style. \$\endgroup\$
    – PlasmaHH
    Sep 3, 2018 at 12:35
  • \$\begingroup\$ You need to go back to Fleming's Right Hand Rule... \$\endgroup\$
    – Solar Mike
    Sep 3, 2018 at 13:24

2 Answers 2


Sorry, your friend is correct.

A permanent magnet dynamo is the same as a permanent magnet motor. If the motor polarity is reversed the motor spins in reverse. Similarly the EMF generated by the dynamo will reverse with rotation reversal.

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Figure 1. A really simple motor/dynamo. Source S-Cool.

The brushes aren't providing rectification of the supply - they're rectifying the windings. Notice that current can flow either direction but that the coil's orientation doesn't matter as the brushes correct it.


Your friend is closer. If the dynamo has permanent magnet, then the brushes would act as rectifier. Or we can say it's a brush DC motor with a permanent magnet.

The rotor is a coil with a commutator, meanwhile the stator has a constant field produced from permanent magnet. If you change polarity on rotor, the motor will spin in the opposite direction.

The dynamo is the same machine, as electrical machine are dual acting they can work as motor or generator. Therefore if you spin the rotor in both directions, the polarity will change.


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