Nikola Tesla on the DC Motor

In our dynamo machines, it is well known, we generate alternate currents which we direct by means of a commutator, a complicated device and, it may be justly said, the source of most of the troubles experienced in the operation of the machines. Now, the currents, so directed cannot be utilized in the motor, but must - again by means of a similar unreliable device - be reconverted into their original state of alternate currents. The function of the commutator is entirely external, and in no way does it affect the internal workings of the machines. In reality, therefore, all machines are alternate current machines, the currents appearing as continuous only in the external circuit during the transfer from generator to motor. In view simply of this fact, alternate currents would commend themselves as a more direct application of electrical energy, and the employment of continuous currents would only be justified if we had dynamos which would primarily generate, and motors which would be directly actuated by, such currents.

- adopted from T.C. Martin