The speed of an electric motor of direct current will depend on the intensity of the magnetic field, the value of the applied voltage and the load. If the field strength decreases, the speed increases, trying to maintain the force against electromotive force. If the field winding were to open, only residual magnetism would remain, and the speed would increase in a dangerous manner, trying to maintain the electromotive force necessary to oppose the applied voltage. With a light load or even no load, an open field circuit could cause a speed rise in such a way that the engine would shatter. The blade of the commutator and other parts of the machine would be thrown away and could cause serious injury to persons near the machine baldor VEM3709T-5 7.5HP.

The motor speed can be controlled by controlling the field current using a rheostat or by controlling the applied voltage using static converters. If an AC voltage source, for example, is rectified, it can be used to convert a fixed alternating power supply into a variable continuous power supply. By modifying the voltage applied to the DC electric motor, it is possible to vary the speed of rotation thereof.