The speed that works on the electric motor of direct current will depend on the intensity of the magnetic field that acts on the armature, as well as of its current. The stronger the field, the lower the degree of rotation required to generate an induced voltage, the broad enough that will survive the applied voltage. For this reason, the speed of the DC motors can be controlled by varying the field current.
As the speed of rotation will control the current flow in the armature, special devices must be used to start the DC motors. When the armature is stopped, it will not actually have resistance, and when applying the normal operating voltage, 471341 timken a large current will occur, which can cause damage to the switch and the armature coils. The natural means of preventing such damages is the use of an ignition resistance connected in series to the armature to decrease the current before the DC electric motor is able to increase the appropriate induced voltage. When the DC motor accelerates, the resistance is gradually reduced, both manually and automatically.