The speed of a DC motor will depend on the strength of the magnetic field, the value of the applied voltage and the load. If the field strength falls, the speed increases, trying to maintain the force against electromotive force. If the field winding were left open, only the residual magnetism would remain, and the velocity would increase in an alarming way, trying to counteract the applied voltage. With a light or no load, an open field circuit could cause an increase in speed that would shatter the engine. The commutator blades and other parts of the machine would be thrown away and could cause serious injury to operators near the machine.
The motor speed can be controlled by controlling the field current using a rheostat or by controlling the applied voltage by applying static converters. If a power supply is rectified, it can be used to convert a fixed alternating power supply into a continuous variable. Through this variation applied to the DC motor, it is possible to vary the speed of rotation of the same. The static converter is well used to perform this speed control by employing rectifiers to correct AC voltage and vary the DC voltage applied to the motor.