[Illustration: Fig. 767.--Cutler-Hammer reversible starter with no voltage release, adapted to start and operate motor at full speed in either direction, such for instance as motors driving auxiliary motions on lathes, planers and other machine tools which may rotate in either direction but always at constant speed. They are not designed to reduce the speed of the motor, but merely to start it and bring it smoothly up to full speed in either direction. Two no voltage release latching devices are provided so that the lever will be held in the full speed position in either direction so long as the voltage of the line remains constant. On failure of voltage a strong centering spring attached to the hub-post of the lever throws the latter to the central, or off position. The shunt field circuit is not opened by starters of this type.] The weakening of the shunt field of a motor by the insertion of resistance in the shunt field circuit causes the armature to revolve more rapidly. One advantage of this method of control is that the motor will inherently regulate to approximately constant speed under widely varying load conditions. Another advantage is found in the fact that all of the current taken from the line is utilized for power, the changes in speed being obtained not by dissipating a portion of the effective energy in the resistance (as in the case of the armature resistance method of control) but by weakening the reverse voltage by inserting resistance in the shunt field circuit. Speed increase by shunt field weakening is limited, however, to about 10 to 15 per cent. above the normal speed in motors of standard construction. Greater ranges of speed can be obtained from motors especially designed for shunt field control but should not be attempted with motors of standard design without first ascertaining from the manufacturer the maximum safe speed. Combined Armature and Shunt Field Control.--Regulation by combined armature and shunt field resistance is by far the easiest way of obtaining a wide range of speeds. Rheostats embodying these methods are known as _compound speed regulators_, one form being shown in fig. 762. Standard regulators can be obtained giving a wide range of speed variation, and special regulators may be constructed giving practically any desired range.