Series (NEETS) modules and the Electrician's Mate Training Manual, NAVEDTRA 12164.
Most of your work with motors, at shore stations especially, will be with a-c motors. Dc motors have certain advantages but a-c power is more widely used and a-c motors are less expensive and on the whole, more reliable.
For example, sparking at the brushes of a dc motor can be very dangerous if there is explosive gas or dust in the surrounding air. On most a-c motors, brushes and commutators are not used and little maintenance is required. They are suited to constant speed applications and are designed to operate at a different number of phases and voltages.
A-c motors are designed in various sixes, shapes, and types such as the induction, series, and synchronous, but as a Construction Electrician in the U. S. Navy, you will be concerned primarily with the induction motors. This type of motor includes, among others, the split-phase, capacitor, repulsion-induction, and the polyphase motors.
A split-phase motor is usually of fractional horsepower. It is used to operate such devices as small pumps, oil burners, and washing machines. It has four main parts. These are the rotor, the stator, the end plates (or end bells, as they are sometimes called), and a centrifugal switch
The rotor consists of three parts. One of these parts is the core which is made up of sheets of sheet steel called laminations. Another part is a shaft on which these laminations are pressed. The third part is a squirrel-cage winding consisting of copper bars which are placed in slots in the iron core and connected to each other by means of copper rings located on both ends of the core. In some motors the rotor has a one-piece cast aluminum winding.
The stator of a split-phase motor consists of a laminated iron core with semiclosed slots, a steel frame into which the core is pressed, and two windings of insulated copper wire that are placed into the slots and are called the running and starting windings.
End bells, which are fastened to the motor frame by means of bolts or screws, serve to keep the rotor in perfect alignment. These end bells are equipped with bores or wells in the center, and are fitted with either sleeve or ball bearings to support the weight of the rotor and thus permit it to rotate without rubbing on the stator.
The centrifugal switch is located inside the motor on one of the end bells. It is used to disconnect the starting winding after the rotor has reached a predetermined speed, usually 75 percent of the full load speed. The action of the centrifugal switch is as follows: the contacts on the stationary part of the switch (the stationary part is mounted on the end bell) are closed when the motor is not in motion and make contact with the starting winding. When the motor is energized and reaches approximately 75 percent of full load speed, the rotating part of the switch (mounted on the rotor) is forced by centrifugal force against the stationary arm, thereby breaking the contact and disconnecting the starting winding from the circuit. The motor is then operating on the running winding as an induction motor. Figure 7-29 shows the two major parts of a centrifugal switch.
Figure 7-29. - Two major parts of a centrifugal switch.Continue Reading