Figure 7-50. - Across-the-line manual controller.
while a polyphase motor requires a two-element overload device (fig. 7-50).
DISCONNECTS. - Disconnects may be used as controllers on motors rated up to 3 horsepower at 220 volts. They must be located within sight of the motor or be able to lock in the open position. A distance of more than 50 feet is considered "out of sight." Double-throw disconnects may be used for reversing three-phase motors if they conform to these requirements.
DRUM CONTROL. - The drum control is a lever-operated, three-position switch. The center position is usually the OFF position with the right and left positions FORWARD and REVERSE, respectively. Normally, it is used to direct the rotation of a three- phase motor. Oil-immersed drum switches are used wherever the air can become charged with corrosive gases or highly flammable dust or lint.
Magnetic starters are made to handle motors from 2 to 50 horsepower. They can be controlled by a start-stop station located locally or remotely. The starter has two different circuits: the control circuit and the load circuit.
CONTROL CIRCUIT. - The control circuit receives its power from the incoming leads to the starter. It is a series circuit (fig. 7-51) going through the start- stop station, the magnetic coil, the overload contacts, and returning to another phase. However, it may return to the ground, depending on the voltage rating of the coil.
LOAD CIRCUIT. - The current flowing through the coil activates a mechanical lever and closes the main line contacts. This closing develops the load circuit and applies power to the motor. The fourth set of contacts provides a shunt around the start button, known as the holding circuit.
Figure 7-51. - Magnetic starter circuit.Continue Reading