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 Full-Voltage Starters
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
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
Figure 7-51.Magnetic starter circuit.