Hand-off Automatic Controls

Objectives

After studying this unit, the student will be able to:

• State the purpose of hand-off-automatic controls

• Connect hand-off-automatic controls

• Read and draw diagrams using hand-off-automatic controls

Hand-off -automatic switches are used to se­ lect the function of a motor controller either man­ ually or automatically. This selector switch may be a separate unit or built into the starter enclosure cover. A typical control circuit using a single­ break selector switch is shown in figure 37-1.

With the switch turned to the HAND (man­ual) position, coil (M) is energized all the time and the motor runs continuously. In the OFF posi­ tion, the motor does not run at all. In the AU­ TOMATIC position, the motor runs whenever the two-wire control device is closed . An operator does not need to be present. The control device may be a pressure switch, limit switch, thermo­ stat, or other two-wire control pilot device.

The heavy-duty, three-position double-break selector switch shown in figure 37-2 is also used for manual and automatic control. When the switch is turned to “hand,” the coil is energized, by-passing the automatic control device in the “auto” position.

FIGURE 37-1 Standard duty, three-position selector switch in control circuit

FIGURE 37-2 Diagram of a heavy duty, three-position selector switch in control circuitFIGURE 37-3

FIGURE 37-4

REVIEW QUESTIONS

1. A selector switch and two-wire pilot device cannot be used to control a large motor directly, but rather must be connected to a magnetic starter. Explain why this is true.

2. Determine the minimum number of wires in each conduit shown in figure 37-3.

3. Complete figure 37-4 to show the wiring diagram of the hand-off-automatic selector switch, pull box, pressure switch and Ll and coil M.


electric equipment 1

Related posts:

Leave a comment

Your email address will not be published. Required fields are marked *