Objectives
After studying this unit, the student will be able to:
• Explain the use of limit switches in the automatic operation of machines and machine tools
• Wire a simple two-wire circuit using a limit switch
• Read and draw normally open (NO) and normally closed (NC) wiring symbols
The automatic operation of machinery re quires the use of switches that can be activated by the motion of the machinery. The repeat accuracy of the switches must be reliable and the response virtually instantaneous.
The size, operating force, stroke, and manner of mounting are all critical factors in the installa tion of limit switches due to mechanical limita tions in the machinery. The electrical ratings of the switches must be carefully matched to the loads to be controlled.
In general, the operation of a limit switch be gins when the moving machine or moving part of a machine strikes an operating lever which ac tuates the switch, figure 21-1. The limit switch, in turn, affects the electrical circuit controlling the machine and its movement.
Limit switches are used as pilot devices in the control circuits of magnetic starters to start, stop, speed up, slow down or reverse electric motors. Limit switches may be used either as control de vices for regular operation or as emergency switches to prevent the improper functioning of machinery. They may be momentary contact (spring return) or maintained contact types.
Limit switch contacts are often drawn differ ently than the symbols shown in figure 21- 1 on control schematics. The contact symbols shown in figure 21-1 are the standard NEMA symbols for normally open and normally closed limit switch contacts. The contact symbol shown in figure 21- 2A shows a limit switch that is “normally open held closed.” This means the contact is wired as a normally open contact, but when the circuit is in its normal off state, some part of the machine holds the contact closed. This symbol can be rec ognized as normally open because the movable contact arm is shown below the stationary contact point.
Figure 21-2B shows a limit switch wired “normally closed held open.” The contact symbol is normally closed because the movable contact arm is drawn above the stationary contact point, but some part of the machine is holding the contact open.
Other contacts such as pressure switches, float switches, and flow switches can also be con nected in this manner. The electrician is more likely to encounter limit switches used in this manner, however, because of the number of limit switches used in industry and the manner in which they are used.
REVIEW QUESTIONS
1. Draw a simple circuit showing how a red pilot light is energized when a limit switch is operated by a moving object.
2. Draw the schematic symbol for a limit switch that is normally open, but held closed.