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GOALS:
- Discuss the operation of a limit switch.
- Connect a limit switch in a circuit.
- Recognize limit switch symbols in a ladder diagram.
- Discuss the different types of limit switches.
Ill. 1 A limit switch is used to detect the position of boxes on a conveyer
line.
Limit switches are used to detect when an object is present or absent from a particular location. They can be activated by the motion of a machine or by the presence or absence of a particular object. Limit switches contain some type of bumper arm that's impacted by an object. The type of bumper arm used is determined by the application of the limit switch. When the bumper arm is impacted, it causes the contacts to change position. Ill. 1 illustrates the use of a limit switch to detect the position of boxes on a conveyer line. This particular limit switch uses a long metal rod that's free to move in any direction when hit by an object. This type of bumper arm is generally called a wobble stick or wiggle stick. Limit switches with different types of bumper arm are shown in Ill. 2.
Limit switches vary in size and contact arrangement depending on the application. Some are constructed of heavy gauge metal and are intended to be struck by moving objects thousands of time. Others are small and designed to fit into constricted spaces. Some contain a single set of contacts and others contain multiple contacts, as shown in Ill. 3. Some limit switches are momentary contact (spring returned) and others are maintained contact.
Generally, limit switches are used as pilot devices to control the coil of relays and motor starters in control circuits. The standard NEMA symbols used to indicate limit switches are shown in Ill. 4. The wedge drawn under the switch symbol represents the bumper arm of the switch.
Ill. 2 Limit switches with different types of activating arms.
Ill. 3 Limit switch with cover removed to show multiple contacts.
Ill. 4 NEMA standard symbols for limit switches.
Micro Limit Switches
Another type of limit switch often used in different types of control circuits is the micro limit switch or micro switch. Micro switches are much smaller in size than the limit switch shown in Ill. 3, which permits them to be used in small spaces that would never be accessible to the larger device. Another characteristic of the micro switch is that the actuating plunger requires only a small amount of travel to cause the contacts to change position. The micro switch shown in Ill. 5 has an activating plunger located at the top of the switch. This switch requires that the plunger be depressed approximately 0.015 inch or 0.38 millimeters. Switching the contact position with this small amount of movement is accomplished by spring loading the contacts, as shown in Ill. 6. A small amount of movement against the spring will cause the movable contact to snap from one position to another.
Electrical ratings for the contacts of the basic micro switch are generally in the range of 250 volts AC and 10 to 15 amperes, depending on the type of switch.
The basic micro switch can be obtained with a variety of different activating arms, as shown in Ill. 7.
Ill. 5 Micro limit switch.
Ill. 6 Spring loaded contacts of a basic micro switch.
Ill. 7 Micro switches can be obtained with different types of
activating arms.
Ill. 8 Cutaway view of a subminiature micro switch.
Ill. 9 Sub-miniature micro switch.
Subminiature Micro Switches
The subminiature micro switch employs a similar spring contact arrangement as the basic micro switch (Ill. 8). The subminiature switches are approximately one-half to one-quarter the size of the basic switch, depending on the model. Due to their reduced size, the contact ratings of subminiature switches range from about 1 ampere to about 7 amperes depending on the switch type. A different type of subminiature micro switch is shown in Ill. 9.
Ill. 10 Platform rises between floors
Ill. 11 Control circuit to raise and lower platform.
Limit Switch Application
Ill. 10 illustrates a common use for limit switches. A platform is used to raise material from a bottom floor to an upper floor. A hydraulic cylinder is used to raise the platform. A limit switch located on the bottom floor detects when the platform is in that position, and a second limit switch on the upper floor detects when the platform has reached the upper floor. A hydraulic pump is used to raise the platform. When the platform is to travel from the upper floor to the lower floor, a solenoid valve opens and permits oil to return to a holding tank. It isn't necessary to use the pump to lower the platform because the weight of the platform will return it to the lower floor.
The schematic diagram for this control circuit's shown in Ill. 11. The schematic shows both limit switches to be normally closed. When the plat form is at the extent of travel in either direction, however, one of the limit switches will be open. If the platform is at the bottom floor, limit switch LS2 will be open. If the UP push button is pressed, a circuit will be completed to M starter, causing the motor to start raising the platform. The M normally closed contact will open to prevent CR from being energized at the same time. When the platform begins to rise, limit switch LS2 will close. The platform will continue up ward until it reaches the top, causing limit switch LS1 to open. This will de-energize M contactor, causing the motor to stop and the normally closed auxiliary contact in series with CR coil to re-close.
When the DOWN push button is pressed, control relay CR will energize. The normally closed CR contacts connected in series with M contactor will open to interlock the circuit, and the normally open CR contact connected in series with the solenoid coil will close.
When the solenoid coil energizes, the platform will start downward, causing limit switch LS1 to re-close.
When the platform reaches the bottom floor, limit switch LS2 will open and de-energize coil CR.
QUIZ:
1. What is the primary use of a limit switch?
2. Why are the contacts of a micro switch spring loaded?
3. Refer to the circuit shown in Ill. 11.
Assume that the platform is located on the bottom floor. When the UP push button is pressed, the pump motor does not start. Which of the following could not cause this problem?
a. The contacts of limit switch LS1 are closed.
b. The contacts of limit switch LS2 are open.
c. Motor starter coil M is open.
d. The overload contact is open.
4. Refer to the circuit shown in Ill. 11.
Assume that the platform is located on the lower floor. When the UP pushbutton is pressed, the platform raises. When the platform reaches the upper floor, however, the pump does not turn off but continues to run until the overload relay opens the overload contacts. Which of the following could cause this problem?
a. The solenoid valve opened when limit switch LS1 opened.
b. The UP pushbutton is shorted.
c. Limit switch LS1 did not open its contacts.
d. Limit switch LS2 contacts did not re-close when the platform began to rise.
5. Refer to the circuit shown in Ill. 11.
Assume that the platform is located at the upper floor. When the DOWN push button is pressed, the platform does not begin to lower. Which of the following could not cause the problem?
a. Control relay coil CR is open.
b. Limit switch LS1 contacts are open.
c. Limit switch LS2 contacts are open.
d. The solenoid coil is open.