How To Repair A Lift Master Limit Switch
What is the Function of Limit Switch?
A limit switch is an electro-mechanical device used to send an electrical bespeak based on a physical interaction. Limit Switches are used to detect the presence of objects, thus allowing the system to accept desired action.
Limit Switches can be found in a number of everyday devices:
- They are used in microwaves to check that the door is closed before functioning is allowed.
- They are used in elevators to notify the system that the cabin has reached the desired floor.
- Limit Switches are used in washing machine lids as a permissive to beginning the wash wheel.
Limit Switches in Manufacturing
Countless limit switches are institute in manufacturing. They are used as control devices and rubber devices for machinery and personnel. In all cases, the limit switches will send a digital signal to the command system. Based on the hardware and software tied to said switches, the system is able to have appropriate action.
Why is the limit switch important?
Limit switches are an cheap fashion to create a link between the concrete and electrical domains. They accept been developed a number of decades ago and the mass adoption of their use significantly lowered their cost for the end user. They thus play an important part in manufacturing due to their simplicity and low price.
Use Cases of Limit Switches
- Product Detection & Count - Equally a product pushes against a limit switch, a point is sent to the control organisation. Through simple PLC ladder logic, the user tin can count the number of times the production goes past the limit switch and display the counter for the operator.
- Personnel Safety - A limit switch can be used to find the opening of a prophylactic guard that stops and de-energizes the motorcar. If the guard is opened during operation, the machine stops. If the guard is opened while the machine is stopped, the limit switch prevents the car from starting. In both cases, the limit switch is used to safeguard the operator from potential harm.
- Auto Prophylactic - A limit switch can be used to protect machinery from unintentional damage. This includes components that are part of changeovers (end of arm tools), components that may wear-out over time (motor clutch) and components that may damage others if they fail (gears, shafts, etc.).
Operation of a Limit Switch
Every bit discussed above, a limit switch is used to translate a change in mechanical country into an electrical signal. How does the device actually accomplish that?
A limit switch is a simple device. The lever is used to close a contact within the switch. When the contact is closed, it allows the electric signal to get through. The mechanical operation is similar to that of a low-cal switch. However, a jump is added to return the limit switch into its original position.
The critical parameter of a limit switch is at which signal it closes the contact within. This parameter volition vary between switch types as well equally brands. It is important to verify the datasheet to ostend this position. It will influence the system design that will utilise the limit switch.
Limit Switch - Electrical Betoken Configurations
As discussed previously, once the actuator is displaced a certain distance, the switch is made. At this indicate, it needs to transmit a signal to an electric device which may exist a Programmable Logic Controller (PLC), relay, control circuit, or an additional limit switch. A limit switch is similar to a relay in the sense that information technology can be purchased in one of the following configurations:
- Normally Open up | The switch will allow the electric current to pass through once it is actuated. In resting country, no electric current volition pass through the switch.
- Normal Closed | The switch will cease the current once it is actuated. In resting state, current will flow freely.
- Combination of Ii | The switch will provide a ready of contacts that can be used in Ordinarily Open (NO) and Normally Airtight (NC) circuits.
There is an statement to be made for either configuration. Even so, when it comes to limit switches being used for safety purposes, it'south e'er advised to take electric current circulating in "resting state". This is of import as during a problem in the circuit, the safe should trigger. Should it be gear up to no-power in the "resting state", the circuit may neglect to prevent injury or damage.
Limit Switch - Real Applications & Datasheet
In this section, we're looking at the Allen Bradley 801 General Purpose Limit Switch configurations. Below, yous'll find the "Contact Arrangement" provided past the manufacturer. Allow's review each section in detail
Department one
A - Normally Open (NO) limit switch symbol.
B - Normally Closed (NC) limit switch symbol.
AC - Alternating Electric current Ratings for light duty limit switches at dissimilar voltage levels. It is important to remember that although your awarding may require a switch to operate within a 24VDC circuit, information technology's possible to use these beyond multiple industries in which voltage level standards are different.
DC - Direct Current Ratings for light duty limit switches.
Section ii
C - Normally Open up (NO) and Normally Closed (NC) set of contacts on a single limit switch.
H - Dual Unremarkably Open up (NO) and Normally Closed (NC) prepare of contacts on a unmarried limit switch.
North - Dual Commonly Open (NO) set of contacts on a single limit switch.
O - Dual Unremarkably Airtight (NC) set of contacts on a unmarried limit switch.
Section 3 - Dual Position Limit Switch
In the outset example of a limit switch nosotros had reviewed, there was a single position at which the switch would trigger the signal. In this instance, there are two points at which the signals will be different. These signals can be picked up in discrete logic, just most normally, this would be done using a PLC. A PLC programmer can utilize such a limit switch to detect a divergence between a partial and a total limit switch closure.
Position of Lever = OFF
- Contact ane = Closed
- Contact two = Open
Position of Lever = Partially Closed
- Contact ane = Open up
- Contact 2 = Open
Position of Lever = Fully Closed
- Contact ane = Open
- Contact 2 = Closed
Department three - Commonly Open (NO) Differential Location Limit Switch
Until now, we've talked about limit switches that would activate the aforementioned 1 way or the other. This is the case for nigh general purpose limit switches. However, it is possible to detect if the switch was pushed in one way or the other through the interlock nowadays in the differential location limit switch. The functioning will exist as follows.
Position of Lever = OFF
- Contact 1 = Open
- Contact 2 = Open up
Position of Lever = Closed to the Left
- Contact i = Open
- Contact 2 = Airtight
Position of Lever = Airtight to the Right
- Contact ane = Closed
- Contact ii = Open
Section 4 - Usually Closed (NC) Differential Location Limit Switch
The Normally Closed (NC) variation of the previous switch volition operate as follows. The only difference is the state of the contacts in the "Off" position.
Position of Lever = OFF
- Contact 1 = Closed
- Contact ii = Closed
Position of Lever = Closed to the Left
- Contact 1 = Open up
- Contact 2 = Closed
Position of Lever = Airtight to the Right
- Contact ane = Closed
- Contact two = Open up
Section 5 - Dual Normally Airtight (NC) & Normally Open up (NO) Differential Location Limit Switch
This variation of a limit switch features four contacts. Two of these contacts tin can be used as Ordinarily Open (NO) connections and two other equally Commonly Closed (NC).
Section vi
Current ratings for regular duty limit switches. Notation that these ratings are higher than the ones found in Section 1.
Limit Switches - Other Types & Applications
A limit switch used in manufacturing is rated for hundreds of thousands of cycles. All the same, there are various flavors of this device that aren't as robust for other everyday applications.
Limit Switches in Elevator Systems
Lift systems usually require limit switches for a variety of purposes by building codes. They are used for position detection, safety and operational country of the elevator. The types used in this application are rated for an even greater number of cycles as they constantly feel actuator movement through the life of the organisation.
Limit Switches in Household Systems
Limit Switches are used in everyday appliances: fridges, tea kettles, mixers, washers, dryers and more. The limit switches in these appliances are the same in principle, but different in reliability. In other words, they operate the same, but may fail much faster then their manufacturing counterparts.
How practice I know if my limit switch is bad?
Using a basic ohmmeter or Digital Multimeter (DMM), it's possible to make up one's mind whether or not a limit switch is bad. Disconnect the limit switch from the system and apply the ohmmeter leads to each terminal. If the limit switch is normally open up (NO), the resistance should be very high. If it'southward normally airtight (NC), the resistance should exist close to cypher. Toggle the limit switch into the active position and measure the resistance. Information technology should exist the reverse in this setting. If the transition does not occur, the limit switch is bad.
Conclusion on Limit Switches
Limit Switches are used in a broad array of applications across the production floor too as our daily lives. A limit switch is one of the almost basic mechanical / electronics components on a product flooring. They are used for object detection and machine / personnel safety. The limit switch will send a point to a control circuit in one case it is in a certain position. Although the function is straight-forward, a wide range of flavors of such switches be in society to provide flexibility to the stop user. In a real-world example, nosotros've looked at the datasheet that specifies multiple functions for full general purpose allen bradley limit switches.
Source: https://www.solisplc.com/tutorials/limit-switch
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