The limit switch is like a 1 NO + 1 NC push button. The push button is actuated by hand whereas the limit switch is operated mechanically. The main function of the limit switch is to open or close an electric circuit when the physical limit of the operation of the controlled device has reached.
It may be operated by a by a cam or lever fitted with the machine to stop, reverse or slow down the operation of the device used for a particular application.
The electrical ratings of the limit switches must be carefully matched to the loads to be controlled. The limit switch contacts should not be overloaded. Contacts must be selected according to the proper load, that is, the proper current and voltage, according to the manufacturer’s specifications.
Micro Limit Switch
Another type of limit switch often used in different types of control circuits is the micro limit switch or microswitch. Micro limit switches are much smaller in size than an ordinary limit switch. This permits them to be used in small spaces that would not be accessible to the larger device.
The actuating lever of micro limit switch requires only a small amount of movement to cause the contacts to change position. The basic micro limit switches are available in different case styles and with different operating levers.
Working of Limit Switch
To understand the working of limit switch, a typical forward – reverse operation of a conveyor is shown in the figure. It should be noted that,
- Both limit switches have two sets of contacts (1 NO + 1 NC).
- Three phase induction motor is used to run the conveyor.
- Contacter A runs motor (and conveyor) in the forward direction.
- Contacter B runts motor in reverse direction (by interchanging two phases of supply).
The operation shown in circuit diagram can be summarized as under:
When START pushbutton is pressed,
- Contacter A is energized.
- Motor runs in the forward direction.
- Striker connected to conveyor moves in the forward direction.
When striker reaches near REV limit switch, it strikes with limit switch and
- R2 opens, de-energies contactor A, the motor stops.
- R1 closes, energies contactor B, the motor starts rotating in reverse direction.
Similarly, when striker reaches near FWD limit switch, it strikes with FWD limit switch and
- F2 opens, de-energies contactor B, the motor stops.
- F1 closes, energies contactor A, the motor starts rotating in forward direction
The process of forward and reverse rotation will continue till the motor is switched off.