LIMIT SWITCHES

Limit switch is an important control element. Limit switch contacts change over their position when its actuating lever or knob is actuated by the mechanical part of a machine. The mechanical  part attached to the machine which actuates the limit switch lever or knob is known as actuator or dog. Limit switches are used to stop a mechanical movement of a machine and may also be used to stop a particular movement, and initiate another movement. The simple application of a limit switch is in producing automatic to and fro movement of a planar machine bed shown in Fig. 2.32. It must be understood here that a limit switch is not used as a mechanical stop. A limit switch controls the electrical signal which is responsible for mechanical stop/movement.

Actual control circuit for achieving this to and fro movement with help of limit switches shown in Fig. 2.32, will be discussed at a later stage.

         

•   Simple Limit Switch

The simplest construction of a limit switch is shown in Fig. 2.33.
When the knob or pin is pushed the plunger attached to the knob move down against
spring pressure. The moving contacts mounted on the plunger also moves down. Thus the terminals 1 and 2, which are normally closed, become open and terminals 3 and 4, which are normally open, get closed. When the pressure on the knob is released the contacts return to their normal position. Another commonly used limit switch is the lever type. This may be single side actuation type or double side actuation type, as shown in Figs. 2.33 (b) and (c).
In a single side actuation type limit switch the contacts operate when the limit switch arm moves in one direction, either the right or to the left. Movement on the other side does not actuate the contacts. The arm returns to the normal position when the actuating force is removed. The limit switch can also be of maintained contact type in which arm has to be brought

 back manually to normal. However this type of limit switches are rarely used. In double side actuation type limit switch, when the arm is moved in one direction, say towards right, contact block A operates; and when the arm is moved towards left, contact block Boperates. This type of a limit switch can serve the purpose of two single side actuation limit switches needed in controlling to and fro movement of a machine. If the contacts of the limit switch change-over independent of the speed of operation, the limit switch is known as snap-action limit switch. At a particular position of the arm, the contacts change-over. If the contacts also move at the same speed as that of the arm of the limit switch, it is called a slow-action limit switch. Most limit switches used are the snapaction type. There are only few applications where slow acting limit switches are preferred.

• Rotary Cam Type Limit Switches

In this type of limit switches, the contacts are mounted on the stationary frame. The cams, which have to actuate the contacts, are mounted on the rotating shaft. The position of the cams is adjustable. The rotating shaft is coupled to the driving motor of the machinery either through chain and sprockets or by gear arrangement. The rotation of the driving motor is thus transmitted to the shaft of the limit switch. The cams mounted on the shaft are shaped and their position adjusted to actuate the contacts at the desired instant. In the machinery, the rotary motion of the driving motor is converted into longitudinal motion through mechanical arrangement. Thus, the cam shaft of the limit switch rotates proportional to the longitudinal motion of the machine. The limits of travel of the machinery, therefore, can be adjusted by changing the cam position. The advantage of a rotary cam limit switch over an ordinary limit switch is that the instant of actuation and hence limits of travel can be varied easily over a wide range by changing the cam positions. In an ordinary limit switch, however, once the limit switch is fixed, only very little adjustment is possible by adjusting the lever position. The reliability of rotary limit switch using sprocket and chain is however low and, therefore, wherever these are used a back-up-protection of ordinary limit switches is generally provided to avoid damage due to over travel, in case the rotary limit switch fails due to slipping or breaking of chain. Rotary limit switches are preferred where frequent adjustment of limits of travel is required. One of its important application is in over-head cranes for hoisting and lowering motion. In over-head crane, for hoisting operation, the rotary motion of the motor is converted into longitudinal motion with the help of a rope wound on the drum attached to the motor shaft. The rope winds on the drum in one direction giving hoisting (raising) operation and unwinds in other direction giving lowering motion. The limits of hoisting and lowering can be easily adjusted by varying the cam positions which have to actuate the hoisting and lowering limit switch contacts.

• Heavy Duty Limit Switches

As the name signifies, these limit switches are used in heavy duty machines such as cranes, conveyors and heavy material handling equipment. The basic difference between an ordinary limit switch and a heavy duty limit switch is that the former is used in the control circuit while the latter is used in the power circuit. Actuation of a heavy duty limit switch cuts off power supply to the motor. Heavy duty limit switch requires a minimum of two normally closed contacts because to stop a motor two phases have to be cut off. A heavy duty limit switch may be either lever type or rotary cam type. They are rugged, and the current rating of their contacts matches with the motor rating. If the rating is quite high, arc chutes are also provided over the contacts. These heavy duty limit switches are used in addition to the control limit switches. They are generally used as back-up-protection for control limit switches. If the control limit switch fails or contactor gets stuck or welded, then the heavy duty limit switch operates to disconnect power supply to the motor. The limit switch contacts are connected in series with the main contactor contacts. These limit switches therefore avoid damage and accidents due to over-travel of machinary like overhead crane and other heavy material handling equipment.

•  Speed Actuating Sensing Switches

These are also commonly known as plugging switches or zero speed switches. These switches have two sets of contacts, one each for either direction or rotation of the motor. The contacts for a particular direction of rotation open or close when a predetermined rotational speed in that direction is achieved. These switches are used in circuits where the motor is to be stopped quickly (i.e.,to be brought to standstill quickly by plugging method of braking). In plugging, the motor is stopped by reversing its two supply leads till motor comes to standstill. The torque developed during reversing causes the motor to come to standstill in a short time. The most commonly used design of this device is the one which use the effect of induced magnetic forces. In this type, a shaft having a permanent magnet on it rotates inside a copper cup. The copper cup can rotate in either direction against spring pressures. The shaft of the switch is coupled with the motor shaft. As the motor shaft rotates, the permanent magnet on the switch shaft rotates inside the cup. Magnetic induction due to its rotation causes the cup to follow the rotation of the shaft. An actuator is connected on the copper cup which can actuate the contacts in either direction. The speed at which the contacts would operate can be adjusted by varying the spring tension which restrain the movement of the cup. The contacts get reset to their original position when the motor speed falls below the value set by the spring tension. If the contact of the speed switch is so adjusted that contacts operate immediately on rotation in either direction and re-close only when zero speed is reached the switch is known as zero speed switch. The plugging switch or zero speed switch is represented in a control circuit as shown in Fig. 2.35.

 

To follow us : 

https://www.youtube.com/channel/UC8uzr-PXQxLpAiVKA7S1rRQ/videos?view_as=subscriber

https://www.facebook.com/Engi.Prog/