SEMI AUTOMATIC AND AUTOMATIC CONTROL OF MODERN MACHINERY

Control of a machine can be semi-automatic or fully automatic. There are probably more machines operated by semi-automatic control than by manual or fully automatic controls. Consider, for example, an over-head tank which supplies drinking water to a factory.

If we provide a manual switch near the pump motor and depute an operator to switch it ON when water level falls, then this is classified as manual control. Here, the operator has to go to the pump site to fill the tank. For the same pump if a magnetic starter is provided near the pump motor and for its starting, a switch is provided near foreman’s desk it may be classified as a semi-automatic control. A lamp indication or a bell can also be provided near the desk to indicate if the tank is full. The foreman can switch ON the pump from his desk without going to the pump site. Over-flow can also be avoided by switching OFF the pump when the lamp glows or the bell rings. If a float switch is provided in the tank to switch ON the pump motor when water level falls below a certain lower limit, and switch it OFF when water level rises beyond a certain upper limit, then the control becomes fully automatic. The cost of installation of an automatic control system will be higher than the other two types of controls. However, an automatic control arrangement relieves the operator from the task of keeping an eye on the water level and operate the pump. Also there is no danger of over-flow from the tank. Thus it is seen that the basic difference in manual, semi-automatic and fully automatic control lies in the flexibility it provides to the system being controlled.
The study of control circuits involves study of the construction and principle of operation of various control components and learning the art of designing control circuits for various functions of machines. In this text, we have first discussed the various control components and then control schemes for ac and dc motors.

Modern machines have large number of operations requiring extensive control circuits consisting of large number of relays. Thus the control panel occupies a lot of space and control circuit design also becomes tedious.

Static control is used for such machines as the control design is easy with static control devices. The static devices used for design of control circuits are the digital logic gates. With much advancement in the field of computers this static control is also becoming obsolete as more and more machines are now being controlled by programmable controllers. Inspite of all these developments as far as single motor control or a machine having few operations is concerned, the magnetic control using contactor and relay will continue to be in use because it is the simplest and cheapest method of control for such applications.

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