Electrical Controls - Chapter 11 PDF

ELECTRICAL CONTROLS by Engr. Alexis T. Belonio Department of Agricultural Engineering and Environmental Management College of Agriculture Central Philippine University Iloilo City [email protected] ...

ELECTRICAL CONTROLS by Engr. Alexis T. Belonio Department of Agricultural Engineering and Environmental Management College of Agriculture Central Philippine University Iloilo City [email protected]  Introduction  The success of any electrical application highly depends upon the proper selection of a control mechanism in operating the equipment. Electric Brooder Dehumidifier Freezer Air Conditioner Food Dehydrator Oven Vacuum Fryer Electric Motor  Classifications of Electrical Control Mechanism  Manually Operated Controls  Switches  AC Relays  Pushbuttons  Motor Starting Switch  Magnetic Starting Switch  Reduce Voltage Starters  Solenoid Valve  Automatically Operated Controls  Thermostat  Time Float Switch  Pressure Switch  Limit Switch  Switches  It is the most basic electrical control mechanism.  It is used to open and close the circuit.  Its contact must open and close quickly “snap-action” the circuit.  Nomenclature for Switches  Single-pole single-throw  Double-pole, single-throw  Triple-pole, single-throw  Single-pole, double-throw  Double-pole, double-throw  Triple-pole, double-throw Single Pole Double Single Pole Throw Single Throw  AC Relays  It is a type of switch having contacts which are operated by magnetism which allows the current to pass through a coil of the wire. Magnetism is removed once the the flow of current through the coil is stopped.  The electrical input to the appliance is controlled by the relay contacts.  The current rating of the contacts is very important, and the proper selection depends upon the current flow to the appliance which is to be controlled. Solid State Relay  The main advantage in using the relay is that the switch and wires which control the current flow to the relay coil do not conduct the current to the appliance.  Relays are adapted for controlling motors, heaters, and large lighting loads for remotely located small-size switches.  Pushbuttons  It is a manually operated type of switch for starting and stopping electric motors  It is also used for operating the electric relay type of circuit.  Motor Starting Switch  It is used to control and protects the electric motor.  It has a built-in overload device that automatically trips the switch in case there is too much current flowing into the motor.  Magnetic Starting Switch  It is used primarily for starting 1-hp and larger size motor.  It can also be used for connecting large heating or lighting loads to the source of power.  This switch incorporates into one complete unit the principle of relay coil, the overhead stripper, and the push button control.  Reduce Voltage Starters  It is used to apply the reduce voltage to the motor when the motor is starting and then apply full voltage after the motor has gained sufficient speed. This allows the motor to start without requiring excessively large current values.  The cost of the control is a disadvantage since its cost is ½ to ¾ of the cost of the motor.  Solenoid Valve  It is operated by electricity but the valve itself does not control the flow of the electricity. Instead it controls the flow of water, oil, or gas.  It is useful for fuel supply line, refrigeration system, humidifier, water supply line, etc.  The only electric power required for this control is about 7 watts hence the cost is negligible.  The switch maybe manually or automatically operated. When current flow the coil, it causes the metal plunger by magnetism to be drawn up into the coil.  Solenoid Valve Automatic Ball Valves Mass Flow Controller Flow Switch  Thermostat  It is a special kind of switch wherein it s contact open and close automatically in response to changes in temperature.  It is an automatic switch which is used for temperature control.  Classifications are:  Bellow Thermostat  Bimetallic Thermostat  Wafer Thermostat Temperature Controller Thermostat Air Temperature Switch Temperature and Humidity Switch Line Voltage Thermostat  Time Switch  It is an electric clock that automatically operates switch contacts at definite time intervals.  The general purpose time switch is designed automatically to close the switch contact two times and open them two times during any 24 hour period.  Usually they are use to switch ON and OFF lighting facility for farm houses, automatic feeders, cooling systems, hay curing, grain drying, and heating systems  Float Switch  It is an automatic control that operates according to some predetermined liquid level.  Float are usually hollow copper ball or cylinder and is connected to the float switch by a rod or chain.  Operation is based on the up and down motion of the float as the liquid level rises and fall.  Float Paddle Level Switch Float Switch  Pressure Switch  It is an automatic control having a switch contacts which are operated according to variations in liquid or gas pressure.  The gas, vapor, or liquid passes through the control, and the pressure of the gas or liquid expands the metal bellows against the force of a spring.  Common application of this type of switch is the pressure tank run by electric pump for domestic water supply.  Differential Pressure Switch Pressure Pressure Switch Switch Vacuum Pressure Switch Gas Pressure Switch Bourdon Pressure Switch  Limit Switch  It is an electric switch which is operated automatically by the movement of some other piece of material or machine.  The switch mechanism is frequently tripped by a knot in a rope, a metal stop fastened to a chin, a cam, or some similar specially rigged arrangement.  Switch contact are normally returned to their normal position by the action of spring.  Reference  Brown, R. 1956. Farm Electrification. McGraw-Hill Book Company. New York. 367pp.