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Industrial Machine Industrial Machine ControlControl

EKT 451EKT 451CHAPTER 7CHAPTER 7

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Basics of Industrial Motor Basics of Industrial Motor ControlControl

When applied to machinery, it involves the starting, acceleration, reversal, deceleration and stopping of a motor and its load.

In this chapter we will study the electrical (not electronic) control of a alternating current motor.

However, the basic principle covered here apply to any system of control.

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Control Control DevicesDevices

Disconnecting switches Isolates the motor from the power

source. 3 knife-switches and 3 line fuses

Manual circuit breaker Opens and closes a circuit. Trip automatically when the

current exceed the predetermined limit.

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Control Control DevicesDevices

Cam switches Has a group of fixed contacts and an

equal number of movable contacts. The contacts can be made to open or

close by rotating a handle or knob. Are used to control the motion and

position of hoist, machine tools and so on.

Pushbuttons Is a switch activated by finger pressure. Usually spring loaded so as to return to

normal position when pressure is removed.

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Control Control DevicesDevices

Control relays Is an electromagnetic switch that

opens and closes when the relay coil is energized.

Mainly used in low-power circuits.

Thermal relays Is a temperature sensitive device. The current flow will increase the

temperature of the relay. The contacts open or close when the

current exceeds a preset limit.

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Control Control DevicesDevices

Magnetic contactors Basically a large control relay

designed to open or close a power circuit.

Possess of a relay coil and a magnetic plunger.

When the relay coil is energized, it attracts the magnetic plunger, thereby closing the power circuit.

Pilot lights Indicates the on/off state of a

remote component in a control system.

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Normally open and normally Normally open and normally close contactsclose contacts

Control circuit diagram always show components in a state of rest, that is, when they are do not energized (electrically) or activated (mechanically).

Normally open contacts (NO) Normally closed contacts (NC)

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Control Control DiagramsDiagrams

A control system can be represented by four types of circuit diagrams.Block diagramOne-line diagramWiring diagramSchematic diagram

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Control DiagramsControl Diagrams Block diagram

Composed of a set of rectangles, each representing a control devices, together with a brief description of its function.

The rectangles are connected by arrow that indicate the direction of power or signal flow.

Block diagram of a combination starter

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Control DiagramsControl Diagrams One-line diagram

Similar to block diagram, except that the components are shown by the symbols rather than rectangles.

The symbols give us an idea of the nature of the components.

Consequently, one-line diagram yield more information.

One-line diagram of a combination starter

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Control DiagramsControl Diagrams

Wiring diagramShows the connection between the componentsThese diagrams are employed when installing

equipment or when troubleshooting a circuit.

wiring diagram of a combination starter

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Control DiagramsControl Diagrams Schematic diagram

Shows all the electrical connections between the components, without regard to their physical location or terminal arrangement.

These diagram is indispensable when troubleshooting a circuit or analyzing its mode of operation.

Schematic diagram of a combination starter

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Starting MethodsStarting Methods A motor control circuit contains two basic

components A disconnecting switch A starter

Manual across-the-line starters Composed of a circuit breaker and either two or three thermal

relay; mounted in appropriate enclosure. The thermal relays trip the circuit breaker whenever the

current exceeds the rated value for a significant length of time.

Manual starter for single phase motor

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Starting MethodsStarting Methods Magnetic across-the-line starters

Whenever a motor has to be controlled from a remote location.

Has 3 main components: a magnetic contactorA thermal relayA control station

Magnetic starter

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We can reverse the direction of rotation of a 3-phase motor by interchanging any two lines.

This can be done by using 2 magnetic contactors and a manual 3-position cam switch.

Schematic diagram of a reversible magnetic starter

Reversing the direction of Reversing the direction of rotationrotation

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Some industrial loads have to be started very gradually.Example : coil winders, printing presses, conveyor

belts. A motor cannot be directly connected to the

line because the starting current is too high. reduce the voltage applied to the motor by

connecting resistor in series with the line.

Reduced-voltage Reduced-voltage startingstarting

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connecting resistor in series

Reduced-voltage Reduced-voltage startingstarting

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With the advent of computers, it is now possible to simulate the behavior of many relay coils and relay contacts.

Instead of using relays, contacts and time delay dashpots , we simply program these device on a computer.

Programmable Logic Controllers (PLCs)

Computer and Computer and ControlledControlled

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The fundamental difference of the electric machine control diagram is the way the control devices are interconnected.

Programmable Logic Programmable Logic ControllersControllers

An automated system is composed of control devices and controlled devices.

Black box is a computer and designed to simulate the relays, relay coils and relay contacts.

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PLCs consists of five basic parts.CPUInput moduleOutput moduleProgramming unitPower supply

Programmable Logic Programmable Logic ControllersControllers

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Programmable Logic Programmable Logic ControllersControllers