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Introduction to Control of Machines

1.1 GENERAL IDEA OF CONTROLSA machine, as we know, is a device consisting of several interconnecting parts which by theirmotion, transform/transmit power to do some work. The motion of various parts of a machinecan be obtained by using two types of devices, viz.

(i) an electrical motor

(ii) a solenoid valve and cylinder piston assembly operated by a compressor or a pump.

A constant rotational motion developed by a motor can be converted into linear mechanicalmovement through rack and pinion arrangement connected to the motor shaft.

In solenoid valve and cylinder piston assembly, fluid or air pressure is applied to cylinderpiston through a solenoid valve. Solenoid valve consists of a mechanical valve operated throughan electrical coil. Fig. 1.1 shows how a cylinder piston is used to get linear to and fro motion of

a machine part. A spring returned solenoid valve is also shown in Fig. 1.1.

Cylinder Piston

A B

D C

Coil

Exhaust

Spring

AirReceiver

Compressor Motor

(a)

Cylinder Piston

A B

D C

ExhaustSpring

Airreceiver

Compressor Motor

(b)

Fig. 1.1 Operation of a solenoid valve (a) solenoid coil de-energised (b) solenoid coil energised

In Fig. 1.1 (a) the solenoid coil is de-energised and ports of the valve are connected asshown by arrows. Port A is connected to exhaust while port B is connected to air receiver in

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dharmd:\N-Mach\Mac1-1.pm5 2

2 CONTROL OF MACHINES

which air is stored at a high pressure. The machine part connected to the piston will be in the

position as shown. This is due to air pressure acting on the right-hand side of the piston.When the solenoid coil is energised, a spool will get shifted in the valve and now the

ports of the valve will get connected as shown by arrows in Fig. 1.1 (b). Port A gets connectedto pressurised air while port B gets connected to exhaust. The piston will thus move forward tothe extreme end due to air pressure on the left-hand side of the piston. In this arrangementalso, of course a motor is used as drive for the air compressor or the pump (if fluid is used) tobuild up air or fluid pressure. Thus it is seen that a motor is the heart of all machinery whetherits output is used directly or indirectly. In this text the main emphasis is on the study ofcontrol of motors. However some control circuits using solenoid valves will also be taken up.Now we will proceed to discuss methods of control of motors.

1.2 DISADVANTAGES OF MANUAL CONTROL

When electric motors were first introduced, simple manual switches were used to start andstop the motor. The only protective device used was the fuse. Progress was subsequently madealong the lines of improving the reliability, flexibility and make-break performance of themanual switches. In those days one large motor was used to drive a line shaft through a beltpulley arrangement. Individual machines were then connected to the line shaft through beltand pulley arrangements. This system of driving a number of individual loads from a commonline shaft and manual switching of motors had many disadvantages as listed below:

Starting, stopping and speed control of motor had to be performed by hand every time.The operator had to move a manual switching device from one position to another.

Switching of large motors required great physical effort.

Operator had to remain continuously alert to watch indicators so as to adjust motorperformance according to drive requirements.

Sequence operations of number of motors could not be accomplished in common lineshaft arrangement.

The varied needs of individual machines like frequent starts and stops, periodic reversalof direction of rotation, high-starting torque requirement, constant speed, variable speed,etc., could not be accomplished in common line shaft arrangement.

1.3 INTRODUCTION TO MAGNETIC CONTROLThe disadvantages of common line shaft arrangement necessitated the use of small motors onindividual machines instead of one large motor in line shaft arrangement. The use of individualmotors on different machines made the machine shop or factory more flexible.

Introduction of electromagnetic CONTACTORS AND RELAYS, that is, devices actuated

by electromagnets and requiring only a small power for actuation as compared to the powerswitched ON through their main contacts, led to the much desired control of machines possible.The word control means to govern or regulate. By control of a motor we mean regulation orgoverning of its various operations like starting, stopping, acceleration, reversal of direction ofrotation, speed variation, protection etc. Fig. 1.2 shows the connections for manual and magneticcontrol of a small squirrel cage induction motor using push buttons.

In a simple manual control the motor starts when the main switch is closed by theoperator. The motor is protected by fuses as shown in Fig. 1.2 (a). In Fig. 1.2 (b) anelectromagnetic contactor is used to switch on the motor. Before we discuss the working of thecircuit, it is essential to be familiar with the working principle of a contactor. Various types ofcontactors, however, will be discussed in detail in the next chapter.

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INTRODUCTION TO CONTROL OF MACHINES 3

Fig. 1.2 Manual and Magnetic control of a small squirrel cage induction motor

In Fig. 1.3 it is seen that a coil is wound on the fixed core while the contacts are mountedon the moveable core, called the armature. The contacts remain in the normal position asshown in Fig. 1.3 (a) i.e., when the coil remains de-energised. The spring (S) keeps the mainand the auxiliary contacts in the position shown. In Fig. 1.3 (b) the contactor is shown with itscoil in energised condition. As soon as the coil is energised, the armature i.e., the movable coreis attracted towards the fixed core against the pressure of spring (S). The contacts thus changetheir positions. The normally open main contacts close. Simultaneously auxiliary contacts alsochange their positions. The normally open contacts close and normally close contacts open.The main contacts are used for switching the power to the motor while auxiliary contacts areused in the control circuit. When the coil is again de-energised armature comes back to itsoriginal position due to the tension of the spring (S).

Now let us refer back again to the starting circuit of motor in Fig. 1.2 (b). In this figurethere are two separate circuits, i.e., the power circuit and the control circuit. Supply to thecontrol circuit is isolated from the main supply using a control transformer T

1. Alternatively,

a phase and neutral can also be used for providing supply to the control circuit. The motor canbe switched ON and OFF with the help of push buttons. These push button switches requiresmall force to actuate their contacts. These contacts remain operated as long as pressure isapplied and they return to their normal position when pressure is released.

In this circuit when the ON-push button is pressed, contact P2

closes. Supply fromsecondary terminal of the control transformer reaches the contactor coil through contacts, P

1

(OFF-push button),P2, (ON-push button) and the over-load relay contact OL. The other terminal

of the coil is connected directly to the terminal b1

of the transformer. The coil is thus energised

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Normally closed

Auxiliarycontacts

Normally open

Normallyopen

Maincontacts

Armature

Spring

Switch

Supply

Switch

SupplyI

(a) (b)

Fig. 1.3 Operation of an electromagnetic type contactor (a) coil de-energised (b) coil energised

and contactor closes its main contacts M1, M2, M3 and the auxiliary contact M4. Closing ofcontact M4 bypasses ON-push button contact P2. Now if pressure on the ON-push button isreleased contactP2 will open but supply to contactor coilMwould reach through closed contactM

4 connected in parallel with contactP2. This contactM4 is known as holding or sealing contact.A bimetallic thermal over-load is also shown connected in the power circuit. If motor drawsmore current than its rated value, thermal relay contact OL opens and de-energises coil M.

De-energisation of coil opens the contactsM1,M2,M3 andM4. Supply to motor stops and holdingof control supply through M

4is also broken. Motor can also be stopped by pressing the OFF-

push button. When OFF-push button contactP1

opens, coilMis de-energised and thus holdingof supply through contactM

4is broken. Motor can be switched on again by pressing the ON-

push button. This circuit that we have discussed is also known as direct-on-line starting ofmotors. This is the simplest control circuit in the field of industrial control.

1.4 ADVANTAGES OF MAGNETIC CONTROLHaving discussed the starting and stopping of a motor by using control devices like push buttons,contactors and over-load relays, we are in a position to discuss the advantages of magneticcontrol over the manual control. The various advantages are listed as follows:

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Magnetic control permits installation of power contacts close to motor whereas the

actuating control device i.e., a push button switch could be located away from the motorin a position most convenient to the operator.

Magnetic control provides safety to the operator as remote operation described aboveminimises the danger to the operator of coming into accidental contact with live partsor being exposed to power arc and flashes at the main contacts.

The most important advantage of magnetic control is the elimination of dependence onoperators skill for control of motor performance. Current and torque peaks could belimited thus resulting in less wear and less maintenance.

Magnetic control also makes interlocking (to be discussed later) between variousoperations of a multi motor drive easy. The various operations can be performed in thedesired sequence automatically.

With the demand for more production in industry, it became necessary to automise themachinery to meet the challenge. Today in our industrial plants most of the machinesare automatic. Once the machine is started most of the operations are carried outautomatically.

1.5 SEMI-AUTOMATIC AND AUTOMATIC CONTROL OF MODERN

MACHINERYControl of a machine can be semi-automatic or fully automatic. There are probably moremachines 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 itON 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 nearthe pump motor and for its starting, a switch is provided near foremans desk it may be classifiedas a semi-automatic control. A lamp indication or a bell can also be provided near the desk toindicate if the tank is full. The foreman can switch ON the pump from his desk without goingto the pump site. Over-flow can also be avoided by switching OFF the pump when the lampglows or the bell rings. If a float switch is provided in the tank to switch ON the pump motorwhen water level falls below a certain lower limit, and switch it OFF when water level risesbeyond a certain upper limit, then the control becomes fully automatic. The cost of installationof an automatic control system will be higher than the other two types of controls. However, anautomatic control arrangement relieves the operator from the task of keeping an eye on thewater level and operate the pump. Also there is no danger of over-flow from the tank. Thus itis seen that the basic difference in manual, semi-automatic and fully automatic control lies inthe flexibility it provides to the system being controlled.

The study of control circuits involves study of the construction and principle of operationof various control components and learning the art of designing control circuits for variousfunctions of machines. In this text, we have first discussed the various control components andthen control schemes for ac and dc motors. Some important industrial control circuits havealso been discussed.

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

Static control is used for such machines as the control design is easy with static controldevices. The static devices used for design of control circuits are the digital logic gates. With

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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 allthese developments as far as single motor control or a machine having few operations isconcerned, the magnetic control using contactor and relay will continue to be in use because itis the simplest and cheapest method of control for such applications.

1.6 DEVELOPMENT OF CONTROL CIRCUIT

1.6.1 Development of Two-wire and Three-wire Control

In this section we will explain the various steps of developing a control circuit. A control circuitis to be drawn in a simple form between two horizontal lines designated by 1, which denotes aphase and by 0, which denotes the neutral wire. In this section also explained are some controlfunctions like remote operation, inter-locking of drives etc.

As shown in Fig. 1.4 a motor is connected to supply through a switch fuse unit, a contactor,and an overload relay. For developing the control circuit, we take a phase say L1 and neutralas shown in the figure. A control fuse is connected in phase L1 and outgoing control supplywire is numbered 1 and neutral is numbered 0.

Fig. 1.4 Development of control circuit

Now we will study how the control circuit for energising a motor is developed. Themotor is energised by closing the contactor and stopped due to opening of the contactor.

One method of designing of the control circuit is to connect a simple selector switch S

and an overload relay control contact in series with control supply wire no. 1 and then connectit to contactor coilMas shown in Fig. 1.5. When switchS is open there is no supply at terminal 2

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and, therefore,Mremains de-energised. When the switch,S is closed supply reaches terminal

3 through the normally closed overload relay contact OL and thus the coilMis energised. Themotor gets supply because of closing of the main contacts of the contactor (not shown in thefigure). This control circuit just developed is known as two wire control. In this type of controlcircuit, however, the motor would automatically start when power supply to the motor is restoredafter a failure. This type of control is only useful for starting of motors at remote places, e.g.,starting of water pump for filling an over-head tank. Here the advantage is that the operatoris not required to switch on the motor when power supply is restored after a failure. This typeof control may however be dangerous in industries and can cause accidents due to suddenrestarting of motors on restoration of power supply. Thus the control circuit of Fig. 1.5 hasvery limited application due to safety reasons.

Fig. 1.5 Two wire control circuit

Now let us develop a control circuit using push button switches. The normally open

contacts of these push buttons remain closed as long as the button is kept pressed by hand.

Each push button has one normally open (NO) and one normally closed (NC) contact. To use a

push button as stop (OFF) switch, the normally closed (NC) contact is generally used and for

start (ON) operation the normally open contact (NO) is generally used. In Fig. 1.6 the stop and

start push buttons and the over-load relay control contact have been connected in series with

the contactor coil.

Under normal conditions as the STOP-push button contact is normally closed supply

would reach upto terminal 2. When START-push button is pressed supply would reach the

contactor coil and hence the contactor would get energised. The coil will remain energised aslong as the START-push button is held pressed. As soon as the pressure on the START-push

button is released, the supply is cut off at terminal 3 and the coil is de-energised. To develop

the circuit further i.e., to ensure that the coil remains energised permanently once the push

button is pressed, we connect an auxiliary contactM1 of contactorMin parallel with the push

button as shown in Fig. 1.6 (b). When the coilMis energised, along with the main contacts the

auxiliary contact M1 also closes. The supply from terminal 2 now has two parallel paths i.e.,

one through the closed contact of the START-push button and the other through the closed

contactM1. Thus even when the pressure on the START-push button is released, supply reaches

coil M through closed contact M1. Thus supply to coilMis held (or sealed) through its own

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1

Stop

Start

2

3

4

M

0 0

M

4

3

Start

2

Stop

1

M1

1

Stop

Start

2

3

4

0

MIndication

lamp

M2

5

M1

(a) (b) (c)

Fig. 1.6 Development of three wire control circuit

contactM1. When it is desired to stop the motor, the STOP-push button is pressed and therefore

its normally closed contact opens and the coilMgets de-energised. De-energisation of coilM

also opens its sealing contactM1. Thus when the pressure on the STOP-push button is released

the coil does not get energised as both the parallel paths i.e., of the START-push button and

that of contactM1 are open. To start the motor again the START-push button has to be pressed.

It is therefore seen that unlike the control circuit of Fig. 1.5, here the control circuit does not

get energised when the power supply gets restored after a failure. Thus the danger of motors

getting restarted on restoration of power supply is eliminated. In case of over loading of the

motor the over-load relay contact opens and supply is disconnected at terminal 4 thereby de-

energising the contactor and subsequently stopping the motor. Even if the over-load protective

device is of auto reset type i.e., if its contact closes when the bimetallic elements cool down the

motor would not start on its own. However, in the two wire control circuit as in Fig. 1.5, if the

overload relay is of auto reset type the motor would restart automatically. Thus the motor may

get damaged due to repeated on-off on over loading. The control, we have just discussed in Fig.

1.6, is known as three wire control. This designation is given because in this control circuit,

three wires lead from a pilot device to the starter. The term two wire and three wires are used

as they describe the simplest application of the two types. Actually in control circuits any

number of wires can start from a pilot device in a three wire control. The control contact M1

used in parallel with the START-push button is called the holding contact or sealing contact.The name is so because it holds/seals the contactor coil in the energised condition even when

START-push button switch is released. The step in further development of the circuit is to

have a motor ON indication. An auxiliary contact M2

of contactor Mis connected in series

with supply wire 1 to feed an indicator lamp terminal. Its other terminal being connected to

the neutral as shown in Fig. 1.6 (c).

1.6.2 Remote Control Operation of a Motor

In further development of circuit of Fig. 1.6 (c) it may be desired that starting and stopping of

the motor should also be possible from a distant place (remote place) while the starter being

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fixed near the motor. In this case, obviously, the

control wire will have to be taken to the remoteSTART and STOP-push buttons from the starter.

Now, it is to be seen as to how the remote START-

STOP push buttons should be connected to the

circuit of Fig. 1.6 (c). The required connections for

remote control have been shown in Fig. 1.7.

The remote STOP-push buttons have been

connected in series with local STOP-push button

and the remote START-push button has been

connected in parallel with local START-push

button. The motor can be started and stopped from

any number of locations by making connectionsas above i.e., by connecting all the STOP-push

buttons in series and all the START-push buttons

in parallel. If the motor is to be started by some

other pilot device like pressure switch, thermostat

etc., the same may be made by connecting such a

device in parallel with START-push button. If such

a device is to be used for stopping also it is to be

connected in series with the STOP-push button.

1.6.3 Interlocking of Drives

We well now see how the control function of interlocking of drives is incorporated into the

circuits. Let us take motorsA andB. It is required that motorB should start only after motor

1

P1 P3

2 5

P2P4

A1 B1

3 6

4 7

A B

0

1

P1

2

P2 A1

3

4

A

0

P3

5

P4 B1

6

A2

7

8

B

P5

9

P6 C1

10

A3

11B2

12

13

C

(a) (b)

Fig. 1.8 Interlocking of drives

Fig. 1.7 Control circuit for remoteoperation of a motor

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A has started. It should however be possible to stop the motor independently. The first step

would be to develop the starters of motor A and motorB independently as shown in Fig. 1.8.In order that contactor B should energise only when contactor A is energised we will

have to insert a normally open contact of contactor A in series with the contactor coil B afterterminal 6 as shown in Fig. 1.8 (b). As shown in the figure a normally open contactA2 has beenconnected after wire No. 6. Thus when contactorA is not energised the contactA2 will be open.If we press the push buttonP4 the supply will reach upto terminal 6 only. The contactor coilB

can be energised only when contactorA is energised i.e., only when its contactA2 is closed.

If in the above circuit it is further required that a motorC should run only if both motorsA andB are running, we will be able to get this function by connecting normally open contactsof contactorA andB in series with the coil of contactor C. This development is also shown inFig. 1.8 (b). When push buttonP

6is pressed the supply will reach coil of contactorC at terminal

13, only if contactsA3

andB2

are closed i.e., only when both contactorsA andB are energised.

When either of the two motors is not running motor C can not be started.

REVIEW QUESTIONS

1. State the disadvantages of using manual control for control operations of electrical motors.

2. Name the devices which led to the use of automatic control for motors.

3. State the advantages of magnetic control over manual control.

4. What is the difference between semi-automatic and automatic control?

5. What will happen if the plunger of a contactor is prevented from completing its stroke?

6. When will a contactor coil take maximum current and why?

7. Under what conditions static control is preferred against magnetic control?

8. Explain why magnetic control is used for control having few operations.

9.State the difference between a two wire and a three wire control.

10. When the plunger of a contactor is in closed position the coil current is:

(a) maximum

(b) minimum

(c) zero.

11. When the contactor coil is de-energised:

(a) the contacts remain closed

(b) are held closed by mechanical latch

(c) gravity and spring tension open the contacts.

12. The purpose of using over load protection in a motor is to protect the motor from:

(a) sustained overcurrent

(b) over voltage

(c) short circuit(d) all the above.

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