11930 11652 model v special machines

8/6/2019 11930 11652 Model v Special Machines

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MODULE - V SPECIAL MACHINES

Special machines - AC and DC servomotors - synchros - constructional features -

working of a tacheo generator. Stepper motors - construction working application and

specifications of stepper motors, universal motors, constructional features typical

applications- Criteria for selection of motors - electromagnetic relays - contactors.

Servomotors

The motors used in automatic control system are called servomotors. The

servomotors are used to convert an electrical signal to an angular displacement of shaft. It is

an output device and is called actuator in a servo system.

Both A.C. and D.C. motors are used. The smaller motors used are a.c. motors

ranging from an output of 1/2 watt to 100 watts. D.C. servomotors vary in copacity from 1/2

HP to few H.P. and are used in large power Servo Mechanisms. In general Servo Motors

should have the following features.

(i) Linear relationship between speed and control signal

(ii) Steady state stability

(iii) Wide range of speed control

(iv) Low mechanical and electrical inertia

(v) Fast - response

vi) The direction of torque is to be determined by the polarity of control voltage

D.C. Servo Motors

The various types of D.C. Servo motors are, series motor, the shunt motor (both armature

controlled or filed controlled) and permanent magnet field excitation shunt motor. These

motors (i) develop high output power for a given size (2) Linearity of output (3) Easy speed

control (4) quick response and (5) has lightweight. In case of field controlled shunt motor,

requires little control power. Radio interference brush wear etc are some of the problems.

Isolation and matching with A.C. circuit is also difficult. The major application is in aircraft

control systems. Since windings are required in the armature, the inertia of D.C. motors is

large and friction drag of the brushes is another problem. Commutation is also a problem for

which inter poles provided. Linearity of flux with field current holds good only for the initial

portion of the magnetization curve and hence the operation in restricted to the linear portiononly.

D.C. motors may be controlled either by controlling the field current or the armature current.

a) Filed controlled D.C. Servo motor

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The armature of the motor is supplied with a constant current from a separate

D.C. source. The field winding is excited from the control voltage, which is the output of a

servo amplifier.

Torque Ia - since Ia is kept constant

Torque )(tor There fore torque of the motor varies linearly with respect to

filed current increase or decrease.

Armature controlled D.C. Servo Motors

This arrangement is more common. In this the armature of the motor is

energised by the control signal and the field current is kept constant. Since flux is constant

torque is as function of the armature current and torque variation is linear with respect tocontrol voltage. This arrangement is suitable for closed loop systems.

A.C. Servo Motor

For low power applications A.C. motors are preferred, became they are light

weight rugged and no brush contacts to maintain . A.C. motors used in control systems are

two phase induction motors.

The stator consists of a distributed winding wound for a two phase supply.

A fixed voltage from a constant voltage source is applied to the reference winding. The other

phase is energized from the control supply. Both the voltages are designed to have a phase

difference of 90

0

. Control voltage supplied from a servo amplifier has a variablemagnitude and polarity. The direction of rotation of the motor rivers if the control voltage

polarity us riversed. The operating principle is same as that of induction motor.

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The torque speed characteristics of an induction motor for different values of

rotor resistance is shown above. When the rotor resistance is very high, characteristics is

(curve for R4) linear. The torque speed characteristics of a servo motor for various control

voltage are also shown above.

Stepper Motor

The name stepper motor is used because this motor rotates through a fixed

angular step in response to a current pulse received by its controller. This type of motors can

be controlled by computers, microprocessors etc. stepper motors are ideally used for precise

positioning or precise speed control. In a stepper motor the output shaft rotates by a number

of steps, one step for each command pulse received. When a definite number of pulses are

supplied, the shaft rotates through a definite angle. This makes the motor for open loop

position control, since no feedback need be taken from output.

Such motors develop torque ranging from Nm1 (in a tiny wrist watch) up

to 40 Nm. Power output range from 1 watt to 2500 watt. The only moving part in a stepper

motor is its rotor which has no winding, no commutator or brushes.

Operating Principles

Let us consider a reluctance motor with a permanent magnet rotor and the

stator poles are magnetized from a D.C. source. The rotor will align itself by the attraction

of the unlike poles and stop in the exact position of alignment. If the current is now riversed

it is possible that the rotor will rotate through 1800 and get aligned in the opposite direction.

But it may be confused in which direction it is to rotate. To avoid this difficulty we can have a

motor with stator poles as in fig 40.5. In this case the coils 1 and 3 are energized first then

coils 2 and 4. Rotor will rotate through 900. Then coils 3 and 1 are energized. Now rotor

will complete 1800 rotation. With suitable control we can obtain very fast rotation with speed

control and exact position of stopping. The control equipment should have two elements.

(i) The number of pulses that determine the number of steps

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(2) Direction data

The practical stepper motor comes in two classes

(i) Variable Reluctance Motor (2) The Hybrid motor

(1) Variable reluctance Motor

The V. R. Motor avoids the use of permanent magnet rotor, by a simple

technology. In this case the number of stator poles are different from number of rotor poles.

Consider a case of stator having 6 poles and rotor having 4 poles as shown in the figure.

If No 1 stator poles are excited the rotor will align in a direction such that the

rotor poles marked come close with No. The unmarked pair of poles will be in a neutral

position. If No 2 pair of poles are energized (moving in clock wise direction), the unmarked

pair or poles in close to this and get aligned as shown in fig 40.7(b). Now the rotor poles

have moved by 300 in anti-clock wise direction (for 600 shifting of stator poles). When No. 3

pair of poles are energised, the rotor move further by an angle of 300. For one complete

rotation of stator poles, the rotor will rotate by 1800. Hence rotor completes one cycle for

every two cycles of stator pole movement. This arrangement is independent of the direction

of excitation current.

By varying the number of poles, the angle of step can be varied from 30 0 to 150, 22.5,

450 etc.

Step Angle =0360

.

)..(

polesrotorofNopolesstatorofNo

polesrotorofNopolesstatorofNo

In order to obtain a smaller angle of step, we have to have a hybrid stepper motor,

which can have a stepping angle as low as 1.800, 2.50 etc.

The Hybrid stepper motor

Instead of just four, six, or eight rotor poles the rotor is having as many as 36 poles oreven more. So they look like rotor teeth. The stator poles also have teeth at the same pitch.

The basic construction of this motor is as shown in fig. 40.8. It may be seen that when teeth

in poles (1) are aligned they

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are at an angle of displacement of 2.50 at stator pole (2). Since the displacement angle is very

small the saliency effect has to be strengthened by introducing a permanent magnet. But it is

not feasible to have a permanent magnet for each pair of poles. For this purpose a special

design is adopted as shown in fig 40 -9

In this case there are two rotors of similar construction mounted on two ends of the

shaft and in between a permanent magnet, having its direction of axis, same as the rotor axis

is placed. Hence all the rotor teeth on the left hand side well act as N pole and all the teeth

onthe right hand side will act as S pole. To balance the double rotor, we require a double

stator. All the stator poles on the left hand side will act as S pole and on the right as N pole.

The working is as follows:- First pole face 1 and 5 are energized (fig 40.8 (a)) on

the left as s pole and on the right as N pole. The rotor teeth get aligned. Now the teeth at

next pole are displaced by 2.50 (equal to step angle). When the next pole pairs 2 and 6 are

energized the rotor steps by 2.50 and gets aligned with 2 and 6. It the process is continued the

rotor can be moved in steps to any desired position. When the rotor is aligned with 1 and 5,

the rotor teeth is in a neutral position as regards 3 and 7. Hence no additional problem is

caused if these poles are also energized along with 1 and 5. In a similar way poles 4 and 8

can also be energized along with 2 and 6. This means that odd poles and even poles can be

energized alternately and this eliminates a complicated switching circuit. But the direction of

current is important, since it determines the direction of rotation.

Step angle =

phasesstatorofNoteethrotorofNo ..

360

Step angles in the example above =0

5.2436

360=

x

Application of stepper motors

Used in operation control in computer periferals textile industry IC fabrication and

robotics. Used in equipments requiring incremental motion such as type writers, line printers,

tape driver, numerically controlled machine tools, X-y plotters etc.

TACHEO GENERATOR

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A techeo generator is an electro mechanical device which produces an output voltage

proportional to its shaft speed. It can be employed as an analog speed indicator. The

direction of rotation of the shaft is indicator by the polarity of the voltage generated. Both

D.C. and A.C. tacheo generators are available.

D.C. Tacheo generator

A D.C. tacheo generator is a small D.c. generator with linear characteristics. It

consists of a stator with a permanent magnet field, a rotating armature circuit with

communator and brushes. The rotor is attached to the shaft whose speed has to be measured.

It delivers approximately 2 to 10 volt / 1000 rpn.

The characteristics of a good tacheo generator are

(1) Output should to proportioned to shaft speed

(ii) Magnitudes should be same for same speed in both direction

(iii) Output should be free from noice, harmonies etc

(iv) High sensitivity, that is appreciable output for small shaft speed.

A.C. Tacheo generator

The A.C. tacheo generator resembles two phase induction motor and is also called

rate generator. It consists of two stator windings. One stator winding is called reference

winding and is excited by an A.C. Voltage. The other winding is called the output winding.

The two windings are placed at 900 apart in space. No electrical connection is made to rotor.

No out put voltage is induced when the rotor is stationary. As the rotor rotates eddy

currents are induced in the rotor, due to the alternating flux produced by the reference

winding. The flux produced die to eddy currents induce an voltage in the output winding.

This voltage will be proportional to the shaft speed and the direction is also dependant on the

direction of rotation. Two types of AC tacheometers are available.

1) Induction type generator using squired cage rotor

(2) Drag cup generator using light conducting cup as rotor. The tacheo generator inertia

must be small and this is much less in case of drag cup construction. Sensitivity of AC

tacheo generators vary from 1/10 volt to 1 volt per 100 rpm

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UNIVERSAL MOTOR

Universal motor is in fact A.C. series motor. It works satisfactorily on both A.C.and D.C. and

hence the name universal motor. If a D.C. series motor is connected to a single phase A.C.

supply of proper voltage it will exert uni directional torque and rotate, since the current

flowing in he armature and field riverse direction at the same time. But the performance will

not be satisfactory.

(i) There will be excessive temp rise in the yoke due to eddy current loss

(ii) Visious sparking at the brushes due to high voltage and current induced in the

short circuited coil

In order to over come the above draw backs the following measures are taken in a

universal motor.

(i) The yoke is also made of laminations

(ii) Field circuit is designed for a low reluctance

(iii) A distributed compensating winding is provided to reduce armature reaction.

Universal motors are designed for commercial frequencies from 60 c/s down to d.c.

and voltage from 250v to 1.5 v. A commercial universal motor has a somewhat weaker series

field and more armature conductors than a D.C. series motor of equivalent copacity. It is

manufactured in ratings up to 3/4 HP and used particularly for vacuum cleaners, mixer

grinders, sewing machines etc. It's no load speed is very high

Advantages of Universal motor

1 High speed from above 3600 rpm to around 25000 rpm

2 High power output for a small size

3 High torque at low and inter mediate speed

4 Variable speed by varying applied voltage

Disadvantages

1 Increased service requirement due to commutator and brushes

2 Relatively high noice level at high speed

3 Radio and television interference

4 Requirement of careful balancing to avoid vibration

5 Requirement of reduction gearing in portable tools.

Application

Used in (a) High speed vacuum cleaners

(b) Swing machines (c) Electric shavers (d) drills (e) machine tools etc.

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Synchros

Synchro is the general name for self synchroning machines, which when electrically

energized and inter-connected exert torques which cause two mechanically independent

shafts either to run in synchronism of the rotor of one unit to follow the position of the other.

Synchros are also known in the trade names selsion or Autosin. Synchros are small

cylinderical motors of diameter varying from 1.5 cm to 10 cm. They are low torque devices

and are widely used in control systems for transmitting shaft position information or for

making two or more shafts to run in synhronism.

Type of Synchros

There are mainly four types of Synchros

1 Control Transmitter (denoted by CX)

2 Control receiver (denoted by CR)

3 Control Transformer (denoted by CT)

4 Control Differential (denoted CD)

i) Control Transmitter (Cx)

It has a 3 phase stator winding similar to that of an alternator. Rotor is of projecting pole

type, dumbell construction as shown in the figure. Rotor is excited from a single phases

supply through slip rings. It produces an alternating flux acting along the rotor axis and

induces e.m.f. in the stator windings by transformer action. It the rotor is aligned in the

direction of the stator coil S2 the voltage induced in the coil is maximum. This position is

defined as the electrical zero position.

(ii) Control receiver (CR)

Its construction is the same as that of a control transmitter. But it has a mechanical viscous

dampers on the shaft which permits. The CR rotor to respond, without over shooting its

mark. The stator windings are excited by the output of a synchro transmitter, where as the

rotor is excited from an external single phase AC supply. When the rotor and stator currents

inter act torque is formed and the shaft rotates.

(iii) Control transformer (CT)

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Stator winding of as control Transformer is similar to that of control Transmitter.

Rotor is cylinderical in shape and has a single phase winding. In this case, electrical zero

position is defined as the position in which the flux linkage of rotor winding minimum with

S2. This is when the rotor axis is at 900 with the axis of the coil S2 as shown in the figure.

(iv) Control Differential (CD)

The control Differential synchro has a balanced three phases distributed winding in both

stator and rotor and the rotor is cylinderical

Application of synchros

Synchros are extensively used in Servo mechanisms for torque transmission, error detection

and for adding and subtracting rotary angles.

a) Torque transmission

Synchros are used to transmit torque over a long distance without mechanical coupling.

Figure shows an arrangement to maintain the alignment between two shafts. It requires a

control transmitter and a control receiver. Stator windings are interconnected. Rotor is

evergised from the same single phone supply. When the rotor of CX is displaced by an angle. from the zero position, voltages are induced in its stator windings which circulates a

current through the stator windings of CR. This establishes a flux, which interacts with rotor

current and a torque in established. The rotor rotates, but when it has moved by the same

angle , the voltage induced in the stator winding due to rotor current will be equal and

opposite to the voltage in the synchro transmitter. Hence there will be no circulating current

and the torque will become zero. Hence we may conclude that the CR rotor will follow the

CX rotor. So this arrangement can be used for indicating shaft position as well as for

transmitting torque.

b) Error Defection

For this purpose one control Transmitter and one control transformer are connected as shown

below.

Rotor of CX is energised from a single phone A.C. supply. Output voltage of CT is a

measure of the shaft position error. The value of R.M.S. voltage in CT rotor winding when

the displacement of CX rotor is and CT rotor is is given by the formula.

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E = E max sin ( )

when = there will be no output voltage.

Electromagnetic Relays

A relay is a device which serves to actuate or control the regime of a high power system by

the action of a relatively small power on the relay. Relays are widely used for protection

against circuit over load, remote switching, riversing etc. Relays are' used in any industrial

circuit in which a large amount of load power is to be controlled by small amount of control

power.

Basic construction and working

Fig shows the basic construction of a typical electromagnetic relay. It essentially

consists of a coil to which the control voltage in applied, the core upon which the coil is

wound, a movable steel armature which is held against one of the contacts by means of a

spring. Before the voltage is applied to the coil, the relay is said to be in its normal de-energised position. Now contacts1-3 is closed (say normally closed contact or NC). When

the coil is energised, the armature is attracted to the core. Contact 1-3 is open and contact 1-2

in closed (1-2 is a normally open contact or NO) The contacts 1-2 remain in the closed

position until the coil remains energised.

Contactors

Magnetic contactors are electro magnetically operated switches that provide a safe

and convenient means for making and breaking branch circuits. The principal difference

between a contactor and a motor starter is that contactors contain no overload relays.

Contactors are used to switch in loads such as lighting, heating, electric motors etc with pilot

control. So that main power leads need not be taken to the remote controlling location. Pilotdevices such as push buttons, pressure switches, float switches, limit switches or thermostats

are used to provide necessary control for operating contactors.

The contactor consists of an electro magnet having a pick up oil, wound over a

ferromagnetic core, an armature which is attracted by the magnet. The main contacts which

make or break the power circuit are actuated by the movement of the armature. Sometimes

the contacts are housed in a blow out chute for quick arc suppression when breaking a circuit.

Contactors are manufactured for AC and DC circuits and may contain one or several poles.

Besides they may have normally open and normally closed contacts, which are switched

instantaneous or with some delay. D.C. contactors have long pick up coil, with small dia so

that more cooling surface is available. A.C. contactors are rated for 220V, 380V or 500V and

for rated current 20A to 600A. The magnetic circuit of a d.c. contactor is to be laminated toreduce eddy current loss.

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Motor Selection Criteria

Selection of a motor for a particular application is a complex process. We may he

looking for a motor to drive some device. For appropriate selection of a motor, it is not

sufficient that we know the output requirement, but there should be an understanding about

the speed requirement, speed control needed, starting, condition; and the environment under

which the motor has to work. The selection of drive motor depends upon the condition under

which the motor has to operate and the type of road it has to handle.

The following factors are to be considered

1. Nature of electric supply available

Available supply may be AC single phase, 3 phase or dc or all. It is better that the

selected motor can operate with the supply available. But some times, supply has to be

modified to suit the motor selected.

2. Nature of load-Different motors have different speed-torque characteristics.

Similarly load also has different types or speed torque. Characteristics Eg: The torque of a

lift is constant irrespective of speed. The torque of a fan is proportional to the square of

speed. The speed torque characteristics of the motor selected should be compatible with the

speed-torque characteristics of the load.

3. Electrical Characteristics

a) Starting characteristics- starting torque and starting current are the main

consideration. It may be necessary to limit the starting current. At the same the starting

torque should be sufficient to start the motor from rest and to accelerate it to rated speed in a

reasonable time. It is to be considered, whether the motor is to be started in no load or with

full load.

b) Running characteristics- This refers to the speed-torque characteristics. The speed

of d.c. shunt-motor and A.C induction motor remains almost constant with increase in load

torque, but the speed of d.c. series motor drops heavily with torque increase. Speed of a

synchronous motor remains constant. Motor has to be chosen according to the speed torque

requirement.

c) Speed Control

In some cases it may be enough that the motor runs at its natural speed. But in most

cases it may be required to control the speed. In such cases, the speed control of the motor

selected should be easy. Speed control of D.C. motors are simpler but that of A.C. inductionmotors are complicated and expensive.

d) Braking characteristics

Braking of the motor become necessary for stopping the motor quickly and also to

stop at the exact point. Mechanical and electrical braking is possible. Electrical braking is

advantageous. Hence in cases, where electrical braking is to be adopted, the motor selected

should be suitable for electrical braking.

4. Mechanical considerations

Type of enclosure, type of transmission arrangement, noice level and heating andcooling time constants are the main factors considered. Type of enclosure is to be decided

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according to the environment in which the motor is to operate. In a textile industry there is lot

of dust and hence a totally enclosed motor has be used. For under water operations

submersible type motor has to be used. The other types are open type, drip proof, pipe

ventilated, flame proof etc. The noice level of the motor is also important.

5. Size and rating of the motor

The size of the motor means its KW capacity and rating describes, the nature of

loading, such as continuous, continuous maximum or short time rating. The size of the motor

is to be chosen based on the output requirement and also the nature of loading. When the

loading is only for a short time the motor can be over loaded. the limiting factor is the

temperature rise and the permissible temperature rise depends on the class of insulation used.

6. Cost

Both initial cost (capital cost) and running cost has to be considered. When a motor

is selected, which has less running cost (cost on energy, maintenance etc.), the initial cost will

increase and hence the interest burden on the capital. Hence a compromise has to be made

between these two and the motor which has the minimum total cost may be selected.

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11930 11652 model v special machines

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