UNIT 5 – Special Machines

7/27/2019 UNIT 5 Special Machines

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UNIT 5 Special Machines


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Content to Discussion

Types of single phase motor

Double revolving field theory

Cross field theory Capacitor start

capacitor run motors

Shaded pole motor Repulsion type motor

Universal motor

Hysteresis motor

Permanent magnet synchronous motor Switched reluctance motor

Brushless D.Cmotor.


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Types of single phase motor

Resistance Split-phase Motor

Capacitor Split-phase Motor

Capacitor-start Motor Capacitor-start and Capacitor-run Motor

Shaded-pole Motor


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Single-phase Induction Motor

winding used normally in the stator ofthe single-phase induction motor (IM) isa distributed one.

unlike that for a three-phase IM. As the

stator winding is fed from a single-phasesupply, the flux in the air gap isalternating only, not a synchronouslyrotating one produced by a poly-phase

This type of alternating field cannot

produce a torque the rotor is stationery if the rotor is initially given some torque

in either direction , then immediately atorque is produced in the motor


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Double field revolving theory


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The flux or field rotating at synchronous speed, say, in the anticlockwisedirection, the same direction, as that of the motor (rotor) taken as positiveinduces emf (voltage) in the rotor conductors. The rotor is a squirrel cageone, with bars short circuited via end rings.

the electromagnetic torque isproduced in the same direction as given above,which is termed as positive (+ve). Theother part of flux or field rotates at thesame speed in the opposite (clockwise) direction,

Two torques are in the opposite direction, and the resultant (total) torque isthe difference of the two torques produced

slip due to forward (anticlockwise) rotating field and Similarly, the slip due tobackward rotating

field is also same . The two torques are equal and opposite, and the resultanttorque is zero. So, there is no starting torque in a single-phase IM.


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Starting Methods

The single-phase IM has no starting torque, but has resultanttorque, when it rotates at any other speed, except synchronousspeed. It is also known that, in a balanced two-phaseIM having twowindings

At a space angle of 90 degree electrical. the rotating magnetic fields

are produced, as in athree-phase IM. The torque-speedcharacteristic is same as that of a three-phase one,having bothstarting and also running torque as shown earlier

in a single-phase IM, if an auxiliary winding is introduced in thestator, in addition to the main winding, but placed at a space angleof 90 (electrical), starting torque is produced.

The various starting methods used in a single-phase IM aredescribed here.


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As detailed earlier, another (auxiliary) winding with a highresistance in series is to be added alongwith the main winding inthe stator. This winding has higher resistance to reactance

R/Xratio as compared to that in the main winding, and is placed ata space angle of 90 degree

The current Ia in the auxiliarywinding lags the voltage ( V) by anangle ,a , which is small, whereas the current (Im) in the mainwinding lags the voltage (V) by an angle, m , which is nearly 90degree . The

phase angle between the two currents is (90 - a ) which should beat least 300

This results in a small amount of starting torque.

The switch, S (centrifugal switch) is in series with the auxiliarywinding. It automatically cuts out the auxiliary or starting winding,when the motor attains a speed close to full load speed.


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Capacitor Split-phase Motor

The motor described earlier, is a simple one, requiring onlysecond (auxiliary) winding placed at a space angle of 900from the main winding

The phase difference between the Ia and Im is responsible

for the starting torque which is a minimum one

To get high starting torque, the phase difference required is900 to achieve such phase difference. The current in themain winding Im is lags the voltage by m and the auxillarycurrent Ia leads the voltage by a so (m

+ a = 900 )

This can be can be achieved by having a capacitor in serieswith the auxiliary winding, which results in additional cost,with the increase in starting torque,

The two types of such motors are described here.


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Capacitor-start Motor


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Capacitor-start Motor

It may be observed that a capacitor along with acentrifugal switch is connected in series with theauxiliary winding, which is being used here as a startingwinding. The capacitor may be rated only for

intermittent duty The switch, S (centrifugal switch) is in series with the

auxiliary winding. It automatically cuts out the auxiliaryor starting winding, when the motor attains a speedclose to full load speed.

This motor is used in applications, such as compressor,conveyor, machine tool drive, refrigeration and air-conditioning equipment, etc.


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Capacitor-start and Capacitor-run

Motor


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Capacitor-start and Capacitor-run

Motor In this motor (Fig. 34.6a), two capacitors Cs

for starting, and CR for running.

The first capacitor is rated for intermittent duty, as described earlier, beingused only for starting

The second one is to be rated for continuous duty, as it is used for running.

The phase difference between the two currents is (m + a

> 900 ) for

both the capacitor is in connection during the starting period The phase difference between the two currents is (m

+ a = 900 ) for

both the capacitor is in connection during the starting period

In the second case, the motor is a balanced two phase one, the twowindings having same number of turns

The efficiency of the motor under this condition is higher. Hence, using

two capacitors, the performance of the motor improves both at the time of

starting and then

running. This motor is used in applications, such as compressor,refrigerator, etc.


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Shaded-pole Motor


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Shaded-pole Motor

Shaded pole motor is a split phase type single phase inductionmotor. It has a salient pole on the stator excited by a single phasesupply and a squirrel cage rotor

They have salient stator poles, with one-coil-per-pole called mainwinding. The auxiliary winding consists of one (or rarely two) short-

circuited copper straps wound on a portion of the pole anddisplaced from the center of each pole

The shaded-pole motor got its name from these shading bands.Induced currents in the shading coil cause the flux in the shadedportion of the pole to lag the flux in the other portion in time.

The result is then like a rotating field moving in the direction from

the unshaded to the shaded portion of the pole. A low startingtorque is produced;


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Repulsion-induction motors

repulsion-start and induction-run machines were themost widely used kind of single-phase motor in therange of 1/3 to 5 hp

This kind of motor has distributed rotor-windingsconnected to a commutator (like a dc machine) withshort-circuited brushes and a distributed single-phasestator winding in the direct axis only.

Brushes are not connected to supply but are short-

circuited consequently, currents are induced in thearmature conductors by transformer action.

R l i i d ti t


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Repulsion-induction motors

CONSTRUCTION

The field structure has non-salient poleconstruction. The field structure has non-salientpole construction.

The field of stator winding is wound like the mainwinding of a split-phase motor and is connecteddirectly to a single-phase source. The armature orrotor is similar to a D.C.motor armature withdrum type winding connected to a commutator

brushes are not connected to supply, Short-circuiting the brushes effectively makes the rotorinto a type of squirrel cage


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PRINCIPLE OF OPERATION

Fig. (1) which shows a two-pole repulsion motor with its two short-circuited brushes. the brush axis is parallel to the stator field. When thestator winding is energized from single-phase supply, e.m.f. is induced inthe armature conductors (rotor) by induction.

By Lenzs law, the direction of the e.m.f. is such that the magnetic effect ofthe resulting armature currents will oppose the increase in flux

The direction of current in armature conductors will be as shown in Fig.(1(i)).With the brush axis in the position shown in Fig. (1 (i)), current willflow from brush B to brush A where it enters the armature and flows backto brush B through the two paths ACB and ADB

With brushes set in this position, half of the armature conductors underthe N-pole carry current inward and half carry current outward. The same

is true under S-pole. Therefore, as much torque is developed in onedirection as in the other and the armature remains stationary. Thearmature will also remain stationary if the brush axis is perpendicular tothe stator field axis. It is because even then net torque is zero.


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If the brush axis is at some angle other than 0 or 90 to theaxis of the stator field, a net torque is developed on therotor and the rotor accelerates to its final speed but thebrushes have been shifted clockwise through some anglefrom the stator field axis.

Now e.m.f. is still induced in the direction indicated in Fig.(1 (i)) and current flows through the two paths of thearmature winding from brush A to brush B.

The direction of rotation of the rotor depends upon thedirection in which the brushes are shifted. If the brushesare shifted in clockwise direction from the stator field axis,the net torque acts in the clockwise direction and the rotoraccelerates in the clockwise direction


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UNIVERSAL MOTOR

A universal motor is defined as a motor which may beoperated either on direct or single phase ac supply atapproximately the same speed and output.

series wound motor, it has high starting torque and a

variable speed characteristics. It runs at dangerouslyhigh speed on no load

Generally universal motors are manufactured in to twotypes 1. Concentrated pole, non compensated type (low power

rating) 2. Distributed field compensated type (high power rating)

Concentrated pole non compensated


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Concentrated pole, non compensated

type (low power rating)

The non compensated motor has two salient

poles and is just like a two pole series motor

expect that whole of its magnetic path is

laminated. The laminated stator is necessarybecause the flux is alternating when motor is

operated from a.c supply

armature is wound type and is similar to thatof a small DC motor

Distributed field compensated type


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Distributed field compensated type

(high power rating)

The distributed field compensated type motorhas a stator core similar to that of a split phasemotor and a wound armature similar to that of asmall dc motor

The compensating winding is used to reduce thereactance voltage present in the armature whenmotor runs on AC supply. This voltage is causedby the alternating flux by transformer action.

Operation of the universal motor is similar to thatof the DC series motor . A force created betweenthe stator flux and current in the rotor conductor


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Hysteresis Motor

The capacity for a body to remain magnetized

after the magnetizing field has


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The rotor is typically a cylinder of magnetically hard steel without any

windings or teeth.

Stator windings are usually a split capacitor type, with the capacitor

chosen to approximate two phase operation as closely as possible. Ora shaded pole salient poles also used in sator

The high retentivity of the rotor material causes its

magneticorientation to lag behind the rotating magnetic field by a

fractionof a rotation.

Interaction between the rotating field and the rotor's magnetic polarity

subjects the rotor to a torque which is constant from standstill to

synchronous speed.

This design allows synchronization of high inertia loads.

Operation is generally smooth and quiet because of the smooth rotorperiphery.

Hysteresis motors are generally used in low power applications

such as clocks.


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Switched-Reluctance Motors

Reluctance - the resistance to magnetic flux offered by a magnetic

circuit.

In principle, a switched-reluctance motor operates like a variable-reluctance

step motor discussed in the previous section.

However, the operation differs mainly in the complicated control mechanism

of the motor.

In order to develop torque in the motor, the rotor position should bedetermined by sensors so that the excitation timing of the phase windings is

precise.

Although its construction is one of the simplest possible among electric

machines, because of the complexities involved in the control and electric

drive circuitry, switched-reluctance motors have not been able to findwidespread applications for a long time.

However, with the introduction of new power electronic and microelectronic

switching circuits, the control and drive circuitry of a switched reluctance

motor have become economically justifiable for many applications where

traditionally dc or induction motors have been used.


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A switched-reluctance motor has a wound stator but has no windings on itsrotor, which is made of soft magnetic material as shown in Figure 12.17.

The change in reluctance around the periphery of the stator forces the rotorpoles to align with those of the stator.

Consequently, torque develops in the motor and rotation takes place.

The total flux linkages of phase-A in the following figure is la = La(q) iaand of phase- B is lb = Lb(q) ib with the assumption that the magnetic

materials are infinitely permeable. Since the magnetic axes of both windings

are orthogonal, no mutual flux linkages

are expected between them.


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Brushless DC Motors

DC motors find considerable applications where controlling a system is a primary

objective.

However, electric arcs produced by the mechanical commutator-brush arrangement

are a major disadvantage and limit the operating speed and voltage.

A motor that retains the characteristics of a dc motor but eliminates the commutator

and the brushes is called a brushless dc motor.

A brushless dc motor consists of a multiphase winding wound on a non-salient statorand a radially magnetized PM rotor.

Figure 12.18 is a schematic diagram

of a brushless dc motor.


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Brushless DC Motors

As can be seen, the operation of a brushless dc motor isvery similar to that of a PM step motor.

The major difference is the timing of the switchingoperation, which is determined by the rotor position to

provide the synchronism between the magnetic field ofthe permanent magnet and the magnetic field produced

by the phase windings.

The rotor position can be detected by using either Hall-effect or photoelectric devices.

The signal generated by the rotor position sensor is sentto a logic circuit to make the decision for the switching,and then an appropriate signal triggers the power circuit

to excite the respective phase winding. The control of the magnitude and the rate of switching of

the phase currents essentially determine the speed-torquecharacteristic of a brushless dc motor, which is shown inFigure 12.19.

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UNIT 5 – Special Machines

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