chapter 1 induction motor

7/24/2019 Chapter 1 Induction Motor

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Dr Gamal SowilamAssociate ProfessorUmm Al-Qura University

Facculty of Engineering & Islamic ArchitectureDepartment of Electrical Engineering

Chapter 1

Induction Motor


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A.C. MotorsClassification of A.C. Motors

Different ac. motors may, however, be classified and divided into various groups

from the following different points of view :

1. AS REGARDS THEIR PRINCIPLE OF OPERATION:

(A) Synchronous motors

(i) plain(ii) super

(B) Asynchronous motors:

(a) I nduction motors : (i) Squirrel cage { single and double}

(ii) Slip-ring (external resistance)

(b)Commutator motors: (i) Series { single phase and universal}

(ii) Compensated { conductively and inductively}

(iii) shunt { simple and compensated}

(iv) repulsion { straight and compensated}

(v) repulsion-start induction

(vi) repulsion induction


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2. AS REGARDS THE TYPE OF CURRENT:(i) single phase

(ii) three phase

3. AS REGARDS THEIR SPEED(i) constant speed.

(ii) variable speed.

(iii) (iii) adjustable speed

4. AS REGARDS THEIR STRUCTURAL FEATURES

(i) open

(ii) enclosed

(iii) semi-enclosed

(iv) ventilated

(v) pipe-ventilated

(vi) riverted frame eye etc.


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INDUCTION MOTORGeneral Principle

As a general rule, conversion of electrical power into mechanical power takes

place in the rotating part of an electric motor.

In d.c. motors, the electric power is conducted directly to the armature (i.e.

rotating part) through brushes and commutator. Hence, in this sense, a d.c.

motor can be called a conduction motor.

However, in a.c. motors, the rotor does not receive electric power by conduction

but by induction in exactly the same way as the secondary of a 2-winding

transformer receives its power from the primary. That is why such motors are

known as induction motors. In fact, an induction motor can be treated as a

rotating transformer i.e. one in which primary winding is stationary but the

secondary is free to rotate.


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Advantages:1. It has very simple and extremely rugged, almost unbreakable construction

(especially squirrel cage type).

2. Its cost is low and it is very reliable.

3. It has sufficiently high efficiency. In normal running condition, no brushes are

needed, hence frictional losses are reduced. It has a reasonably good power

factor.

4. It requires minimum of maintenance.

5. It starts up from rest and needs no extra starting motor and has not to be

synchronized. Its starting arrangement is simple especially for squirrel-cage

type motor.

Disadvantages:1. Its speed cannot be varied without sacrificing some of its efficiency.

2. Just like a dc shunt motor, its speed decreases with increase in load.

3. Its starting torque is somewhat inferior to that of a dc shunt motor.


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Construction

An induction motor consists essentially of two main parts :

(a) a stator

The stator carries a 3-phase windingand is

fed from a 3-phase supply. It is wound for

a definite number of poles, the exact

number of poles being determined by the

requirements of speed.

Greater the number of poles, lesser the

speed and vice versa.

When supplied with 3-phase currents,

produce a magnetic flux, which is of

constant magnitude but which revolves (or

rotates) at synchronous speed.

Synchronous speed Ns = 120 f/P

This revolving magnetic flux induces an emf in the rotor by mutual induction.


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The number of poles P, produced in the rotating f ield is P = 2n where n is the

number of stator slots/pole/phase.

(b) a rotor.

(i ) Squirrel-cage rotor:Motors employing this type of rotor are known as squirrel-cage induction

motors.

(ii ) Phase-wound or wound rotor :

Motors employing this type of rotor are variously known as phase-wound

motors or woundmotors or as slip-ringmotors.

Squirrel-cage rotor with copper bars and alloy brazed end-rings


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Squirrel-cage Rotor

The rotor consists of a cylindrical laminated core with parallel slots for carrying

the rotor conductors which, it should be noted clearly, are not wires but consist of

heavy bars of copper, aluminum or alloys. One bar is placed in each slot, rather thebars are inserted from the end when semi-closed slots are used.

The rotor bars are brazed or electrically welded or bolted to two heavy and stout

short-circuiting end-rings, thus giving us, what is so picturesquely called, a

squirrel-case construction.

It should be noted that the rotor bars are permanently short-circuited on

themselves, hence it is not possible to add any external resistance in series with the

rotor circuit for starting purposes.

This is useful in two ways :(i) it helps to make the motor run quietly by reducing the magnetic hum and

(ii) it helps in reducing the locking tendency of the rotor i.e. the tendency of the

rotor teeth to remain under the stator teeth due to direct magnetic attraction

between the two.


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Other results of skew which may or may not be desirable are:

(i) increase in the effective ratio of transformation between stator and rotor.

(ii) increased rotor resistance due to increased length of rotor bars.

(iii) Increased impedance of the machine at a given slip.

(iv) increased slip for a given torque.


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Rotor bars (slightly skewed)

End ring


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Phase-wound Rotor

The rotor is provided with 3-phase, double-layer, distributed winding consisting of

coils as used in alternators. The rotor is wound for as many poles as the number of

stator poles and is always wound 3-phase even when the stator is wound two-phase.

The other three winding terminals are brought out and connected to three insulated

slip-rings mounted on the shaft with brushes resting on them.

These three brushes are further externally connected to a 3-phase star-connected

rheostat. This makes possible the introduction of additional resistance in the rotor

circuit during the starting period for increasing the starting torque of the motor.for

changing its speed-torque/current characteristics.

When running under normal conditions, the slip-rings are automatically short-

cir cui ted by means of a metal col lar, which is pushed along the shaf t and

connects all the rings together. Next, the brushes are automatically lifted from the

slip-rings to reduce the frictional losses and the wear and tear.

Hence, it is seen that under normal running conditions, the wound rotor is short-

circuited on itself just like the squirrel-case rotor.


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Slip-ring motor with slip-ringsbrushes and short-circuiting devices


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1. Frame. Made of close-grained alloy cast iron.

2. Stator and Rotor Core. Built from high-quality low-loss silicon steel

laminations and flash-enamelled-on both sides.

3. Stator and Rotor Windings. Have moisture proof tropical insulation

embodying mica and high quality varnishes. Are carefully spaced for most

effective air circulation and are rigidly braced to withstand centrifugal forces

and any short-circuit stresses.

4. Air-gap. The stator rabbets and bore are machined carefully to ensure

uniformity of air-gap.

5. Shafts and Bearings. Ball and roller bearings are used to suit heavy duty,

trouble-free running and for enhanced service life.

6. Fans. Light aluminum fans are used for adequate circulation of cooling air and

are securely keyed onto the rotor shaft.

7. Slip-rings and Slip-ring Enclosures. Slip-rings are made of high quality

phosphor-bronze and are of moulded construction.


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(a) represents stator (b) rotor (a) represents stator (b) stator

(c) bearing shields (d) fan (c) bearing shields (d) fan

(e) venti lation grill a (f ) terminal box. (e) venti lation gri ll

(f ) terminal box

(g) sl ip-r ings

(h) brushes and brush holders.

squirrel-cage rotor. slip-ring motor


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Production of Rotating Field

It will now be shown that when three-

phase windings displaced in space by

120, are fed by three-phase currents,

displaced in time by 120, they produce

a resultant magnetic flux, which rotates

in space as if actual magnetic poles were

being rotated mechanically.

The flux (assumed sinusoidal) due to

three-phase windings.


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A+

A-

B+

B-

C+

C-

A BC


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The resultant flux r, at any instant, is given by the

vector sum of the individual fluxes, 1, 2 and 3 due

to three phases. We will consider values of r at four

instants 1/6th time-period apart corresponding to points

marked 0, 1, 2 and 3(i) When = 0 i. e. corresponding to point 0:

(i i) when = 60 i .e. corresponding to point 1:


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(i i i ) When = 120 i.e. corresponding to point 2

Hence, we conclude that:

1. The resultant flux is of constant value =m i.e. 1.5 times the maximum

value of the fl ux due to any phase.2. The resul tant flux rotates around the stator at synchronous speed given by :

Ns = 120 f/P.


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Mathematical Proof

The resultant flux is of constant magnitude and does not change with time t.

Expanding and adding the above equations, we get

Taking the direction of flux due to phase 1 as reference direction, we have


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BASIC INDUCTION MOTOR CONCEPTS

The Development of Induced Torque in an Induction Motor

(a) The rotating stator field s induces a voltage in the rotor bars;

(b) The rotor voltage produces a rotor current flow which lags behind the voltage

because of the inductance of the rotor;

(c) The rotor current produces a rotor magnetic field R lagging 90behind itself.and R interacts with res to produce a counterclockwise torque in the

machine.


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This results in a current flow out of the upper bars and into the lower bars.

However, since the rotor assembly is inductive, the peak rotor current lags behind

the peak rotor voltage. The rotor current flow produces a rotor magnetic fieldR.

Finally, since the induced torque in the machine is given by:

the resulting torque is counterclockwise. Since the rotor induced torque iscounterclockwise, the rotor accelerates in that direction.

Note that in normal operation both the rotor and stator magnetic f ields BR and

Bsrotate together at synchronous speed nsyncwhi le the rotor itself tuns at a slower

speed.


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A three-phase set of voltages has been applied to the stator, and a three-phase set of

stator currents is flowing. These currents produce a magnetic field Bs, which is

rotating in a counterclockwise direction. The speed of the magnetic field's rotation

is given by

This rotating magnetic field Bs passes over the rotor bars and induces a voltage in

them. The voltage induced in a given rotor bar is given by the equation;

where v = velocity of the bar relative to the magnetic field

B = magnetic flux density vector

I = length of conductor in the magnetic field

It is the relative motion of the rotor compared to the stat or magnetic field that

produces induced voltage in a rotor bar. The velocity of the upper rotor bars

relative to the magnetic field is to the right, so the induced voltage in the upper

bars is out of the page, while the induced voltage in the lower bars is into the page.


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The Concept of Rotor Slip

The voltage induced in a rotor bar of an induction motor depends on the speed of

the rotor relative to the magnetic fields. Since the behavior of an induction motor

depends on the rotor's voltage and current, it is often more logical to talk about this

relative speed. Two terms are commonly used to de fine the relative motion of the

rotor and the magnetic fields.

One is slip speed, defined as the difference between synchronous speed and rotor

speed:

The other term used to describe the relative motion is slip, which is the relative

speed expressed on a per-unit or a percentage basis. That is, slip is defined as:


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Notice that if the rotor turns:

at synchronous speed,s = 0,

If the rotor is stationary, s = 1.

All normal motor speeds fall somewhere between those two limits.

The Electrical Frequency on the Rotor:

An induction motor works by inducing voltages and currents in the rotor ofthe machine, and for that reason it has sometimes been called a rotating

transformer. But unlike a transformer, the secondary frequency is not necessarily

the same as the primary frequency.

At nm= 0 r/min, the rotor frequency fr = fe, and the slip s = 1.

At nm = nsync the rotor frequencyfr= 0 Hz, and the slip s = 0.For any speed in between, the rotor frequency is directly proportional to the

difference between the speed of the magnetic field nsyncand the speed of the rotor

nm. Since the slip of the rotor is defined as


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Several alternative forms of this expression exist that are sometimes useful.

andand

THE EQUIVALENT CIRCUIT OF AN INDUCTION MOTOR:

The transformer model or an induction motor. with rotor and stator connected byan ideal transformer of turns ratio aeff


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The stator resistance will be called R1. and the stator leakage reactance will be

calledX1.

Also, like any transformer with an iron core,

the flux in the machine is related to theintegral of the applied voltage E1. The curve

of magneto motive force versus flux

(magnetization curve) for this machine is

compared to a similar curve for a power

transformer in Figure.

This is because there must be an air gap in an induction motor, which greatly

increases the reluctance of the flux path and therefore reduces the coupling

between primary and secondary windings. The higher reluctance caused by the airgap means that a higher magnetizing current is required to obtain a given flux

level.

Therefore, the magnetizing reactance XM in the equivalent circuit will have a

much smaller value (or the susceptance BMwil l have a much larger value) than

it would in an ordinary transformer.


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The primary internal stator voltage E1 is coupled to the secondary ERby an ideal

transformer with an effective turns ratio aeff . The effective turns ratio aeff is fairly

easy to determine for a wound-rotor motor- it is basically the ratio of the conductors

per phase on the stator to the conductors per phase on the rotor, modified by anypitch and distribution factor differences.

An induction motor equivalent circuit differs from a transformer equivalent

circuit primarily in the effects of varying rotor frequency on the rotor voltage ER

and the rotor impedancesRRand jXR

The Rotor Circuit Model

The magnitude of the induced rotor voltage at

locked-rotor conditions is called ER0, The

magnitude of the induced voltage at any s lip will

be given by the equation:

and the frequency of the induced voltage at any slip will be given by the equation


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This voltage is induced in a rotor containing both resistance and reactance. The

rotor resistance RR is a constant (except for the skin effect), independent of slip,

while the rotor reactance is affected in a more complicated way by slip.

where XR0 is the blocked-rotor

rotor reactance.

The equivalent rotor impedance from this point of view is:


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The Final Equivalent Circuit:

To produce the final per-phase equivalent circuit for an induction motor, it is

necessary to refer the rotor part of the model over to the stator side.

The transformed rotor voltage and current become:

And

The per-phase equivalent circuit of an induction motor.

whereR2/s


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The rotor impedance becomes:

The rotor resistance RR and the locked-rotor rotor reactance XR0 are very difficultor impossible to determine directly on cage rotors, and the effective turns ratio aeff

is also difficult to obtain for cage rotors.

POWER AND TORQUE IN INDUCTION MOTORS:

Losses and the Power-Flow Diagram:

and


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The stator copper losses in the three phases are given by:

The core losses are given by:

so the air-gap power can be found as:

And

The actual resistive losses in the rotor circuit are given by the equation:

The power converted , which is sometimes called developed mechanical power, is

given by:


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The rotor copper losses are equal to the air-gap power times the slip::

Note that if the rotor is not turning, the slip S = 1 and the air-gap power isentirely consumed in the rotor. This is logical, since if the rotor is not turning, the

output power Pout (= "Tload m,) must be zero. Since Pconv= PAG- PRCL,

There another relationship between the air-gap power and the power converted

from electrical to mechanical form:

Finally, if the friction and windage losses and the stray losses are known, the output

power can be found as:


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The relationship between air gap power , rotor copper losses and

the conventional power are given by:


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The induced torque rind in a machine was def ined as the torque generated by

the internal electric-to-mechanical power conversion. This torque differs from

the torque actually available at the terminalsof the motor by an amount equal

to the friction and windage torques in the machine.

The induced torque (developed torque )is given by the equation:

Separating the Rotor Copper Losses and the Power Converted in an

Induction Motor 's Equivalent Circuit

Part of the power coming across the air gap in an induction motor is consumed in

the rotor copper losses, and part of it is converted to mechanical power to drive the

motor's shaft. It is possible to separate the two uses of the air-gap power and to

indicate them separately on the motor equivalent circuit.


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The difference between them isPconvwhich must therefore be the power consumed

in a resistor of value:

The per-phase equivalent circuit with rotor losses and Pconv separated.

Per-phase equivalent circuit with the rotor copper losses and the power converted to

mechanical form separated into distinct elements is shown in Figure.


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The Derivation of the Induction Motor Induced-Torque Equation

the air-gap power supplied to one

phase of the motor can be seen to beTherefore, the total air-gap power is


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Thevenin's theorem states that any linear circuit that can be separated by two

terminals from the rest of the system can be replaced by a single voltage source in

series with an equivalent impedance.

Find VTH

I

Since the magnetization reactanceXM X and XM

R1, the magnitude of the Thevenin voltage is

approximately


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Find RTH

This impedance reduces to

Because XM X1and (XM + X1)R1the TI levenin r esistance and reactance areapproximately given by

And


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The resulting equivalent circuit

From this circuit, the current I2is given by

The magnitude of this current is


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The air-gap power is therefore given by

The rotor-induced torque is given by


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The induction motor torque-speed characteristic curve


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Induction motor torque-speed characteristic curve. showing the extended operating

ranges (braking region and generator region).


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Comments on the Induction Motor Torque-Speed Curve

1. The induced torque of the motor is. zero at synchronous speed.

2. The torque- speed curve is nearly linear between no load and full load. In thisrange, the rotor resistance is much larger than the rotor reactance, so the

rotor current , the rotor magnetic field, and the induced torque increase linearly

with increasing slip.

3. There is a maximum possible torque that cannot be exceeded. this torque, calledthe pul lout torque or breakdown torque, is 2 to 3 times the rated full load

torque of the motor.

4. The starting torque on the motor is slightly larger than its full -load torque,

so this motor will start carrying any load that it can supply at full power.

5. Notice that the torque on the motor for a given slip varies as the square of

the applied voltage.


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7. If the motor is turning backward relative to the direction of the magnetic

fields, the induced torque in the machine will stop the machine very

rapidly and will try to rotate it in the other direction. Since reversing the

direction of magnetic field rotation is simply a matter of switching any two

stator phases, this fact can be used as a way to very rapidly stop an induction

motor. The act of switching two phases in order to stop the motor very rapidly

is called plugging.

6. If the rotor of the induction motor is driven faster than synchronous speed,

then the direction of the induced torque in the machine reverses and the

machine becomes a generator, converting mechanical power to electric

power.


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Notice that the peak power supplied by the induction motor occurs at a different

speed than the maximum torque; and, of course, no power is converted to

mechanical form when the rotor is at zero speed.


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Maximum (pullout) Torque in an Induction Motor

Since the air-gap power is equal to the power consumed in the resistor R2/s, the

maximum induced torque wil loccur when the power consumed by that resistor is

maximum.

The maximum power transfer theorem states that maximum power transfer to the

load resistor R2/s wi ll occur when the magnitude of that impedance is equal to

the magni tude of the source impedance. The equivalent source impedance in the

circuit is

So the maximum power transfer occurs when:

we see that the sl ip at pul lout torque is given by:


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Notice that the referred rotor resistance R2appears only in the numerator, so

the slip of the rotor at maximum torque is directly proportional to the rotor

resistance.

The resulting equation for the maximum or pullout torque is

This torque is proportional to the square of the supply voltage and is alsoinversely related to the size of the stator impedances and the rotor reactance.

Note that sl ip at which the maximum torque occurs is dir ectly proportional to

rotor resistance, but the value of the maximum torque is independent of the

value of rotor resistance

The torque-speed characteristic for a wound-rotor induction motor is shown in

following Figure. Recall that it is possible to insert resistance into the rotor circuit

of a wound rotorbecause the rotor circuit is brought out to the stator through slip

rings. Notice on the figure that as the rotor resistance is increased, the pullout

speed of the motor decreases, but the maximum torque remains constant.


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From here we can say:

a) Torque is related to the square of the applied voltage

b) Torque is also inversely proportional to the machine impedances

c) Slip during maximum torque is dependent upon rotor resistance

d) Torque is also independent to rotor resistance as shown in themaximum torque equation.

By adding more resistance to the machine impedances, we can

vary:

a) Starting torque

b) Max pull out speed


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It is possible to take advantage of this characteristic of wound-rotor induction

motors to start very heavy loads.

Variations in Induction Motor Torque-Speed Characteristics


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q p


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A torque-speed characteristic curve combining high-resistance effects at low

speeds (high slip) with low-resistance effects at high speed (low slip).


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Control of Motor Characteristics by Cage Rotor Design

Leakage reactance X2 represents the referred form of the rotors

leakage reactance (reactance due to the rotors flux lines that do notcouple with the stator windings)

If the bars of a cage rotor are placed near the surface of the rotor,

they will have only a small leakage flux and the reactance X2 willbe small in the equivalent circuit.

On the other hand, if the rotor bars are placed deeper into therotor surface, there will be more leakage and the rotor reactance

X2will be larger.

Laminations from typical cage induction motor rotors. showing the cross


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(a) NEMA design class A-

large bars near thesurface;

(b) NEMA design class B-

large, deep rotor bars;

(c) NEMA design class C-

double-cage rotor

design;

(d) NEMA design class D-

small bars near the

surface.

yp g g

section of the rotor bars:

Deep-Bar and Double-Cage Rotor Designs


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Deep Bar and Double Cage Rotor Designs

How can a variable rotor resistance be produced to combine the high starting

torque and low starting current of Class D, with the low normal operating slip and

high efficiency of class A?

Use deep rotor bars (Class B) or double-cage rotors (Class C).

(a) For a current flowing in the top of the bar. the flux is tightly linked to the stator.and leakage inductance is small;

(b) for a current flowing in the bottom of the bar. the flux is loosely linked to the

stator. and leakage inductance is large;

(c) resulting equivalent circuit of the rotor bar as a function of depth in the rotor.


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Typical torque-speed curves for different rotor designs.

NEMA (National Electrical Manufacturers Association) class A


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NEMA (National Electrical Manufacturers Association) class A

Rotor bars are quite large and are placed near the surface of the rotor.

Low resistance (due to its large cross section) and a low leakage reactance X2(due to the barslocation near the stator).

Because of the low resistance, the pullout torque will be quite near synchronous

speed.

Motor will be quite efficient, since little air gap power is lost in the rotor

resistance.

However, since R2 is small, starting torque will be small, and starting

current will be high.

This design is the standard motor design.

Typical applications driving fans, pumps, and other machine tools.

NEMA Class B


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NEMA Class BAt the upper part of a deep rotor bar, the current flowing is tightly coupled to

the stator, and hence the leakage inductance is small in this region.

Deeper in the bar, the leakage inductance is higher.

At low slips, the rotorsfrequency is very small, and the reactances of all the

parallel paths are small compared to their resistances. The impedances of all

parts of the bar are approx equal, so current flows through all the parts of the bar

equally. The resulting large cross sectional area makes the rotor resistancequite small, resulting in good efficiency at low slips.

At high slips (starting conditions), the reactances are large compared to the

resistances in the rotor bars, so all the current is forced to flow in the low-

reactance part of the bar near the stator. Since the effective cross section is lower,

the rotor resistance is higher. Thus, the starting torque is relatively higher and

the starting current is relatively lower than in a class A design

Applications similar to class A, and this type B have largely replaced type A.


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NEMA Class C

It consists of a large, low resistance set of bars buried deeply in the rotor

and a small, high-resistance set of bars set at the rotor surface. It is similar tothe deep-bar rotor, except that the difference between low-slip and high-slip

operation is even more exaggerated.

At starting conditions, only the small bars are effective, and the rotor

resistance is high. Hence, high starting torque. However,

At normal operating speeds, both bars are effective, and the resistance is

almost as low as in a deep-bar rotor.

Used in high starting torque loads such as loaded pumps, compressors, and

conveyors.

NEMA class D


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Rotor with small bars placed near the surface of the rotor (higher-resistance

material)

High resistance (due to its small cross section) and a low leakage reactance X2

(due to the barslocation near the stator)

Like a wound-rotor induction motor with extra resistance inserted into the

rotor.

Because of the large resistance, the pullout torque occurs at high slip, and

starting torque will be quite high, and low starting current.

Typical applications extremely high-inertia type loads.

In addition to these four design classes, NEMA used to recognize design

classes E and F,which were called soft-start induction motors .

These designs were distinguished by having very low starting currents and

were used for low-starting-torque loads in situations where starting currents

were a problem. These designs are now obsolete.


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Rotor cross section. showing the construction of the former design class F

induction motor. Since the rotor bars are deeply buried. they have a very high

leakage reactance. The high leakage reactance reduces the starting torque and

current of this motor. so it is called asoft-start design.

STARTING INDUCTION MOTORS


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STARTING INDUCTION MOTORS

For wound-rotor induction motors, starting can be achieved at relatively low

currents by inserting extra resistance in the rotor circuit during starting. This extra

resistance not only increases the starting torque but also reduces the startingcurrent.

For cage induction motors, To estimate the rotor current at starting conditions, all

cage motors now have a starting code letter (not to be confused with their design

class letter) on their nameplates.The code letter sets limits on the amount of

current the motor can draw at starting conditions.

These limits are expressed in terms of the starting apparent power of the

motor as a function of its horsepower rating. The following table containing

the starting kilo-volt-amperes per horsepower for each code letter.

To determine the starting current for an induction motor, read the rated voltage,

horsepower, and code letter from its nameplate. Then the starting apparent

power for the motor will be:

Sstart= (rated horsepower)(code letter factor)


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Table of NEMA code letters. indicating the starting kilovolt amperes per

horsepower of rating for a motor. Each code letter extends up to, but does not

include, the lower bound of the next higher class.

(Reproduced by permission from Motors and Generators. NEMA Publi cation MG-I . copyright 1987by NEMA.)

The starting current can be found from the equation


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The starting current can be found from the equation

Example What is the starting current of a 15-hp, 208-V, code-Ietter-F, three-

phase induction motor?

According to table, the maximum kilo-volt-amperes per horsepower is 5.6.

Therefore, the maximum starting kilo-volt-amperes of this motor isSstart= (15 hp)(5.6) = 84 kVA

The starting current is thus

it is seen that to start an induction motor, there is a need for high starting current.

For a wound rotor type induction motor, this problem may be solved by

incorporating resistor banks at the rotor terminal during starting (to reduce

current flow) and as the rotor picks up speed, the resistor banks are taken out.

For a squirrel cage rotor reducing starting current may be achieved by varying the


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For a squirrel cage rotor, reducing starting current may be achieved by varying the

starting voltage across the stator terminal. Reducing the starting terminal voltage

will also reduce the rated starting power hence reducing starting current.

One way to achieve this is by using a step down transformer during thestarting sequence and stepping up the transformer ratio as the machine spins

faster (refer figure below).

Starting sequence:

(a) Close I and 3

(b) Open I and 3(c) Close 2

An autotransformer starter

for an induction motor.

It is important to realize that while the starting current is reduced in direct

proportion to the decrease in terminal voltage, the starting torque decreases as

the square of the applied voltage.Therefore, only a certain amount of current

reduction can be done if the motor is to start with a shaft load attached.

Induction Motor Starting Circuits


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g

Operation:


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p

When the start button is pressed, the relay or contactor coil M is energized,

causing the normally open contacts M1, M2 and M3 to shut.

Then, power is applied to the induction motor, and the motor starts.

Contact M4 also shuts which shorts out the starting switch, allowing the

operator to release it without removing power from the M relay.

When the stop button is pressed, the M relay is reenergized, and the M

contacts open, stopping the motor.

A magnetic motor starter of this sort has several built in protective features:

a) Short Circuit protectionprovided by the fuses

b) Overload protection provided by the Overload heaters and the overload

contacts (OL)

c) Undervoltage protectionreenergizing of the M relays.

A STEP RESISTIVE STARTER FOR AN INDUCTION MOTOR.


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Operation:


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Operation:

Similar to the previous one, except that there are additional components present

to control removal of the starting resistor. Relays 1TD, 2TD and 3TD are called

time-delay relays.

When the start button is pushed, the M relay energizes and power is applied to

the motor.

Since the 1TD, 2TD and 3TD contacts are all open, the full starting resistor arein series with the motor, reducing the starting current.

When M contacts close, the 1TD relay is energized. There is a finite delay

before the 1TD contacts close. During that time, the motor speeds up, and the

starting current drops.

After that, 1TD close, cutting out part of the starting resistance and

simultaneously energizing 2TD relay and finally 3TD contacts close, and the

entire starting resistor is out of the circuit.

SPEED CONTROL OF INDUCTION MOTOR


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SPEED CONTROL OF INDUCTION MOTOR

Until the advent of modern solid-state drives, induction motors in general were

not good machines for applications requiring considerable speed control. The

normal operating range of a typical induction motor (design classes A, B, and C) isconfined to less than 5 percent slip, and the speed variation over that range is

more or less directly proportional to the load on the shaft of the motor.

Since PRCL= sPAG, if slip is made higher, rotor copper losses will be high as well.

There are basically 2 general methods to control induction motorsspeed:

a) Varying stator and rotor magnetic field speed

b) Varying slip s

Varying the magnetic field speed may be achieved by varying the electrical

frequencyor by changing the number of poles.

Varying slip may be achieved by varying rotor resistance or varying the

terminal voltage.

1. Induction Motor Speed Control by Pole Changing


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1. Induction Motor Speed Control by Pole Changing

There are 2 approaches possible:

a) Method of Consequent Poles (Old Method)

b) Multiple Stator Windings Method

Method of Consequent Poles

It relies on the fact that the number of poles in the stator windings of an

induction motor can easily be changed by a factor of 2: 1 with only simplechanges in coil connections.

General Idea:

Consider one phase winding in a stator. By changing thecurrent flow in one portion of the stator windings as such that

it is similar to the current flow in the opposite portion of the

stator will automatically generate an extra pair of poles.

A close-up view of one phase of a pole-


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A close-up view of one phase of a pole-

changing winding.

(a) In the two-pole configuration.

one coil is a north pole and the

other one is a south pole.

(b) When the connection on one ofthe two coils is reversed. they

are both north poles. and the

magnetic flux returns to the

stator at points halfway between

the two coils. The south poles

are called consequent poles.

and the winding is now a four

pole winding.


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Possible connections of the stator coils in a pole-changing motor. together with

the resulting torque-speed characteristics:

(a) Constant-torque connection- the torque capabilities of the motor remain

approximately constant in both high-speed and low-speed connections.

(b) Constant horse power connection--the power capabilities of the motor

remain approximately constant in both high-speed and low-speed connections.

(c) Fan torque connection---the torque capabilities of the motor change with

speed in the same manner as fan-type loads.


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By applying this method, the number of poles may be maintained (no

changes), doubled or halfed, hence would vary its operating speed.

In terms of torque, the maximum torque magnitude would generally

be maintained.

This method will enable speed changes in terms of 2:1 ratio steps,hence to obtained variations in speed, multiple stator windings has

to be applied. Multiple stator windings have extra sets of

windings that may be switched in or out to obtain the required

number of poles.Unfortunately this would an expensive alternative.

Disadvantage:

2. Speed Control by Changing the Line Frequency


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p y g g q y

Changing the electrical frequency will change the synchronous speed of the

machine.

Changing the electrical frequency would also require an adjustment to the terminal

voltage in order to maintain the same amount of flux level in the machine core. If

not the machine will experience:

a) Core saturation (non linearity effects)b) Excessive magnetization current.

Varying frequency with or without adjustment to the terminal voltage may

give 2 different effects:

a) Vary frequency, stator voltage adjusted generally vary speed and maintain

operating torque.

b) Vary Frequency, stator voltage maintained able to achieve higher speeds but

a reduction of torque as speed is increased.


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Variable-frequency speed control in an induction motor:

(a) The family of torque-speed characteristic curves for speeds below base

speed. assuming that the line voltage is derated linearly with frequency.

(b) The family of torque-speed characteristic curves for speeds above base

speed. assuming that the line voltage is held constant.


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The torque-speed characteristic curves for all frequencies.


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There may also be instances where both characteristics are needed in

the motor operation; hence it may be combined to give both effects.

With the arrival of solid-state devices/power electronics, line

frequency change is easy to achieved and it is more versatile to a

variety of machines and application.

3. Speed Control by Changing the Line Voltage


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Varying the terminal voltage

will vary the operating speed

but with also a variation of

operating torque. In terms ofthe range of speed variations,

it is not significant hence this

method is only suitable for

small motors only.

The torque developed by an induction motor is proportional to the

square of the applied voltage.

4. Speed Control by Changing the Rotor Resistance


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In wound-rotor induction motors, it is possible to change the shape

of the torque- speed curve by inserting extra resistances into the

rotor circuit of the machine.

Such a method of speed

control is normally used

only for short periodsbecause of this efficiency

problem.

chapter 1 induction motor

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