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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.