ELE-351
Electrical Energy Conversion
Chapter 7
I d ti Motors-1
Induction
M t
1
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Outline
Basic characteristics of induction motors.
motors
The induced torque phenomenon.
The slip speed and the rotor speed.
The equivalent model of an induction
motor
Power & torque in induction motors.
characteristics
Torque x speed characteristics.
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Induction Machines
Usually employed as motors.
Advantages: Cheaper, lighter and easier to
maintain than synchronous machines.
Disadvantages: Speed, for motor operation,
varies with load torque.
Stator: Identical to that of a synchronous
machine.
Rotor: No need for a dc field current (cage
rotor or three
three-phase
phase wound rotor).
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Stator of a 2 poles ac machine
Generation of
the rotating
magnetic field
The rotating magnetic
field as created by a
rotating pair of
magnets
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Induction machine rotors
Wound rotor: A set of
three-phase windings
usually Y-connected,
accessible at the stator
through slip rings &brushes.
Not very common.
Cage rotor (Squirrel
g ) Conducting
g bars
Cage):
short-circuited at both ends.
y rugged.
gg Workhorse of
Very
industry.
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WoundrotorIM
SquirrelCageIM
Induction Machine
WoundrotorIM
Squirrel Cage IM
Squirrel-Cage
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Induction motor concepts
The
h rotating
i magnetic
i field
fi ld (BS)
( S) is
i createdd by
b the
h stator
winding with a speed:
A voltage is induced in the rotor conductors being cut by
the rotating magnetic field (BS) (Faradays Law):
The rotor magnetic field (BR) is created by the rotor
current that
h results
l from
f
an induced
i d d voltage.
l
The induced torque:
This means that the rotor and the rotating field can not
h
have
th
the same speed!
d!
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Induction motor concepts
Phenomena related to a moving permanent
magnet placed
l d above
b
a conducting
d i ladder:
l dd
(a) Voltage induction and
(b) force on the conductor.
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The concept of the rotor slip
The voltage induced in the rotor depends on the
speed of the rotor relative to the magnetic fields.
p
Slipp speed:
Slip:
p
Mechanical
ec a ca speed of
o the
t e rotor.
oto .
Rotor frequency:
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Example
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10
Equivalent model of an induction motor
An induction motor is like a rotating
transformer But the rotor (sec) frequency is
transformer.
different from the stator (prim).
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11
The final equivalent model
Referring the rotor to the stator:
These values can be measured by specific tests
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12
Power flow in induction motors
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13
Power & torque in induction motors
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14
Equivalent circuit of an induction motor
Shows the power dissipated/converted at each
point of the model.
model
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15
Equivalent circuit of an induction motor
Shows the power dissipated/converted at each point of
the model.
The magnitude of the currents in the rotor side can be
calculated from this circuit but the frequency
q
y is still
given by: fr = s fe
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16
Derivation of the induced torque equation
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17
Torque x speed characteristics
start > rated
Maximum (pullout)
(p
)
torque.
q
For load torques
smaller than rated, the
slip varies linearly with
the
h load
l d torque!!
ind= 0 for nm =nsync
y
What is the rotor speed at no-load condition?
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18
Torque x speed characteristics
The complete curve.
If the rotor is driven
at a speed greater
than nsync, the
machine becomes a
generator
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19
Comments on the IM -n curve
1. Induced torque of motor is zero at syn. speed
2. Torque-speed curve is nearly linear between no load and
full load.
load In this range rotor resistance is much larger than its
reactance so rotor current, rotor magnetic field & induced
torque increase linearly with increasing slip
3. There is a maximum possible torque that cannot be exceeded
(pullout torque) is 2 to 3 times rated full-load torque of
motor (calculated in next section)
4. Starting torque on 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. Torque on motor for a given slip varies as square of applied
voltage. This is useful in one form of induction motor speed
control that will be described.
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20
Comments on the IM -n curve
6. If rotor of induction motor driven faster than sync.
speed,
p , direction of ind reverses and the machine
becomes a generator converting Pmech to Pelec
7. If motor turningg backward relative to direction of
magnetic fields, induced torque will stop machine
very rapidly & will try to rotate it in other direction
Since reversing direction of magnetic field rotation is
simply a matter of switching any two stator phases,
this fact can be used as a way
y to very
y rapidly
p y stopp an
induction motor.
The act of switching two phases in order to stop motor
very rapidly
idl iis called
ll d plugging.
l
i
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21
Comments on the IM -n curve
Power converted to mechanical in an induction motor:
Pconv = ind m
Note: peak power supplied by induction motor occurs at a
different speed than maximum torque; and of course no power
is converted to mechanical form when rotor is at zero speed
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22
Maximum (Pullout) Torque in IM
Since ind=PAG/sync , maximum possible torque occurs when
air gap power is maximum.
Since air-gap
Si
i
power = power consumedd in
i R2/s
/ maximum
i
induced torque will occur when power consumed by this
resistor is maximum
Maximum power transfer to R2/s will occur when the
magnitude of that impedance = source impedance magnitude
Equivalent source impedance:
Zsource=RTH + jXTH + jX2
Maximum power transfer occurs when:
R2
2
2
= RTH + ( X TH + X 2 )
s
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23
Maximum (Pullout) Torque in IM
Maximum power transfer occurs when:
R2
2
2
= RTH + ( X TH + X 2 )
s
Solving for the slip
smax =
R2
2
RTH
+ ( X TH + X 2 )
Note: smax ~ R2
Applying this value of slip to torque equation
max =
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2
3VTH
2 sync RTH +
2
TH
+ ( X TH + X 2 )
24
Maximum (Pullout) Torque in IM
The maximum torque is
proportional to the square of the supply voltage and
inversely related to the size of the stator impedances
and the rotor reactance
The smaller a machine
machiness reactance the larger its
maximum torque
Note: smax ~ R2 , however maximum torque is
independent of R2
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25
Maximum (Pullout) Torque in IM
Effect of varying rotor resistance on -n of
wound rotor
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26
Maximum (Pullout) Torque in IM
As the value of external resistor connected to rotor
circuit of a wound rotor through slip rings is
increased the pullout speed decreased
decreased, however the
maximum torque remains constant.
Advantage can be taken from this characteristic of
wound-rotor induction motors to start very heavy
loads.
If a resistance inserted into rotor circuit, max can be
adjusted to occur at starting conditions.
And while load is turning, extra resistance can be
removed from circuit, & max move up to near
synchronous speed for regular operation
operation.
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27
Start-up schemes
The starting current in IM
is 55-7
7 times the full load
current which causes a dip
in the system voltage.
It is sometimes required
to reduce the starting
current.
current
The starting schemes are
based on the principle of
applying a reduced
voltage during the start-up
and
d then
h full
f ll voltage.
l
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An autotransformer starter for induction motor
28
Speed control of induction motors
Line frequency variation:
To prevent saturation: V/f
Power derating.
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29
Speed control of induction motors
Line voltage variation:
V = 00.5V
5Vrated
start= 0.25start_rated
Used for small motors only
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30
Speed control of induction motors
Speed
p
can be controlled
by inserting a resistance
in the rotor circuit of
woundd rotor IM
IM.
This method reduces the
efficiency
ffi i
off the
th motor
t
as the rotor resistance
increases
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31
Determining the circuit model parameters
No-load test: Magnetization current and
rotational losses
losses.
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32
Determining the circuit model parameters
The DC test for stator resistance (R1). No induced
voltage no rotor current,
voltage,
current no induced torque,
torque no
rotation
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33
Determining the circuit model parameters
The locked-rotor test: Stator and rotor impedances
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