Single Phase Induction Motor1
Transcript of Single Phase Induction Motor1
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Single Phase Induction Motor
The single-phase induction machine is the most
frequently used motor for refrigerators, washing
machines, clocks, drills, compressors, pumps, and so
forth.
The single-phase motor stator has a laminated iron
core with two windings arranged perpendicularly.
One is the main and The other is the auxiliary winding orstarting
winding
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Single Phase Induction Motor
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Single Phase Induction Motor
This single-phase
motors are truly two-
phase machines.
The motor uses asquirrel cage rotor,
which has a laminated
iron core with slots.
Aluminum bars are
molded on the slots andshort-circuited at both
ends with a ring.
Stator with laminated
iron core Slots with winding
Bars
Ring to short
circuit the barsStarting winding
+
_
Main winding
Rotor with
laminated
iron core+
_
Figure 7.42 Single-phase
induction motor.
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Single Phase Induction Motor
Figure 7.10 Squirrel cage rotor
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Single Phase Induction Motor
The single-phase induction motor operation
can be described by two methods:
Double revolving field theory; and
Cross-field theory.
Double revolving theory is perhaps the
easier of the two explanations to understand
Learn the double revolving theory only
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Single Phase Induction Motor
7.6.1 Double revolving field theory
A single-phase ac current supplies the main windingthat produces a pulsating magnetic field.
the pulsating field could be divided into two fields,which are rotating in opposite directions.
The interaction between the fields and the currentinduced in the rotor bars generates opposing torqueSo the resultant torque is equal zero.
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Single Phase Induction Motor
The interaction betweenthe fields and the currentinduced in the rotor barsgenerates opposing
torque. Under these conditions,
with only the main fieldenergized the motor willnot start
However, if an external
torque moves the motor inany direction, the motorwill begin to rotate.
Starting winding
Main winding
+t-t
Main winding flux
Figure 7.43 Single-phase motor main winding
generates two rotating fields, which oppose and
counter-balance one another.
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*Tomake the single phase
induction motor self start: By adding extra winding to the main winding .
These two winding (main winding and auxiliary
winding )are perpendicular . And are connected inparallel with the single phase supply.
To produce torque to rotate the motor there must be
phase angle between the current down by main
winding (Im)and the current drown by the startwinding.
To make this there are many methods:
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Split phase machine: Main winding has low
resistance and highreactance .
Auxiliary winding hashigh resistance and lowreactance.
To increase the resistanceof the start winding
connect resistance inseries with the startwinding.
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Is lags volt by small angle butIm lags volt by larger angle sothere is larger phase angle
between the two currents sothe torque of split phasedepend on sin(alpha)
The center fugal switch isdisconnect the starting
winding after the motorreaches to 70 to 80 percentageof its full load speed.
Center fugal switch
connected in series with thestarting winding.
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We can use electromagnetic relay
instead of center fugal switch:
The relay coil us connected to themain winding
The pair of contacts witch is
normally open is included in the
startingwinding. At start Im is very larg the relay
contacts close and allow to Is to
flow and the motor start as usual .
After speed reaches to 7percentage of the full load speed
Im drop to low value enough to
open the contacts of the relay.
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*capacitor start induction-run motors: At this type we connect
capacitor in series with thestarting winding and alsocenter fugal switch.
the capacitor connected out of
the motor. The phase angle at this type
reaches to 80 degrees at(200W,50Hz motor) but at splitphase type phase angle reaches
to 30 degrees. This means thatthe torque developed in case ofcapacitor start type is twice thevalue at split phase type.
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*capacitor start capacitor run:
We get greater phase angle and greater
torque than capacitor start by adding
capacitor in parallel with the capacitor inthe previous type .
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Single Phase Induction Motor
Forward
rotating field
Xsta/2
Vsta
Ista
Rrot
(1-spos
)/(2spos
)
Rsta/2
Rc/2 X
m/2
Xrot/2 R
rot/2
Reverse
rotating field
Xsta/2
Rrot(1-sneg)/(2sneg)
Rsta/2
Rc/2 Xm/2
Xrot/2 R
rot/2
Ipos
Ineg
Figure 7.44 Equivalent circuit of a single-phase motor in running condition.
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7.6 Single Phase Induction Motor
The results of the calculations are:
Input power:
Developed or output power:
*
stastain IVS
neg
negrot
pos
posrot
devs
sR
s
sRP
1
2
1
2
22
negpos II
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7.6 Single Phase Induction Motor
nm 400rpm 410rpm 1780rpm
400 600 800 1000 1200 1400 1600 18000
100
200
300
400
500
Pmech nm W
Pmot_dev nm W
Pmech nrated W
nm
rpm
Pdev_max
nmax
Operating Point
Pmech_max
Stable
Operating
Region
Figure 7.47 Single-phase motor mechanical output power and electrically
developed power versus speed.
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Single Phase Induction Motor
Operating Point
nm 0.1rpm 1rpm nsy
0 500 1000 15000
2
4
6
8
10
12
Tsm nm N m
Tm nm N m
T nm N m
Tnom
N m
nm
rpm
Starting winding
is disconnected
Main winding
generated torque
Combined main and
starting windings
generated torque Figure 7.51Torquespeed
characteristic of
a small single-
phase induction
motor.
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Single Phase Induction Motor
A less effective but more economical method usingshaded pole motors
The motor has two salient poles excited by ac
current.
Each pole includes a small portion that has a short-circuited winding. This part of the pole is called theshaded pole.
The main winding produces a pulsating flux thatlinks with the squirrel cage rotor.
This flux induces a voltage in the shorted winding.
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Single Phase Induction Motor
The induced voltage produces a current in the
shorted winding.
This current generates a flux that opposes the mainflux in the shaded pole (the part of the pole that
carries the shorted winding).
The result is that the flux in the unshaded and
shaded parts of the pole will be unequal.
Both the amplitude and the phase angle will be
different.
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Single Phase Induction Motor
These two fluxes generate an unbalanced rotatingfield. The field amplitude changes as it rotates.
Nevertheless this rotating field produces a torque,which starts the motor in the direction of the shadedpole.
The starting torque is small but sufficient for fansand other household equipment requiring small
starting torque.
The motor efficiency is poor but it is cheap
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Single Phase Induction Motor
The motor has twosalient poles excitedby ac current.
Each pole includes asmall portion that hasa short-circuitedwinding.
This part of the poleis called the shadedpole
Main winding
Shorted
coil
Unshaded
pole
Unshaded
pole
Shaded
pole
Shaded
pole
Shorted
coil
Squirrel cage
rotor
Figure 7.52 Concept of single-
phase shaded pole motor.
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Single Phase Induction Motor
Figure 7.53
Shaded polemotor for
household fan.