2. DOUBLE FIELD REVOLVING THEORY
• AC current flows through the Stator two fields
are generated
1.φf- forward rotating field
2.φb-backward rotating field
• Pulsating field- due to various magnitudes of
field at diff time
2
5. Why 1 phase induction motor is not
self starting
• Rotor = squirrel cage type rotor
• Alternating flux – not required to produce rmf
5
6. Split phase induction motor
• Two winding
• Main winding & auxiliary
winding
• 90 degree phase angle
• Main winding – low
resistance and high
reactance
• Aux winding- high
resistance and low
reactance
• Switch = motor pickup its
75 % of its rated speed.
6
7. Working….
7
• Is lags by V because low
reactance and high resistance
• Im lags V by very large angle due
to high reactance
• Starting torque proportional to
sin α
• Disconnect switch after reach its
75 % of rated speed
8. Capacitor induction motor
1. Capacitor start motor
2. Permanent capacitor motor
3. Capacitor start capacitor run motor
8
9. Capacitor start motor
9
• Electrolytic capacitor
• 90degree phase angle
• Im and Is
• Im lags V due to high
reactance of main
winding
• Due to capacitor Is lead V
• Im and Is angle 90
• High starting torque
• Apps: Lathes, drilling
machiine,fan etc
11. Synchronous motor
• Machine which
converts electrical
energy into mechanical
energy that rotates at a
constant speed equal
to synchronous speed
• Alternator can runs as a
synchronous motor if
AC -> armature winding
DC-> field winding
11
12. Synchronous motor
12
Parts of 3 phase synchronous
motor
1. Laminated stator core
with 3 phase armature
winding
2. Rotating field structure
complete with damper
winding and slip rings
3. Two end shields to house
the bearings that support
the rotor shaft
14. Starting of synchronous motor
1. Dc motor method
2. Pony motor method
3. Damper winding method
4. Starting as a slip ring induction motor
14
15. Dc motor method
• Attaching an external motor to it to bring syn.
Machine up to full speed
• Then syn. Machine be paralleled with its power
system as a generator
• Now starting motor can be detached from
machine shaft, then its slow down
• machine change its mode to be motor
• Once paralleling completed syn. Motor can be
loaded down in an ordinary fashion
15
16. Pony motor method
• Since starting motor should overcome inertia of syn.
machine without a load & starting motor can have
much smaller rating
• since most syn. motors have brushless excitation
systems mounted on their shaft, often these exciters
can be used as starting motors
• For many medium-size to large syn. motors, an external
starting motor or starting by using exciter may be the
only possible solution , because the connected power
system source may not be able to feed the required
starting current for amortisseur winding
16
17. Damper winding method
• most popular method is to employ amortisseur
or damper winding
• armortisseur windings are special bars laid into
notches carved in face of a syn. motor’s rotor &
then shorted out on each end by a large shorting
ring
• pole face shown in next slide
• To understand what a set of amortisseur windings
does in a syn. motor, examine salient 2 pole rotor
shown next
17
19. Starting as a slip ring induction motor
• Damper winding method of starting doesn’t
provide high starting torque
• To get high starting torque instead of shorting
the damper winding – designed a three phase
star winding , end brought to slip ring
• External rheostat connected in series with
rotor circuit.
19
25. Applications
• Synchronous motors are particularly attractive for low speeds (< 300
r.p.m.) because the power factor can always be adjusted to unity and
efficiency is high.
• Overexcited synchronous motors can be used to improve the power factor
of a plant while carrying their rated loads.
• They are used to improve the voltage regulation of transmission lines.
• High-power electronic converters generating very low frequencies enable
us to run synchronous motors at ultra-low speeds. Thus huge motors in the
• 10 MW range drive crushers, rotary kilns and variable-speed ball mills.
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