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L19 M3 AC Motors EN

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TSI
APAC
ELECTRICAL ENGINEERING
М3 Electrical Fundamentals
SUBJECT 19.
AC MOTORS
2014
AC motors. Designation
Used in automated electrical drives, that provide
speed and position control of different compressors,
pumps, fans, lifts, gyroscope devices and others.
Main principals of magnetism and electromagnetic
inductance, used in DC motors, relate to AC motors
as well.
AC motors do not need a commutater (collector) for
rotor electrical supply, because AC changes its
direction with magnetic flow direction.
Three main types of AC electrical engines:
• asynchronous motors
•
synchronous motors
•
universal motors
2
AC motor principle
Brushes
Axis
Rotation
momentum
Contact rings
Force
When a loop is
connected to the
source of
alternating EMF
with the help of two
contact rings,
electrical current
flows through it,
creating a magnetic
field around it.
This field interacts
with the excitation
magnetic field
(stator) and creates
a rotation
momentum.
3
AC motor principle
Excitation current (stator)
Motion is created by the rotating
magnetic field
Motion direction of the conductor depends on
the direction of this current relatively to the
magnetic field and can be defined by the “lefthand rule”.
4
Asynchronous (inductance) motor operation
principle
For
Contact ce
rings
Rotor
coil
Asynchronous is an electrical
motor, the armature coil
frequency of which differs from
rotation frequency of the rotor
shaft.
Shaft
Brushes
Starting
rheostats
The principal of operation of the
asynchronous motor is based on
interaction between the rotating
stator magnetic field with rotor
coil currents, that appear as a
result of coil EMF inductance.
Asynchronous motor coil are
SHORT CIRCUITED
5
Asynchronous motor. Construction
If a fixed conductor crosses by a rotating stator
magnetic field, then voltage is induced.
Asynchronous motor in the moment of the start acts
like a fixed TRANSFORMER.
Induced voltage is called electromotive force (EMF).
Earmature = 4,44 ⋅ kA ⋅ wA ⋅ f1 ⋅ ΦE
Points of mounting
Then in a short circuited rotor coil AC
flows and creates its rotating magnetic
field.
Armature
Enclosure
I armature = E armature / Z A ,
Rotor shaft
where Z A =
R 2A + X 2A
Stator coil
This field interacts with the stator magnetic
field and creates a rotation momentum.
МЭ = СМА ⋅ Φ E ⋅ IA ⋅ Cos(ϕ A )
6
Asynchronous motor. Armature construction
Bearings
Stator
Rotor coil
Rings
Fan
Rotor core
Rotor
Contact rings
Bearings
Phase short-circuited rotor
Starting
rheostats
Starting rheostats output, while starting, R=мах, and then with their help
momentum and speed can be regulated up to short circuit mode (R=0) (used for
high-power systems)
7
Rotor construction “squirrel cage”
а) Copper “squirrel cage”, poured
with aluminum (or hollow) for
low-power motors
Stator
Short-circuited rotor
b) Aluminum plates in laminated
steel slots
Rotor construction “squirrel cage” has its main advantage – absences of electrical
sliding contact with the rotor (this provides simplicity, reliability, low cost)
8
Asynchronous motor. Sliding
Main parameter (asynchronicity measure) of the asynchronous motor is
a measure, called sliding S.
Collector
brushes
Щетки
токосъемников
Rotor
Ротор
Контактные
Contactкольца
rings
Статор
Stator
Sliding:
Sliding shows, how the rotor rotation
frequency n2 lags behind the rotating
excitation magnetic field (stator) n1.
Where the
synchronous
frequency is:
S=
n1 − n2
n1
n1 =
(%)
60 ⋅ f
p
The motor mode is only available if S > 0.
9
Asynchronous motor characteristics
Motor mode
Режим
двигателя
Breaking mode by
concurrent
Режим
торможения
connection
противовкючением
Mmax
-Sk
-1
n1
nK
R1<R2
R2
S
0
Generator
mode
Режим генератора
n
Speed
М
Sk
1
- Mmax
Mechanical characteristic
0
М
МП
Момент
Мmax Momentum
Mechanical (speed) characteristic
fSYNCHR =
n⋅p
60
[ Hz ]
n = n 1 ⋅ (1 − S )
S K ≈ 0 ,1 ÷ 0 ,15
S 0 ≈ 0 ,005 ÷ 0 ,01
S H ≈ 0 ,04 ÷ 0 ,05
Work characteristic
Power when
10
Asynchronous motor start
Phase rotor asynchronous
motor start
Stator
Starting rheostat, when
starting, are set to the
maximum position, and
then after the start – into
the short circuit mode (R=0)
Speed
Скорость
Rings
Rotor
Starting
rheostats
Short-circuited rotor
asynchronous motor start
Stator
Short-circuited rotor asynchronous motor start methods
exist:
1.
Direct switching (relative to the available power in
the mains)
2.
Mains switching with a preemptive decreasing of
voltage on the stator coils:
•
•
n2 = n1 ⋅ (1 − S)
n2 =
Momentum
60 ⋅ f1
⋅ (1 − S)
p
Using a reactor (capacitor block)
Using a autotransformer
Asynchronous motor control:
Conducted by changing the value of voltage
f, the number of pole pairs p, parameters,
that affect sliding S
11
Three-phase asynchronous motor operation in a singlephase mains
Stator
Stator
Short-circuited
rotor
Short-circuited
rotor
Stator
Three-phase asynchronous motor can
work in a single-phase mains, if its
coils are connected with a phase
shifting capacitor to 900 according to
the schematics (i.e. convert it to a
single-phase motor)
Short-circuited
rotor
12
Single-phase asynchronous motor
A single-phase motor has a main coil, and
a support coil.
The starting capacitor is switched on to
create a starting rotation momentum.
L1
L2(N)
After the motor start, the capacitor is
switched off using the time relay or
centrifugal contact.
Контакт
Relay
contact
реле
The capacity of phase-shifting capacitor
С is picked in a way, to make the currents
in the main and support coils differ by a
phase of 900.
С
w
z
Support
Вспомогательная
обмотка coil
u
v
Реверс
Reverse
М
Главная
Main
coil
обмотка
To create a reverse in a single-phase
asynchronous motor, the starting
capacitor must be connected between
different outputs (u and v terminals
switched)
13
Asynchronous single-phase motor with a split phase
(capacitor start)
Mains
The formation of a rotating field: using a
capacitor, occurs in a motor with a
capacitor start.
This motor with a split phase, that the
support coil Б with a capacitor is
connected only during the start time.
Capacitors creates a phase-shift of 900
between currents of coils А and Б, axis
of which are shifted in space.
The biggest rotating momentum is
gained, when the phase-shift is equal to
90°, but its’ amplitudes are chosen so
that the rotating field becomes circular.
When starting the capacitor asynchronous motor, both of the capacitors must be
switched on, and after its acceleration, one of its capacitors is switched off,
because during nominal rotation frequency, a substantially lower capacitance is
required, than during the start. Used in low-power electrical gears.
14
Asynchronous motor with shielded poles in a singlephase mains
Short-circuited
copper ring
Shielded poles are used to
start the motors in
lowreserve power systems.
Shielded pole
“Squirrel cage”
rotor
Short-circuited copper rings are placed
around the shielded poles, and play the role
of transformers, forming a support coil, used
during engine start.
Two magnetic fluxes, created by two currents, shifted by phase one relative to
another, create a common rotating field, which allows the rotor to rotate. A
single-phase motor, which works this way, functions as a motor with a “split
phase”.
15
AC Synchronous motor
The naming of the synchronous motor is connected with
the fact that in static mode its rotor rotates with the same
speed, as the magnetic field, i.e. synchronously.
As opposed to the asynchronous motor the rotation
frequency of the synchronous motor is constant during
different loads.
Synchronous motors are used as gears for constant speed
machines (pumps, compressors, fans).
There are no constructional differences between the
synchronous motor and generator:
• A coil supplied by three-phase
current and forming a rotating
magnetic field is placed on the stator.
• The motor rotor is comprised from
the core with an excitation coil or a
permanent magnet
16
Reactive synchronous motor.
A synchronous motor, that has no excitation coil
on its rotor, is called a reactive synchronous motor.
A rotor of this motor is made from ferromagnetic
materials and must have pronounced poles.
Stator
The stator rotating magnetic field magnetizes the
rotor.
Rotor
This kind of rotor has different magnetic
resistances longitudinally and transversally to the
pole axis.
n1 = n2
n1 =
60 ⋅ f
p
Magnet field deformation causes, because of the
resilient characteristics of the force lines, reactive
momentum, that rotates the rotor synchronously
with the stator field.
n1 – rotating magnetic field frequency
n2 – rotor rotation synchronous frequency
17
Synchronous AC motor
The rotors of the synchronous motors
are made as salient pole (with
pronounced poles) and non-salient
Communica
tion box
Poles
Three-phase synchronous
motor (1 pole pair)
Brushes
Core
Excitation
coils
Rotor
shaft
Contact rings
Salient pole rotor (used for quiet machines)
Synchronous motor (2 pole pairs)
18
Synchronous motor with a non-salient pole rotor
Ferromagnetic
cylinder
Excitation
coil
Rotors of synchronous motors are made
non-salient poled and are used for highspeed machines.
Armature coils
Brushes
Rotor shaft
Non-salient pole
rotor
Rotor
shaft
Core with slots
Contact rings
19
Synchronous motor start
Synchronous motors do not have a start momentum, because the electromagnetic rotation
momentum, that affects a fixed rotor, changes its direction two times in one period of AC Т.
Because of inertia the rotor does not budge in time and и can’t reach the required rotation
value, that is why a special start is required.
1. Acceleration motor start. A special starting DC electric motor starts the
synchronous motor up to the synchronous speed and then turns off.
Short-circuited rotor
rods “squirrel cage”
Stator core
Excitation
coils
2. Asynchronous start. Is done by an
additional short-circuited rotor coil. In
the synchronous mode it does not
work.
Permanent
magnet
Rotor
core
3. Excitation current frequency increase
start. Used, if there is a possibility to
smoothly regulate the frequency of the
source voltage from 0 to nominal.
20
Synchronous motor working characteristics
• cosφ can reach 1 while regulating the rotor excitation, М2 lineary
dependent from shaft power Р2, because n2 = const, i.e. does not
depend on Р2. While overexcited (IE > IEN) for the external mains, the
motor presents a capacitive load (φ<0).
• Efficiency relatively high with a permanent magnet, because there are
no losses on the rotor excitation.
• The consumed current, while the Р2 increases, changes insignificantly.
21
Synchronous motor advantages and disadvantages
Advantages:
• The ability to work, whe cosφ = 1, what improves the cosφ mains, as well as to a
decrease of the motor size, because its current is lower than the current of the
asynchronous motor of the same power.
• Smaller sensitivity to voltage fluctuations, because the rotation momentum is
proportional to first degree voltage, and not the square voltage, as the
asynchronous motor has.
• Frequency constancy of the rotation independently from mechanical load on the
shaft.
Disadvantages:
• More complex construction, than a short-circuited asynchronous motor: a
synchronous motor must have an exciter or another device for supplying the
excitation coil with DC.
• Complex start.
• Difficulties with regulating rotation frequency, that may be used only while
changing the supply voltage frequency.
22
DC terminals
AC terminals
Universal (collector) synchronous motor
A motor, that is able to work from both AC and DC
sources
EC1
Unlike DC motors, universal motors consist of a laminated
stator and a rotor core, that decreases eddy currents while
supplied by AC.
The motor operation on AC is possible only then, when
currents in the stator coil and in the armature coil change
synchronously, when supplying AC to the universal motor.
UM
There are two excitation coils on the motor poles: the first
one – to operate from the AC mains, the other is
connected in serial with the first when the motor operates
from DC.
EC2
Universal motors are used in vacuum cleaners, in hand
operated drilling tools and other devices, that require
small amounts of power.
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