Upload
devesh
View
219
Download
0
Embed Size (px)
Citation preview
8/8/2019 25471 Energy Conversion 14
1/17
8/8/2019 25471 Energy Conversion 14
2/17
1 Starting by Reducing ElectricalFrequency
If stator B rotate at low enough speed,
there will be no problem for rotor toaccelerate & will lock in with stator
Speed of BS then can be increased graduallyto normal 50 or 60 Hz
Shortcoming: how to provide a variableelectrical frequency source, this needs adedicated generator
This requirement is obviously impractical
8/8/2019 25471 Energy Conversion 14
3/17
Today, (as described in ch. 3) rectifier-inverter &cycloconverter can be used to convert a constantfrequency to any desired output frequency
With modern solid-state variable frequency drivepackages, it is perfectly possible to continuouslycontrol electrical frequency applied to motor from a
fraction of Hz up to and above rated frequency If such a variable-frequency drive unit included inmotor-control circuit to achieve speed control, thenstarting syn. motor is very easy
When syn. Motor operated at a speed lower than ratedspeed, its internal generated voltage EA=K will be
smaller than normal & If EA reduced, voltage applied to motor mustreduced to keep stator current at safe levels
Voltage in any variable-frequency drive (or variable-frequency starter cct) must vary roughly linearly withapplied frequency
8/8/2019 25471 Energy Conversion 14
4/17
2- Starting With an External Prime Mover
Attaching an external motor to it to bring syn.Machine up to full speed
Then syn. Machine be paralleled with itspower system as a generator
Now starting motor can be detached from
machine shaft, then its slow down BR fall behind Bnet & machine change its mode
to be motor
Once paralleling completed syn. Motor can be
loaded down in an ordinary fashion
8/8/2019 25471 Energy Conversion 14
5/17
Since starting motor should overcome inertiaof syn. machine without a load & startingmotor can have much smaller rating
since most syn. motors have brushlessexcitation systems mounted on their shaft,often these exciters can be used as startingmotors
For many medium-size to large syn. motors,an external starting motor or starting by usingexciter may be the only possible solution ,because the connected power system sourcemay not be able to feed the required startingcurrent for amortisseur winding (next)
8/8/2019 25471 Energy Conversion 14
6/17
3- Starting by Using Amortisseur Windings
most popular method is to employ amortisseuror damper winding
armortisseur windings are special bars laid intonotches carved in face of a syn. motors rotor& then shorted out on each end by a large
shorting ring pole face shown in next slide
To understand what a set of amortisseurwindings does in a syn. motor, examine salient
2 pole rotor shown next
8/8/2019 25471 Energy Conversion 14
7/17
Simplified diagram of salient 2 polemachine
Not a way normalmachines work,
-however, illustrate reason
for its application
8/8/2019 25471 Energy Conversion 14
8/17
Assume initially main rotor field winding isdisconnected & that a 3 phase set of voltagesapplied to stator
assume when power is first applied at t=0, BSis vertical as shown, & as BS sweeps along incounter-clockwise direction, it induces avoltage in bars of amortisseur winding :
eind=(v x B) . l
v=velocity of bar relative to B
B=magnetic flux density vector
l=length of conductor magnetic field
8/8/2019 25471 Energy Conversion 14
9/17
Development of a unidirectional torque withsyn. Motor amortisseur windings
8/8/2019 25471 Energy Conversion 14
10/17
1- at t=0Bars at top of rotor moving to right relative tomagnetic field of stator, so induced voltage is out ofpage
And similarly induced voltage in bottom bars into page Voltages produced a current flow out of top bars & into
bottom bars, therefore this winding (bars) magneticfield Bw pointing to right
Employing induced torque equation: Tind=k BW x BS
Direction of resulting torque on bars (& rotor)counterclockwise2- at t=1/240 s,
BS now rotated 90, while rotor has barely moved(simply can not speed up in short a time), since v is inparallel with B no induced voltage & current is zero
8/8/2019 25471 Energy Conversion 14
11/17
3- at t=1/120 s
stator magnetic field rotated 90 and isdownward, and rotor still not moved
Induced voltage in damping winding out ofpage in bottom bars & into page in top bars
Resulting current also out of page in bottom
bars & into page at top bars which cause BW
pointing to left
Induced torque : Tind=kBW x BS
is counterclockwise
8/8/2019 25471 Energy Conversion 14
12/17
4- at t=3/240 s Here as t=1/240 induced torque is zero Note: during these four steps, sometimes
torque is counterclockwise & sometimes zero,
however always unidirectional and the net isnonzero, motor speed up Although rotor speed up, never reach syn.
Speed Since if rotor turn at syn. Speed, there would
be no relative motion between rotor and BSconsequently induced voltage and passingcurrent in bars zero and no torque will bedeveloped to maintain rotor rotating, howeverit get close to syn. Speed, then regular fieldcurrent turned on, and rotor will pull into stepwith stator magnetic fields
8/8/2019 25471 Energy Conversion 14
13/17
In Real Machines, field windings not open-circuited duringstarting procedure If field windings were O.C. then very high voltages would be
produced during starting If field winding be sh. cct. During starting no dangerous
voltage developed, and induced field current contribute
extra torque to motor- Starting procedure for machines with amortisseur
winding: 1- disconnect field windings from dc power source & sh.
Them
2- apply a 3 phase voltage to stator winding, let rotoraccelerate up to near-syn. Speed, motor should have noload to get close to nsyn
3- connect dc field circuit to its power source, after thismotor get to syn. Speed and loads then may be added toshaft
8/8/2019 25471 Energy Conversion 14
14/17
Effects of Amortisseur Windings on MotorStability There is another advantage when there is an
armortisseur winding, i.e. increase machine stability Stator magnetic field rotates at a constant speed nsyn
which varies only when system frequency varies
If rotor turns at nsyn amortisseur winding have noinduced voltage If rotor turn slower than nsyn there will be relative
motion between rotor & BS & a voltage will be induced,consequently current pass and magnetic fieldproduced that develop a torque which tend to speed
machine up again On the other hand if rotor turn faster than BS a torque
develop to slow rotor down These windings dampen out load or other transients
on machine and this the reason that this windingnamed Damping Winding
8/8/2019 25471 Energy Conversion 14
15/17
Motors and Generators1- syn. Gen.: EA lies ahead ofV while for motor:
EA lies behind V2- machine supplying Q have EA cos > V (regardless
of being motor or generator) and machine consumingreactive power Q hasEA cos < V
Synchronous motors commonly used for lowspeed , high power loadsWhen connected to power system, frequency and
terminalvoltage of syn. motor is fixednm= nsync=120 fe/pPmax=3 V EA / XS
this is maximum power of machine and if exceeded,motor slip poles
8/8/2019 25471 Energy Conversion 14
16/17
Phasor Diagrams of generation &consumption
8/8/2019 25471 Energy Conversion 14
17/17
SYNCHRONOUS MOTOR RATINGS
One major difference is that a large EAgives a leading PF, instead of lagging in syn.Gen.