Bab 7 MOTOR AC SINKRON - Bab 7 Motor AC

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  Edit by: Ir. Harlianto Tanudjaja, M.Kom,MM.

  

MOTOR

LISTRIK

Motor Listrik merupakan sebuah perangkat elektromagnetik

yang mengubah energi listrik menjadi energi mekanik.

  

Energi mekanik ini digunakan untuk, misalnya, memutar

impeller pompa, fan atau blower, menggerakan kompresor,

mengangkat bahan, dll di industri dan digunakan juga pada

  

Tree

Sumber : Lessons In Electric Circuits Volume II – AC, Sixth Edition, last

HUKUM FISIKA The Motor

  

F = B

  I L [Newtons] North pole

  B = Density of the magnetic flux in Teslas I = Induced current in Amps

  L L = Length of conductor in field in metres

  F Force Example 1 B If a conductor of length 0.4m carrying a current of

  I

  10.6A is placed in a magnetic field with a flux density of 0.03T, determine the force experienced by this conductor in newtons.

  South pole

F =

0.03 x 10.6 x

  0.4 = 0.1272 N

  

Tangan Kiri

Fleming’s Left Hand Rule North pole

  Each digit of your hand must be at right angles to both of the other two thumb motion L se c ond finger

  Force current f irst finger B field

  I South pole If the current is reversed, the direction of motion will change

The Motor Effect

  Each digit of your hand must be at right North pole angles to both of the other two se c ond finger

  F current Force thu m b motion B

  I f irst finger field

  South pole If the current is reversed, the direction of motion will change

The Motor Effect

  Each digit of your hand must be at right North pole angles to both of the other two se c ond

  F finger current Force thu m b motion B

  I f irst finger field

  South pole If the field is reversed, the motion will be in the opposite direction

The Motor Effect

  Each digit of your hand must be at right South pole angles to both of the other two se c ond field finger

  F f irst finger current Force thu m b

  B motion

  I North pole If the field is reversed, the motion will be in the opposite direction

  field is clockwise Current into page field is anticlockwise Current out of page

Using the following convention, we can show why Fleming’s left hand rule works

  The Motor Effect

  

Field lines in the same direction cause repulsion, field lines in opposite directions

cause attraction

Force

Force attraction repulsion repulsion attraction The Motor Effect

  South Pole North Pole South Pole North Pole

AC Machines

  1. Select the proper ac motor type for various applications.

  2. State how torque varies with speed for various ac motors.

  3. Compute electrical and mechanical quantities for ac motors.

  4. Use motor nameplate data.

  5. Understand the operation and characteristics of three-phase induction motors, three-phase synchronous machines, various types of single-phase ac motors, stepper motors, and brushless dc motors.

  

Klasifikasi Motor Listrik Berdasarkan

Pasokan Input, Konstruksi, dan Mekanisme

Operasi

Sumber : http://dunia-listrik.blogspot.com/

  AC Motors 

  AC motors can be divided into two main forms:  synchronous motors

   induction motors

   High-power versions of either type invariably operate from a three- phase supply , but single-phase

  

Perbedaan Motor Sinkron dan asinkron

(Induksi)

  Machine 

  Synchronous machines are ac machine that have a field circuit supplied by an external dc source.

  

  DC field winding on the rotor ,

  

  AC armature winding on the stator

  

  Origin of name: syn = equal, chronos = time

  

  Synchronous machines are called ‘synchronous’ because

  their

mechanical shaft speed is directly

MOTOR AC - SINKRON

  

Motor sinkron adalah motor AC yang bekerja pada

  kecepatan tetap pada sistem frekuensi tertentu. Motor

  

ini memerlukan arus searah (DC) untuk pembangkitan

daya dan memiliki torque awal yang rendah, dan oleh

karena itu motor sinkron cocok untuk penggunaan awal

dengan beban rendah , seperti kompresor udara ,

  

Prinsip Kerja Motor AC Sinkron

Pada motor sinkron, supply listrik bolak-balik (AC ) membangkitkan fluksi medan

putar stator (B ) dan suplai listrik searah (DC) membangkitkan medan rotor (B ).

s s

  

Rotor berputar karena terjadi interaksi tarik-menarik antara medan putar stator dan

medan rotor. Namun dikarenakan tidak adanya torka-start pada rotor, maka motor

sinkron membutuhkan prime-mover yang memutar rotor hingga kecepatan sinkron

agar terjadi coupling antara medan putar stator (B ) dan medan rotor (B ) s r Sebuah motor sinkron dapat dinyalakan oleh sebuah motor dc pada satu

sumbu.Ketika motor mencapai kecepatan sinkron, arus AC diberikan kepada belitan

stator. Motor dc saat ini berfungsi sebagai generator dc dan memberikan eksitasi

medan dc kepada rotor. Beban sekarang boleh diberikan kepada motor sinkron. Motor

sinkron seringkali dinyalakan dengan menggunakan belitan sangkar tupai (squirrel-

cage) yang dipasang di hadapan kutub rotor. Motor kemudian dinyalakan seperti

halnya motor induksi hingga mencapai –95% kecepatan sinkron, saat mana arus

searah diberikan, dan motor mencapai sinkronisasi. Torque yang diperlukan untuk

  

.

  Seperti diketahui, rotor motor sinkron terkunci dengan medan putar dan

harus terus beroperasi pada kecepatan sinkron untuk semua keadaan

beban. Selama kondisi tanpa beban (no-load), garis tengah kutub medan

putar dan kutub medan dc berada dalam satu garis (gambar di bawah

bagian a).

  Seiring dengan pembebanan, ada pergeseran kutub rotor ke

belakang, relatif terhadap kutub stator (gambar bagian b). Tidak ada

perubahan kecepatan. Sudut antara kutub rotor dan stator disebut sudut

torque

  

sinkron

Motor sinkron adalah motor ac yang memiliki kecepatan konstan, namun

kecepatan dapat diatur karena kecepatannya berbanding lurus dengan

frekuensi. Motor sinkron secara khusus sangat baik digunakan untuk

kecepatan rendah . Kelebihan dari motor sinkron ini antara lain, dapat

dioperasikan pada faktor daya lagging maupun leading, tidak ada slip yang

dapat mengakibatkan adanya rugi-rugi Sedangkan kelemahan dari motor

sinkron adalah tidak mempunyai torka mula daya sehingga motor ini memiliki

efisiensi tinggi. , sehingga untuk starting diperlukan cara-cara tertentu. Bila

metode starting telah dapat dikembangkan kemudian hari, maka motor ini

Prinsip kerja 3 Phasa

  1. Bila sumber tegangan tiga phasa dipasang pada kumpara stator, maka pada kumparan stator akan timbul medan putar dengan kecepatan

  ns = kecepatan sinkron f = frekuensi sumber p = jumlah kutup

  P f n s

  120  P f n s

  120 

  2. Medan putar stator akan memotong konduktor yang terdapat pada sisi rotor, akibatnya pada kumparan rotor akan timbul tegangan induksi ( ggl ) sebesar

  E = tegangan induksi ggl f = frekkuensi N = banyak lilitan Q = fluks

   E fN s

  4 ,

  44

  2  DC Machine & AC machine

  DC motor - ends of the coil connect to a

mechanical rectifier called commutator to

'rectify' the emf produced.

  AC motor - No need rectification, so don’t

  AC Motor 

  As in the case, a current is passed through the coil, generating a torque on the coil.

  

  Since the current is alternating, the motor will run smoothly only at the frequency of the sine wave.

  Synchronous Machine

  Construction

  

  Energy is stored in the inductance

  

  As the rotor moves, there is a change in the energy stored

  

  Either energy is extracted from the magnetic field (and becomes mechanical energy – motor)

  

  Or energy is stored in the magnetic field and eventually flows into the electrical circuit that powers the

  Synchronous Machine 

  

DC field windings are mounted on

the (rotating) rotor - which is thus a rotating electromagnet

   AC windings are mounted on the (stationary) stator resulting in three-phase AC stator voltages and currents

   The main part in the synchronous

  Synchronous Machine

Rotor

  

  There are two types of rotors used in synchronous machines: cylindrical (or round) rotors and salient pole rotors.

  

  Salient pole rotors are less expensive than round rotors.

  

  Cylindrical ( round) rotor – low speed machines (hydro-turbines)

   Salient-Pole rotor - high speed

  machines (steam-turbines)

Synchronous Machine

  Construction-Rotor

  

  i) Cylindrical (or round) rotor

  

  i) Salient-pole rotor

  

Salient-Pole Synchronous Generator

  

1. Most hydraulic turbines have to turn at low speeds

(between 50 and 300 r/min)

  N uniform air-gap

  D » 10 m S S q-axis

  Turbin N e

  

Hydro

(water)

Hydrogenerator

  Salient-Pole Synchronous Generator Stator t-pole rotor

  

Cylindrical-Rotor Synchronous Generator

D » 1 m Turbine

  » 10 m L Steam d-axis

  Stator winding 

  N High speed

  Uniform air- gap Þ 2-pole  3600 r/min

  Stato r  1800 r/min Þ 4-pole

  Rotor winding q-axis

   Direct-conductor cooling (using Roto hydrogen or water as coolant) r

   Rating up to 2000 MVA S

  Turbogenerator

  Cylindrical-Rotor Synchronous Generator Stator

Synchronous Machine

  

  Synchronous machine rotors are simply rotating

  electromagnets built to have as many poles as are produced by the stator windings.

  

DC currents flowing in the field coils surrounding

each pole magnetize the rotor poles.

  

  The magnetic field produced by the rotor poles locks in with a rotating stator field, so that the shaft and the stator field rotate in synchronism.

  

  Salient poles are too weak mechanically and develop too much wind resistance and noise to be used in large, high-speed generators driven by steam or gas turbines. For these big machines, the rotor must be a solid, cylindrical steel forging to

Synchronous Machine

  

  Axial slots are cut in the surface of the cylinder to accommodate the field windings.

  

  Since the rotor poles have constant polarity they must be supplied with direct current.

  

  This current may be provided by an external dc generator or by a rectifier. In this case the leads from the field winding are connected to insulated rings mounted concentrically on the shaft. Stationary contacts called brushes ride on these slip rings to carry current to the rotating field windings from the dc supply. The brushes are made of a carbon compound to provide a good contact with low mechanical friction. An external dc generator used to provide current is called an “exciter.

Synchronous Machine Stator

   The stator of a synchronous machine carries the armature or load winding which is a three-phase winding.

   The armature winding is formed by interconnecting various conductors in slots spread over the periphery of the machine’s

stator. Often, more than one independent

three phase winding is on the stator. An arrangement of a three-phase stator winding is shown in Figure below. Notice

that the windings of the three-phases are

Synchronous Machine – Stator construction

  3 p has e AC Synchronous Motor Operation

  ELO 2.6 – Describe how an AC synchronous motor is started and its operating characteristics.

  • Synchronous motors are like induction motors in that they both have stator windings that produce rotating magnetic field
  • Unlike an induction motor, an external DC source excites the synchronous motor rotor and therefore requires slip rings and brushes

  Figure: AC Synchronous Motor AC Synchronous Motor Operation

  • Synchronous motors use wound rotor, shown in figure
  • Slip rings and brushes needed
  • Rotor is much more expensive to manufacture than squirrel cage rotor associated with AC induction motors

  Figure: Wound Rotor Operation

Starting a Synchronous Motor

  • Not self-starting, torque only developed when running at synchronous speed
  • Needs some type of device to bring rotor to synchronous speed

  Two ways of starting a synchronous motor:

  1. Use a separate DC motor

  2. Embed squirrel-cage windings on the face of the rotor

  • If starting a synchronous motor with a separate DC motor, both motors may share a common shaft
  • Upon bringing DC motor to synchronous speed, stator windings receive AC current, DC motor then acts as a DC generator and supplies DC field excitation to rotor of synchronous motor
  • Once operating at synchronous speed, synchronous motor is ready for load

  AC Synchronous Motor

Starting a Synchronous Motor

  • More commonly, a squirrel-cage winding is embedded in face of rotor poles to start motor
  • Starts as an induction motor, speed increases to ≈95% of synchronous speed, then direct current is applied and motor begins to pull into synchronism
  • Pull-in torque is term for torque required to pull motor into synchronism
  • Because of more complex nature and higher manufacturing cost, designers rarely specify synchronous motors
  • However, large industrial applications sometimes use synchronous motors:
    • – to accommodate large loads
    • – to improve power factor of transformers in large industrial applications
    • – in applications where a constant speed is important
    AC Synchronous Motor Knowledge Check Select all of the statements about AC synchronous motors that are true.

  

A. Synchronous motors are the most commonly used motors in large

industrial applications.

  B. Synchronous motors develop starting torque in the same manner as induction motors.

  C. Synchronous motors are not often used due to their more complex nature and higher manufacturing costs.

  

D. Synchronous motors can continue to operate with more slip than

induction motors, thus producing more torque.

  Correct answer is C.

Synchronous Machine

  

Magnetomotive Forces (MMF’s) and Fluxes Due to Armature and Field Windings

  Flux produced by a stator winding Two Cycles of MMF around the Stator

  Phase AC Motor Knowledge Check

Select all of the statements about single-phase AC motors that are true.

  A. The starting winding in a single-phase AC motor is always energized while the motor is running.

  

B. Single-phase AC motors have a starting winding that is out of

phase with the main winding.

  C. The resistance and reactance of the starting winding must be the same as the main winding.

  

D. Single-phase AC motors are used for small load applications.

  Correct answers are B and D. Rotating the stator produces a net magnetic field

  Sistem 3 Phase

Further Study - AC Motor Performance

  Synchronous Wound induction Cage Induction

  

Speed Speed

  

Synchronous Generator

Equivalent circuit model – synchronous

generator

Synchronous Speed

  2 P s

    

  

P

f n s

  120 

  Matematis Motor AC sinkron Slip dari motor AC dihitung dengan : Di mana :

  N = Kecepatan putar, dalam r rpm S = Slip normal, 0 sampai 1.

  Sebagai contoh, sebuah motor dengan empat kutub beroperasi pada 60 Hz bisa memiliki plat nama 1725 RPM pada beban penuh, sedangkan bila dihitung kecepatannya 1800 RPM.

  Machine

The frequency of the induced voltage is related to the rotor speed by:

  where P is the number of magnetic poles fe is the power line frequency.

  Typical machines have two-poles, four- poles, and six-poles

  

Matematis Motor AC

sinkron

Motor ini berputar pada kecepatan sinkron, yang diberikan oleh

persamaan berikut :

  

Ns = 120 f / P

di mana : N = kecepatan serempak, dalam rpm s

  F = frekuensi daya AC p = jumlah kutup per lilitan phase

Slip and Slip Frequency

  n n     s m s m s   n

   s s s

  

  

  slip

Effect of Rotor Inductance on Torque

  c Ic

  Rjs Lc c

   If the generator operates at a terminal voltage V while

  T Ia supplying a load corresponding to an armature current , then;

   In an actual synchronous machine, the reactance is much greater than the armature resistance, in which case;

   Among the steady-state characteristics of a synchronous generator, its voltage regulation and power-angle characteristics are the most important ones. As for transformers, the voltage regulation of a synchronous generator is defined at a given load as;

Penggunaan Motor AC sinkron

  Motor ini memerlukan arus searah (DC) untuk pembangkitan daya dan memiliki torque awal yang rendah, dan oleh karena itu motor sinkron cocok untuk penggunaan awal dengan beban rendah, seperti kompresor udara, perubahan frekuensi dan generator motor.

Synchronous Generator Example 2

   A three-phase, wye-connected 2500 kVA and 6.6 kV generator operates at full-load. The per- phase armature resistance R and the a

synchronous reactance, X , are (0.07+j10.4).

d

  Calculate the percent voltage regulation at (a)

  0.8 power-factor lagging, and (b) 0.8 power-factor leading.

Proteksi Motor AC sinkron Ada banyak metode kendali motor AC (motor induksi, motor

  

sinkron) dengan kelebihan dan kekurangannya. Namun secara

umum metode ini dapat dikelompokkan sebagai berikut:

  1.Kendali Skalar (v/f Konstan)

  2.Kendali Berorientasi Medan (Field Oriented Control, FOC)

  3.Kendali Torsi Langsung (Direct Torque Control, DTC)

  

Proteksi Motor AC sinkron

  Sesuai dengan namanya proteksi motor ini menggunakan panas sebagai

pembatas arus pada motor. Alat ini sangat banyak dipergunakan saat ini.

  

Biasanya disebut TOR, Thermis atau overload relay. Cara kerja alat ini adalah

dengan mengkonversi arus yang mengalir menjadi panas untuk

mempengaruhi bimetal.

  Bimetal inilah yang menggerakkan tuas untuk menghentikan aliran listrik

pada motor melalui suatu control motor starter. Pembatasan dilakukan dengan

mengatur besaran arus pada dial di alat tersebut. Jadi alat tersebut memiliki

range adjustment misal TOR dengan range 1 ~ 3,2 Amp disetting 2,5 Amp.

  Artinya, kita membatasi arus dengan TOR pada level 2,5 Amp saja.

Proteksi Motor AC Sinkron

  Overload Motor Protection, yang dimaksud motor ini adalah electric motor

yang oleh orang awam disebut dinamo. Dan disini dikhususkan yang terjadi pada

motor AC 3 phase. Fungsi dari motor ini adalah sebagai penggerak atau untuk

mengkonversi energi listrik menjadi mekanik / gerak seperti lift, conveyor, blower,

crusher dll.

  Dalam dunia industri saat ini peran yang dilakukan motor ini sangat vital.

Untuk itu proteksi sangat diperlukan untuk menjaga kelancaran suatu proses.

Sistem proteksi motor ini sudah lama dikenal dan berkembang seiring kemajuan

teknologi. Mulai dari penggunaan eutic relay, thermal, sampai elektronik. Secara

umum sistem kerja alat tersebut dapat dibagi menjadi dua yaitu dengan thermal

dan elektronik. THERMAL OVERLOAD

  sinkron

Pembangkitan Torka

s r

  Interaksi antara medan putar stator (B ) dan medan rotor (B ) yang

  • membangkitkan torka seperti terlihat dalam persamaan berikut :

  

T = B x B (sin δ)

s s δ disebut sudut beban karena besarnya tergantung pembebanan. Pada saat

  • beban nol nilai δ = 0. Jika dibebani, medan rotor tertinggal dari rotor sebesar δ,

    o

    kemudian berputar sama lagi. Beban maksimum tercapai pada δ = 90 . Jika

    beban dinaikkan terus melebihi batas itu, maka motor akan kehilangan

    sinkronisasi dan akhirnya akan berhenti.

  sinkron

Pembangkitan Medan Putar

  

Pada Motor sinkron 3 fasa, mengalir arus seimbang pada tiap fasa dengan beda sudut

o fasa 120

i = I sin ωt

a m o

i = I sin (ωt-120 )

b m o

i = I sin (ωt-240 )

c m

  Tiap arus fasa membangkitkan ggm F yang merupakan fungsi sudut ruang  seperti :

i à F .cos θ. Dengan F =F . sin ωt

a a a m

  Maka ggm F tiap fasa yang dibangkitkan :

F = F sin ωt.cos θ

a m o o

F = F sin (ωt-120 ).cos (θ-120 )

b m o o

F = F sin (ωt-240 ) .cos (θ-240 )

c m

  Resultan ketiga ggm, F =F + F +F r a b c

  Pengukuran Motor AC sinkron

  Jika kemudian disederhanakan dengan persamaan trigonometri akan diperoleh:

F(θ,t) = 3/2 F .cos (θ-ωt)

m

  

Yang berarti resultan-mmf adalah medan putar sebagai fungsi dari ruang dan

waktu, seperti terlihat dalam gambar berikut :

  Karakteristik Motor AC sinkron

  

Gambar di atas memperlihatkan bahwa Torka (Torque) adalah fungsi sin δ,

dengan δ adalah sudut daya. Pada motor sinkron nilai δ negatif dan nilainya

o

positif pada generator sinkron. Torka maksimum dicapai pada δ= +/- 90 . Jika

melebihi batas itu, maka motor atau generator akan kehilangan stabilitas dan

sinkronisasi sehingga pada akhirnya akan berhenti.

  

Karakteristik Motor AC

sinkron

  Gambar Model Motor Sinkron (Model dan Diagram Fasor) Pengaruh Penguatan Medan Untuk membangkitkan fluksi dibutuhkan daya reaktif yang bersifat induktif.

  • Pada motor sinkron, ggm dibangkitkan arus medan (DC) pada belitan rotor. Jika arus medan ini
  • cukup, maka motor tidak membutuhkan supply energi reaktif dari sisi stator yang bersumber dari jaringan listrik. Sehingga motor bekerja dengan faktor daya = 1. Jika penguatan arus medan kurang, maka motor sinkron akan menarik daya reaktif yang bersifat
  • induktif dari sisi stator. Sehingga motor bekerja dengan factor daya (pf) terbelakang (lagging). Artinya motor menjadi pembangkit daya reaktif yang bersifat induktif. Kebalikannya jika kelebihan penguatan arus medan, maka motor sinkron akan menarik daya reaktif
  • yang bersifat kapasitif dari sisi stator. Sehingga motor bekerja dengan factor daya (pf) mendahului (leading). Artinya motor menjadi pembangkit daya reaktif yang bersifat kapasitif.

  

Synchronous Generator

Phasor diagram of a synchronous generator

  The phasor diagram is to shows the relationship among the voltages within a phase (E ,V , jX I and R I ) and

  φ φ S A A A the current I in the phase. A

  Unity P.F (1.0)

  Leading P.F. Lagging P.F

Synchronous Generator

Synchronous Generator Power and Torque

  In generators, not all the mechanical power going into a synchronous generator becomes electric power out of the machine The power losses in generator are represented by difference between output power and input power shown in power flow diagram below

Synchronous Generator

  Rotor

  • - resistance; iron parts moving in a magnetic field

    causing currents to be generated in the rotor body
  • - resistance of connections to the rotor (slip rings)

    Stator - resistance; magnetic losses (e.g., hysteresis) Mechanical - friction at bearings, friction at slip rings Stray load losses
  • due to non-uniform current distribution

Synchronous Generator

  The input mechanical power is the shaft power in the generator given by equation:

The power converted from mechanical to electrical form internally

is given by

The real electric output power of the synchronous generator can be

expressed in line and phase quantities as and reactive output power

Synchronous Generator

  In real synchronous machines of any size, the armature resistance RA is more than 10 times smaller than the

synchronous reactance XS (Xs >> RA). Therefore, RA can

be ignored

  Synchronous Generator

   Power flow

  

Synchronous Generator

  

FIGURE 8–26 A schematic showing the motor controls for a Lexus RX 400h. Note

the use of the rear motor to provide 4WD capability.

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