Elektrik: Memulakan dan Membrek Motor AT

Introduction

Motor AT  meluas digunakan dalam pemacu pelbagai kelajuan dan kedudukan sistem kawalan di mana tindak balas dinamik yang baik dan prestasi keadaan mantap diperlukan.
Sebagai contoh penggunaan seperti pemacu robot, pencetak, alat mesin, proses  kilang mencanai, kertas dan tekstil industri, dan lain-lain lagi. Kawalan motor AT, terutama jenis ujaan berasingan , adalah sangat mudah, terutamanya kerana penukar-tertib (commutator) dalam motor. 

Berus penukar tertib membolehkan daya kilas motor dibangunkan sebagai berkadar dengan arus angker jika arus medan yang malar. Teori kawalan klasik kemudiannya mudah digunakan untuk reka bentuk daya kilas dan gelung kawalan lain dalam sistem memacu.

Kelebihan P emacuDC


• pelbagai julat kelajuan
• daya kilas permulaan Tinggi
• kawalan kelajuan Sangat tepat
• Kebolehpercayaan dan kawalan mudah
• Kos yang lebih rendah

Memulakan  Motor AT

A DC motor teruja berasingan dimulakan oleh satu reostat angker. Semasa medan dikekalkan pada nilai tertinggi sebanyak 1.6 Amp dalam kes ini. Arus angker kadaran adalah 10 A.

 layari juga:  kerjaya-sebagai-pendidik
                     9-ciri-ciri-pendidik-yang-hebat
                     ciri2-yang-baik-sebagai-pendidik
                      kisah-pemimpin-islam-zainab-al-ghazali

Biasanya arus angker (armature current) Ia = (Vdc-Eb) / (Ra+Rext)

di mana Eb is the emf balikan dan Vdc adalah voltan angker yang dikenakan. Semasa motor dimulakan dari keadaan pegun, emf balikan adalah sifar kerana kelajuan sifar dan oleh itu arus angker,

Ia = Vdc/ (Ra +Rext)

Di mana, Ra adalah arus angker

Dayakilas terjana oleh motor Te = KфIa

Di mana, K =  pemalae emf balikan  motor
              Ф =  flux utama motor
              Ia = arus angker

The back emf developed by the motor at any speed of ω rad/sec is

             Eb = Kфω

The power output is the product of Eb and Ia.

Initially the current will be very large if no external resistance is included due to the back emf being zero. So, the motor starter generally consists of a large resistance in series with the armature circuit which is cut down slowly as the motor picks up speed. This is being emulated in this experiment with the help of a series rheostat Rext.

 

Speed Control

Speed can be controlled by any of the following methods:

i.   Armature voltage control
ii.  Field flux control
iii. Armature resistance control

For speed control below base speed the armature voltage control is employed and for above base speed field flux control is used.

In armature resistance control, speed is varied by adding an external resistance (Rext) in series with the armature. Since the energy was wasted in resistor, it is an inefficient method of speed control and used only in intermittent load applications where the duration of low speed operation forms only a small proportion of total running time (Eg. traction application).

Electric Braking

Electric Braking of DC motor can be done by three methods

i.   Regenerative braking if the speed exceeds no-load value or when TL=0
ii.  Dynamic braking or rheostatic braking by including an external resistance across the armature in place of DC supply
iii. Plugging or reverse current braking by connecting the power supply Vdc in reversed mode.

The first method allows the mechanical energy stored in the rotor to be fed back to the battery by converting the kinetic energy into electrical energy. The second method, though, makes the machine work as a generator but it dissipates the power in the external resistance connected. The third method draws extra power from the external power supply and wastes both- energy drawn from the power supply as well as the kinetic energy stored in the rotor. The last two methods can be used for stopping the motor whereas the first one can bring it up to no-load speed which is (Vdc/Kф).