islam atya
رئيس ادارة المنتدى
عدد الرسائل : 129
العمر : 37
تاريخ التسجيل : 02/09/2008
 |  موضوع: The AC induction motor  الخميس أكتوبر 30, 2008 12:10 pm | |
| The AC induction motor is a rotating electric machine designed to operate from a three-phase source of alternating voltage. The stator is a classic three phase stator with the winding displaced by 120°. The most common type of induction motor has a squirrel cage rotor in which aluminum conductors or bars are shorted together at both ends of the rotor by cast aluminum end rings. When three currents flow through the three symmetrically placed windings, a sinusoidally distributed air gap flux generating the rotor current is produced. The interaction of the sinusoidally distributed air gap flux and induced rotor currents produces a torque on the rotor. The mechanical angular velocity of the rotor is lower then the angular velocity of the flux wave by so called slip velocity.
In adjustable speed applications, AC motors are powered by inverters. The inverter converts DC power to AC power at the required frequency and amplitude. The inverter consists of three half-bridge units where the upper and lower switch are controlled complimentarily. As the power device's turn-off time is longer than its turn-on time, some dead-time must be inserted between the turn-off of one transistor of the half-bridge and turn-on of it's complementary device. The output voltage is mostly created by a pulse width modulation (PWM) technique.The 3-phase voltage waves are shifted 120° to each other and thus a 3-phase motor can be supplied.
Page Contents
Key Characteristics
Parameters
Applications
Products
Documentation
Tools
Key Characteristics
Medium construction complexity, multiple fields on stator, cage on rotor
High reliability (no brush wear), even at very high achievable speeds
Medium efficiency at low speed, high efficiency at high speed
Driven by multi-phase Inverter controllers
Motor EMI good but…terrible EMI from Inverter
Sensorless speed control possible
Low cost per horsepower, though higher than for 1-ph AC induction motor
Higher start torque than for 1-ph, easy to reverse motor
Inverter ‘shoot-through’ possible, requires ‘dead-time’ circuits & compensation
3-PH A.C. INDUCTION MOTOR CONTROL
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