21-09-2012, 05:33 PM
. DYNAMIC MODELING OF THE INDUCTION MOTOR
2.dynamic modeling.doc (Size: 172.5 KB / Downloads: 42)
Introduction:
Till recently, in applications where high performance variable speed operation was required, only DC motors were extensively used due to the ease with which one could control them. Separately excited DC motors were particularly popular in applications where fast torque response was required. However, DC motors have some generic disadvantages like,
Requirement of periodic maintenance.
Unusable in explosive or corrosive environments due to sparking problem.
Commutation problems at higher powers and hence its use is limited to low power, low speed motors.
These problems can be overcome by using induction motors, which have a simple and rugged structure. Further, they have a high torque to weight ratio as compared to the same with their DC counterparts.
The field and the armature currents respectively can control the flux and torque independently in the case of DC motors. It is because of this inherent decoupling between the field flux and the armature current; one is able to achieve very good torque dynamics from DC machines. Unlike DC machines, there is no inherent decoupling between flux and torque producing components of the stator currents in AC machines. Therefore, achieving good torque dynamics in AC machines is not easy. However, nowadays, field oriented control or vector control techniques have evolved which results in good torque dynamics of AC motors.
Review of Induction Motors:
In this project, all discussions relating to induction motors are with respect to squirrel cage type induction motors only. The operation of the 3phase induction motor is based on Faraday’s law and Lorentz force on a conductor. According to Faraday’s law, if the flux linking a conductive loop varies as a function of time, then an e.m.f proportional to the rate of change of flux is induced in the loop, thereby forcing a current to flow in the conductive loop. In the case of induction motor, the induced current in the rotor conductors interact with the air gap flux to produce the torque. As the rotor tries to catch up with the magnetic flux in accordance with the Lenz’s law, the rate at which conductors are cut by the magnetic flux is reduced. Consequently the induced rotor currents decrease and the Lorentz force on the rotor conductors reduce. Evidently if the rotor conductors were to catch up with the magnetic flux then there would be no relative motion between the conductors and the flux.
Space Phasor Representation of Induction Motor Variables:
Induction motor is a 3 phase machine. Therefore, all its variables like the stator currents, rotor currents etc. are three phase quantities each having 3 components. Consider for the moment the stator current components isa, isb and isc. Referring the fig.2.1. one can note that isa component of the stator current is along an axis in space which is represented by the sa-axis. Similarly isb and isc components of the stator current are represented by the sb and sc-axes in space respectively. As these current components are represented as phasors in spatial coordinates, they are called current space phasors. Considering the sa-axis as the reference, it is evident from the fig 2.1 that the sb-axis and sc-axis are positioned spatially at 2/3 and 4/3 radians respectively, measured anticlockwise from the sa-axis.