Page 193 - Electric Drives and Electromechanical Systems

188 Electric Drives and Electromechanical Systems

fixed-frequency/variable-voltage and variable-voltage/variable-frequency supplies; this
approach is termed scalar control. In order to achieve the performance required by servo
applications, induction motors have to be controlled using vector controllers.
The key features that differentiate between scalar and vector control are:

Vector control is designed to operate with a standard a.c., squirrel-cage, asynchro-
nous, induction motor of known characteristics. The only addition to the motor is
a rotary position encoder.
A vector controller and its associated induction motor form an integrated drive;
the drive and the motor have to be matched to achieve satisfactory operation.
A vector-controlled induction motor and drive is capable of control in all four
quadrants through zero speed, without any discontinuity. In addition, the drive is
capable of holding a load stationary against an external applied torque.
The vector-controlled-induction-motor’s supply currents are controlled, both in
magnitude and phase in real time, in response to the demand and to external
disturbances.

7.1 Induction motor characteristics

Traditionally, a.c. asynchronous induction motors operated under constant speed,
open-loop conditions, where their steady-state characteristics are of primary importance
(Bose, 2006). In precision, closed-loop, variable-speed or position applications,
the motor’s dynamic performance must be considered; this is considerably more
complex for induction motors than for the motors which have been considered
previously in this book. The dynamic characteristics of a.c. motors can be analysed by
the use of the two-axis d-q model.
The cross section of an idealised, a.c., squirrel-cage induction motor is shown in
Fig. 7.1. As with sine-wave-wound permanent-magnet brushless motors, it can be shown
that if the effects of winding-current harmonics caused by the non-ideal mechanical
construction of the motor are ignored, and if the stator windings (a s ,b s ,c s ) are supplied
with a balanced three-phase supply, then a distributed sinusoidal flux wave rotates
1
within the air gap at a speed of N e rev min , which is given by,
60f e
N e ¼ (7.1)
p
where f e is the supply frequency and p is the number of pole pairs. The speed, N e , is the
induction motor’s synchronous speed. If the rotor is held stationary, the rotor conduc-
tors will be subjected to a rotating magnetic field, resulting in an induced rotor current
with an identical frequency. The interaction of the air gap flux and the induced rotor
current generates a force, and hence it generates the motor’s output torque. If the rotor is
rotated at a synchronous speed in the same direction as the air-gap flux, no induction
will take place and hence no torque is produced. At any intermediate speed, N r , the