an investigation of dual stator winding induction machines

The actual rotor mechanical speed is obtained from a speed sensor coupled on the
rotor shaft. The error of the reference rotor speed and the actual rotor speed is applied to a
slip controller whose output represents a reference slip frequency signal. Then the
reference electric frequency is obtained by adding the reference slip frequency to the
actual rotor speed. A small voltage boost is necessary to compensate the relatively large
voltage drop on stator resistance at the low speed region and to improve the
controllability. The disadvantages of this control method are the low speed accuracy and
poor torque responses. In [10.1], currents and speed feedback are used to regulate the
frequency and the magnitude of applied voltages. A current regulated PWM inverter was
found in [10.2]. The stability issue of an induction machine driven by constant V/Hz
inverter has been studied in [10.3].
Scalar torque control is another scalar control method, in which the magnitude and
frequency of the stator current are regulated so that the steady state torque of the machine
is controlled. In this case, the feedback currents are required and the current-controlled
type of inverter is used. The block diagram of a scalar torque control system is shown in
Figure 1.6.
The current controller performs the computation of the reference torque current *
I T
and the reference flux-producing current
linkage command
I
*
T
*
I φ from the torque command
25
*
T and rotor flux
*
λ r . The expression of reference torque current calculation is given as:
*
T
= (1.1)
*
1. 5Pλr
where, P is the pole number of the machine.