Environmental Engineering Reference
In-Depth Information
B
Series
Parallel
A
C
Figure 5.38 IM stator winding changeover method
mobile applications. Furthermore, the inverter controller must coordinate the
changeover very accurately to allow time for stator drive removal, time for the
mechanical contactor opening or closing and time to re-excite the IM. Mechanical
changeover is accomplished with the inverter in its high impedance state so that it
is not damaged by overvoltage transients due to persistence of the rotor flux.
There have also been numerous designs of winding changeover that employ ac
switches such as the thyristor combinations described in this chapter. Many of those
schemes follow the same procedure in terms of inverter protection and application
as the mechanical changeover techniques. The most common again are delta-wye,
delta-2delta etc.
5.4.3 Pole changing
With IMs, it is possible to establish rotor flux having arbitrary pole number and in
doing so obtain discrete steps in rotor mechanical speed. The cage rotor of an IM
can be viewed as either a continuous conducting surface into which eddy current
patterns can be established via excitation from the stator pole number or an
m
-phase
winding where
m
equals the number of rotor bar circuits around the periphery of the
machine. In either case, the pole pattern established in rotor flux is that due to the
stator impressed pattern.
There have been numerous attempts in the past to produce discrete speed
control using an IM such as the 2:1 pole change technique developed by Dahlander
in which the entire winding is utilized. Unlike conventional tapped windings and
winding reconfiguration techniques, pole changing provides discrete steps in
mechanical speed of 2:1, 3:1 or at arbitrary pole number ratios. Pole-amplitude
modulation is another technique employed in synchronous machines for large fan
drives in which the speed could be reduced by some fraction of the designed syn-
chronous speed via a winding change.
In this section, it will be shown that pole-phase modulation (PPM) is the more
general class of discrete speed change for which both pole number and phase
