As with any thyristor circuit, once a thyristor is
turned on, the current must be reduced to zero
before the thyristor returns to the non-conducting
state. With an excited synchronous motor as the
load, the commutation is accomplished using the
open circuited winding to generate opposing
voltages in the machine. (Figures 56 and 57.) To
provide reliable operation at low speeds, encoders
are sometimes used to indicate the instantaneous
position of the rotor to the control so that
succeeding thyristors can be turned on at the proper
instant. Variations in technologies also exist which
use no encoders, but work by sensing the back
voltages from the machine. Whichever method is
used, a rotating magnetic field is established.
The simplest type of LCI uses a six-step
approach. Refer to Figure 59 and note the
progression of the resultant vector as dc is applied
in sequence and at alternating polarity to each phase
of the machine. The vector, representing the
magnetic field, progresses around the stator in one
complete cycle.
The six-step approach, while the simplest, is
not the smoothest. The pulsations associated with
the finite progression of magnetic fields around the
stator are reflected in torque pulsations and
magnetic noise. Care must be exercised to avoid
critical frequencies in the motor shaft and other
elements which could respond to pulsations set up
by the inverter action. In some cases, special
couplings must be applied to avoid premature
failures. The six-step approach is commonly used
for horsepower in the 500-2500 range, sometimes
up to 6000HP.