A method of providing ride-through capability in a chiller/refrigeration
system employs a variable speed drive with an active converter stage, a
DC link stage and an inverter stage for providing variable frequency and
voltage to power at least one motor. An induction motor is coupled to the
output of the inverter stage for driving a compressor in the
chiller/refrigeration system. The ride-through method comprises operating
the active converter to regulate the DC link voltage of the DC link stage
to a predetermined voltage level until the current through the active
converter equals a predetermined current limit, then transferring
regulation of the DC link to the inverter upon reaching the current limit
of the converter. The compressor is unloaded, and the power flow through
the inverter is reversed to maintain the voltage level of the DC link
stage. Pre-rotation vanes, slide valve, or check valve are used to unload
the compressor.