Motor-generators Used To Increase Ride-Through
Motor-generators have even been used where the input and output currents are essentially the same. In this case, the mechanical inertia of the M-G set is used to filter out transients in the input power. The output's electric current can be very clean (noise free) and will be able to ride-through brief blackouts and switching transients at the input to the M-G set. This may enable, for example, the flawless cut-over from mains power to AC power provided by a diesel generator set.
The motor-generator set may contain a large flywheel to improve its ride-through; however, consideration must be taken in this application as the motor-generator will require a large amount of current on re-closure, if prior to the pull-out torque is achieved, resulting in a shut down. The in-rush current during re-closure will depend on many factors, however. As an example, a 250 kVA motor generator operating at 300 ampere of full load current will require 1550 ampere of in-rush current during a re-closure after 5 seconds. This example used a fixed mounted flywheel sized to result in a 1/2 Hz per second slew rate. The motor-generator was a vertical type 2 bearing machine with oil bath bearings.
Motors and generators may be coupled by a non-conductive shaft in facilities that need to closely control electromagnetic radiation, or where high isolation from transient surge voltages is required.
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