Editing Motor–generator (section)

==Motor–generators used to increase ''ride-through''==
{{main article|Uninterruptible power supply#Rotary|l1=Rotary}}
Motor–generators have even been used where the input and output currents are essentially the same. In this case, the mechanical inertia of the MG 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 MG set.  This may enable, for example, the flawless cut-over from mains power to AC power provided by a [[Diesel engine|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 lose speed and may draw a large current when power returns or the circuit-breaker is re-closed. If the speed loss is excessive (the power outage is too long), the re-closure current will trip the protection circuit-breakers, 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 {{frac|1|2}}&nbsp;Hz per second [[slew rate]]. The motor–generator was a vertical type two-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,<ref>''Physical Security Standard for Construction of Sensitive Compartmented Information Facilities '', DIANE Publishing, 1994 {{ISBN|0-941375-87-0}}, page 27</ref> or where high isolation from transient surge voltages is required.