The purpose of standby generators are basically to provide power when the mains utility supply fails. To prevent the two sources of power from being mis-synchronised together, standby generators are typically connected to the load via an Automatic Transfer Switch (ATS). Under normal operating conditions the mains utility powers the load via the ATS. When the utility supply fails, the ATS detects the loss of power and sends a signal to start the generator and transfer the power source from the utility to the standby alternator once at rated speed and voltage.

There are three common types of ATS switches used:-

  1. Open-transition transfer devices: when the source fails, the open-transition transfer device opens the connected source before engaging the new source. This causes an interruption of power for a small period of time.
  2. Fast closed-transition transfer devices: operates similar to the Open-transition transfer devices but uses a “make-before-break” switching action on return of original source by paralleling both sources for approximately 100 milliseconds. This device limits power interruption upon returning to mains utility.
  3. Soft closed-transition devices: operates like an open-transition transfer switch but when transferring power from one generator to utility, the control system will synchronise the sources and gradually transfer the load.

The type of ATS is selected based on the application requirements. Most ATS systems include a bypass switch which can be used for maintenance purposes.

Mission Critical Standby Generators

From the different types of ATS, we understand that there is momentary interruption of power from a few milliseconds to several minutes affecting the reliability of power supply. In certain applications standby alternators play a vital role in providing power to critical power applications known as ‘mission critical’. Every effort is made to reduce the interruption of power to the loads as certain energy consumers cannot accept loss of power even for a few milliseconds due to the criticality of the application. Mission critical is separated into two classes Private and Public Safety. The Private sector are business related such as data centres, whereas the Public Safety sector includes applications such as hospitals and emergency services.

To address this problem an Uninterruptible Power Supply (UPS) is connected between the load and the ATS. When the mains fail the UPS will continue to power the critical loads until the ATS transfers the load to the Standby alternator.

There are two main types of UPS systems – Static and Rotary.

Static UPS System

The static UPS uses power electronics and utilises batteries to deliver power for short periods. The UPS has a rectifier at the input stage to rectify the ac to dc where the batteries are connected and then through an inverter to provide an ac output. Under normal operating conditions the batteries are charged by the dc. During loss of power the batteries take over and supply the inverter. A bypass switch is included for maintenance purposes.

Rotary UPS Systems

A typical rotary UPS systems consists of a diesel engine, electromagnetic clutch, synchronous machine and a flywheel all coupled in line. The flywheel is used to provide short term power during utility failures.

During normal mode of operation, tthe utility drives the synchronous machine as a motor which spins the flywheel. When the Utility fails the kinetic energy stored within the flywheel is used to drive the synchronous machine until the diesel engine is started and the electromagnetic clutch is engaged to drive the synchronous machine.


Network design engineers must conduct a feasibility study on performance vs cost when making the choices to select the right standby alternator, ATS and maybe UPS to suit the application. As an alternator manufacturer, Cummins Generator Technologies have a proven track record for supplying alternators for these demanding and challenging applications in the standby market. The technical knowledge and expertise of Cummins Generator Technologies enable us to provide reliable machines, work in partnership with network designers and select the correct machine to meet all load demands by providing reliable power around the clock.

About the author

Mohammed Yusuf joined Cummins Generator Technologies in 2012 as an Applications Engineering graduate after completing his Masters degree in Electrical Engineering. Now in an Application Engineer role, Mohammed is responsible for supporting key customers in all technical enquiries on Cummins Generators operating capabilities and application, provides technical support to other cross functional BU functions including commercial, marketing and service and provide training for both internal & external customer.
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