| More than ever, today's uninterruptible power supplies | | | | Loads, like Blade servers, with a leading power factor |
| (UPS) must be highly reliable and cause as little | | | | have a current waveform that 'leads' the voltage |
| disturbance as possible upstream and downstream of | | | | waveform by a factor equal to the reactance of the |
| their supply. Some manufacturers believe Rotary UPS | | | | load. |
| to be the answer but others disagree, stating that | | | | Rotary uninterruptible power supplies offer a number |
| new, improved designs of static online UPS make the | | | | of significant disadvantages too. A far higher capital, |
| added expense of Rotary UPS unjustifiable. | | | | installation and environmental cost, for example, |
| What are Rotary UPS? | | | | which can be as much as 40% higher than a |
| Rotary UPS are mechanical uninterruptible power | | | | comparable static online UPS design. |
| supplies that convert Kinetic energy into electrical | | | | Higher Costs and Complex Installation |
| energy to power connected loads. There are two | | | | Installing Rotary UPS is much more complex than a |
| competing designs: Rotating Transformer, based on a | | | | static online UPS. They are a motor generator-based |
| regulated isolated rotating transformer; and Induction | | | | device and their installation presents high ventilation, |
| Coupling, comprising a diesel engine, two-speed | | | | vibration, noise and removal of exhaust gas issues. |
| concentrically-mounted induction coupling and | | | | Some Rotary UPS designs operate at 30dBA higher |
| alternator. | | | | noise levels than static online designs, which can |
| Whatever their design, Rotary UPS start at around | | | | cause noise pollution on site. |
| 500kVA in size and range up to 2MVA (or more) | | | | Increased Size and Weight |
| when configured in parallel operation. For this reason, | | | | Rotary uninterruptible power supplies are heftier than |
| unsurprisingly, their application is reserved for large | | | | their static online cousins and can demand as much as |
| installations. | | | | 20% more footprint area within a plant room or |
| Compared to a static online uninterruptible power | | | | dedicated UPS area. This also makes system |
| supply, manufacturers argue, Rotary UPS generate | | | | expansion difficult. |
| little in the way or harmful harmonics and can achieve | | | | Increased Service Costs |
| higher MTBF (Meantime Between Failure) values. | | | | As a mechanical device, Rotary UPS contain bearings |
| Harmonics can result in poor power quality, which can | | | | in their workings, which require lubrication and this can |
| be as hazardous to critical equipment as a partial or | | | | make them sensitive when operating in Ambient |
| complete mains supply failure. It can lead to | | | | temperatures. As a result, they can require more |
| intermittent data corruption and hardware failure. | | | | frequent maintenance and have a higher MTTR |
| The actual quality of mains power supply is measured | | | | (Meantime to Repair) than static online designs. MTTR |
| in terms of its waveform, voltage and frequency and | | | | is a measure of the average time it takes to return a |
| the presence (or not) of a variety of power | | | | UPS to normal operation from shutdown in the event |
| problems including blackouts and momentary | | | | of a system failure. As Rotary UPS contain many |
| interruptions. Harmonics are voltage or current | | | | bearings, their average MTTR is higher as these can |
| waveforms, the frequencies of which are multiples of | | | | take days to replace. |
| the fundamental. In Europe this is 50Hz (50 cycles per | | | | Static online UPS designs offer many advantages |
| second) and the multiples are ordered into a specific | | | | over their Rotary counterparts. Firstly, they are far |
| sequence. For example, the 2nd harmonic is 100Hz | | | | less expensive to install and maintain and can be |
| (2x50Hz), 3rd harmonic 150Hz (3x50Hz) and so forth. | | | | easily used in parallel and redundant configurations to |
| MTBF is a standard indicator of the reliability of an | | | | increase reliability and resilience and improve MTBF. |
| uninterruptible power supply. It represents the | | | | Manufacturers have also made great strides in |
| average operational time between powering-up and | | | | mitigating harmonic pollution in uninterruptible power |
| system shutdown due to failure. The figure is usually | | | | supplies with the inclusion of rectifiers and filters. |
| presented in hours. | | | | They are also introducing designs specifically for |
| Rotary UPS manufacturers also believe their machines | | | | high-end computing environments, that offer |
| to have better fault clearance capabilities and that | | | | extremely high reliability, cost-effectively, but with |
| they are more suitable to loads with a leading power | | | | the minimal footprint and ability to support leading |
| factor. A typical example of this is high-end server | | | | power factor loads and minise harmonics. |
| loads, such as Blade servers, which have a modular | | | | The greatest advantage of a static online |
| electronic circuit board often dedicated to a single | | | | uninterruptible power supply over a Rotary design is |
| application and housed within server chassis. Blade | | | | their modularity and flexibility. It is far easier and |
| servers allow more processing power within less rack | | | | cost-effective to expand a modern static online UPS |
| space than traditional or older server designs. They | | | | design by adding another module into the parallel |
| are being widely adopted within large data centre and | | | | system, for example, than it is to rip-out-and-replace |
| telecommunications environments. | | | | a huge, noisy Rotary unit. |