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