Data Center Design: Battery-based / Static UPS or Rotary UPS?

UPSs vary greatly in physical size, weight, form factor (e.g., standalone vs. rack-based), capacity, supported input power source (e.g., single phase vs. 3-phase), technological design, and cost.

When considering the procurement of a UPS for a data center or other mission critical facility, there are a number of design and acquisition decisions to make such as:

1. The size of the load to be protected
2. The battery runtime required
3. The proper input and output voltages
4. The right type of system (i.e. on-line, line-interactive, etc.)
5. Pricing and performance seen within manufacturer product portfolios
6. The advances in technologies
7. The ideal level of redundancy (i.e., N, N+1, 2N, 2N+1, etc.)
8. The required output distribution

This time our focus will be limited to comparing 3-phase battery-based UPS and rotary UPSs that support data centers. Some of the common 3-phase UPS architectures are listed below:


- Double Conversion On-Line UPS
- Delta Conversion On-Line UPS
- Engine-coupled Rotary UPS
- Flywheel UPS


Other Single phase UPS systems (will be discussed later):

• Standby UPS (single-phase load ranges from 100 to 1,500 VA)
• Line Interactive UPS (single-phase load ranges from 0.5 to 10 kVA)
• Standby Ferro UPS (single-phase load ranges from 3 to 15 kVA)

Double Conversion On-Line UPS

This design is by far the most common type for loads above 10 kVA. A double conversion UPS is considered ‘on-line’ because its nearly ideal output waveform is derived completely from battery power through its inverter. As such, this isolates the output from the input. The input primarily serves to continually charge the battery.

Therefore, during an AC power failure, on-line operation results in no transfer time. Since 100% of the load power is converted twice, once from AC to DC (to charge the battery) and a second time from DC to AC (for the output), a double conversion UPS is inherently less efficient than offline UPS types.

These UPSs are available in a broad range of sizes for three-phase loads from 10 to 1,000 kVA. Furthermore, these UPSs can be arranged in a vast array of configurations to achieve high availability for very large three-phase data center loads.

Delta Conversion On-Line UPS

That design is a more recent topology that was originally patented and utilized exclusively by APC for its Silcon-series 3-phase UPS line.

The advantage of this topology is its energy efficiency. It achieves high efficiency by not processing 100% of the power, 100% of the time, as is the case with a double conversion UPS.

Rather, it processes only the portion of the power that is outside of the ‘window’ of an acceptable power waveform, while at the same time having an output that is on-line with load such that there is only a load step change from steady-state to full battery-load operation. As such, this technology is best suited for the highest power applications and well above 1,000 kW.

Engine-coupled Rotary UPS

A rotary UPS is one that unlike the prior approaches does not rely on electronics to pass through or recreate an output AC waveform. Rather, a rotary UPS relies on a mechanical motor and generator to create a pure sine wave output without the need for filter capacitors. Additionally, the resulting low input impedance allows the ability to handle any type of load no matter how ‘dirty’ it may be.

Also, this approach yields an ability to handle high fault currents and provide isolation from high harmonic load input currents. Finally, a rotary UPS can be configured to use direct diesel bypass. By comparison, a static (or battery-based) UPS, can only source its load from a generator in a ‘break before make’ fashion. Due to the nature of its ability to handle ‘difficult’ loads, rotary UPSs are large in size and expensive. As such, they are reserved for large loads in excess of 1,000 kW.

The advantages of Rotary UPS:

1. Most energy efficient UPS/CPS system
2. Use of stored kinetic energy
3. No batteries required, that means no chemical waste!
4. Save energy for battery room's ventilation and cooling 
5. Lowest Total Cost of Ownership (TCO)
6. Highest power factor
7. No conditioned battery room required, that means saving space
8. Long life time
9. Efficiency about 96%

Flywheel UPS

Flywheel UPS system is similar to the settings of battery-based UPS system. The rotary UPS is called “rotary” because rotating components (such as a motor-generator) within the UPS are used to transfer power to the load. The true definition of a rotary UPS is any UPS whose output sine wave is the result of rotating generation. Therefore, the UPS in Figure below, although it utilizes a flywheel as a rotating temporary energy storage source in case the utility fails, is not, by definition, a rotary UPS.

About The Blogger


Strategic Media Asia (SMA, www.stmedia-asia.com) is a leading technical training and event organizer for corporations specialized in data center design & build, E&M facilities, telecom, ICT, finance and colocation. Currently, SMA delivers a series of data center trainings and qualification programs in Hong Kong, Taiwan and Macau.

All these events / training seminars are designed to support the leadership needs of senior executives (Chief Information Officers, IT Directors / Managers, Facilities Managers, company decision makers, etc.) and to provide useful and applicable knowledge.

For detail, please visit our data center courses & training seminars at http://www.stmedia-asia.com/trainings.html.