You must weigh the pros and cons before choosing a rotary over static UPS system.

Rotary or static? That's the dilemma many specifiers and end-users of UPS systems continue to face. In some instances, space and weight constraints tilt the choice in favor of static UPS systems. But other highly critical applications make the rotary UPS a very attractive option-regardless of its space requirements. To make a knowledgeable choice, you should be aware of some key rotary characteristics.

How the rotary works. A rotary UPS system uses a motor-generator set, with its rotating inertia, to ride through brief power interruptions. Power goes to critical loads by means of a generator driven by an AC or DC motor. The generator provides true isolation of the power so no abnormalities pass through the UPS (other than some slight harmonics due to the characteristics of the generator's windings).

When you lose regular AC power, the DC motor provides conversion from battery power to rotational energy (See Fig. 1, on page 55, original article). The rectifier changes the normal AC power to DC to serve the motor and charge the batteries.

Another version uses the rectifier output to charge the batteries as well as feed an inverter that powers an AC motor (See Fig. 2, on page 55, original article). With the latter version, you can bypass (via a static transfer switch) the rectifier/battery/inverter system, should one of these malfunction. Then the AC source directly powers the motor. As with static UPS systems, the rotary UPS also has an online topology. Here, the generator continuously serves the critical load.

Advantages and disadvantages of rotary UPS systems. As with almost any design, you'll need to evaluate the pros and cons before selecting a rotary UPS. For example, the additional rotating mass of a rotary's flywheel extends ride-through capability from fractions of a second to several seconds. However, this extended ride-through comes at the expense of added weight, increased size, reduced efficiency, shorter bearing life, and higher initial cost. Here are some other points to consider:

Advantages. Due to its low impedance, the rotary UPS' overload capability is approximately 10 times its continuous rating. Therefore, a rotary system will have superior fault clearing capability. It can also supply currents for high inrush loads, such as transformers. Another advantage is the rotary unit offers isolation from harmonic distortion generated by nonlinear loads connected to the normal utility service. In contrast to a static UPS system, a rotary UPS sends almost no harmonics back into the power system that's supplying input power. Compared to a static unit, a rotary unit will have a much higher efficiency. It's also able to operate reliably at higher ambient temperatures.

Disadvantages. A rotary UPS unit is usually larger and heavier than its static counterpart. It also produces more audible noise than certain types of static systems. In addition, a rotary unit requires routine maintenance. For example, you must replace bearings periodically, but bearing failure is predictable with routine testing for deterioration.

You can choose the type of motor you want. You have a choice of motors to drive the AC generator: AC induction, AC synchronous, or DC.

AC induction motor. On the plus side, this type of motor has the lowest initial cost per horsepower. You can also start it under load. On the negative side, this motor has the lowest operating efficiency.

AC synchronous motor. Advantages include power factor correction capabilities and higher efficiency. The disadvantage is: You have to bring a synchronous motor up to operating speed before you can apply the load.

DC motor. This motor provides excellent frequency regulation. The major disadvantage is its need for regular maintenance and brush replacement.

Variations on the same theme. As reported in EC&M's May '98 issue, there's a unique flywheel-driven rotary DC power supply that uses a vacuum chamber and hybrid mechanical/magnetic bearings. The unit operates with a static UPS system. It has a completely integrated combination of motor, generator, and flywheel-all of which sit in a single rotating unit. The device provides power during brief interruptions of utility service, and it can provide tens of seconds of ride-through at higher capacities, such as 40kW for 120 sec or 400kW for 15 sec. When coupled to a static UPS, it minimizes battery requirements, enhancing the flywheel's return-on-investment.

A variation of the AC motor gen-set is a design that uses a common stator for both the motor and generator functions. This system is available from 35kVA to 1000kVA, and it provides 100ms of ride-through. This unique design offers these advantages:

  • Reduced size and weight,

  • No brushes,

  • Improved efficiency,

  • Reduced internal impedance, and

  • Increased reliability.

The common stator system provides slightly less isolation between the power source and load than the traditional rotary design.

There's still another innovative UPS solution available: a packaged system that directly couples a compact diesel engine to the shaft of a rotary UPS unit having a common stator. Upon loss of normal power to the UPS, the diesel prepares to take over the function normally performed by the motor portion of the rotary UPS. A clutch automatically engages the diesel shaft to the motor-generator's shaft at the appropriate moment-when the speed of the input (engine) shaft overtakes the other shaft. Within 15 sec of losing normal utility power, the diesel engine provides the rotational force that drives the generator portion of the UPS. This design is an alternative to connecting a standby engine gen-set as an emergency backup source at the input of the UPS.