Alternatives to solid-state invertors or UPS?

There is yet another unique solution that is still popular for critical installations like life support, airport signalling, etc., beyond about 50 KVA in load and for industrial loads of all values. It combines the benefits of all invertor-based solid-state battery-backed UPS systems. Called Rotary UPS, No-Break generator, short-break generator or MG Set, depending on the exact application-specific configuration, it is available in power ratings from fractional HP to Megawatts.

In larger power or high-current requirements, the battery bank used in inverter and UPS involves multiple joints. Even at few ohms, a high current can drop several volts from the line. Such drops add with each joint, causing high line loss. Next, the plates of lead-acid batteries (most commonly used) under hi-drain and hi-charge conditions tend to harden, especially in applications like telephone exchanges, which drain all stored current in 5 hours from C-5 Duty Class batteries. This means either total replacement of the battery bank in 3 years or 30% battery replacement per year. The only alternative is to use excess battery capacity by a higher Duty Class, say C-25 to C-100, but that means filling up a room instead of a stool as at present.

Either way, all this means high running cost. No inverter/UPS manufacturer will advise a buyer of this but will promptly blame the battery vendor in case of a failure. Invertor/UPS warranties exclude responsibility from battery performance. Likewise, the fine print also puts limits on the acceptable levels of EMI or Noise, transients and frequency changes, all of which together lower the reliability of inverter-UPS systems, even though SMPS and PWM-based circuits and IGBT bridges have brought in improvements. Besides, in India, these often need expensive airconditioned rooms.

On the other hand, a rotary UPS uses a motor to drive an alternator or dynamo via a flywheel. Despite the worst in interrupted power supply, flywheel momentum drives the generator steadily, delivering a smooth voltage and frequency as needed. During short failures too, flywheel momentum drives the generator but at gradually lowering voltage or frequency. However, if the supply from the mains fails for longer than a preset time limit, an engine, pneumatic or hydraulic motor gets ‘kicked’ by a clutch fitted on the flywheel. This prime mover in turn revs up the entire motor-flywheel-generator system, delivering the desired power output without a break.

A variety of prime mover choices as well as motor and alternator or dynamo choices can be exercised to match about any set of line and load regulation requirements. Petrol engines suit rapid starts and short runs, while diesel engines are heavy-duty workhorses. Hydraulic and pneumatic motors are used where fuel storage is infeasible, besides in small ratings. Likewise, DC and synchronous motors provide a precisely regulated voltage-frequency output, while squirrel-cage induction motors save on cost. In few cases, battery-banks are also used to drive DC motors during short interruptions. Larger No-break generator systems also use PLCs and microprocessor controls for sensing conditions and acting on them since all uninterrupted supplies can be hazardous in the face of a fault.

The race between solid-state and rotary UPS thus continues…