How do you spend your free time? Ask people that question and you’ll get a variety of replies. Some like binge-watching Netflix series, while others prefer playing football. But I know someone who loves to tinker with electrical machines. At some point, he set out to make a rotary UPS system. He built it in his barn and even sold several units to a telecom operator. He is an excellent engineer, and his machines worked just fine.
His rotary UPS was deceptively simple, comprising an electrical motor that drove an alternator. He connected a flywheel between them to bridge power interruptions. This simple solution is effective and reliable, but it has one flaw: the flywheel and alternator are interconnected and thus run at the same speed. When the flywheel runs down, so does the alternator and consequently the frequency of the output.
This was not a big problem for the telecom operator’s equipment, as it worked on a wide range of voltages and frequencies. Usually, however, this is not the case. Therefore, rotary UPS systems need to keep the output voltage and frequency within tight limits, making a simple rotary UPS with a fixed flywheel unsuitable. Such a UPS has a limited range of speed, so it can use only a small portion of the stored kinetic energy. To be effective, the flywheel must be large and heavy.
Rotary UPS manufacturers therefore had to develop more sophisticated designs. Heemaf, a company that later evolved into Hitec Power Protection, patented such a system in 1970. Today, several manufacturers produce rotary UPS systems based on that invention. The main principle is that the flywheel is not coupled directly to the alternator. It is connected via a magnetic coupling, allowing the flywheel to run at a higher speed. A controller regulates the coupling strength and keeps the alternator frequency constant. As long as the flywheel is running faster than the alternator, it can support it.
This is a big improvement. Remember that, in the formula for the amount of kinetic energy in a flywheel, the energy is proportional to the square of the speed. Thus, if the generator is running at 1500 rpm, you could run the flywheel at twice that speed (3000 rpm). At that speed, the kinetic energy is four times higher. Slowing down from 3000 to 1500 rpm releases 75% of the stored energy. Not bad.
IEM Power Systems applies both principles in its Rotabloc, which has two flywheels. One is mounted on the alternator axis. The main energy store, however, is the drum – a magnetically coupled flywheel that runs at a higher speed than the alternator. The video clip below, taken from the full IEM Power Solutions Rotabloc video, explains how it works.
The Rotabloc design elegantly combines the pony motor, flywheel and magnetic coupling in a compact assembly.
Hitec Power Protection uses the same core principles in its latest PowerPRO series of diesel UPS systems. The picture below shows Hitec’s Kinetic Energy Module (KEM).
The KEM in the PowerPRO 2700 elegantly combines pony motor, flywheel and magnetic coupling. Its flywheel runs up to 2400 rpm faster than the alternator.
Hitec’s KEM is relatively small when compared to the Rotabloc. A PowerPRO diesel UPS uses its KEM only to bridge the diesel starting time, which is about 2 seconds. A Rotabloc can support the load longer, for around 12 seconds. This requires more kinetic energy and thus a larger flywheel.
The Hitec PowerPRO 3600 series of diesel UPS systems provides the largest units. Pony motor, flywheel and magnetic coupling are separate components here.
It may not be as compact as its smaller siblings, but it achieves the world’s highest power rating per unit. As Johan Cruyff, our famous legendary football player, used to say, “Every disadvantage has its advantage”.
By now, you should have a good understanding how a rotary UPS stores and retrieves energy effectively from a flywheel. In the next article in the UPS Fundamentals series, I will explain how a UPS keeps the voltage constant.