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Flywheel UPS technology

By Don Markley

While big generator capability is pretty well known in the industry, a short-term power supply source in the UPS business is quietly becoming popular for broadcast stations. The temporary energy source is the flywheel.

Flywheel history

As far back as the early '40s in Europe, motor-generator sets with big flywheels were used to provide continuous electrical energy. In the tube era, minor glitches weren't the panic that they are for solid-state electronics. Still, it was preferred that large radar and communication systems not go dark at inopportune moments.

To facilitate that operation, large motors were connected to a common shaft with a large flywheel. When normal electrical service was available, the motor simply kept the flywheel spinning. That was a small load given the use of good bearings. If the primary source failed, the flywheel kept spinning, turning the motor into a generator for a short while. This allowed a standby generator plant to start and be switched onto the line.

Obviously, it wasn't that simple because the connection to the primary source had to be broken and the generator connected and synchronized. Still, it was workable and provided standby power to many loads during World War II and shortly thereafter. Today the flywheel is being used in broadcast facilities in the place of batteries to power UPS systems.

Standard UPS systems

The first UPS systems used little packages for smaller loads such as small computer systems or light loads in areas such as medicine. Now, UPS systems take much higher loads, including full-power television transmitters.

A standard configuration consists of installing a large transfer panel immediately downstream from the station's main circuit breaker. That transfer panel is sometimes installed after some noncritical loads are split off from the main input distribution bus. The transfer panel then feeds the UPS, which, in turn, is connected to the remainder of the power distribution circuitry for the station.

The UPS does more than just provide continuous power. It provides clean, glitch-free power, which helps to improve the overall dependability of the solid-state systems in modern stations.

UHF stations need to ensure that the UPS can tolerate the enormous short-term load caused by a crowbar circuit. This is mainly handled now by circuitry that simply switches out the UPS and allows the main electrical service to bypass directly to the load until the high load passes. Then, the UPS returns to the task of providing power.

If the power fails, the UPS continues to provide energy for a short period of time while the generator is started and placed online. A bank of batteries provides the power to the UPS. Remember, the batteries only have to provide that energy for a few seconds because a modern generator system is capable of picking up the load quickly. The disadvantage of this system is the necessity of a huge battery bank with the attendant service requirements.

The flywheel-powered UPS

Today's systems use a modern UPS with a flywheel generator for the short-term energy source. The heart of these systems is a flywheel supported by magnetic bearings. This greatly reduces the wear factor over years of operation. A motor operates the flywheel until the energy stored there is needed. Then, for a short period of time, the flywheel is used to generate energy for the UPS until the main generator comes online.

The flywheels are the wild technology here. Some manufacturers still use steel flywheels weighing up to 600lbs. Others use materials such as carbon fiber composites to build a lighter flywheel that can spin extremely fast, sometimes exceeding 40,000rpm. Remember, the energy stored in flywheels is a function of the mass and rotational speed. Therefore, a huge amount of energy can be stored in a fairly light and small flywheel if it is spun very fast.

The numbers for these devices are really interesting. Most have a rating of around 200kVa per flywheel, with the ability to parallel multiple flywheel systems for greater capacity up to 1000kVa or more for periods of around 10 to 12 seconds.

The actual flywheels are enclosed in a sealed housing at the factory, and no field repairs are needed. In the case of a failure, the entire assembly is simply returned to the factory. Remember, these are normally sealed with extremely delicate bearing and seal assemblies, and operate in a partial or near full vacuum. You don't fix that on the back bench with a pair of Channellocks. On the other hand, the flywheel assemblies are not huge. Some single flywheel models measure less then 3ft × 3ft × 6ft. They are quiet and can be installed immediately adjacent to the UPS equipment.

Coordinating the entire system

Before you get too excited, realize that these power sources and UPS systems aren't worth squat without the generators to go with them. It is imperative that the design of the power systems be carefully coordinated between the flywheel power system, the UPS and the generator manufacturers. Most manufacturers will have already done much of the work, including the design of coordinated control systems between the parts of the overall network. Small accommodations can made, however, for each particular project concerning loads and physical space available.

Stations don't need a big rack full of batteries with the attendant chargers, cables, buckets of acid, etc. The flywheel systems are both clean and quiet, causing only an acceptable low hum in the electrical equipment room. The new flywheel assemblies are small and appear to be quite dependable. Still, I flinch at the idea of a broken flywheel, still spinning at 30,000rpm, bouncing around inside the back room in the station. Such an occurrence is not likely, but wouldn't it be fun to watch?

Editor’s note:

For an update on flywheel technology, see "Weathering the storm: Flywheels light the stage in greening up power protection."