Some electrical equipment such as computers can be severely disrupted if their supply of electrical power is interrupted for even a few seconds. Uninterrupted power supply (UPS) systems are in common use to prevent the disruption of operations when a normally used electric power line falters or fails. In some UPS systems, a standby internal-combustion engine is automatically started upon failure of the utility power line. The standby internal-combustion engine drives an AC electric dynamo, which supplies electrical power to the load until normal power is restored.
When the utility power fails the internal-combustion engine requires a few seconds to start and to accelerate to a speed fast enough to drive the dynamo to produce the desired electrical output frequency. Thus there is a delay, which could result in a harmful interruption of power to the load if the delay were not bridged.
To bridge the delay upon utility power failure, an already spinning flywheel has been used in typical prior art systems to drive the electrical dynamo until the utility power is restored or, in case of a prolonged utility power failure, until the internal-combustion engine has been brought up to speed. The dynamo is usually a synchronous machine that can operate as a generator and a motor. While the utility power is normal, the dynamo acts as a motor; it idles on the power line and keeps the flywheel spinning at a high speed. Upon a utility power failure the emergency dynamo acts as a generator; it is driven by the flywheel, which has stored kinetic energy.
During a utility power outage and before the internal combustion engine takes over the load, and while the flywheel drives the dynamo, the speed of the flywheel gradually diminishes. The flywheel alone cannot drive the dynamo at a constant speed. A variable hydraulic transmission, connected mechanically between the flywheel and the dynamo, has been used in the prior art to keep constant the speed at which the flywheel drives the dynamo. The hydraulic transmission converts the varying speed of the flywheel to a constant speed at the dynamo's shaft.
After the engine has come up to full speed it is connected by a main clutch to the dynamo, so the flywheel is relieved of providing the main power. The speed of the engine is maintained constant thereafter by a governor or electronic control loop.
A first important problem in prior UPS art is low efficiency when the system is on standby, i.e., when the dynamo and flywheel are spinning in readiness to supply power if and when the utility power line fails. The system is in standby mode most of the time, so it is very desirable to minimize parasitic standby losses. These losses include flywheel and dynamo "windage" losses, friction losses, and various hydraulic losses.
Some of the power losses occur in the hydraulic variable-ratio transmission that connects the flywheel to the dynamo. The hydraulic "windage" loss in a pump is generally proportional the square of the pump's speed. A typical value for a six-cubic-inch-per-revolution pump attached to a flywheel would be about 11 kilowatts (kW) at 3,600 rpm. If the standby pump speed was 5,400 rpm, the hydraulic windage loss would be about 25 kW. Conversely, if the pump attached to the flywheel were to have a standby speed of only 1,800 rpm, the loss would be less than 3 kW. To minimize hydraulic losses, the hydraulic devices should be operated at the lowest practical speed and the highest practical pressure, and a gear box is desirable.
A second important problem in prior UPS art is that the flywheel cannot deliver enough power when it reaches a low speed, even though it may still have a useful amount of kinetic energy left. Therefore, the "ride-through time" provided by the flywheel is not long enough. The "ride-through time" is the interval in which the flywheel supplies power to the dynamo, starting when the main power source fails and ending when the utility power is restored or the emergency engine takes over the load. It is desirable to extend the ride-through time by extending a UPS system's ability to deliver power at lower flywheel speeds.
A typical 500 kW UPS system might require as many as five hydraulic motors and five pumps. Because the standby losses of so many hydraulic elements would be very great, they should be decoupled from the rotating elements during standby, either mechanically or hydraulically. When the hydraulic elements are mechanically decoupled, it is desirable also to provide a braking action on shaft of a pump or motor to prevent its rotation due to fluid manifold pressure. Otherwise, since fluid pressure creates a torque proportional to a hydraulic device's displacement, a free-shaft device having little inertia could be accelerated to high speeds, creating high losses.