UPS OPERATIONAL TYPES
The operation of UPS's can be divided into two basic types: On-line or double conversion unit and off-line or standby power unit. The on-line type continuously changes the input AC into DC and then converts the DC to clean regulated AC which is used to power the load continuously. The off-line unit powers the load from the input AC normally starting the inverter only when the input AC is lost.
The standby power unit is a good choice when the load is not extremely critical and when the power is relatively clean, free from spikes, and has infrequent momentary interruptions. Since the off-line unit requires some time to turn on, it offers little or no protection from spikes and flickers which are passed directly through to the load. Even a 1 millisecond transfer time will not protect the load from damaging voltage spike. When an off-line unit fails, the failure only becomes apparent when the unit is required to supply backup power and does not. The most stressful period of operation, and therefore the most likely time to have a failure, is during the shock of starting up the unit which is exactly the time that the unit is needed most.
The standby power unit should never be used when it is to be powered by a backup engine-generator unit. The generator's frequency deviations will be passed directly to the critical load or will cause the standby UPS to operate from battery even when the generator is up and running.
In the on-line double conversion unit by contrast, the DC bus absorbs any input spikes, flickers, sags, surges, brownouts, or frequency variations and continues providing clean regulated power to the load without discharging the batteries. In a blackout, the inverter continues to supply the load from the batteries without having to switch on. Failures in the unit, if they occur, will most likely occur when the AC is available and the load will be transferred automatically to the input AC without interruption.
For the most critical loads, where loss of power or damage from voltage spikes can result in costly down time and repair, only the on-line UPS should be chosen.

TECHNOLOGIES
There are four basic technologies used to make UPS units: ferroresonant, pulse width modulation (PWM), PWM/ferroresonant, and step wave.
With the ferroresonant circuit, a 60 Hz square wave is generated from the DC bus and supplied to the ferro transformer. The ferro transformer generates a clean sine wave and provides the regulation, isolation, and the current limit, making for a greatly simplified control circuit. The power SCR's are used to turn each other off, eliminating the commutating SCR's and reducing the power circuit components count.
The PWM circuit generates a series of high frequency pulses that are varied in width to form a regulated 60 Hz sine wave after the high frequency has been filtered out. The control must also provide a high speed current limit since any overload or substantial nonlinear load (such as a switching power supply) will pass through the filter and stress the semiconductors. Therefore, the control must form the sine wave, regulate the output, and provide a current limit circuit making for a far more complex circuit. The PWM power circuit requires at least two commutating SCR's and two power SCR's per phase. Each of these SCR's require a separate drive circuit. This greatly increases the component count, thus reducing the reliability.
In order to reduce the cost of the ferroresonant transformer, some companies vary the pulse width of the 60 Hz wave feeding their ferro transformer. Because the volt seconds across the primary can remain constant and lower by reducing the pulse width as the primary voltage increases, the amount of transformer iron can be greatly reduced. However, this circuit requires current limit circuitry and regulation circuitry. Because the ferro is not hard into resonance, a voltage spike on the primary can cause the primary to saturate, sometimes blowing fuses, and can allow the spike to appear on the output.
In both the PWM circuits, some companies have switched to transistors, BI-MOS's or IGBT's, again to reduce cost. Our Inverter uses the far more reliable SCR's instead of Transistors, BI-MOS's or IGBT's. An SCR can take a 1000% current surge without damage while these other devices die if the current exceeds 100%. These devices are fine for computer applications where some failures can be tolerated.
A Dependable Power Systems ferroresonant three phase unit uses only 4 SCR's and two drive circuits in the inverter while a PWM uses a minimum of 12 SCR's and 12 drive circuits. A transistorized PWM reduces the component count in small units but, in larger systems, transistors increase the parts count because of the required paralleling. The transistor is not as rugged as an SCR, and is therefore, far more likely to be damaged. The step wave unit uses a series of voltage steps to generate a stair step wave which is then filtered to form a sine wave output. The control circuitry is at least as complicated as the PWM control circuit. This circuit, as with the PWM, has a much larger components count, and therefore, a higher failure rate than the Dependable Power Systems design. While the PWM and step wave can sometimes offer a smaller package, lower noise, and higher efficiency at 50% and lower loads, they also require far more complex control and power circuits which often necessitates a microprocessor controlled diagnostics program for trouble shooting. All this complexity reduces the dependability of these UPS's. The PWM, PWM/Ferro, or step wave is rated at .8pf versus the Dependable Power Systems unity (1.0) pf, i.e. a 25 KV A PWM is only a 20 KW unit, where as a 25 KV A Dependable Power Systems unit is rated at 25 KW. This means that a PWM or step wave unit must be oversized by 20% to be equivalent to our unit in real power output. This requirement to oversize PWM and step wave units for high power factor applications can more than offset their somewhat higher efficiency at partial loads.
Many companies have gone to great lengths to obscure what technology approach they are using. We are very competitive when all the facts are known to the customer!