The Facts About PWM vs FERRO Based UPS
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Design: Simple transformer design.
Efficiency of Operation: Highly efficent, even at light loads.

PWM UPS Response to Voltage Fluctuations:
Excellent voltage response to sudden load changes up to 100% stepped on or off.

Location Foot-Print:
Good floor space utilization.

Sensitivity to Frequency Variations:
PWM UPS are not sensitive to frequency variations in the alternate source, over a wide range.

Power Factor Sensitivity:
Not sensitive to the power factor of the load.

Flexibility of Adjusting to Output Voltage Change:
Adjustable output voltage in field situations.

PWM Compatibility with Loads:
Compatible with any load.

Stability in Operation:
Three phase systems designed using full three phase inverter transformers for unbalanced load and independent control of each phase.

Handling Load Changes:
AC Static Switch(s) uses controls which sense voltage waveform over the entire cycle.

Reaction Speed of Static Switches:
Inverter control interface sends overcurrent signal to static switch for fast transfer.

Noise Level of Operation:
Quiet.  Magnetic lamination vibration may be above the audible hearing range.

AC Voltage Balance:
Individual rectifier phase adjustment not required to maintain AC voltage balance.

Efficiency of Operation: Lower efficiency at all loads, but especially poor at part load.  Very high no-load lossses.  Larger battery required. 

History of very high internal losses at no-load leading to high transformer losses, and overheating.

Ferro UPS Response to Voltage Fluctuations:
Poor transient response to load changes.  Oscillatory at load removal.  Jump mode oscillation possibilities in three phase applications.

Location Foot-Print:
Larger and heavier.  More floor loading per square feet.

Sensitivity to Frequency Variations:
Ferro UPS output voltage amplitude is sensitive to frequency.  Change of 1% in frequency creates a 1% change in voltage in same direction.

Power Factor Sensitivity:
Volatage is a function of load power factor.

Flexibility of Adjusting to Output Voltage Change:
No field adjustments without complex tap, and/or addded transformer.

Ferro Compatibility with Loads:
May not work with loads containing internal ferroresonant transformers or which cycle loads on/off.

Stability in Operation:
Three phase systems designed using two hybrid transformers in a "scott-tee'.  Poor transient response and poor unbalance load capability.

Handling Load Changes:
Ferro design can exhibit long periods of jump mode voltage oscillation during load changes.

Reaction Speed of Static Switches:
AC Static Switch uses controls which sense only a portion of the cycle.  Uses a coined term "no-break" transfer to describe this control.

Noise Level of Operation:
Acoustically noisy due to requirment to drive the magnetics into saturation.

AC Voltage Balance:
Older technology rectifier requires individual phase adjustment to maintain balance.
Comparison Conclusion and Results
UPS Systems have evolved over the years.  The advanges of modern PWM versus older Ferro Resonance based UPS technology is quite clear.   PWM handles a wide range of conditions and fluctuations without having to be replaced, redesigned or add additional components (When new loads are added).  PWM is state-of-the-art technology that by the study comparison above shows that it is stable, more reliable, rugged, adaptable, quieter and the best engineered choice for your next UPS project.

* Think about Diebold Power Solutions when considering your next project!
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Design: Very complex inverter transformer design with tight design margins on construcion and gaps..
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