Page 45 - Energize February 2022

TECHNICAL

Figure 2: UPS waveforms (Kstar)

• Pure sinewave: The requirement for a pure sinewave comes from
motors and some switched thyristor input equipment. Neither
Figure 1: UPS configurations of these equipment types works well with any waveform other
than pure sinewave. The pure sinewave is the required output
Furthermore, there may be a difference of loads on different for industrial UPS systems. The other two are found in smaller
phases of a 3-phase system, meaning that the central UPS will be systems or systems that are only required to drive resistive loads
sized for the heaviest loaded phase on all three phases, whereas or loads that are essentially DC and use converters at the front
using single phase UPS systems will reduce sizes. Three-phase UPSs end. This includes most IT equipment. The pure sinewave is
are generally the preferred choice for high kVA applications, which generally produced using pulse width modulation techniques.
are typically more sophisticated and have high computer densities.
Large multi-story buildings, data centres and industrial facilities Motor requirements
protecting high-power processes, are typical three-phase UPS Both synchronous and induction motors require a smooth rotating
applications, as they need to distribute large amounts of power electric field at the fundamental frequency. Any waveform other
over relatively long distances. than pure sine contains harmonics which will set up rotating fields
Single-phase UPSs tend to be a sensible and economical option at odd multiples of the fundamental frequency, which interfere
for simpler, smaller applications with low kVA requirements, which with the fundamental field. The effect of using non sinewave
are typically seen in the home, small businesses and in remote or output on AC motors includes erratic speed, overheating or simple
satellite offices where the load is less than 20 kVA. failure to work at all. Obviously the square wave output is the
worst.
Transformer vs. transformerless designs
Transformer-based UPS systems Thyristor fired circuits requirements
There has been a move to the use of transformerless UPS Thyristor circuits make use of a controlled variable firing angle
systems in the commercial sector, but the demands of industrial to control the power drawn by a device. This relies upon a pure
loads favour transformer-based UPSs. Although more expensive sinewave to function correctly. Any other waveform will cause
and generally less efficient than transformerless systems, malfunction.
transformer-based UPS systems are more robust for harsh loads
on the output. UPS sizing for specific industrial loads
The UPS has to be sized to cater for a variety of loads, from static
Output wave shape constant loads to dynamic motor start and inrush loads. Typical
The output wave shape is critical for industrial UPS systems. Three industrial loads include both induction and synchronous motors,
types of waveforms are used in UPS systems (Figure 2). and the UPS must be sized to cater for the start-up current of these
loads. Today’s industrial systems also incorporate capacitive input
• Modified square wave: This is the cheapest version to produce converters as well as transformers and the inrush current of these
and simply consists of a single reversed pulse per cycle. The devices needs to be considered, as well as that due to motor start-
harmonic content is high. up operations.
• Modified sinewave or pseudo sinewave: This consists of a
stepped pulse per cycle and attempts to simulate the sinewave. Motors
The number of steps will determine the cost and effectiveness Motors have two states: the normal running state under load, and
of the unit. The harmonic content of the output is less than the the start-up state. The start-up state will determine the size of both
square wave but is still of a level that can affect operation of the inverter, the battery and the transformer of the UPS. The start–
equipment. up current will determine the kVA rating of the UPS. The inverter

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