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Maintaining Power and Protection of Electronic Equipment by Choosing the
Correct UPS System
By: Steven Sadler
April 3, 2009
Abstract
Often times the power from the wall socket is neither stable nor uninterrupted.
Many abnormalities such as blackouts, spikes, surges, and noise can occur. Under the
best conditions, these power interruptions can be an inconvenience. At their worst, they
can cause loss of data in computer systems or cause damage to electronic equipment.
The primary function of an Uninterruptible Power Supply (UPS) system’s primary
function is to provide short-term back-up power allowing time for critical data to be
saved and sensitive operating systems to be properly shut down. Another feature of the
UPS is that it can provide clean and reliable power to electronic equipment that may be
vulnerable to the abnormalities that may occur if your available power source is not
reliable.
The basic concept of a UPS is to store energy during normal operation, by
charging a battery, or bank of batteries, connected to the UPS and release the energy
stored in the batteries, through DC to AC conversion, in the event of a power failure. This
application note describes how to correctly select a UPS based on the environment you
plan to use the system in, the total wattage of the appliances you plan to connect to the
UPS and the desired functions of the UPS system.
Keywords: Uninterruptable Power Supply, UPS, Battery Backup, On-line Power System,
True UPS, Off-line Power System, Standby UPS
Background
UPS systems can be categorized as either on-line or off-line (standby). An on-line
UPS is constantly engaged, converting all incoming power to DC current that passes
though the battery circuit and is then converted back to AC again. Equipment connected
to the on-line UPS is electrically isolated from utility power and receives continuous
highly regulated power. This design prevents any output power disturbance or gap when
there is utility power loss since the battery circuit is always engaged. An on- line UPS
can accept as low as 50% normal incoming voltage without drawing battery power,
which in turn greatly increases battery life. A figure showing the circuitry of an on-line
UPS system is provided in Figure 1 at the end of this document.
Many smaller UPS systems, usually below 5 KVA, are off-line and are typically
selected as a cheaper alternative to the on-line system. In an off-line UPS system’s
functionality can be described as a switch that routes power directly to your equipment
under normal operation. The voltage-sensing switch goes from bypassing the UPS to
using the battery when there is a power outage. An off-line UPS does little or no power
conditioning and primarily acts as a switch to draw power from the batteries in the result
of a power failure. A minor surge in output voltage occurs when it switches from utility
power to battery power resulting in a decrease in battery life. Off-line UPS batteries
typically last around (2-3) years and are sometimes not replaceable. A figure showing the
circuitry of an off-line UPS system is provided in Figure 2 at the end of this document.
In addition to choosing between an on-line and off-line system, you must also
decide if you need an indoor and outdoor UPS system. An outdoor UPS system is usually
resembles a circuit breaker box that you would see in a home, and is available as a
wall/pole mounted UPS or an in-ground UPS. The outdoor UPS circuitry is contained
within an enclosure that is designed to protect it from common weather conditions, such
as direct sunlight, rain and snow, thus allowing it to be mounted outside. Outdoor UPS
systems are typically used for cellular base stations and home security and surveillance
systems.
The indoor UPS systems are not equipped with protection from weather and are
more portable than the outdoor UPS system. The indoor systems can vary significantly in
size depending on the size of the battery; the larger the batter the longer your UPS system
will be able to power your appliances while operating on battery backup before shutting
down. Indoor UPS systems are more common than outdoor systems and can be used to
support a wide range of electronics from a single computer to an entire data center.
Choosing The Correct UPS System
The first question to ask when choosing a UPS system is whether or not the unit
will be in a controlled environment. UPS systems are available for various environmental
conditions, however you must be certain to select the correct system for your intended
usage. If you were to place an indoor UPS in a non-controlled environment, the usable
lifetime of the battery and the system itself may be shortened considerably and also
backup of the equipment you are protecting, and data stored on that equipment, may also
be sacrificed.
Upon determining the environment you wish to place your UPS system in you
must also decide the appliances that you wish to connect to the UPS. In order to provide
the desired protection you must ensure that the total wattage of the desired appliances
does not exceed the capabilities specified by the UPS system. For instance if you choose
to provide protection to a computer, with a full processor and hard drive power draw
equal to 120 watts, a 17” LCD screen, with a power draw of 20 watts, and a satellite
router, with 70 watts of power draw, you would need a UPS system capable of handling
up to 210 watts of total load.
After determining the total wattage of your load it is necessary to decide on the
desired amount of runtime, the amount of time the UPS battery backup will be able to
provide power and protection to the connected appliances total wattage in the event of
power loss from your regular power source. The runtime for a UPS is proportional to the
total wattage of the load connected to the UPS. The runtime will also vary based on the
maximum wattage rating of the UPS. If you are using a UPS that is capable of handling
1500 watts of total load, your runtime will be significantly larger if you only have 750
watts connected to the UPS instead of the full 1500 watts. Each model of UPS will have
different specifications for runtime and a chart is typically available on the manufacturers
website to help you choose the correct amount of runtime for the load that you plan to
connect to the UPS. A typical rule of thumb is to choose a UPS system that is capable of
handling double your expected total load wattage, this will usually allow you at least 30
minutes of battery backup from your UPS.
It is important to note that most UPS manufacturers will describe the handling
capabilities of the system in both watts and Volt-Amperes (VA). The VA rating is the
product of voltage and the amperage that can be drawn on the UPS. Since you are dealing
with alternating current, this number is not the same as wattage. For most types of loads,
wattage can be expressed as the result of the product of voltage, amps and a "load factor”.
The “load factor” is a calculation used to account for the difference in phase between the
voltage (Volts) and the current (Amperes) and can be determined by taking the
trigonometric cosine of this difference. When the load factor is multiplied by the apparent
power, the VA rating given on the UPS, the result is the “true” power, in watts, being
used by the load. The VA rating of a power supply negates the load factor and is used
mainly to determine heat generation by equipment under use and for sizing wires and
circuit breakers. UPS manufacturers design the system for a load factor of between 0.6
and 0.7. So for instance a UPS that is rated at 1000 VA can handle a total load of between
600-700 watts.
Another item worth noting when selecting a UPS is output management software.
This software is typically available for larger UPS systems, those rated 750VA and
above. Most major UPS manufacturers, such as APC and Tripplite, will provide their
customers with a copy of software, upon the purchase of their UPS, that is capable of
managing the multi-output ports of the UPS. This software can be very useful and, in
most cases, will allow you to shut down each of your available outputs at specific levels
of remaining battery life. This software will, again in most cases, allow the user to view
and create a report containing the line power voltage, load wattage attached to the UPS,
the output voltage of the UPS system and the results of many self tests that are conducted
internally by the UPS system.
Conclusion
In this application note I have outlined the basic needs and considerations of a
UPS system, including differences in UPS circuitry, environmental protection of the
system, and a brief background on how to select the correct for your intended use. Each
section of this document includes trade offs to consider for common, or popular, UPS
selection and its counterpart. The method of selecting a UPS provided by this application
note is intended to give a general background of UPS system and present the customer
with the most viable options to consider when selecting a UPS system.
Attached Figures
Figure 1: Schematic Diagram of On-line (“True”) UPS
Figure 2: Schematic Diagram of Off-line (“Standby”) UPS. The solid path shows
operation with steady input power. Dashed line is operation when power from the input
source has been terminated.
References
All schematic diagrams included in the Attached Figures section were obtained from The
PC Guide which can be accessed at http://www.pcguide.com/ref/power/ext/ups/index.htm