What
is the difference between three phase and single phase power?
What is a UPS?
UPS stands for uninterruptible power supply. A UPS is
a back-up power system used to prevent power loss or damage
to a computer or other critical piece of electrical equipment.
Top of page
Why do I need a UPS?
There are many answers to that question.
A good UPS system can help prevent down time, by providing
clean power to critical systems when there is a power
anomaly.
A UPS can also protect equipment and help it last longer
by acting as a power conditioner, filtering out surges,
sages, spikes and outages.
A UPS increases the productivity of computer users by
keeping their computers functioning more reliably and
preventing work loss.
A Liebert UPS can provide back up power in a remote or
powerless situation with the dark start capability, utilizing
the batteries as a power source.
Top of page
What kind of UPS do I need?
The type of UPS needed depends on how critical the system
is which needs protection. There are three main types
of UPS: off-line or standby, line interactive, and on-line
(double conversion).
1. An off-line or standby UPS is used as an alternative
source of power when there has been a sudden loss of utility
power. An off-line UPS generally provides a few minutes
of back-up power to ride through the outage or in the
event of an extended outage, allow a gradual shutdown
of the connected equipment. An off-line UPS has few power-conditioning
features, and will rely on battery power whenever utility
power is unacceptable for the critical load. An off-line
UPS is best for desktop computers, point-of-sale terminals
or other applications that need some power protection
but are not mission critical. An off-line UPS uses a voltage-sensing
switch to activate the DC rectifier and draw power from
the batteries when incoming power is not ideal. A standby
UPS does little or no power conditioning and primarily
acts as a switch to draw power from the batteries. Some
surge in output voltage occurs when it switches from utility
power to battery power. An off-line UPS has no user replaceable
batteries and lasts 3 years, on average.
2. A line-interactive UPS is useful for large desktop
systems or a set of rack-mounted computers with up to
2200VA of power. While similar in functional layout to
the off-line UPS, line-interactive technology also includes
a buck and boost capability. This feature compensates
for power surges and sags of +/- 25% of the normal incoming
voltage without using batteries to regulate the voltage.
A greatly reduced battery duty cycle means that the batteries
in the UPS will last longer than in the off-line style
and that the connected equipment has a greater degree
of protection because the back-up power will be more reliable.
A line-interactive UPS has about 5 minutes of back-up
time when fully loaded, which is enough to ride out 90%
of power anomalies and for extended outages, can shut
down connected equipment gradually to prevent equipment
damage. A line interactive UPS uses a voltage sensing
switch to draw power from the batteries when incoming
power is outside of the buck and boost input voltage range.
A minimal power loss and a surge in the outgoing power
occurs at the time of transfer to the batteries. The PowerSure
Interactive series of line-interactive UPS have user replaceable
batteries and last 3 years, on average.
3. An on-line UPS is the third and best type of UPS.
True on-line UPS systems are only those that employ double
conversion topology. On-line delta conversion or ferroressonant
systems are not on-line UPS systems. Those types of UPS
are line-interactive. Liebert on-line UPS systems range
from 700 VA to 1,100 KVA in size. An on-line UPS is designed
for use with mission critical systems that cannot go down
without causing significant work and or financial loss.
The typical applications for an on-line UPS are manufacturing
equipment, telecommunications systems, Internet service
providers, financial networks, data centers and other
critical networks or systems. An on-line UPS is constantly
active, hence the name. On-line UPS systems convert all
incoming power to DC, allows that DC power to pass across
the battery circuit, called the DC bus, and then converts
the power back into AC power and out to the protected
equipment. The critical load is electrically isolated
from utility power and receives continued highly regulated
power. There is no output power disturbance or gap when
there is a power loss or anomaly in the incoming power
since the batteries are always connected to the DC bus
from which the inverter draws its current. An on-line
UPS can also accept as low as 50% of the normal incoming
voltage without drawing power from the batteries. This
allows the battery systems to have a longer run time and
require less maintenance. In single phase UPS, on-line
UPS batteries are all hot swappable and will last five
years, on average.
Top of page
What is a VA?
VA is a unit of measurement for power, similar to a watt
or horsepower. VA stands for volt-amps. VA is used to
signify the total power requirements of a piece of equipment.
However, it is generally preferred to use Watts to measure
power, since VA can be relative to the type of equipment
and Watts is not. VA is the apparent power that is being
used, whereas Watts is the actual power being used. Without
getting too complicated, the difference between Watts
and VA is the power factor. VA = Volts * Amps, Watts =
Volts * Amps * Power Factor.
Top of page
What is Power Factor?
Power factor is a measurement of how the incoming power
is being used by a piece of equipment. Most computer equipment
has a power factor of 0.7. This basically means that the
equipment is using 70% of the incoming VA. For example,
a computer designed to use 100 VA at a 0.7 PF is using
70 Watts. Liebert UPS systems are designed to output power
with a 0.7 power factor so that the connected equipment
does not waste power by not using it. Some equipment is
power factor corrected, meaning its power factor is very
close to 1.0. This type of equipment is more efficient
because it is utilizing more of the incoming power and
for all practical purposes the VA rating is the same as
the Watts. All of Liebert's on-line UPS systems are power
factor corrected to work with any type of equipment and
to be as efficient as possible.
Top of page
What is the difference between
a surge suppressor and a UPS?
The biggest difference is that a UPS has battery backup
power, and a surge suppressor or TVSS device does not.
However a UPS is much more than a surge protector with
batteries. A surge protector is designed to protect a
sensitive electrical appliance from being destroyed by
power surges and spikes. A UPS has this capability, but
also conditions power so that the connected equipment
always receives an acceptable voltage. A surge protector
only protects the equipment from extreme voltage spikes
and surges. A UPS can also protect equipment from brown
outs, frequency variation, waveform distortion, outages,
as well as voltage spikes and surges, and can shut down
a computer safely in an extended blackout.
Top of page
What size of UPS do I need?
The best way to determine what size of UPS is needed is
to measure the amount of amps that are being used by the
equipment to be protected. Look on the back of the equipment
near the input power cord for a power rating plate. On
this plate or label will typically be a maximum amp draw
for one or more voltages. For example: a computer may
be rated at 8/4 Amps and 110/220 Volts. This computer
uses either 4 amps at 220 volts or 8 amps at 110 volts.
4 * 220 = 880 VA, and 8 * 110 = 880 VA, so this computer
uses a maximum of 880 VA.
Another consideration is the potential for future expansion.
Top of page
How much back-up time do I need?
Back-up time is entirely dependant upon the application.
If there is an alternative power source, such as a generator,
to supply back-up power, a few minutes is probably fine,
since a well maintained generator will come on in 30 seconds
or less. More critical systems may need protection, to
protect against the possibility that the alternative power
source fails or isn't immediately available. If no alternative
power source is available, back up times can be extended
with additional batteries. However, batteries are expensive,
heavy, and large, so be reasonable with expectations,
and know that most power anomalies are over in five minutes
are less.
Top of page
What is the difference between
three phase and single phase power?
Single-phase power is what most computers and telecommunication
systems run on, as well as most things in your home or
office. Single phase refers to the fact that one sine
wave of voltage and current is being supplied and used.
Three-phase power is used when large amounts of power
are required for industrial systems, large electric motors
and industrial equipment or a facility wide UPS. Three-phase
refers to the fact that three offset synchronous sine
waves of current and voltage are being used to obtain
approximately 1.7 times as much power each of the individual
phases.
Top of page
Power - Frequently Asked Questions