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					UPS - How To Rate A UPS?

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Summary:
The UPS is rated in kVA or for smaller models VA, this is basically the
product of the UPS rate output voltage and current. However due to the
nature of the electrically complex load that uninterruptible power
supplies are required to support manufacturers often derate the UPS. It
is often stated for example that a 20kVA UPS has a power factor of 0.8,
i.e. 20kVA at 0.8PF, this will give the true output rating in kW and we
have the following equation for a typical 20kVA 3 phas...


Keywords:
uninterruptible power supplies,diesel   generators, load bank


Article Body:
The UPS is rated in kVA or for smaller models VA, this is basically the
product of the UPS rate output voltage and current. However due to the
nature of the electrically complex load that uninterruptible power
supplies are required to support manufacturers often derate the UPS. It
is often stated for example that a 20kVA UPS has a power factor of 0.8,
i.e. 20kVA at 0.8PF, this will give the true output rating in kW and we
have the following equation for a typical 20kVA 3 phase input, 3 phase
output UPS: -

20000 x 0.8 (kW) /3 (power per phase) / 230 (output voltage) = output
current per phase which in this case would be 23A maximum.

Uninterruptible power supplies with ratings of 10kVA and below the output
power factor is often stated as being 0.7. Therefore a 10kVA single phase
input; single phase output UPS at 0.7PF would have a maximum output
current capacity of 30A. Manufacturers have many different ways of
calculating there power factor rating, some state 0.9, 0.95,0.99 etc.
only by carefully comparing their methodology can the true factor be
calculated. The figures themselves shouldn't be taken on face value
without further investigation.

General

During normal operation the UPS system will provide a 'clean supply' to
its load; 'clean supply' can be clarified by describing the operation of
the UPS. The utility mains supply from the local electricity supplier can
be distorted by the load that it is experiencing, e.g. if there are a
number of motors be stopped and started due to a manufacturing process
spikes of extremely high voltages can be produced. These can have an
adverse effect on delicate computer equipment.

Also as large numbers of the computer equipment are made overseas these
days the power supplies associated with these equipments may not be
designed to accept the voltage ranges experienced in this country.
Typically voltages can be as high as 260V, 230V being the nominal supply,
this is especially prevalent during the summer months where there can be
a high demand for air conditioning and the local sub station transformer
taps are raised to cope with demand, then, when the demand is lower
during the night the supply voltage will increase.

Conventional UPS systems operate in the following way and can be broken
down into a number of building blocks for clarity of description.

Mains Input

With UPS systems greater than 6kVA there is normally provision for two
separate utility mains inputs, for maximum security they should be
separately protected by fuses or circuit breakers. One input is used for
the rectifier/charger and hence the inverter, the other is used for the
reserve supply, which the load will be switched to in the case of
controlled switching or in the event of an inverter failure or an
overload condition.

Should you require a Diesel Generator as standby power protection then
this needs to be rated at least twice the size of the UPS System.

PowerContinuity Means Business Continuity

For further information regarding uninterruptible power supplies please
visit our website at http://www.powercontinuity.co.uk

				
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posted:3/2/2010
language:English
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