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Watts (W) and Voltammetry (VA): Two concepts that are confusing

2016-10-17

Many people do not know should be used in watt or UPS should be used to express the capacity. Many UPS manufacturers can not tell the difference between these two concepts, even the W and VA two terms equated, which adds to the understanding of the confusion.

Large-capacity UPS capacity is always expressed in VA; small-capacity UPS (less than 1000VA) with W said capacity.

Voltages in the 1KVA - 500KVA UPS with VA instead of W to represent the capacity. W with small capacity UPS may be due to small capacity, UPS users are more familiar with the concept of watt. However, with the VA can more accurately show the degree of matching UPS and load capacity, because the most fundamental decision of the UPS output capacity of the current value (A), so the natural VA that is more appropriate.

The W value is always less than or equal to the VA value

Conversion relationship is as follows:

Watts = VA * Power Factor = Volts * Amps * Power Factor

Typical Volts = 120 or 230V

Amps = load current

Power Factor: The power factor, which is between 0 and 1

The power factor is between 0 and 1, which represents the percentage of useful current (Watts) of the load current.

Only electric heaters and bulbs and other power factor of 1; for other devices, some of the load current cycle in the load, there is no work done. This part of the current is harmonic or reactive current, which is caused by the load characteristics. It is important to understand that VA is greater than W because of this current, and W is considered to be a special case of VA when the power factor is unity.

The Watt value of the computer is 60 to 70% of its VA value,

In fact all of today's modern computer chargers are capacitive, the power factor value of 0.6 - 0.7. The personal machine tends to 0.6, the mainframe tends to 0.7. Recently developed a power factor called a self-correction function of the new power supply, its power factor value of 1. The future is likely to be a wide range of use of this power, but the market is still rare to see this power.

For a computer load, the W value of the UPS is 60 to 70% of its VA value

Since all computers have a power factor of 0.6-0.7, the W value of the UPS for the computer load is 60-70% of the VA value.

Most UPS manufacturers use W for capacity, and in fact they refer to VA

When the UPS manufacturer indicates the rated W value without the indication of the power factor and the VA rating, the user can assume that this is the W value (equal to the VA value) at the power factor of 1 and the manufacturer actually refers to the VA rating of the UPS. The actual load on the computer W value of the marked value of 60 - 70%, so a rating of 100

W can drive a 100W bulb, but can only drive a 65W computer.

Most computer equipment use VA for capacity

Most computer equipment capacity is represented by VA, and some computers have recently begun to use W for capacity (most notably DEC and IBM). But in general with the VA or more. So VA VA with UPS capacity to better reflect the degree of matching and load. APC All UPSs provide both W and VA values. The model of the product includes the VA value, or it can be converted to a W value by multiplying the VA value by 0.65.

The UPS is rated VA to avoid confusion

When a UPS is marked with a W value, its actual W capacity for the computer load is 60--100% (typically 60%) of the marked value and the VA value is a calibrated W value of 100-130%. And when a UPS rated VA value, it is the actual VA load on the computer is equal to the marked value, W value of the marked value of 60 - 70%.

For example:

A standard Compaq386 machine with the following features: NEC Color Display, 120M hard drive, one streaming tape drive, one Ethernet card, and one Logitech

Bus mouse, measured in the 120VAC power supply under its W, current AVA values are as follows:

Total Watts = 230W

The total Amps = 3.04A

AC voltage = 120V

The total V-A = 365 VA

Power factor = 0.63

The computer power factor values at 230 VAC and other configurations are similar (see PC Magazine 9/16/1986)

To learn more about the power factor of non-linear loads, refer to the following:

IEEE GUIDE TO HARMONIC CONTROL AND REACTIVE

COMPESATION OF STATIC

POWER CONVWRTERS (IEEE std 519-1981)

The instiute of Electrcal and Electrcs Engineers, lnc., 345

E47th Street, New York, NY10017

GUIDELINE ON ELECTRICAL POWER FOR ADP INSTALLATIONS (FIP PUB

94 September 21, 1983) U.S. Dept. Of Commerce, National Technical information Service, 5285 Port

Royal Road, Springfield, VA 22161

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