外文翻译 - 瓦特和VA之间的差异是一个较大的困惑.doc

1、外文文献及译文 文献、资料题目：Watts and Volt-Amps: Powerful Confusion文献、资料来源：DCSCSchneider-E文献、资料发表（出版）日期：院 （部）： 专 业： 班 级： 姓 名： 学 号： 指导教师： 翻译日期： 外文文献一：Watts and Volt-Amps: Powerful ConfusionExecutive summarywhite papers are now part of the Schneider Electric white paper library produced by Schneider Electrics Dat

2、a Center Science Center DCSCSchneider-E ，Watts and Volt-Amps: Powerful Confusion Schneider Electric Data Center Science Center White Paper 15 Rev 1 2.IntroductionThis note helps explain the differences between watts and VA and explains how the terms are correctly and incorrectly used in specifying p

3、ower protection equipment. Many people are confused about the distinction between the watt (W) and volt-amp (VA) measures for UPS load sizing. Many manufacturers of UPS and load equipment add to this confusion by failing to distinguish between these measures. BackgroundThe power drawn by computing e

4、quipment is expressed in watts or volt-amps (VA). The power in watts is the real power drawn by the equipment. Volt-amps are called the apparent power and are the product of the voltage applied to the equipment times the current drawn by the equipment. Both watt and VA ratings have a use and purpose

5、. The watt rating determines the actual power purchased from the utility company and the heat loading generated by the equipment. The VA rating is used for sizing wiring and circuit breakers. The VA and watt ratings for some types of electrical loads, like incandescent light bulbs, are identical . H

6、owever, for computer equipment the watt and VA ratings can differ significantly, with the VA rating always being equal to or larger than the watt rating. The ratio of the watt to VA rating is called the Power Factor and is expressed either as a number (i.e., 0.7) or a percentage (i.e. 70%).Watts may

7、 not equal VAAll information technology equipment including computers uses an electronic switching power supply. There are two basic types of computer switching power supplies, which are called 1) Power Factor Corrected (PFC) supplies or 2) capacitor input supplies. It is not possible to tell which

8、kind of power supply is used by inspection of the equipment, and this information is not commonly provided in equipment specifications. PFC supplies were introduced in the mid 1990s and have the characteristic that the watt and VA ratings are equal (power factor of 0.99 to 1.0). Capacitor input supp

9、lies have the characteristic that the watt rating is in the range of .55 to .75 times the VA rating (power factor of 0.55 to 0.75).All large computing equipment such as routers, switches, drive arrays, and servers made after about 1996 uses the PFC supply and consequently for this type of equipment

10、the power factor is 1.Personal computers, small hubs, and personal computer accessories typically have capacitor input supplies and consequently for this type of equipment the power factor is less than one, and is ordinarily in the range of 0.65. Larger computer equipment made prior to 1996 also typ

11、ically used this type of power supply and exhibited a power factor less than one.The power rating of the UPSUPS have both maximum watt ratings and maximum VA ratings. Neither the watt nor the VA rating of a UPS may be exceeded.It is a de-facto standard in the industry that the watt rating is approxi

12、mately 60% of the VA rating for small UPS systems, this being the typical power factor of common personal computer loads. In some cases, UPS manufacturers only publish the VA rating of the UPS. For small UPS designed for computer loads, which have only a VA rating, it is appropriate to assume that t

13、he watt rating of the UPS is 60% of the published VA rating.For larger UPS systems, it is becoming common to focus on the watt rating of the UPS, and to have equal watt and VA ratings for the UPS, because the watt and VA ratings of the typical loads are equal. For a further discussion of the issues

14、of power factor of larger systems and data centers, see White Paper 26, Hazards of Harmonics and Neutral Overloads.Examples where a sizing problem can occurExample 1: Consider the case of a typical 1000 VA UPS. The user wants to power a 900 W heater with the UPS. The heater has a watt rating of 900

15、W and a VA rating of 900 VA with a power factor of 1. Although the VA rating of the load is 900 VA, which is within the VA rating of the UPS, the UPS will probably not power this load. That is because the 900 W rating of the load exceeds the watt rating of the UPS, which is most likely 60% of 1000 V

16、A or around 600 W.Example 2: Consider the case of a 1000 VA UPS. The user wants to power a 900 VA file server with the UPS. The file server has a PFC power supply, and so has a watt rating of 900 W and a VA rating of 900 VA. Although the VA rating of the load is 900 VA, which is within the VA rating of the UPS, the UPS will not power this load. That is because the 900