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| An electric generator is a device used to convert mechanical energy into electrical energy. Electric generator |
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| Making Electricity |
| Main Generator and Support Systems |
| Electric motor |
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| Watt (W) |
| About Voltage Converters / Transformers |
| Portable vs. Fixed-In-Place Generators |
| Direct current (DC) or alternating current (AC) |
| How to find correct Voltage Converter? |
| Guide to Standard Uints Kilo Volt Amperes kVA KiloWatts (1000 watts = 1 kW) kW Ampere (Volt-Amperes or Current) I Volts E Power Factor PE Percent Efficiency %EFF Horse Power HP |
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Have you ever wondered why some power ratings are expressed in WATTS, some in AMPERES or AMPS, some in VOLTS, and some in KVA? This page will explain in simple terms the difference between the power ratings and describe when each should be used in your data center and network architecture planning.
A KVA is simply 1,000 volt amps. A volt is electrical pressure while an amp is electrical current. A term called apparent power (the absolute value of complex power, S) is equal to the product of the volts and amps. On the other hand, a Watt (W) is a measurement of real power. Real power is the amount of actual power that can be drawn from a circuit. When the voltage and current of a circuit coincide, the real power is equal to the apparent power. However, as waves of current and voltage coincide less, less real power is transferred, even though the circuit is still carrying current. Differences between real and apparent power, and thus Watts and volt amps, arise because of inefficiencies in electrical transmission. The resulting inefficiency of electrical transmission can be measured and expressed as a ratio called the power factor. The power factor is a ratio (a number from 0 to 1) of real power and apparent power. In the case of a 1.0 power factor, the real power equals the apparent power. In the case of a 0.5 power factor, real power is approximately half that of the apparent power. Deploying systems that have higher power factors result in less electrical loss and can help improve your PUE. Most UPS units will list the average power factor and real-time load capacity of the UPS, in addition to the KVA. Example: You own a 500 KVA UPS unit (apparent power) with a 0.9 power factor. The resulting real power is 450 kilowatts. Some Useful Conversion Factors and Formulas * VA = Voltage x Amps * Watts = Voltage (root-mean-squared) x Amps (root-mean-squared) x Power Factor (PF) (a three-phase circuit would multiply the voltage by the square root of 3 or approximately 1.732) * 1 BTU (British thermal unit) = Watts x 3.413 * 1 BTU = 1,055.053 joules (J) * 1 watt = 3.413 BTU/hour * 1 ton = 200 BTU/minute * 1 ton = 12,000 BTU/hour * 1 ton = 3.517 kilowatts Electrical Unit Conversions The purpose of this document is to provide information, formulas and documentation to take certain electrical values and convert them into other electrical values. The formulas below are known and used universally but we use them here in association with computer, network, telecom and other IT equipment. To Find Watts To Find Volt-Amperes To Find Kilovolt-Amperes To Find Kilowatts To Convert Between kW and kVA TO Find kBTUs from Electrical Values Background It is often necessary to turn voltage, amperage and electrical "nameplate" values from computer, network and telecom equipment into kW, KVA and BTU information that can be used to calculate overall power and HVAC loads for IT spaces. The following describes how to take basic electrical values and convert them into other types of electrical values. * NOTE #1: The informational nameplates on most pieces of computer or network equipment usually display electrical values. These values can be expressed in volts, amperes, kilovolt-amperes, watts or some combination of the foregoing. * NOTE #2: If you are using equipment nameplate information to develop a power and cooling profile for architects and engineers, the total power and cooling values will exceed the actual output of the equipment. Reason: the nameplate value is designed to ensure that the equipment will energize and run safely. Manufacturers build in a "safety factor" when developing their nameplate data. Some nameplates display information that is higher than the equipment will ever need - often up to 20% higher. The result is that, in total, your profile will "over engineer" the power and cooling equipment. Electrical and mechanical engineers may challenge your figures citing that nameplates require more power than necessary. * NOTE #3: Our advice: Develop the power and cooling profile using the nameplate information and the formulas below and use the resultant documentation as your baseline. Reasons: (1) it's the best information available without doing extensive electrical tests on each piece of equipment. Besides, for most projects, you are being asked to predict equipment requirements 3-5 years out when much of the equipment you will need hasn't been invented yet. (2) the engineers will not duplicate your work; they do not know what goes into a data center. They will only challenge the findings if they appear to be to high. If the engineers want to challenge your figures, it's OK but have them do it in writing and let them take full responsibility for any modifications. If you must lower your estimates, do so. But, document everything. There will come a day in 3-5 years when you will need every amp of power you predicted. We've had projects where it was very evident within six months that what we predicted would come true - sometimes even earlier than we estimated. * NOTE #4 If you are designing a very high-density server room where you will have racks and racks (or cabinets and cabinets) of 1U and 2U servers tightly packed, you need to read our article entitled "IT Pros - Don't be Left in the Dust on IT Server Room Design". To Find Watts 1. When Volts and Amperes are Known POWER (WATTS) = VOLTS x AMPERES * We have a small server with a nameplate shows 2.5 amps. Given a normal 120 Volt, 60 hz power source and the ampere reading from equipment, make the following calculation: POWER (WATTS) = 120 * 2.5 ANSWER: 300 WATTS To Find Volt-Amperes (VA) 1. Same as above. VOLT-AMPERES (VA) = VOLTS x AMPERES ANS: 300 VA To Find kilovolt-Amperes (kVA) 1. SINGLE PHASE KILOVOLT-AMPERES (kVA) = VOLTS x AMPERES 1000 Using the previous example: 120 * 2.5 = 300 VA 300 VA / 1000 = .3 kVA 2. 208-240 SINGLE-PHASE (2-POLE SINGLE-PHASE) * Given: We have a Sun server with an amp rating of 4.7 and requiring a 208-240 power source. We'll use 220 volts for our calculations. KILOVOLT-AMPERES (kVA) = VOLTS x AMPERES 1000 220 x 4.7 = 1034 1034 / 1000 = 1.034 kVA 3. THREE-PHASE * Given: We have a large EMC Symmetrix 3930-18/-36 storage system with 192 physical volumes. EMC's website shows a requirement for a 50-amp 208 VAC receptacle. For this calculation, we will use 21 amps. Do not calculate any value for the plug or receptacle. KILOVOLT-AMPERES (kVA) = VOLTS x AMPERES x 1.73 1000 208 x 21 x 1.73 = 7,556.64 7,556.64 / 1000 = 7.556 kVA To Find Kilowatts * Finding Kilowatts is a bit more complicated in that the formula includes a value for the "power factor". The power factor is a nebulous but required value that is different for each electrical device. It involves the efficiency in the use of of the electricity supplied to the system. This factor can vary widely from 60% to 95% and is never published on the equipment nameplate and further, is not often supplied with product information. For purposes of these calculations, we use a power factor of .85. This arbitrary number places a slight inaccuracy into the numbers. Its OK and it gets us very close for the work we need to do. 1. SINGLE PHASE Given: We have a medium-sized Compaq server that draws 6.0 amps. KILOWATT (kW) = VOLTS x AMPERES x POWER FACTOR 1000 120 * 6.0 = 720 VA 720 VA * .85 = 612 612 / 1000 = .612 kW 2. TWO-PHASE * Given: We have a Sun server with an amp rating of 4.7 and requiring a 208-240 power source. We'll use 220 volts for our calculations. KILOWATT (kW) = VOLTS x AMPERES x POWER FACTOR x 2 1000 220 x 4.7 x 2 = 2068 2068 x .85 = 1757.8 1757.8 / 1000 = 1.76 kW 3. THREE-PHASE * Given: We have a large EMC Symmetrix 3930-18/-36 storage system with 192 physical volumes. EMC's website shows a requirement for a 50-amp 208 VAC receptacle. For this calculation, we will use 22 amps. Do not calculate the value of the plug or receptacle. Use the value on nameplate. KILOWATT (kW) = VOLTS x AMPERES x POWER FACTOR x 1.73 1000 208x22x1.73 = 7,916.48 7,916.48 * .85 = 6,729.008 6,729.008/1000=6.729 kW To Convert Between kW and kVA * The only difference between kW and kVA is the power factor. Once again, the power factor, unless known, is an approximation. For purposes of our calculations, we use a power factor of .85. The kVA value is always higher than the value for kW. kW to kVA kW / .85 = SAME VALUE EXPRESSED IN kVA kVA TO kW kVA * .85 = SAME VALUE EXPRESSED IN kW To Find BTUs From Electrical Values * Known and Given: 1 kW = 3413 BTUs (or 3.413 kBTUs) * The above is a generally known value for converting electrical values to BTUs. Many manufacturers publish kW, kVA and BTU in their equipment specifications. Often, dividing the BTU value by 3413 does not equal their published kW value. So much for knowns and givens. Where the information is provided by the manufacturer, use it. Where it is not, use the above formula. 1. What is the difference between kW and kVa? 2. What is a power factor? 3. What is the difference between standby, continuous, and prime power ratings? 4. If I am interested in a generator that is not the voltage I need, can the voltage be changed? 5. What does an Automatic Transfer Switch do? 6. Can a generator I am looking at parallel with one I already own? 7. Can you convert a 60 Hz generator to 50 Hz? 8. How do I determine what size Generator I need? What is the difference between kW and kVa? The primary difference between kW (kilowatt) and kVA (kilovolt-ampere) is the power factor. kW is the unit of real power and kVA is a unit of apparent power (or real power plus re-active power). The power factor, unless it is defined and known, is therefore an approximate value (typically 0.8), and the kVA value will always be higher than the value for kW. In relation to industrial and commercial generators, kW is most commonly used when referring to generators in the United States, and a few other countries that use 60 Hz, while the majority of the rest of the world typically uses kVa as the primary value when referencing generator sets. To expand on it a bit more, the kW rating is essentially the resulting power output a generator can supply based on the horsepower of an engine. kW is figured by the horsepower rating of the engine times .746. For example if you have a 500 horsepower engine it has a kW rating of 373. The kilovolt-amperes (kVa) are the generator end capacity. Generator sets are usually shown with both ratings. To determine the kW and kVa ratio the formula below is used. .8 (pf) x 625 (kVa) = 500 kW What is a power factor? The power factor (pf) is typically defined as the ratio between kilowatts (kW) and kilovolt amps (kVa) that is drawn from an electrical load, as was discussed in the question above in more detail. It is determined by the generators connected load. The pf on the nameplate of a generator relates the kVa to the kW rating (see formula above). Generators with higher power factors more efficiently transfer energy to the connected load, while generators with a lower power factor are not as efficient and result in increased power costs. The standard power factor for a three phase generator is .8. What is the difference between standby, continuous, and prime power ratings? Standby power generators are most often used in emergency situations, such as during a power outage. It is ideal for applications that have another reliable continuous power source like utility power. It’s recommend usage is most often only for the duration of a power outage and regular testing and maintenance. Prime power ratings can be defined as having an “unlimited run time”, or essentially a generator that will be used as a primary power source and not just for standby or backup power. A prime power rated generator can supply power in a situation where there is no utility source, as is often the case in industrial applications like mining or oil & gas operations located in remote areas where the grid is not accessible. Continuous power is similar to prime power but has a base load rating. It can supply power continuously to a constant load, but does not have the ability to handle overload conditions or work as well with variable loads. The main difference between a prime and continuous rating is that prime power gensets are set to have maximum power available at a variable load for an unlimited number of hours, and they generally include a 10% or so overload capability for short durations. If I am interested in a generator that is not the voltage I need, can the voltage be changed? Generator ends are designed to be either reconnectable or non-reconnectable. If a generator is listed as reconnectable the voltage can be changed, consequently if it is non-reconnectable the voltage is not changeable. 12-lead reconnectable generator ends can be changed between three and single phase voltages; however, keep in mind that a voltage change from three phase to single phase will decrease the power out put of the machine. 10 lead reconnectable can converted to three phase voltages but not single phase. What does an Automatic Transfer Switch do? An automatic transfer switch (ATS) transfers power from a standard source, like utility, to emergency power, such as a generator, when the standard source fails. An ATS senses the power interruption on the line and in turn signals the engine panel to start. When the standard source is restored to normal power the ATS transfers power back to the standard source and shuts the generator down. Automatic Transfer Switches are often used in high availability environments such as data centers, manufacturing plans, telecommunication networks and so forth. Can a generator I am looking at parallel with one I already own? Generator sets can be paralleled for either redundancy or capacity requirements. Paralleling generators allows you to electrically join them to combine their power output. Paralleling identical generators will not be problematic but some extensive thought should go into the overall design based on the primary purpose of your system. If you are trying to parallel unlike generators the design and installation can be more complex and you must keep in mind the affects of engine configuration, generator design, and regulator design, just to name a few. For more information and details on paralleling standby generators in critical systems take a look at this informative article. Can you convert a 60 Hz generator to 50 Hz? In general, most commercial generators can be converted from 60 Hz to 50 Hz. The general rule of thumb is 60 Hz machines run at 1800 Rpm and 50 Hz generators run at 1500 Rpm. With most generators changing the frequency will only require turning down the rpm’s of the engine. In some cases, parts may have to be replaced or further modifications made. Larger machines or machines already set at low Rpm are different and should always be evaluated on a case by case basis. We prefer to have our experienced technicians look at each generator in detail in order to determine the feasibility and what all will be required. How do I determine what size Generator I need? Getting a generator that can handle all your power generation needs is one of the most critical aspects of the purchasing decision. Whether you are interested in prime or standby power, if your new generator can't meet your specific requirements then it simply won't be doing anyone any good because it can put undue stress on the unit and even damage some of the devices connected to it. Determining exactly what size of generator to get is often very difficult and involves a number of factors and considerations. To get more detailed information on this subject, please visit our expanded article on Sizing a Generator. What is a Watt? What is 400HZ Electric Power? What does 400HZ mean? Why isn't 400HZ power more common? What is an inverter? How big is an inverter ? Can you use the inverter to eliminate the requirement of a gearbox by reducing the RPM of the motor? Why not use Air Power instead of 400HZ power? How about hydraulic power? How many KW generator or inverter do I need to run a 25HP motor? What is a Watt? Here are some definitions of watts on the Web: * A measure of the amount of work done by a certain amount or amperage of electric current at a certain pressure or voltage. * A watt is a measurement of total electrical power. Volts x amps = watts. * A measure of power or the rate of energy consumption by an electrical device when it is in operation, calculated by multiplying the voltage at which an appliance operates by the current it draws (Watts = Volts X Amperes). * Watts is the measurement of the amount of electrical power drawn by the load. * A measure of electricity. * The power required to maintain one ampere of current at a pressure of one volt when the two components are in phase with each other. - What is 400HZ Electric Power? - What does 400HZ mean? - Why isn't 400HZ power more common? - What is an inverter? - How big is an inverter? - Can you use the inverter to eliminate the requirement of a gearbox by reducing the RPM of the motor? - Why not use Air Power instead of 400HZ power? - How about hydraulic power? - How many KW generator or inverter do I need to run a 25HP motor? Q: What is 400 HZ Electric Power? A: It is the type of electric power that is the standard of the commercial aircraft and aerospace industry because of its lightweight, its high power, and its proven reliability. Every time you fly in commercial airliners, the 400HZ power produced by the alternators on each engine power the overhead lights, air conditioning, heats your food, moves the landing gear up and down, rolls the wing flaps in and out, flushes the toilets, powers the radar, TV screens, radios, etc., etc. It is the primary power on all commercial and military aircraft. Q: What does 400HZ mean? A: The commercial power producers in the United States (the “Edison” Co’s) provide alternating current (A/C) power for home and industry. This power limits the RPM of the fastest induction motor to a maximum of 3,600 RPM. The maximum speed of a 400HZ induction motor is 24,000 RPM, approximately seven times faster than is possible with a 60HZ motor. This higher speed and the use of higher quality wire and lamination steel make it possible to produce motors with 10 times the power for the same weight and same size as a 60HZ motor. Q: Why isn't 400HZ power more common? A: Several Reasons! (1) The only way to get 400HZ power from 1945 – 1970 was with an engine-generator or a motor-generator set. (2) The cost of a 400HZ motor or generator was extremely high because of the aerospace specifications and necessary paper trail. (3) Most people did not recognize that 400HZ power is basically identical in wiring and operation to a 3 phase – 240V Edison Company style power system. (4) Since inverters have become available to convert 60HZ power to 400HZ power, 400HZ is growing much more quickly. Q: What is an inverter? A: An inverter is an electronic device that takes in 50HZ or 60HZ power and rectifies it to D.C. and then chops it back up into a different frequency, which in our case is 400HZ. Q: How big is an inverter? A: Today it’s very small. Our 15 KW inverters in their protective box with extra cooling fans are approximately the size of a large loaf of bread and weigh approximately two pounds per horsepower of output. They also provide ground fault protection, soft start capability, overload protection and variable motor RPM. Q: Can you use the inverter to eliminate the requirement of a gearbox by reducing the RPM of the motor? A: Yes and No! If you reduce the inverter frequency you can slow the motor but you reduce the horsepower of the motor. If you run your motor at 200HZ instead of 400HZ you will reduce its horsepower by half. It is better to use gears and keep full power to work faster. Q: Why not use Air Power instead of 400HZ power? A: (1) It takes a 100 horsepower compressor to make approximately 10 horsepower with an air motor. (2) Lots of line loss when working far away from the compressor. (3) Lots of noise from the motor exhaust. (4) Lots of oil from the exhaust air. (5) You will probably need an operating engineer to start and stop your air compressor if it is a portable unit. Q: How about hydraulic power? A: (1) Better power than air if work is close to power pack. (2) Needs two heavy, oily hoses. (3) Lots of line loss, long hoses suck up power. (4) Machinery gets hot, oily and difficult to handle. (5) Doctors do not like 55 gallons of hot hydraulic oil on the floor of the operating room. Q: How many KW generator or inverter do I need to run a 25HP motor? A: You can run a 25HP motor from a 20KW generator or 20 KW inverter. The generator will be in overload for a few seconds to start a 25HP motor, but it should start unless your power cord is too small or too long, lowering the motor starting voltage. An inverter will start your motor more easily as they have a built in motor starting ramp (soft start). Most three phase induction motors take 3-5 times running amperage to start them without a “soft start”. |