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The Nature Of Reactive Energy
All inductive machines i.e. electromagnetic and devices that operate on AC systems convert electrical energy from the powersystem generators into mechanical work and heat. This energy is measured by kWh meters, and is referred to as active or wattful energy. In order to perform this conversion, magnetic fields have to be established in the machines, and these fields are associated with another form of energy to be supplied from the power system, known as reactive or wattless energy.
The reason for this is that inductive plant cyclically absorbs energy from the system (during the build-up of the magnetic fields) and re-injects that energy into the system (during the collapse of the magnetic fields) twice in every power-frequency cycle.
The effect on generator rotors is to (tend to) slow them during one part of the cycle and to accelerate them during another part of the cycle. The pulsating torque is stricly true only for single-phase alternators. In three-phase alternators the effect is mutually cancelled in the three phases, since, at any instant, the reactive energy supplied on one (or two) phase(s) is equal to the reactive energy being returned on the other two (or one) phase(s) of a balanced system. The nett result is zero average load on the generators, i.e. the reactive current is “wattless”.
An exactly similar phenomenon occurs with shunt capacitive elements in a power system, such as cable capacitance or banks of power capacitors, etc. In this case, energy is stored electrostatically. The cyclic charging and discharging of capacitive plant reacts on the generators of the system in the same manner as that described above for inductive plant, but the current flow to and from capacitive plant is in exact phase opposition to that of the inductive plant. This feature is the basis on which powerfactor improvement schemes depend.
It should be noted that while this “wattless” current (more accurately, the wattless component of a load current) does not draw power from the system, it does cause power losses in transmission and distribution systems by heating the conductors.
In practical power systems, wattless components of load currents are invariably inductive, while the impedances of transmission and distribution systems are predominantly inductively reactive. The combination of inductive current passing through an inductive reactance produces the worst possible conditions of voltage drop (i.e. in direct phase opposition to the system voltage).
Fig. 1 : An electric motor requires active power P and reactive power Q from the power system
For these reasons, viz:
- Transmission power losses and
- Voltage drop
The power-supply authorities reduce the amount of wattless (inductive) current as much as possible. Wattless (capacitive) currents have the reverse effect on voltage levels and produce voltage-rises in power systems.
The power (kW) associated with “active” energy is usually represented by the letter P. The reactive power (kvar) is represented by Q. Inductively-reactive power is conventionally positive (+ Q) while capacitively-reactive power is shown as a negative quantity (- Q). S represents kVA of “apparent” power.
Figure 1 shows that the kVA of apparent power is the vector sum of the kW of active power plus the kvar of reactive power.
Alternating current systems supply two forms of energy:
- Active energy measured in kilowatt hours (kWh) which is converted into mechanical work, heat, light, etc
- Reactive energy, which again takes two forms:
- “Reactive” energy required by inductive circuits (transformers, motors, etc.),
Plant and appliances requiring reactive energy
All AC plant and appliances that include electromagnetic devices, or depend on magnetically-coupled windings, require some degree of reactive current to create magnetic flux. The most common items in this class are transformers and reactors, motors and discharge lamps (i.e. the ballasts of).
The proportion of reactive power (kvar) with respect to active power (kW) when an item of plant is fully loaded varies according to the item concerned being:
- 65-75% for asynchronous motors
- 5-10% for transformers
SOURCE: Schneider Electric
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Cost benefits of AC drives
In addition to their technical advantages, AC drives also provide many cost benefits. In this chapter, these benefits are reviewed, with the costs divided into investment, installation and opera- tional costs.
At the moment there are still plenty of motors sold without variable speed AC drives. This pie chart shows how many motors below 2.2 kW are sold with frequency converters, and how many without. Only 3% of motors in this power range are sold each year with a frequency converter; 97% are sold without an AC drive.
This is astonishing considering what we have seen so far in this guide. Even more so after closer study of the costs of an AC drive compared to conventional control methods. But first let’s review AC drive technology compared to other control methods.
How many motors below 2.2 kW are sold with and without frequency converters
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Technical differences between other systems and AC drives
AC drive technology is completely different from other, simpler control methods. It can be compared, for example, to the dif- ference between a zeppelin and a modern airplane.
We could also compare AC drive technology to the develop- ment from a floppy disk to a CD-ROM. Although it is a simpler information storage method, a floppy disk can only handle a small fraction of the information that a CD-ROM can.
The benefits of both these innovations are generally well known. Similarly, AC drive technology is based on a totally different technology to earlier control methods. In this guide, we have presented the benefits of the AC drive compared to simpler control methods.
Technical differences between other systems and AC drives
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No mechanical control parts needed
To make a proper cost comparison, we need to study the configurations of different control methods. Here we have used pumping as an example. In traditional methods, there is always a mechanical part and an electrical part.
In throttling you need fuses, contactors and reactors on the electrical side and valves on the mechanical side. In On/Off control, the same electrical components are needed, as well as a pressure tank on the mechanical side. The AC drive provides a new solution. No mechanics are needed, because all control is already on the electrical side.
Another benefit, when thinking about cost, is that with an AC drive we can use a regular 3-phase motor, which is much cheaper than the single phase motors used in other control methods. We can still use 220 V single phase supply, when speaking of power below 2.2 kW.
| Conventional methods: | AC drive: |
| • Both electrical and mechanical parts | • All in one |
| • Many electrical parts | • Only one electrical component |
| • Mechanical parts need regular maintenance | • No mechanical parts, no wear and tear |
| • Mechanical control is energy consuming | • Saves energy |
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Factors affecting cost
This list compares the features of conventional control methods with those of the AC drive, as well as their effect on costs. In conventional methods there are both electrical and mechanical components, which usually have to be purchased separately. The costs are usually higher than if everything could be pur- chased at once. Furthermore, mechanical parts wear out quickly. This directly affects maintenance costs and in the long run, maintenance is a very important cost item. In conventional methods there are also many electrical components. The installation cost is at least doubled when there are several different types of components rather than only one.
And last but not least, mechanical control is very energy con- suming, while AC drives practically save energy. This not only helps reduce costs, but also helps minimise environmental impact by reducing emissions from power plants.
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Investment costs: Mechanical and electrical components
Price Comparison For Pumps
In this graph, the investment structure as well as the total price of each pump control method is presented. Only the pump itself is not added to the costs because its price is the same regardless of whether it’s used with an AC drive or valves. In throttling, there are two possibilities depending on whether the pump is used in industrial or domestic use. In an industrial environment there are stricter requirements for valves and this increases costs.
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The motor
As can be seen, the motor is much more expensive for traditional control methods than for the AC drive. This is due to the 3-phase motor used with the AC drive and the single phase motor used in other control methods.
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The AC drive
The AC drive does not need any mechanical parts, which reduc- es costs dramatically. Mechanical parts themselves are almost always less costly than a frequency converter, but electrical parts also need to be added to the total investment cost.
After taking all costs into account, an AC drive is almost always the most economical investment, when compared to differ- ent control methods. Only throttling in domestic use is as low cost as the AC drive. These are not the total costs, however. Together with investment costs we need to look at installation and operational costs.
| Throttling | AC drive | |
| Installation material | 20 USD | 10 USD |
| Installation work | 5h x 65 USD = 325 USD | 1h x 65 USD = 65 USD |
| Commissioning work | 1h x 65 USD = 65 USD | 1h x 65 USD = 65 USD |
| TOTAL: | 410 USD | 140 USD |
| Savings in installation: 270 USD! | ||
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Installation costs: Throttling compared to AC drive
Because throttling is the second lowest investment after the AC drive, we will compare its installation and operating costs to the cost of the AC drive. As mentioned earlier, in throttling there are both electrical and mechanical components. This means twice the amount of installation material is needed.
Installation work is also at least doubled in throttling compared to the AC drive. To install a mechanical valve into a pipe is not that simple and this increases installation time. To have a mechanical valve ready for use usually requires five hours compared to one hour for the AC drive. Multiply this by the hourly rate charged by a skilled installer to get the total installation cost.
The commissioning of a throttling-based system does not usu- ally require more time than commissioning an AC drive based system. One hour is usually the time required in both cases. So now we can summarise the total installation costs. As you can see, the AC drive saves up to USD 270 per installation. So even if the throttling investment costs were lower than the price of a single phase motor (approximately USD 200), the AC drive would pay for itself before it has even worked a second.
| Throttling | AC drive | |
| Power required | 0.75 kW | 0.37 kW |
| Annual energy 4000 hours/year | 3000 kWh | 1500 kWh |
| Annual energy cost with 0.1 USD/kWh | 300 USD | 150 USD |
| Maintenance/year | 40 USD | 5 USD |
| TOTAL COST/YEAR: | 340 USD | 155 USD |
| Savings in installation: 185 USD! | ||
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Operational costs: Maintenance and drive energy
In many surveys and experiments it has been proved that a 50% energy saving is easily achieved with an AC drive. This means that where power requirements with throttling would be 0.75 kW, with the AC drive it would be 0.37 kW. If a pump is used 4000 hours per year, throttling would need 3000 kWh and the AC drive 1500 kWh of energy per year.
To calculate the savings, we need to multiply the energy con- sumption by the energy price, which varies depending on the country. Here USD 0.1 per kWh has been used.
As mentioned earlier, mechanical parts wear a lot and this is why they need regular maintenance. It has been estimated that whereas throttling requires USD 40 per year for service, maintenance costs for an AC drive would be USD 5. In many cases however, there is no maintenance required for a frequency converter.
Therefore, the total savings in operating costs would be USD 185, which is approximately half of the frequency convert- er’s price for this power range. This means that the payback time of the frequency converter is two years. So it is worth considering that instead of yearly service for an old valve it might be more profitable to change the whole system to an AC drive based control. To retrofit an existing throttling system the pay-back time is two years.
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Total cost comparison
Total Savings Over 10 Year - USD 1562
In the above figure, all the costs have been summarised. The usual time for an operational cost calculation for this kind of investment is 10 years. Here the operational costs are rated to the present value with a 10% interest rate.
In the long run, the conventional method will be more than twice as expensive as a frequency converter. Most of the savings with the AC drive come from the operational costs, and especially from the energy savings. It is in the installation that the high- est individual savings can be achieved, and these savings are realised as soon as the drive is installed.
Taking the total cost figure into account, it is very difficult to understand why only 3% of motors sold have a frequency con- verter. In this guide we have tried to present the benefits of the AC drive and why we at ABB think that it is absolutely the best possible way to control your process.
SOURCE: ABB Drives
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The Best Applications For VFDs
The most commonly used motor in building HVAC applications is the three-phase, induction motor, although some smaller applications may use a single-phase induction motor. VFDs can be applied to both.
While VFD controllers can be used with a range of applications, the ones that will produce the most significant benefits are those that require variable speed operation. For example, the flow rate produced by pumps serving building HVAC systems can be matched to the building load by using a VFD to vary the flow rate. Similarly, in systems that require a constant pressure be maintained regardless of the flow rate, such as in domestic hot and cold water systems, a VFD controlled by a pressure setpoint can maintain the pressure over most demand levels.
The majority of commercial and institutional HVAC systems use variable volume fan systems to distribute conditioned air. Most are controlled by a system of variable inlet vanes in the fan system and variable air volume boxes. As the load on the system decreases, the variable air volume boxes close down, increasing the static pressure in the system. The fan’s controller senses this increase and closes down its inlet vanes. While using this type of control system will reduce system fan energy requirements, it is not as efficient or as accurate as a VFD-based system.
Another candidate for VFD use is a variable refrigerant flow systems. Variable refrigerant flow systems connect one or more compressors to a common refrigerant supply system that feeds multiple evaporators. By piping refrigerant instead of using air ducts, the distribution energy requirements are greatly reduced. Because the load on the compressor is constantly changing based on the demand from the evaporators, a VFD can be used to control the operating speed of the compressor to match the load, reducing energy requirements under part-load conditions.
Additional VFD Applications
While the primary benefit of both of these VFD applications is energy savings, VFDs are well suited for use in other applications where energy conservation is of secondary importance. For example, VFDs can provide precise speed or torque control in some commercial applications.
Some specialized applications use dual fans or pumps. VFDs, with their precise speed control, can ensure that the two units are operated at the desired speed and do not end up fighting each other or having one unit carry more than its design load level.
Advances in technology have increased the number of loads that can be driven by the units. Today, units are available with voltage and current ratings that can match the majority of three-phase induction motors found in buildings. With 500 horsepower units or higher available, facility executives have installed them on large capacity centrifugal chillers where very large energy savings can be achieved.
One of the most significant changes that has taken place recently is that with the widespread acceptance of the units and the recognition of the energy and maintenance benefits, manufacturers are including VFD controls as part of their system in a number of applications. For example, manufacturers of centrifugal chillers offer VFD controls as an option on a number of their units. Similarly, manufacturers of domestic water booster pump systems also offer the controls as part of their system, providing users with better control strategies while reducing energy and maintenance costs.
A Few Cautions
When evaluating the installation of a VFD, facility executives should take into consideration a number of factors related to the specifics of the application. For example, most VFDs emit a series of pulses that are rapidly switched. These pulses can be reflected back from the motor terminals into the cable that connects the VFD to the motor. In applications where there is a long run between the motor and the VFD, these reflected pulses can produce voltages that exceed the line voltage, causing stresses in the cable and motor windings that could lead to insulation failure. While this effect is not very significant in motors that operate at 230 volts or less, it is a concern for those that operate at 480 volts or higher. For those applications, minimize the distance between the VFD and the motor, use cabling specifically designed for use with VFDs, and consider installing a filter specifically designed to reduce the impact of the reflected pulses.
Another factor to consider is the impact the VFD may have on the motor’s bearings. The pulses produced by the VFD can generate a voltage differential between the motor shaft and its casing. If this voltage is high enough, it can generate sparks in the bearings that erode their surfaces. This condition can also be avoided by using a cable designed specifically for use with VFDs.
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SOURCE: facilitiesnet
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How Wind Turbines Work
Wind is a form of solar energy. Winds are caused by the uneven heating of the atmosphere by the sun, the irregularities of the earth’s surface, and rotation of the earth. Wind flow patterns are modified by the earth’s terrain, bodies of water, and vegetation. Humans use this wind flow, or motion energy, for many purposes: sailing, flying a kite, and even generating electricity.
The terms wind energy or wind powmegawatts.er describe the process by which the wind is used to generate mechanical power or electricity. Wind turbines convert the kinetic energy in the wind into mechanical power. This mechanical power can be used for specific tasks (such as grinding grain or pumping water) or a generator can convert this mechanical power into electricity.
So how do wind turbines make electricity? Simply stated, a wind turbine works the opposite of a fan. Instead of using electricity to make wind, like a fan, wind turbines use wind to make electricity. The wind turns the blades, which spin a shaft, which connects to a generator and makes electricity. Take a look inside a wind turbine to see the various parts. View the wind turbine animation to see how a wind turbine works.
Wind turbines operate on a simple principle. The energy in the wind turns two or three propeller-like blades around a rotor. The rotor is connected to the main shaft, which spins a generator to create electricity. Wind turbines are mounted on a tower to capture the most energy.
At 100 feet (30 meters) or more above ground, they can take advantage of faster and less turbulent wind.
Wind turbines can be used to produce electricity for a single home or building, or they can be connected to an electricity grid (shown here) for more widespread electricity distribution.
This aerial view of a wind power plant shows how a group of wind turbines can make electricity for the utility grid. The electricity is sent through transmission and distribution lines to homes, businesses, schools, and so on.
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Types of Wind Turbines
Modern wind turbines fall into two basic groups: the horizontal-axis variety, as shown in the photo, and the vertical-axis design, like the eggbeater-style Darrieus model, named after its French inventor.
Horizontal-axis wind turbines typically either have two or three blades. These three-bladed wind turbines are operated “upwind,” with the blades facing into the wind.
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Sizes of Wind Turbines
Utility-scale turbines range in size from 100 kilowatts to as large as several megawatts. Larger turbines are grouped together into wind farms, which provide bulk power to the electrical grid.
Single small turbines, below 100 kilowatts, are used for homes, telecommunications dishes, or water pumping. Small turbines are sometimes used in connection with diesel generators, batteries, and photovoltaic systems.
These systems are called hybrid wind systems and are typically used in remote, off-grid locations, where a connection to the utility grid is not available.
Many wind farms have sprung up in the Midwest in recent years, generating power for utilities. Farmers benefit by receiving land lease payments from wind energy project developers.
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GE Wind Energy's 3.6 megawatt wind turbine is one of the largest prototypes ever erected. Larger wind turbines are more efficient and cost effective.
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Inside the Wind Turbine
Inside the Wind Turbine
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Anemometer: - Measures the wind speed and transmits wind speed data to the controller.
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- Blades:
- Most turbines have either two or three blades. Wind blowing over the blades causes the blades to “lift” and rotate.
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- Brake:
- A disc brake, which can be applied mechanically, electrically, or hydraulically to stop the rotor in emergencies.
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- Controller:
- The controller starts up the machine at wind speeds of about 8 to 16 miles per hour (mph) and shuts off the machine at about 55 mph. Turbines do not operate at wind speeds above about 55 mph because they might be damaged by the high winds.
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- Gear box:
- Gears connect the low-speed shaft to the high-speed shaft and increase the rotational speeds from about 30 to 60 rotations per minute (rpm) to about 1000 to 1800 rpm, the rotational speed required by most generators to produce electricity. The gear box is a costly (and heavy) part of the wind turbine and engineers are exploring “direct-drive” generators that operate at lower rotational speeds and don’t need gear boxes.
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- Generator:
- Usually an off-the-shelf induction generator that produces 60-cycle AC electricity.
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- High-speed shaft:
- Drives the generator.
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- Low-speed shaft:
- The rotor turns the low-speed shaft at about 30 to 60 rotations per minute.
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- Nacelle:
- The nacelle sits atop the tower and contains the gear box, low- and high-speed shafts, generator, controller, and brake. Some nacelles are large enough for a helicopter to land on.
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- Pitch:
- Blades are turned, or pitched, out of the wind to control the rotor speed and keep the rotor from turning in winds that are too high or too low to produce electricity.
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- Rotor:
- The blades and the hub together are called the rotor.
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- Tower:
- Towers are made from tubular steel (shown here), concrete, or steel lattice. Because wind speed increases with height, taller towers enable turbines to capture more energy and generate more electricity.
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- Wind direction:
- This is an “upwind” turbine, so-called because it operates facing into the wind. Other turbines are designed to run “downwind,” facing away from the wind.
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- Wind vane:
- Measures wind direction and communicates with the yaw drive to orient the turbine properly with respect to the wind.
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- Yaw drive:
- Upwind turbines face into the wind; the yaw drive is used to keep the rotor facing into the wind as the wind direction changes. Downwind turbines don’t require a yaw drive, the wind blows the rotor downwind.
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- Yaw motor:
- Powers the yaw drive.
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SOURCE: U.S. Department Of Energy | How Wind Turbines Work
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