ELECTRIC MOTORS

Electric motors, both ac motors and dc motors, come in many shapes and sizes. Some are standardized electric motors for general-purpose applications. Other electric motors are intended for specific tasks. In any case, electric motors should be selected to satisfy the dynamic requirements of the machines on which they are applied without exceeding rated electric motor temperature. Thus, the first and most important step in electric motor selection is determining load characteristics -- torque and speed versus time. Electric motor selection is also based on mission goals, power available, and cost.

Motors are divided into  two types  a.c and d.c

1.     AC Motors

2.     Linear AC Motors

3.     Self-Protecting Starters

4.     Synchronous Motors

5.     Motor Starters

6.     Protecting Motors

7.     Definite and Special-Purpose Motors

8.     Induction Motors

D.C  motors

1.     Coreless DC Motors

2.     Linear DC Motors

3.     Selecting DC Motors

4.     DC Motors

5.     Linear DC Motors

Starting and running torque are the first parameters to consider when sizing electric motors. Starting torque requirements for electric motors can vary from a small percentage of full load to a value several times full-load torque. Starting torque varies because of a change in load conditions or the mechanical nature of the machine, which the electric motor is installed in. The latter could be caused by the lubricant, wear of moving parts, or other reasons.

Electric motors feature torque supplied to the driven machine, which must be more than that required from start to full speed. The greater the electric motor's reserve torque, the more rapid the acceleration.

Electric motor drive systems that use gear reducers have parts that rotate at different speeds. To calculate acceleration torque required for these electric motors, rotating components must be reduced to a common base. The part inertias are usually converted to their equivalent value at the drive shaft. Equivalent inertia W₂K₂² of the load only is found from:

W₂K₂² =(W₁K₁²)(N₁/N₂)²

where W₁K²₁ = load inertia in lb-ft², N₁ = load speed in rpm, and N₂ = electric motor speed in rpm.

Electric motors have bodies, which have a straight-line motion are often connected to rotating driving units by rack-and-pinion, cable, or cam mechanisms. For these electric motor parts, the equivalent WK² is found from:

WK² = W(S/2ΠN)²

where W = load weight, S = translation speed in fpm, Π is pi, and N = rotational speed in rpm.

Electric Motors - Acceleration time: Acceleration time for electric motors is directly proportional to total inertia and inversely proportional to the electric motor torque. For electric motors with constant acceleration torque, acceleration time is: where WK2 = rotational inertia in lb-ft2, (N2 - N1) = the speed difference, and Tx = acceleration torque in lb-ft. For translating bodies, acceleration time is: where W = weight of the load in lb, (S2 - S1) = the translation speed difference, and Fx = translation force in lb. An approximation method is necessary to find the electric motor's acceleration time if acceleration torque is not linear during speed increase. The quickest method is to break up the speed versus torque curves of the electric motor and the driven machine into segments and calculate acceleration time for each segment. Accurate electric motor acceleration times usually result. Electric Motors - Power rating

Electric motors offer the horsepower required to drive a machine, which is typically referred to as electric motor load. The most common equation for power based electric motors on torque and rotational speed is: hp = (torque X rpm)/5,250.

If the electric motor's load is not constant and follows a definite cycle, a horsepower versus time curve for the driven machine is helpful. From this curve both peak and rms the electric motor's horsepower can be determined. Rms load horsepower indicates the necessary continuous electric motor rating. Peak load horsepower is not necessarily an indication of the required electric motor rating. However, when a peak load is maintained for a period of time, electric motors feature a rating, which usually should not be less than peak load horsepower.

Duty cycle - Electric Motors:

Continuous steady-running loads over long periods are demonstrated by fans and blowers. On the other hand, electric motors installed in machines with flywheels may have wide variations in running loads. Often, electric motors use flywheels to supply the energy to do the work, and the electric motor does nothing but restore lost energy to the flywheel. Therefore, choosing the proper electric motor also depends on whether the load is steady, varies, follows a repetitive cycle of variation, or has pulsating torque or shocks.

For example, electric motors that run continuously in fans and blowers for hours or days may be selected on the basis of continuous load. But electric motors located in devices like automatically controlled compressors and pumps start a number of times per hour. And electric motors in some machine tools start and stop many times per minute.

Duty cycle is a fixed repetitive load pattern over a given period of time which is expressed as the ratio of on-time to cycle period. When operating cycle is such that electric motors operate at idle or a reduced load for more than 25% of the time, duty cycle becomes a factor in sizing electric motors. Also, energy required to start electric motors (that is, accelerating the inertia of the electric motor as well as the driven load) is much higher than for steady-state operation, so frequent starting could overheat the electric motor.

For most electric motors (except squirrel-cage electric motors during acceleration and plugging) current is almost directly proportional to developed torque. At constant speed, torque is proportional to horsepower. For accelerating loads and overloads on electric motors that have considerable droop, equivalent horsepower is used as the load factor. The next step in sizing the electric motor is to examine the electric motor's performance curves to see if the electric motor has enough starting torque to overcome machine static friction, to accelerate the load to full running speed, and to handle maximum overload.

lectric Motors - Service factors: A change in NEMA standards for electric motor service factors and temperature rise has been brought about because of better insulation used on electric motors. For instance, a 1.15 service factor -- once standard for all open electric motors -- is no longer standard for electric motors above 200 hp. Increases in electric motor temperature are measured by the resistance method in the temperature rise table. Electric motors feature a nameplate temperature rise, which is always expressed for the maximum allowable load. That is, if the electric motor has a service factor greater than unity, the nameplate temperature rise is expressed for the overload. Two Class-B insulated electric motors having 1.15 and 1.25 service factors will, therefore, each be rated for a 90°C rise. But the second electric motor will have to be larger than the first in order to dissipate the additional heat it generates at 125% load. Electric motors feature a service factor, which indicates how much over the nameplate rating any given electric motor can be driven without overheating. NEMA Standard MGI-143 defines service factor of an ac motor as "...a multiplier which, when applied to the rated horsepower, indicates a permissible horsepower loading which may be carried under the conditions specified for the service factor..." In other words, multiplying the electric motor's nameplate horsepower by the service factor tells how much electric motors can be overloaded without overheating. Generally, electric motor service factors: Handle a known overload, which is occasional. Provide a factor of safety where the environment or service condition is not well defined, especially for general-purpose electric motors. Obtain cooler-than-normal electric motor operation at rated load, thus lengthening insulation life.

Electric Motors - Efficiency: Small universal electric motors have an efficiency of about 30%, while 95% efficiencies are common for three-phase machines. In less-efficient electric motors, the amount of power wasted can be reduced by more careful application and improved electric motor design. Electric motor's feature an efficiency level, which also depends on actual electric motor load versus rated load, being greatest near rated load and falling off rapidly for under and overload conditions.

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working of solar panel

Most of people don't think much about where our electricity comes from, only that it's available and plentiful. Electricity generated by burning fossil fuels such as coal, oil and natural gas, emits carbon dioxide, nitrogen oxides and sulfur oxides -- gases scientists believe contribute to climate change. Solar thermal (heat) energy is a carbon-free, renewable alternative to the power we generate with fossil fuels like coal and gas. This isn't a thing of the future, either. Between 1984 and 1991, the United States built nine such plants in California's Mojave Desert, and today they continue to provide a combined capacity of 354 megawatts annually, power used in 500,000 Californian homes. Reliable power, at that. In 2008 when six days of peak demand buckled the power grid and brought electricity outages in California, those solar thermal plants continued to produce at 110 percent capacity.

Wondering where the technology's been since then? In the 1990s when prices of natural gas dropped, so did interest in solar thermal power. Today, though, the technology is poised for a comeback. It's estimated by the U.S. National Renewable Energy Laboratories that solar thermal power could provide hundreds of gigawatts of electricity, equal to more than 10 percent of demand in the United States.Shake the image of solar panels from your head -- that kind of demand is going to require power plants. There are two main ways of generating energy from the sun. Photovoltaic (PV) and concentrating solar thermal (CST), also known as concentrating solar power (CSP) technologies.

PV converts sunlight directly into electricity. These solar cells are usually found powering devices such as watches, sunglasses and backpacks, as well as providing power in remote areas.

Solar thermal technology is large-scale by comparison. One big difference from PV is that solar thermal power plants generate electricity indirectly. Heat from the sun's rays is collected and used to heat a fluid. The steam produced from the heated fluid powers a generator that produces electricity. It's similar to the way fossil fuel-burning power plants work except the steam is produced by the collected heat rather than from the combustion of fossil fuel

Solar Thermal Systems

Parabolic troughs, like these used in Colorado, concentrate the sun's energy to great temperatures. There are two types of solar thermal systems: passive and active. A passive system requires no equipment, like when heat builds up inside your car when it's left parked in the sun. An active system requires some way to absorb and collect solar radiation and then store it.

Solar thermal power plants are active systems, and while there are a few types, there are a few basic similarities: Mirrors reflect and concentrate sunlight, and receivers collect that solar energy and convert it into heat energy. A generator can then be used to produce electricity from this heat energy.

The most common type of solar thermal power plants, including those plants in California's Mojave Desert, use a parabolic trough design to collect the sun's radiation. These collectors are known as linear concentrator systems, and the largest are able to generate 80 megawatts of electricity. They are shaped like a half-pipe you'd see used for snowboarding or skateboarding, and have linear, parabolic-shaped reflectors covered with more than 900,000 mirrors that are north-south aligned and able to pivot to follow the sun as it moves east to west during the day. Because of its shape, this type of plant can reach operating temperatures of about 750 degrees F (400 degrees C), concentrating the sun's rays at 30 to 100 times their normal intensity onto heat-transfer-fluid or water/steam filled pipes. The hot fluid is used to produce steam, and the steam then spins a turbine that powers a generator to make electricity.

While parabolic trough designs can run at full power as solar energy plants, they're more often used as a solar and fossil fuel hybrid, adding fossil fuel capability as backup.

Solar power tower systems are another type of solar thermal system. Power towers rely on thousands of heliostats, which are large, flat sun-tracking mirrors, to focus and concentrate the sun's radiation onto a single tower-mounted receiver. Like parabolic troughs, heat-transfer fluid or water/steam is heated in the receiver (power towers, though, are able to concentrate the sun's energy as much as 1,500 times), eventually converted to steam and used to produce electricity with a turbine and generator.

Power tower designs are still in development but could one day be realized as grid-connected power plants producing about 200 megawatts of electricity per tower.A third system is the solar dish/engine. Compared to the parabolic trough and power towers, dish systems are small producers (about 3 to 25 kilowatts). There are two main components: the solar concentrator (the dish) and the power conversion unit (the engine/generator). The dish is pointed at and tracks the sun and collects solar energy; it's able to concentrate that energy by about 2,000 times. A thermal receiver, a series of tubes filled with a cooling fluid (such as hydrogen or helium), sits between the dish and the engine. It absorbs the concentrated solar energy from the dish, converts it to heat and sends that heat to the engine where it becomes electricity.

Solar Thermal Heat

Solar thermal systems are a promising renewable energy solution -- the sun is an abundant resource. Except when it's nighttime. Or when the sun is blocked by cloud cover. Thermal energy storage (TES)systems are high-pressure liquid storage tanks used along with a solar thermal system to allow plants to bank several hours of potential electricity. Off-peak storage is a critical component to the effectiveness of solar thermal power plants.

Three primary TES technologies have been tested since the 1980s when the first solar thermal power plants were constructed: a two-tank direct system, a two-tank indirect system and a single-tank thermocline system.

In a two-tank direct system, solar thermal energy is stored right in the same heat-transfer fluid that collected it. The fluid is divided into two tanks, one tank storing it at a low temperature and the other at a high temperature. Fluid stored in the low temperature tank runs through the power plant's solar collector where it's reheated and sent to the high temperature tank. Fluid stored at a high temperature is sent through a heat exchanger that produces steam, which is then used to produce electricity in the generator. And once it's been through the heat exchanger, the fluid then returns to the low temperature tank.

A two-tank indirect system functions basically the same as the direct system except it works with different types of heat-transfer fluids, usually those that are expensive or not intended for use as storage fluid. To overcome this, indirect systems pass low temperature fluids through an additional heat exchanger.Unlike the two-tank systems, the single-tank thermocline system stores thermal energy as a solid, usually silica sand. Inside the single tank, parts of the solid are kept at low to high temperatures, in a temperature gradient, depending on the flow of fluid. For storage purposes, hot heat-transfer fluid flows into the top of the tank and cools as it travels downward, exiting as a low temperature liquid. To generate steam and produce electricity, the process is reversed.

Solar thermal systems that use mineral oil or molten salt as the heat-transfer medium are prime for TES, but unfortunately without further research, systems that run on water/steam aren't able to store thermal energy. Other advancements in heat-transfer fluids include research into alternative fluids, using phase-change materials and novel thermal storage concepts all in an effort to reduce storage costs and improve performance and efficiency.

Solar Thermal Greenhouses

The idea of using thermal mass materials -- materials that have the capacity to store heat -- to store solar energy is applicable to more than just large-scale solar thermal power plants and storage facilities. The idea can work in something as commonplace as a greenhouse.

All greenhouses trap solar energy during the day, usually with the benefit of south-facing placement and a sloping roof to maximize sun exposure. But once the sun goes down, what's a grower to do? Solar thermal greenhouses are able to retain that thermal heat and use it to warm the greenhouse at night.

Stones, cement and water or water-filled barrels can all be used as simple, passive thermal mass materials (heat sinks), capturing the sun's heat during the day and radiating it back at night.

Apply the same ideas used in solar thermal power plants (although on a much smaller level) and you're on your way to year-round growing. Solar thermal greenhouses, also called active solar greenhouses, require the same basics as any other solar thermal system: a solar collector, a water storage tank, tubing or piping (buried in the floor), a pump to move the heat-transfer medium (air or water) in the solar collector to storage and electricity (or another power source) to power the pump.

In one scenario, air that collects in the peak of the greenhouse roof is drawn down through pipes and under the floor. During the day, this air is hot and warms the ground. At night, cool air is drawn down into the pipes. The warm ground heats the cool air, which in turn heats the greenhouse. Alternatively, water is sometimes used as the heat-transfer medium. Water is collected and solar heated in an external storage tank and then pumped through the pipes to warm the greenhouse.

Solar Thermal Chimneys

Solar thermal power has great potential because the technology is already all there.Just as solar thermal greenhouses are a way to apply solar thermal technologies to an everyday need, solar thermal chimneys, or thermal chimneys, also capitalize on thermal mass materials. Thermal chimneys are passive solar ventilation systems, which means they are nonmechanical. Examples of mechanical ventilation include whole-house ventilation that uses fans and ducts to exhaust stale air and supply fresh air. Through convective cooling principles, thermal chimneys allow cool air in while pushing hot air from the inside out. Designed based on the fact that hot air rises, they reduce unwanted heat during the day and exchange interior (warm) air for exterior (cool) air.

Thermal chimneys are typically made of a black, hollow thermal mass with an opening at the top for hot air to exhaust. Inlet openings are smaller than exhaust outlets and are placed at low to medium height in a room. When hot air rises, it escapes through the exterior exhaust outlet, either to the outside or into an open stairwell or atria. As this happens, an updraft pulls cool air in through the inlets.

In the face of global warming, rising fuel costs and an ever-growing demand for energy, energy needs are expected to increase by nearly the equivalent of 335 million barrels of oil per day, mostly for electricity. Whether big or small, on or off the grid, one of the great things about solar thermal power is that it exists right now, no waiting. By concentrating solar energy with reflective materials and converting it into electricity, modern solar thermal power plants, if adopted today as an indispensable part of energy generation, may be capable of sourcing electricity to more than 100 million people in the next 20 years. All from one big renewable resource: the sun.