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| An '''induction generator''' or '''asynchronous generator''' is a type of AC [[electrical generator]] that uses the principles of [[induction motor]]s to produce power. Induction generators operate by mechanically turning their rotor faster than the synchronous speed, giving negative slip. A regular AC asynchronous motor usually can be used as a generator, without any internal modifications. Induction generators are useful in applications such as [[minihydro]] power plants, wind turbines, or in reducing high-pressure gas streams to lower pressure, because they can recover energy with relatively simple controls.
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| An induction generator must be excited with a leading voltage. This is usually done by connection to an electrical grid; sometimes, however, they are self-excited by using phase correcting capacitors.
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| Induction generators cannot usually "[[black start]]" a de-energized distribution system.
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| ==Principle of operation==
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| Induction generators and motors produce electrical power when their rotor is turned faster than the ''synchronous speed''. For a typical four-pole motor (two pairs of poles on stator) operating on a 60 Hz electrical grid, the synchronous speed is 1800 rotations per minute. The same four-pole motor operating on a 50 Hz grid will have a synchronous speed of 1500 RPM.
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| In normal motor operation, stator flux rotation is faster than the rotor rotation. This causes the stator flux to induce rotor currents, which create a rotor flux with magnetic polarity opposite to stator. In this way, the rotor is dragged along behind stator flux, at a value equal to the slip.
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| In generator operation, a [[Engine|prime mover]] (turbine, engine) drives the rotor above the synchronous speed. The stator flux still induces currents in the rotor, but since the opposing rotor flux is now cutting the stator coils, an active current is produced in stator coils and the motor now operates as a generator, sending power back to the electrical grid.
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| An induction machine requires externally supplied a.c. armature current; it cannot start on its own as a generator. Because the rotor field always lags behind the stator field, the induction machine always "consumes" reactive power, regardless of whether it is operating as a generator or a motor.
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| ==Excitation==
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| [[File:Asynch-esb-2.svg|thumb|right|Equivalent circuit of induction generator]]
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| Note that a source of excitation current for magnetizing flux (reactive power) for the stator is still required, to induce rotor current.
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| This can be supplied from the electrical grid or, once it starts producing power, from the generator itself. The rotating magnetic flux from the [[stator]] induces currents in the rotor, which also produces a [[magnetic field]]. If the rotor turns slower than the rate of the rotating flux, the machine acts like an induction motor. If the rotor is turned faster, it acts like a generator, producing power.
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| ==Active power==
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| Active power delivered to the line is proportional to slip above the synchronous speed. Full rated power of the generator is reached at very small slip values (motor dependent, typically 3%). At synchronous speed of 1800 rpm, generator will produce no power. When the driving speed is increased to 1860 rpm, full output power is produced. If the prime mover is unable to produce enough power to fully drive the generator, speed will remain somewhere between 1800 and 1860 rpm range.
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| ==Required capacitance==
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| A capacitor bank must supply reactive power to the motor when used in stand-alone mode.The reactive power supplied should be equal or greater than the reactive power that the machine normally draws when operating as a motor. Terminal voltage will increase with capacitance, but is limited by iron saturation.
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| ==Grid and stand-alone connections==
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| [[File:Kondensatorerregter-asynchrongenerator-mit-last.svg|thumb|right|Typical connections when used as a standalone generator]]
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| In induction generators, the reactive power required to established the air gap magnetic flux is provided by [[Power factor correction unit|capacitor bank]] connected to the machine in case of stand alone system and in case of grid connection it draws reactive power from the grid to maintain its air gap flux.
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| For a grid connected system, frequency and voltage at the machine will be dictated by the electric grid, since it is very small compared to the whole system.
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| For stand-alone systems, frequency and voltage are complex function of machine parameters, capacitance used for excitation, and load value and type.
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| [[File:DFIG in Wind Turbine.svg|thumbnail|Induction generator in grid connected operation]]
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| ==Use of induction generators==
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| Induction generators are often used in [[wind turbines]] and some [[micro hydro]] installations due to their ability to produce useful power at varying rotor speeds. Induction generators are mechanically and electrically simpler than other generator types. They are also more rugged, requiring no brushes or [[Commutator (electric)|commutators]].
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| Induction generators are particularly suitable and usually used for wind generating stations as in this case speed is always a variable factor, and the generator is easy on the gearbox. Unlike synchronous motors, induction (asynchronous) motors are load-dependent and cannot be used for load frequency control. Induction motors are not in sync with the grid frequency.
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| ==Torque vs. Slip==
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| The basic fundamental of induction generators is the conversion between mechanical energy to electrical energy. In addition, a negative slip value is required for the induction motor to act as a generator. The slip is negative as this requires a higher shaft rotating speed than the synchronous speed. To receive a greater a shaft speed, An external torque is required to be applied to the rotor, which generates electric power. However, infinite torque doesn't lead to an infinite amount of power generation. The rotating magnetic field torque excited from the armature works to counter the motion of the rotor and prevent overspeed because of induced motion in the opposite direction. As the speed of the motor increases the counter torque reaches a max value of torque (breakdown torque) that it can operate until before the operating conditions become unstable. Ideally, induction generators work best in the stable region between the no-load condition and maximum torque region.
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| ==Example application==
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| We must use 10 hp, 1760 r/min, 440 V, 3 phase induction motor as an asynchronous generator. Full-load current of the motor is 10 A and full-load power factor is 0.8.
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| Required capacitance per phase if capacitors are connected in delta:
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| :[[Apparent power]] S = √3 E I = 1.73 * 440 * 10 = 7612 VA
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| :Active power P = S cos θ = 7612 * 0.8 = 6090 W
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| :[[Reactive power]] Q = <math>\sqrt{S^2-P^2}</math> = 4567 VAR
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| For machine to run as an asynchronous generator, capacitor bank must supply minimum 4567 / 3 phases = 1523 VAR per phase. Voltage per capacitor is 440 V because capacitors are connected in delta.
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| :Capacitive current Ic = Q/E = 1523/440 = 3.46 A
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| :Capacitive reactance per phase Xc = E/I = 127 Ω
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| Minimum capacitance per phase:
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| :C = 1 / (2*π*f*Xc) = 1 / (2 * 3.141 * 60 * 127) = 21 microfarads.
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| If load also absorbs reactive power, capacitor bank must be increased in size to compensate.
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| Prime mover speed should be used to generate frequency of 60 Hz:
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| Typically, slip should be similar to full-load value when machine is running as motor, but negative (generator operation):
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| :if Ns= 1800 we can choose N=Ns+40 rpm
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| :Required prime mover speed N = 1800 + 40 = 1840 rpm.
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| ==See also==
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| *[[Electrical generator]]
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| ==References==
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| *Electrical Machines, Drives, and Power Systems, 4th edition, Theodore Wildi, Prentice Hall, ISBN 0-13-082460-7, pages 311-314.
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| ==External links==
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| *[http://www.educypedia.be/electronics/generatorasync.htm Collection of papers about asynchronous generators at educypedia.be] {{dead link|date=October 2012}}
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| *[http://www.arthropodsystems.com/AsynchronousGenerator1/AsynchronousGenerator1.html Testing of stand-alone and grid connected asynchronous generator]
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| *[http://www.philica.com/display_article.php?article_id=316 Electronic approaches to direct drive an induction generator; without mechanical gearbox]
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| {{Electricity generation}}
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| [[Category:Electrical generators]]
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I would like to introduce myself to you, I am Andrew and my spouse doesn't like it at all. What me and my family members love is doing ballet but I've been taking on new things lately. My wife and I live in Kentucky. Distributing production is how he makes a living.
My page - real psychic [Cpacs.org]