Induction Motor Braking and Speed Control Methods

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Induction Motor Braking


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Braking Methods

Regenerative BrakingPlugging or reverse voltage braking

Dynamic ( or rheostatic ) braking :a) ac dynamic brakingb) Self-excited braking using capacitor

c) dc dynamic brakingd) zero-sequence braking


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Regenerative Braking

If an induction motor is forced to run at speeds inexcess of the synchronous speed , the loadtorque exceeds the machine torque and the slip

is negative , reversing the rotor induced EMF androtor current. In this situation the machine willact as a generator with energy being returned tothe supply.

If the AC supply voltage to the stator excitation issimply removed, no generation is possiblebecause there can be no induced current in therotor.


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Regenerative braking

In traction applications, regenerative braking isnot possible below synchronous speed in amachine fed with a fixed frequency supply . If

however the motor is fed by a variable frequencyinverter then regenerative braking is possible byreducing the supply frequency so that thesynchronous speed becomes less than themotor speed.AC motors can be microprocessor controlled toa fine degree and can regenerate current downto almost a stop


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Regenerative brakingapplied to overhauling loads;it can only be used to reduce the speed to the no-load or synchronous value.

If you overspeed the motor, the motor is returning thepower back to its power supply. When you use aninverter that has a variable frequency capability, you turnthe frequency down and your motor is a generator,returning its power to the controller, until your newreduced speed is reached. Or you can have a over running load, like when you drive your AC motor powered tractor down a hill, and your motor is agenerator.


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Regenerative brakingproblem in industry: Most AC inverters cannot return theexcess power to the power line. There is a diode bridgethat takes AC power and charges a capacitor bank. (There are a very few specialized AC invertersthat use a bi-directional bridge of transistors to chargethe capacitors or return power to the AC line.) The

capacitor bank is connected to the motor via a bridge of transistors or gate turn off SCRs. This bridge is bi-directional. So whenever the motor is a generator, thecapacitor bank is charged up and potentiallyovercharged. Some drives turn off to protect thecontroller. Some drives switch on a resistor to dump theexcess energy. Here we are not really regenerative,

If you were to replace the capacitor bank withbatteries, like some electric cars, you now have theability to absorb a lot of energy from the motor, and youcan easily be regenerative over the entire speed rangeof the motor.


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Regenerative braking

Advantage : Generated power is usefullyemployed

Disadvantage : It can not be employedbelow synchronous speed when fed fromconstant frequency source.Speed Range : Between synchronousspeed and the speed for which brakingtorque is maximum.


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PluggingThe phase sequence of the supply (in the case of 3-phase) is reversed by interchanging two leads. Theusual arrangement for squirrel-cage motors is by areverse contactor which closes when the stop button isoperated. The supply to the motor must be disconnectedas the speed nears zero. This is effected by having areverse-rotation relay coupled to the motor shaft andconnected to open the braking-contactor coil before themotor reverses. The motor takes heavy current from the

supply unless resistance is added to the stator circuit.Rotor resistance may be employed to ensure that a highbraking torque is obtained.


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PluggingInduction motors can be brought rapidly to astop (and / or reversed) by reversing one pair of leads which has the effect of reversing therotating wave. This is known as "plugging".

The motor can also be stopped quickly bycutting the AC supply and feeding the stator windings instead with a DC (zero frequency)

supply. With both of these methods, energy isnot returned to the supply but is dissipated asheat in the motor. These techniques are knownas dynamic braking.


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Dynamic braking

Dynamic braking, obtained bydisconnecting the machine from the mainsand establishing a fixed magnetic fieldfrom a DC supply which causes e.m.f.s. tobe induced in the rotor windings. Powerfuldynamic braking may be obtained by

introducing capacitors into an induction-motor circuit, with or without DC injection.


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Direct current is injected into the stator windingafter this winding has been disconnected fromthe supply. This sets up a stationary field,inducing e.m.f.s. and hence currents in the rotor circuits. This method provides a high-brakingtorque with low losses, but the braking effect issmall at high speeds for machines of normal slipand it is usually necessary to use heavy DCexciting currents to effect a quick-stop. Someimprovement of the braking characteristic can bemade by increasing the rotor-circuit resistance athigh speeds and reducing it as the speed falls.


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Capacitor dynamic braking uses the ability of theinduction motor to self-excite if sufficientcapacitance is connected across its terminalswhen the supply is removed. The motor thenruns as an induction generator and dissipatespower with subsequent braking effect.If the motor terminals are then short-circuited,magnetic braking follows. A typical basicconnection diagram is shown in Figure .Capacitor braking followed by simultaneousmagnetic and DC-injection braking may beapplied when load inertia is very high.


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Operation of stop buttonopens supply lines and inserts capacitors 'C' acrossmotor terminals.Voltage relay 'VR' provides

the necessary delay beforeoperating to short-circuit the

motor terminals.Instead of 'VR' a timer or limit switches maybe used R1 areresistors for controlling braking torque.

R2 discharge resistors and R3 control resistor for relay drop-out voltage.


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DC Injection Braking

DC injection braking is a method of braking in whichd irect current (DC) is applie d to the stationarywin d ings of an AC motor after the AC voltage isremove d . This is an efficient and effective method of braking most AC motors. DC injection braking provides aquick and smooth braking action on all types of loads,including high-speed and high-inertia loads.

Recall that opposite magnetic poles attract and likemagnetic poles repel. This principle, when applied toboth AC and DC motors, is the reason why the motor shaft rotates.


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The only thing that can keep the rotor fromstopping with the first alignment is the rotationalinertia of the load connected to the motor shaft.

However, because the braking action of thestator is present at all times, the motor is brakedquickly and smoothly to a standstill.Because there are no parts that come inphysical contact during braking, maintenance iskept to a minimum.


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Induction Motor Braking and Speed Control Methods

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