Transcript
7/21/2019 Speedcontrolinduction Motor
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Stator side
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V/f control
V / f control or frequency control - Wheneverthree phase supply is given to three phaseinduction motor rotating magnetic eld is
produced which rotates at synchronous speedgiven by
In three phase induction motor emf is inducedby induction similar to that of transformerwhich is given by
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V/f control
Where K is the winding constant ! is the number ofturns per phase and f is frequency" #ow if wechange frequency synchronous speed changes butwith decrease in frequency $u% will increase and
this change in value of $u% causes saturation ofrotor and stator cores which will further causeincrease in no load current of the motor " So itsimportant to maintain flu% & constant and it is onlypossible if we change voltage " i"e if we decreasefrequency $u% increases but at the same time if wedecrease voltage $u% will also decrease causing nochange in $u% and hence it remains constant" Sohere we are 'eeping the ratio of V/ f as constant"
(ence its name is V/ f method" )or controlling the
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V/f control
Where K is the winding constant ! is the number ofturns per phase and f is frequency" #ow if wechange frequency synchronous speed changes butwith decrease in frequency $u% will increase and
this change in value of $u% causes saturation ofrotor and stator cores which will further causeincrease in no load current of the motor " So itsimportant to maintain flu% & constant and it is onlypossible if we change voltage " i"e if we decreasefrequency $u% increases but at the same time if wedecrease voltage $u% will also decrease causing nochange in $u% and hence it remains constant" Sohere we are 'eeping the ratio of V/ f as constant"
(ence its name is V/ f method" )or controlling the
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Inverter Block Diagram
Diodes rectify AC into DC
DC link feeds into inverter stage
Inverter stage chops up DC into 3 phase, variable frequency output to motor
V/f control
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Supply voltage control
*t low values of slip
Since rotor resistance +, is constant"We 'now that Vth . Vs"
So ! . sVs"
s R
V
T th
synmech **
1
2
2
′≅ ω
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Supply voltage control
! . sVs
)rom the equation above it is clear that if we
decrease supply voltage torque will alsodecrease" ut for supplying the same load thetorque must remains the same and it is onlypossible if we increase the slip and if the slipincreases the motor will run at reduced speed "
!his method of speed control is rarely usedbecause small change in speed requires largereduction in voltage and hence the currentdrawn by motor increases which cause over
heating of induction motor"
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Supply voltage control
! . sVs
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Adding rheostat in the statorcircuit
In this method of speed control of three phaseinduction motor rheostat is added in the statorcircuit due to this voltage gets dropped"
In case of three phase induction motor torqueproduced is given by ! . sVs
" If we decrease
supply voltage torque will also decrease" utfor supplying the same load the torque must
remains the same and it is only possible if weincrease the slip and if the slip increase motorwill run reduced speed"
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+otor resistance control
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+otor resistance control#ow if we increase rotor resistance + torque
decreases but to supply the same load torque mustremains constant" So we increase slip which willfurther results in decrease in rotor speed" !hus byadding additional resistance in rotor circuit we can
decrease the speed of three phase induction motor"
Advantage0 with addition of e%ternal resistancestarting torque increasesDisadvantages 01"!he speed above the normal value is not possible""2arge speed change requires large value ofresistance and if such large value of resistance isadded in the circuit it will cause large copper loss
and hence reduction in e3ciency"
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Speed control by cascading
!his cascaded set of two motors will now run at new speed havingnumber of poles 751 8 59" In the above method the torque producedby the main and au%iliary motor will act in same direction resulting innumber of poles 751 8 59" Such type of cascading is calledcumulative cascading" !here is one more type of cascading in whichthe torque produced by the main motor is in opposite direction to that
of au%iliary motor" Such type of cascading is called di:erentialcascading; resulting in speed corresponds to number of poles 751 -59"In this method of speed control of three phase induction motor fourdi:erent speeds can be obtained1"When only main induction motor wor' having speed corresponds to
#S1 < 1= ) / 51""When only au%iliary induction motor wor' having speedcorresponds to #S < 1= ) / 5"4"When cumulative cascading is done then the complete set runs at aspeed of # < 1=) / 751 8 59"6"When di:erential cascading is done then the complete set runs at a
speed of # < 1=) / 751 - 59"
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