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INDUCTION MOTOR Scalar Control (squirrel cage) MEP 1523 ELECTRIC DRIVES

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Scalar control of induction machine: Control of induction machine based on steady-state model (per phase SS equivalent circuit): R r ’/s +Vs–+Vs– RsRs L ls L lr ’ + E ag – IsIs Ir’Ir’ ImIm LmLm

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Scalar control of induction machine rr s T rated Pull out Torque (T max ) TeTe ss smsm rated rotor TLTL TeTe Intersection point (T e =T L ) determines the steady –state speed

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Given a load T– characteristic, the steady-state speed can be changed by altering the T– of the motor: Scalar control of induction machine Pole changing Synchronous speed change with no. of poles Discrete step change in speed Variable voltage (amplitude), frequency fixed E.g. using transformer or triac Slip becomes high as voltage reduced – low efficiency Variable voltage (amplitude), variable frequency Using power electronics converter Operated at low slip frequency

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Variable voltage, fixed frequency Lower speed slip higher Low efficiency at low speed e.g. 3–phase squirrel cage IM V = 460 V R s = 0.25 R r =0.2 L r = L s = 0.5/(2*pi*50) L m =30/(2*pi*50) f = 50Hz p = 4

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Variable voltage, variable frequency At low slip Constant V/f operation

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Variable voltage, variable frequency – Constant V/f If Φ ag is constant T e α slip frequency

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Approximates constant air-gap flux when E ag is large E ag = k f ag = constant Speed is adjusted by varying f - maintaining V/f to approximate constant air-gap flux How do we make constant ? Variable voltage, variable frequency – Constant V/f

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Characteristic with constant

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V rated f rated VsVs f Variable voltage, variable frequency Constant constant V/f Constant slope

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Constant V/f – open-loop VSI Rectifier 3-phase supply IM Pulse Width Modulator s*s* + Ramp f C Variable voltage, variable frequency V rate limiter is needed to ensure the slip change within allowable range (e.g. rated value)

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Constant V/f – open-loop Variable voltage, variable frequency Simulation example : 415V, 50Hz, 4 pole, R s = 0.25 , R r = 0.2 , L r =L s = H, L m = , J = kgm 2, Load: k 2

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Constant V/f – open-loop Variable voltage, variable frequency Signal 1 Time (sec) constant_vhz_withoutBoost/Signal Builder : Group 1 Simulation example : 415V, 50Hz, 4 pole, R s = 0.25 , R r = 0.2 , L r =L s = H, L m = , J = kgm 2, Load: k 2

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Constant V/f – open-loop Variable voltage, variable frequency Simulation example : 415V, 50Hz, 4 pole, R s = 0.25 , R r = 0.2 , L r =L s = H, L m = , J = kgm 2, Load: k 2

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Constant V/f – open-loop Variable voltage, variable frequency Simulation example : 415V, 50Hz, 4 pole, R s = 0.25 , R r = 0.2 , L r =L s = H, L m = , J = kgm 2, Load: k 2 With almost no rate limiter

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Constant V/f – open-loop Variable voltage, variable frequency Simulation example : 415V, 50Hz, 4 pole, R s = 0.25 , R r = 0.2 , L r =L s = H, L m = , J = kgm 2, Load: k 2 With 628 rad/s 2

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Problems with open-loop constant V/f At low speed, voltage drop across stator impedance is significant compared to airgap voltage - poor torque capability at low speed Solution: (i) Voltage boost at low frequency (ii) Maintain I m constant stator current control Variable voltage, variable frequency Constant V/f – open-loop low speed problems

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Variable voltage, variable frequency Torque deteriorate at low frequency – hence compensation commonly performed at low frequency In order to truly compensate need to measure stator current – seldom performed Constant V/f – open-loop low speed problems (i) voltage boost

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Variable voltage, variable frequency Torque deteriorate at low frequency – hence compensation commonly performed at low frequency In order to truly compensate need to measure stator current – seldom performed With voltage boost of I rated* R s Constant V/f – open-loop low speed problems (i) voltage boost

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Voltage boost at low frequency V rated f rated Linear offset Non-linear offset – varies with I s Boost Variable voltage, variable frequency Constant V/f – open-loop low speed problems (i) voltage boost

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VSI Rectifier 3-phase supply IM Pulse Width Modulator V boost s*s* + + V Ramp f C Variable voltage, variable frequency I dc + V dc - Constant V/f – open-loop low speed problems (i) voltage boost

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Variable voltage, variable frequency Constant V/f – open-loop low speed problems (i) Constant I m ag, constant → E ag /f, constant → I m, constant (rated) R r ’/s +Vs–+Vs– RsRs L ls L lr ’ + E ag – IsIs Ir’Ir’ ImIm LmLm maintain at rated Controlled to maintain I m at rated

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Variable voltage, variable frequency Constant V/f – open-loop low speed problems (i) Constant I m Current is controlled using current- controlled VSI The problem of stator impedance drop is solved Dependent on rotor parameters – sensitive to parameter variation From per-phase equivalent circuit,

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VSI Rectifier 3-phase supply IM ** + + |I s | slip C Current controller ss PI + Variable voltage, variable frequency rr - Current reference generator Tacho Constant V/f – open-loop low speed problems (i) Constant I m

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Constant V/f Variable voltage, variable frequency Poor speed regulation Problems with open-loop constant V/f Solution: (i) Slip compensation (ii) Closed-loop control

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Constant V/f – poor speed regulation: (i) slip compensation Variable voltage, variable frequency T ω r (rad/s) ω slip1 ω r1 T1T1 ω r2 ≈ω s1 * T2T2 Motor characteristic AFTER slip compensation ω s2 *=ω s1 *+ω slip1 ω slip1 ω s1 * T load Motor characteristic BEFORE slip compensation

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Constant V/f – poor speed regulation: (i) slip compensation VSI Rectifier 3-phase supply IM Pulse Width Modulator V boost Slip speed calculator s*s* V V dc I dc Ramp f C Variable voltage, variable frequency I dc + V dc -

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Variable voltage, variable frequency How is the slip frequency calculated ? P dc = V dc I dc P motor,in = P dc – P inv,losses P air-gap P motor,in Stator Copper lossess Stator Core losses ROTORSTATOR + V dc I dc INV Constant V/f – poor speed regulation: (i) slip compensation

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Variable voltage, variable frequency How is the slip frequency calculated ? P air-gapc = T e syn T e = P air-gap / syn For constant V/f control, Constant V/f – poor speed regulation: (i) slip compensation

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Variable voltage, variable frequency Require speed encoder Increase complexity Constant V/f – poor speed regulation: (i) closed-loop speed

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