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Understanding Space Vector Modulation in AC Drives

Dive into Space Vector Modulation (SVM) in AC drives, learn its definition, synthesis of sinusoidal voltage with Voltage Source Inverter (VSI), sector analysis, sampling intervals, and comparison with Sinusoidal Pulse Width Modulation (SPWM).

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Understanding Space Vector Modulation in AC Drives

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  1. Space Vector Modulation (SVM)

  2. Closed loop current-control AC motor iref PWM VSI if/back PWM – Voltage Source Inverter Open loop voltage control AC motor VSI PWM vref

  3. + va - Vdc a + vb - b + vc - n c N PWM – Voltage Source Inverter S1 S3 S5 S4 S6 S2 va* Pulse Width Modulation vb* S1, S2, ….S6 vc*

  4. Vdc q vtri vc q Vdc Pulse width modulator vc PWM – Voltage Source Inverter PWM – single phase

  5. Va* Pulse width modulator Vb* Pulse width modulator Vc* Pulse width modulator PWM – Voltage Source Inverter PWM – extended to 3-phase  Sinusoidal PWM

  6. PWM – Voltage Source Inverter SPWM – covered in undergraduate course or PE system (MEP 1532) In MEP 1422 we’ll look at Space Vector Modulation (SVM) – mostly applied in AC drives

  7. Space Vector Modulation Definition: Space vector representation of a three-phase quantities xa(t), xb(t) and xc(t) with space distribution of 120o apart is given by: a = ej2/3 = cos(2/3) + jsin(2/3) a2 = ej4/3 = cos(4/3) + jsin(4/3) x – can be a voltage, current or flux and does not necessarily has to be sinusoidal

  8. Space Vector Modulation

  9. Space Vector Modulation Let’s consider 3-phase sinusoidal voltage: va(t) = Vmsin(t) vb(t) = Vmsin(t - 120o) vc(t) = Vmsin(t + 120o)

  10. Space Vector Modulation Let’s consider 3-phase sinusoidal voltage: At t=t1, t = (3/5) (= 108o) va = 0.9511(Vm) vb = -0.208(Vm) vc = -0.743(Vm) t=t1

  11. b a c Space Vector Modulation Let’s consider 3-phase sinusoidal voltage: At t=t1, t = (3/5) (= 108o) va = 0.9511(Vm) vb = -0.208(Vm) vc = -0.743(Vm)

  12. Three phase quantities vary sinusoidally with time (frequency f)  space vector rotates at 2f, magnitude Vm

  13. Space Vector Modulation How could we synthesize sinusoidal voltage using VSI ?

  14. + va - Vdc a + vb - b + vc - n c N va* vb* vc* Space Vector Modulation S1 S3 S5 S4 S6 S2 We want va, vb and vc to follow v*a, v*b and v*c S1, S2, ….S6

  15. + va - Vdc a + vb - b + vc - n c N Space Vector Modulation S1 S3 S5 S4 S6 S2 van = vaN + vNn vbn = vbN + vNn From the definition of space vector: vcn = vcN + vNn

  16. = 0 vaN = VdcSa, vaN = VdcSb, vaN = VdcSa, Sa, Sb, Sc = 1 or 0 Space Vector Modulation

  17. (1/3)Vdc (2/3)Vdc Sector 1 Sector 3 Sector 4 Sector 5 Sector 2 Sector 6 Space Vector Modulation [010] V3 [110] V2 [100] V1 [011] V4 [101] V6 [001] V5

  18. 110 V2 100 V1 Space Vector Modulation Reference voltage is sampled at regular interval, T Within sampling period, vref is synthesized using adjacent vectors and zero vectors If T is sampling period, V1 is applied for T1, Sector 1 V2 is applied for T2 Zero voltage is applied for the rest of the sampling period, T0 = T  T1 T2

  19. T0/2 T1 T2 T0/2 V0 V1 V2 V7 va vb vc T T Vref is sampled Vref is sampled Space Vector Modulation Reference voltage is sampled at regular interval, T Within sampling period, vref is synthesized using adjacent vectors and zero vectors If T is sampling period, V1 is applied for T1, V2 is applied for T2 Zero voltage is applied for the rest of the sampling period, T0 = T  T1 T2

  20. Space Vector Modulation How do we calculate T1, T2, T0 and T7? They are calculated based on volt-second integral of vref

  21. q 110 V2 100 V1 d Space Vector Modulation Sector 1 

  22. where Space Vector Modulation Solving for T1, T2 and T0,7 gives:

  23. a o b c amplitude of line-line = Space Vector Modulation Comparison between SVM and SPWM SPWM vao Vdc/2 For m = 1, amplitude of fundamental for vao is Vdc/2 -Vdc/2

  24.  15% Line-line voltage increased by: Space Vector Modulation Comparison between SVM and SPWM SVM We know max possible phase voltage without overmodulation is amplitude of line-line = Vdc

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