VSIs and normal SVM

Question 1

What defines the switching frequency of the devices

Answer 1

Ts, because each device only switches on and off once per Ts

Question 2

What is the amplitude of the fundamental component of the output voltage?

Answer 2

Vref

Question 3

How do you prove that the peak value of the fundamental component of output voltage is 2/Pi root(3)Vd

(not SVM)

Answer 3

Find VAN(wt) using fourier series formula. Go up to about the 5th harmonic to show the pattern. Use VAN(wt) to find VBN(wt) by adding a phase shift.
Get the phase-phase (she calls it that but it is same as line-line) voltage by doing VAN(wt)-VBN(wt)
Select the first term
Replace (sin(wt)-sin(wt-2Pi/3)) with root(3)
- works because of some vector maths wizardry

Question 4

Explain the purpose of a given freewheel diode question - what do you need to include

Answer 4

Sketch VAN and ia (or relevant phase)

Draw circuit diagrams to show current path

Question 5

How can you change the amplitude and frequency of the output voltage in an SPWM controlled system?

Answer 5

Change the sine wave modulation signal

Question 6

How can you change the amplitude of the output voltage in a 6-step mode system

Answer 6

The only way is to change Vdc

Question 7

What to include when explaining the effect of deadtime on VAN

Answer 7

Draw S1 and S4 ideal
Draw VAN ideal (on when S1 on, at height of Vd)
If required, draw VAZ ( follows pattern of VAN but is either Vdc/2 or -Vdc/2)
Draw S1 and S4 real (delayed on rising edge)
Draw 2 VAN graphs (one for each current direction)
When ia is +ve, VAN follows the INSIDE timing lines
When ia is -ve, VAN follows the OUTSIDE lines
Label what is happening (in terms of which are devices are on) on the different parts of the VAN waves.
Draw error waveform - height is Vd

Draw equivalent circuits of deadtime if required
- either goes A->load->GND or source->load->A

Question 8

What does ‘output phase voltage’ refer to by default

Answer 8

VAZ

Therefore use VAZ to get ma

Question 9

What to remember when drawing SPWM generation circuit diagram

Answer 9

Include the mathematic definitions of the signals coming out of the comparator (m1 = 1.1 sin(wot) etc.)
Also connect to S1 and S4 (via a NOT gate)

Question 10

What to include when explaining how the ampl and freq of the fund comp of output V can be changed

Answer 10

Graph of Vph vs ma, including saturation region

Can only reach saturation with 6SM

Question 11

What is the phase amplitude at saturation

Answer 11

Vph(pk) = 2/Pi*Vd

Question 12

How to sketch VAZ harmonic spectrum

Answer 12

• Scale as VAZ/(Vdc/2)
• Include 3rd harmonic if exists
• high freq value peak is at mf
• fh = (jmf+- k)fc

Question 13

How to sketch VAB harmoic spectrum

Answer 13

Like VAZ spectrum, but:

• No 3rd harmonic
• No mf (the ones around it still exist though)
• Scaled by VAB/(root(3)*Vdc/2)

Question 14

How to derive fundamental component of Verror(wt)

Answer 14

Integrate over 0 to 2pi (and split into 2 to make maths easy)
But add phi into each of the limits and pretend to use it (doesn’t actually make a difference).
Also add -phi phase shift