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ELEC2530 - Power electronics > Inverters > Flashcards

Inverters Flashcards

1
Q

Draw the circuit for an inverter

A

[Picture22]

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2
Q

Describe the operation of this inverter circuit [Picture22]

A

> Vi is a constant input voltage.

> When S1 and S4 are on vAB = Vi

> When S2 and S3 are on vAB = -Vi

> When S1 and S4 are on, S2 and S3 are off and when S1 and S4 are off, S2 and S3 are on

> We have a ground point half way between the + and – points of VI

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3
Q

How is the RMS output voltage calculated from the RMS input voltage?

A

VAB = 0.9Vi

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4
Q

What can be done to smooth the output current?

A

A large inductor

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5
Q

What is the purpose of the diodes for the inverter circuit?

A

When we add an inductor, it allows it to discharge

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6
Q

For the output current waveform [Picture23], what describe each stage

A

t1:

> S1 and S4 are on

> IO is negative because the inductor is discharging

> This forward biases D1 and D4 and so current flows through D1 and D4

> This returned energy can be used to charge the input terminal capacitors

t2:

> S1 and S4 are on

> IO is positive

> The inductor is charging

> D1 and D4 are reverse

t3:

> S2 and S3 are on

> IO is positive because the inductor is discharging

> This forward biases D2 and D3 and so current flows through D2 and D3

> This returned energy can be used to charge the input terminal capacitors

t4:

> S2 and S3 are on

> IO is negative

> The inductor is charging

> D2 and D3 are reverse

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7
Q

How can a better signal be made?

A

By altering the timings for the PWM signal for the switches S1, S2, S3, S4

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8
Q

What is bipolar timing?

A

This is using two signals and comparing them to produce an output PWM signal that produces a better sine wave for an inverter

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9
Q

What are the features of this graph? [Picture24]

A

> vm modulating signal

> vc carrier signal

> Vm magnitude of vm

> Vc magnitude of vc

> fm frequency of vm

> fc frequency for vc

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10
Q

How is an output formed from these two signals? [Picture24]

A

> When vm > vc

  • S1 and S4 control = 1
  • S2 and S3 control = 0

> When vm

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11
Q

What is the equation for frequency modulation ratio?

A

mf = fc / fm

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12
Q

What is the equation for amplitude modulation ratio?

A

ma = Vm / VC

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13
Q

What is the equation for the peak output voltage when bipolar PWM signal is applied?

A

VAB[peak] ≈ ma × Vi

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14
Q

What is the equation for the RMS output voltage when bipolar PWM signal is applied?

A

VAB ≈ ma × Vi / √2

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15
Q

What is the maximum RMS value that the output voltage can have when compared to the input voltage?

A

70% because (1/√2) × ma

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16
Q

What is bipolar timing?

A

This is when two inverted signals are compared with a carrier signal to produce an output PWM signal that produces a better sine wave for an inverter

17
Q

Describe all the scenarios for this graphic [Picture26]

A

When vm1 > vc

  • S1 control = 1
  • S3 control = 0

When vm1 < vc

  • S1 control = 0
  • S3 control = 1

When vm2 > vc

  • S2 control = 1
  • S4 control = 0

When vm2 < vc

  • S2 control = 0
  • S4 control = 1
18
Q

What is the rough output current graph? Why is it like that?

A

[Picture27]

> When S1 and S4 are on & S2 and S3 are off, iO increases

> When S2 and S3 are on & S1 and S4 are off, iO decreases

19
Q

What is the equation for the voltage across the inductor in the inverter circuit?

A

vAB - iORO = Lf × dIO/dt

20
Q

What happens to the output voltage when the inductor is very large?

A

It can be assumed to be sinusoidal

21
Q

What is the equation for the output voltage? why?

A

VAB = VO + jXfIO Because of the inductor, the output voltage is complex

22
Q

What is the equation to calculate the phase angle of the output signal when it is purely active power?

A

θ = tan-1(⁡(Xf × IO) / VO)

23
Q

What is the equation to calculate the phase angle of the output signal when the power is active and reactive?

A

θ = tan-1(⁡(Xf × IO × cosϕ) / VO + XfIOsinϕ)

ELEC2530 - Power electronics (10 decks)

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