09-08-2012, 12:12 PM
Inverter Circuits
Inverters.ppt (Size: 231 KB / Downloads: 70)
Provide a variable voltage, variable frequency AC output from a DC input
Very important class of circuits. Extensively used in variable speed AC motor drives for example (see H5CEDR)
We have already seen how the fully controlled thyristor converter can operate in the inverting mode ( > 90O) - however that is limited:
Can only invert into an existing AC supply
Voltages must already be present to provide natural commutation of thyristors
The circuits we will look at here are much more versatile and can provide an AC output into just about any kind of load
Three phase and single phase versions are possible - principles are the same
Basic Inverter Leg (1)
Capacitor does not have to be split - O provides a convenient place to reference voltages to for understanding
Obviously never gate Q1 and Q2 at the same time! - “shoot through” causes destruction
Normal mode is to use complementary gating for Q1 and Q2
In practice a small delay must be introduced between turning Q1 off and Q2 on (and vice versa) to avoid “shoot through” due to finite switching times
We will ignore the effect of this and assume perfect switching
Single Phase Inverter H-bridge (2)
VXO, VYO are 2-level waveforms (E), VXY can be a 3-level waveform
Note: this is called a “2-level” circuit since each leg is a 2-level leg
Circuit can produce +E, 0 and -E in response to gating commands, regardless of current direction
We can synthesize (on average) any waveform we like by switching for varying amounts of time between +E, 0, -E
For example, for variable DC we could use:
Q1, Q4 gated 0 < t < dT, Q2, Q3 gated dT < t < T
Single Phase Inverter H-bridge (3)
To get AC output, we could operate like described previously, but dynamically vary the duty cycle (d) to follow an AC demand
This is called Pulse-Width Modulation (PWM) - see lhandout for what the waveform looks like
For this to be effective, the switching frequency has to be an order of magnitude greater than the demand frequency
PWM produces an output waveform with a spectrum consisting of the wanted component + distortion components clustered (sidebands) around the switching frequency and its multiples
Single Phase Inverter Square wave operation
Return to PWM later - simplest method of voltage/frequency control is “quasi-squarewave”
Used to be very popular when power devices were slow and high switching frequencies were not possible
Gate each side of the bridge with a squarewave at the desired output frequency
Adjust phase shift between the two sides to get voltage control
Natural Sampling 2
Frequency ratio (FR) can be integer (synchronous PWM) or non-integer (asynchronous PWM).
It is normal now to keep the carrier frequency fixed as the modulating frequency is varied – hence most PWM today is asynchronous.
Modulation Index (MI) tells us how large the modulating frequency component at the inverter output will be for a given DC link voltage.
Modulation Depth (MD) tells us how much we have modulated the pulses by (compared to an unmodulated 50% duty cycle carrier frequency squarewave).
For Natural Sampling MI = MD (provided MD < 1)
Hence control of amplitude and frequency of the modulating wave, provides direct frequency and voltage control at the inverter output.
Spectrum of 2-level PWM: Modulating component + sidebands around carrier frequency + sidebands around 2 times carrier frequency etc – see Handout