Can any one kindly help me in providing me the techniques available of obtaining a pure sinusoidal output voltage from a 3-phase Inverter.
Use three phase pulse width modulation technique, which can be easily implemented using Texas DSP ( library is available). Choose high carrier frequency in the range of 20 to 50 KHz . Use appropriate LC filter to remove harmonic components and get sinusoidal voltage.
There are so many methods to do this, like Sinusoidal PWM, space vector PWM. Depending upon your objective or application you have to decide which will give optimum result. Switching frequency is always limited by the switches we use. high switching frequency results in less filter requirements.
A pure sinusoidal wave? Impossible without a rotary converter. But yes, the PWM techniques as mentioned by my learned learned friends, can take you very close to a pure sinusoidal wave.
- Switching losses are proportional to switching frequency. In small inverters using MOSFETs perhaps you can go beyond the ultrasonic limit of about 16 kHz. There is no advantage to go to 30 kHz unless you want to use the inverter to heat your house in winter ...
- Using IGBTs in medium to high power inverter would give too high losses at ultrasonic frequencies. Usually the frequency is limited to 5 to 8 kHz.
If the switching frequency is randomly changed, the noise perceived by people decreases a lot. Some switching strategies do this automatically , as delta modulation, with impressive results.
- Centred pulses, as I repeat later, give double ripple and noise frequency. Your filter can be smaller.
PWM strategy:
Thousands of papers are written on the subject, but the difference in practical results is minimal. But remeber: "publish or perish".
You do not need a DSP to get excelent three phase modulation. Most of low cost microcontrollers have PWM outputs that can do the job. Many years ago I designed a range of drives using just a 8051 with variable frequency (constant V·t so constant ripple).
And you may also get rid of microcontrollers: a counter, a memory (i.e. EPROM) a DAC and 3 S&H (just 3 CMOS switches and 3 high OpAmps inputs) may provide and excellent three phase modulator.
Two important aspects are:
- The possibility to reach full voltage: the voltage to the motor has to be the same as the supply voltage at rated frequency. This is not possible if you use standard PWM with a sine wave as a reference.
It can be done by space vector modulation or by standard PWM adding 1/6 of third harmonic to the reference wave. The first option needs more computing power, so perhaps you will need a DSP. Adding third harmonic to the sine wave reference you get the same result but if your micro-controller has PWM outputs it is a very simple and effective solution.
- In any case try that the three phases are not switched at the same time, that is, that the pulses are approximately centred. This will double the effective frequency to the motor.
Using PWM built it in a microcontroller, the values of the sine+3d harmonic table may have olnly 8 bit lengh. For half wave symmetry the number of steps N has to be a multiple of 2, and to be exactly 1/3 out of phase, a multiple of 3. So it has to be a multiple of 6. In a byte the highest multiple of 6 is 252 which has to be the length of the table.
Then for a "pure" sine wave you have to add a 3-phase LC filter and how pure is the output depends of the values of the filter and your switching frequency. If you use elctrolytic capacitors in the filter, clamp their voltage with a diode to the positive and another to the negative rail.
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