Sine + 3rd Harmonic PWM Technique
Sine 3rd Harmonic PWM Technique:
Fig: The modulating signal for Sine 3rd harmonic modulation
- The idea of Sine 3rd harmonic modulation technique is based on the fact that the 3-phase inverter-bridge feeding a 3-phase ac load does not provide a path for zero-sequence component of load current.
- Only three output points are brought out from a three-phase inverter-bridge. These output points are connected to the three supply terminals of the load. Such an arrangement does not cause any confusion for the delta connected load but for a star connected load the neutral point remains floating.
- However for a balanced, three-phase, star-connected load this should not be a drawback as the fundamental component in the load phase voltage is identical to the fundamental component of inverter’s pole voltage.
- In fact, the floating neutral point has the advantage that no zero sequence current (which includes dc, third and integer multiples of third harmonics) will be able to flow through the load and hence even if the pole voltage is distorted by, say, 3rd and integral multiples of third harmonics the load side phase and line voltages will not be affected by these distortions.
- In the Sine 3rd harmonic PWM technique is a modification over the SPWM technique deliberately some amount of third harmonic voltage is introduced in the pole voltage waveform. Accordingly a suitable amount of third harmonic signal is added to the sinusoidal modulating signal of fundamental frequency.
- Now, the resultant waveform (modified modulating signal) is compared with the high frequency triangular carrier waveform. The comparator output is used for controlling the inverter switches exactly as in SPWM inverter. Thus, the low frequency component of the pole voltage will be a replica of the modified modulating signal provided
(i) The instantaneous magnitude of the modified modulating signal is always less than or equal to the peak magnitude of the carrier signal and
(ii) the carrier frequency is significantly higher than the frequency of modulating signal.
- Accordingly, the pole voltage of Sine 3rd harmonic PWM inverter has same composition of fundamental and third harmonic as in the modified modulating signal. However, as per the earlier discussion, the third harmonic component of pole-voltage will not appear in the load phase and line voltages.
- The advantage of adding small amount of third harmonic in the modulating waveform is that it brings down the peak magnitude of the resultant modulating waveform. The modified modulating waveform appears more flat topped than its fundamental component. Thus if the fundamental sinusoidal modulating wave had a peak magnitude equal to the peak magnitude of the triangular carrier wave (corresponding to modulation index ‘m’ = 1.0), the addition of small percentage of 3rd harmonic to the fundamental wave causes the peak magnitude of the combined signal to become lower than triangle wave’s peak magnitude.
- In other words, a fundamental frequency signal having peak magnitude slightly higher than the peak magnitude of the carrier signal, if mixed with suitable amount of 3rd harmonic may result in a modified signal of peak magnitude not exceeding that of the carrier signal. Thus the peak of the modulating signal remains lower than the peak of triangular carrier signal and still the fundamental component of output voltage has a magnitude higher than what a SPWM can output with m = 1.0.
- The load sees only the fundamental component of pole voltage (and not the third harmonic) and thus the achievable load (output) voltage magnitude is higher than that of SPWM inverter. Higher output voltage is achieved without compromising on the quality of the output waveform.
- is the modulating waveform with a resultant peak magnitude of just 1.0. A higher amount of third harmonic will cause the magnitude limit to be exceeded. Thus the fundamental voltage output by the inverter employing Sine 3rd harmonic modulation technique can be higher by nearly 15.47% than a simple SPWM inverter. The maximum output voltage can now go to 347*1.1547 volts = 400 volts and the peak voltage requirement of the drive will be met.