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While learning 3-Phase scalar control of ACIM using STM32F407 MCU I could not clearly understand the diffrence between the following terms: 1. SPWM 2. 6-step PWM 3. SVMPWM

How do they differ from each other? Do we get the same 3-phase sinusoidals from each of the above PWMs and they are 3 different ways to implement it OR Do they differ in the output that we get from them also? Someone told me that SPWM is used to generate Phase-voltages while the other 2 techniques are used to generate Line-voltages. Is this the only difference between them?

scico111
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2 Answers2

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With simple sine-PWM you get output voltages with sinusoidal fundamental. For all modulation functions shown below, the voltage output fundamental will be sinusoidal as well.

Simply adding a third harmonic ("mod_3rdH") is a very powerful way to improve the inverter perfomance concerning minimum current ripple. Another big advantage of the 3rd harmonic is that the inverter ouput voltage can be higher than in case of simple sine-PWM (by about 15%).

There are many more options for inverter optimization (current ripple at given switching losses) by adding all kind of functions to the sine-PWM, e.g. discontinuous modulation patterns (the last two).

If you add a function which is based on space-vector calculus derived optimization, we speak of space vector modulation (not shown here) which is performance-wise slightly better than "mod_SIN".

The best modulation is also dependent on the modulation index (ratio of DC link voltage and output voltage).

enter image description here

UweD
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  • You mean that SVPWM can be used to add third harmonic to get the above benefits. Will it not increase the audible noise of the Inverter also? – scico111 Feb 04 '19 at 10:18
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    No. The audible noise goes with the switching frequency which does not change (this is important because we don't want to increase the switching losses by manipulating the modulation function). – UweD Feb 04 '19 at 10:29
  • Can you suggest if SVPWM technique is only good for PMAC (PMSM) and PMDC (BLDC) motors only or this is equally good for ACIM also? – scico111 Feb 04 '19 at 11:10
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    It is equally good for IM. There is no difference concerning the PWM generation and the resulting sinusoidal voltage fundamental. – UweD Feb 04 '19 at 11:18
  • Mostly i have seen SVPWM is used in vector control. If i am doing simple scalar V/F control then do i need to use SVPWM in it? – scico111 Feb 04 '19 at 12:03
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In order to run a 3 phase induction motor we have to use 3 phase inverter.The inverter consists of 6 mosfets/igbts.The image is given below for better understanding

enter image description here

We have to supply 6 individual pwm pulses from the microcontroller to s1,s2.. s6 gate inputs.Then we can generate 3 phase wave form at the terminals A,B and C.

The following modulation techniques are used to generate sine wave. SPWM(Sinusoidal Pulse width modulation) SVPWM(Space Vector Pulse Width Modulation) enter image description here

Sinusoidal pulse width modulation is easy to implement but space vector pulse width modulation is bit difficult to implement,SVPWM has more advantages over SPWM.

Shameer N
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