PWM Inverter Drive explanation please

[Physicist3]

Hi, I understand that the PWM inverter drive system first uses a diode bridge rectifier in order to convert the AC incoming into DC before using an inverter to convert this DC into PWM for use on the induction motor. Could someone please explain in simple terms how the PWM inverter section works, (i.e. Pairing the wave with a triangle wave) etc and what the original sine wave and dc waves are used for?? Also, how does the mark-space ratio of the output signal change with a change in frequency?? How is the frequency changed?? Is it by changing the triangle wave frequency or what?? Thanks

 

[Windadct]

There are a few questions in your post that I'll try to straighten out..

I think you are mention "Original Sine" relative to the AC power TO the drive. The operation fof the VFD is independent of the sources AC.

The DC really should not be called a "wave" - ideally it is a perfectly flat DC source of power for the VFD Inverter. ( in the real world this is not perfectly flat - between 2-10% ripple - an undesirable result from both the Rectifier and the VFD inverter current)

OK - then the common way to generate the PWM Sine wave is a Triangle Wave operating at the Switching Frequency ( Fsw) and the width (modulation) of the pulse (output from the inverter) is determined by the the ratio of the triangle wave to the desired output (typically the Voltage). (That is not the best wording )

There are LOTS of very good references on this : http://www.ab.com/support/abdrives/documentation/techpapers/PWMDrives01.pdf

It looks like you are making a common mistake and tying to look at the whole system at once, the simpler answer is that the PWM wave is really using pulses to synthesize a wave- this can be done to make really any waveform from DC to just under 1/2 the FSW ( but then heavily distorted - high harmonics)

 

[Physicist3]

So am I right in thinking that the DC link output signal which goes to the inverter is 'chopped' using the IGBTs in order to reproduce an artificial sine wave? Is this then paired with the carrier triangle wave and then a PWM wave is created from the points of intersection between the two waves, with the PWM wave being 'on' in the area between two intersection points where the recreated sine wave passes over the triangle wave, and 'off' in the area where the wave passes under the point of one of the triangle points? How is the height of the PWM wave determined??

 

[Windadct]

Sorry for the delay - traveling for work. I (personally) really do not like referring to the DC as a signal, the word signal to me implies that it has some information or meaning, and DC in this case is JUST a power source. (in a DC analog feedback - this is a DC signal)

After that - yes, the Inverter chops in the manner you have described. The height of the PWM - is technically the full DC voltage, this is then filtered to develop an average of the ON (full V) and the Off (0 V)... so if the duty cycle of the output is 50%--- the average voltage is 1/2 of the DC link voltage.

 

[alphy]

Sure, I'd be happy to explain the PWM (Pulse Width Modulation) inverter drive system in simpler terms.

First, let's start with the purpose of the PWM inverter drive. Its main function is to control the speed and torque of an induction motor. This is done by converting the incoming AC power into DC power, and then using an inverter to convert that DC power into a series of pulses with varying widths and frequencies.

The first step is the diode bridge rectifier, which is used to convert the AC power into DC power. This is necessary because the inverter can only work with DC power.

Next, the inverter takes the DC power and converts it into a series of pulses. These pulses are generated by pairing a high frequency wave (typically a square wave) with a lower frequency triangle wave. The high frequency wave is used to control the width of the pulses, while the lower frequency triangle wave determines the frequency of the pulses.

The original sine wave and DC wave are used to create the triangle wave. The sine wave is used as a reference, while the DC wave is used to adjust the amplitude of the triangle wave. This allows for precise control over the frequency of the pulses.

The mark-space ratio of the output signal refers to the ratio of the pulse width to the pulse period. As the frequency changes, the mark-space ratio also changes. This is because the inverter adjusts the width of the pulses to maintain a consistent frequency.

The frequency can be changed by adjusting the frequency of the triangle wave. This can be done manually or through a control system. By changing the frequency of the triangle wave, the inverter will adjust the frequency of the pulses accordingly.

I hope this helps to clarify how the PWM inverter drive system works. It's a complex system, but it allows for precise control over the speed and torque of induction motors, making it a valuable tool in various industrial and scientific applications.