PULSE WIDTH MODULATION 1

[Chris33]

I was wondering if anyone out there can tell me how a PWM AC VFD works. In that I mean whats used in the drive to create the pulses (SCR's etc.). Could somone explain how this works.

Thanks in advance

 

[redtrumpet]

It used to be that PWM pulses were created by modulating a low frequency sinusoidal wave with a high frequency triangle wave. The points where the sawtooth wave intersected with the low frequency wave determined the switching points and pulse width. These pulses were usually generated by microprocessors.

I believe the latest drives technology uses sophisticated digital processing algorithms to generate the pulses and/or determine the IGBT firing points directly. The algorithms usually resolve the output current into magnetizing and torque producing current vectors to obtain optimum performance from the motor.

 

PWM VFD's are mostly controlled by IGBT's now-a-days. IGBT's are "Insulated Gate Bipolar Transistors." They are used for several reasons, the most important being their high switching speed.

In PWM VFD's, you take the incoming three-phase power and convert it to a fixed-level DC. This DC is stored on the "DC link" in capacitors, mainly. The "link" connects the AC-DC converter from the "DC-AC" converter.

You convert this DC to AC in the "inverter." I will not cover inversion, but it is essentially chopping up the DC back into AC.. Call it "un-rectifying" if it helps.

The older systems used to generate a triangle wave (leading edge and trailing edge slopes equal, coming to a point) that was the height of the full-voltage output sine wave, and use the crossovers between the two to generate the pulse widths. Draw this for clarity. This triangle wave was the "carrier" that was to be modulated by a sine wave. The frequency and amplitude of the sine wave (which should be at LEAST ten times less than the carrier frequency) will vary with the desired output.

The newer systems, which are usually digitally controlled, use computers to determine the amount of current needed to magnetize the motor, and the amount of current needed to produce torque. These are discrete numbers measured in amperes. Now, the magnetizing current is plotted vertically on what is the Y-axis on a Cartesian system. The torque producing current is plotted along the X-axis. You now have two magnitudes married to two directions. "Magnitude and direction" is a "vector." Hence, this system is called "vector modulation," as once one determines the two vectors needed, he can generate the third vector, the resultant, which is applied to the motor. Essentially, the resultant is a voltage level (magnitude) separated from the current level by a given angle (direction). The current is NOT controlled, but IS limited.... I won't go into that further here.

So, you now have a desired voltage and phase angle... and apply that to your motor (load). You will need an encoder in most cases to maintain the angle accurately, however, systems exist that allow you to "fudge" this. These are "encoderless vector controlled" systems. They mostly use the feedback from the load to calculate the vectors. There is an inherent error in this in that the drive is responding to events that have already occurred, as opposed to calculating responses to measurements that are occurring now.

A few notes about PWM in general:
In PWM, you only approximate the output. You are calculating an "average" for a given time-slice. This time slice is determined by your modulation frequency. The more often you switch the inverter devices, the more accurately you approximate the needed (continuous) output. The more times you can switch, the more accurately you can approximate the desired output. However, devices in the real world do not switch instanteously, so every time you switch, you introduce error. The faster you can switch the device, the less error you introduce, and the more often you can do so. Hence, inverter device switching speed in a PWM system is of paramount importance.

Hope this helps,
Don, they guy who doesn't know anything about drives, but makes them work anyway.