Drive PWM/Carrier Frequency Question

D

dirtyc

I found this statement in a VFD tutorial and it seems to fly in the face of what I thought I understood about Carrier Frequency:

"Lower resultant voltage is created by <b>more</b> and narrower pulses. Higher resultant voltage is created by <b>fewer</b> and wider pulses."

I agree with the statement about the width of the pulses. Wider pulses = a larger volt-seconds area = more voltage to the motor.

However, what I don't understand is the more/less pulses part. I was under the impression that the number of pulses was dictated entirely by the carrier frequency. For any given amount of time, you will always have the same number of pulses. What changes is the amount of time during that carrier frequency period that the pulse is high.

I am clearly missing something here. I know that carrier frequency is fixed because its a parameter that you set in any VFD. You set it and it doesn't change.

And yet I also know that the fundamental basis of VFD is that in V/Hz mode it must maintain a fixed ratio: if they are sending a lower voltage, they must also be sending a lower frequency. And yet, if the carrier frequency is constant (as I believed it was) then how could you get a variable frequency from the drive?

Can someone tell me which piece of the puzzle I have wrong here?

J

JRaef2

You are conflating the output frequency that is going to the motor with the carrier frequency. Two different things, one a part of the other. The root of this confusion is that, for simplicity, most texts simplify the discussion of PWM output to describe it as if it is a series of monolithic "block" pulses and gaps of varying widths, making up the changes in the RMS values.

That's not exactly true. The ON "blocks" are in actuality a series of much tinier pulses, all at the same rate. THAT rate is the Carrier Frequency. So each PWM "pulse" is really a series of thousands of much faster pulses, firing at the CF rate, for a period of time commensurate with the width of that PWM pulse. Each PWM "gap" is NO pulses at the CF rate for that period of time. The individual tiny pulses are all on for the same amount of time within the "on" portion of the PWM pulse.

By the way, this too is kind of an over simplification, but it's more descriptive of the issue you were questioning.

D

dirtyc

That answers my question perfectly. That oversimplification that so many use is exactly what was tripping me up. Thank you very much.