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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?
"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?