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## Re: [mach1mach2cnc] Re: Question on index pulse

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• Hi Andy, If you average a steady state PWM signal (apply a low pass filter) you get a DC voltage. At 0% PWM you get 0V, at 50% PWM you get 1/2 voltage and at
Message 1 of 15 , Mar 1, 2004
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Hi Andy,
If you average a steady state PWM signal (apply a low pass filter) you
get a DC voltage. At 0% PWM you get 0V, at 50% PWM you get 1/2 voltage
and at 100% PWM you get full voltage and so on.
Now if you slowly change the pulse width your DC voltage will slowly
change to follow it.

A VFD changes the pulse width in a sinusoidal pattern and the motor
inductance acts as the filter. As long as the PWM frequency is much
higher than the sine wave frequency (say 10kHz or highter) all the motor
will see is the sine wave. By varying the pattern the VFD can control
the sine wave frequency and voltage.

Les

Andy Wander wrote:

> Les:
>
> Thanks for the explanation-what I still don't understand about it is how the
> "Sine Wave", which is generated by the "filtering" of the motor, will vary
> in frequency, if the only thing that is done to it is to change the PWM duty
> cycle. This would seem to me to get us to a "Sine wave" which has different
> values for it's Positive and Negative half-cycles, but I can't see how the
> frequency would change unless the frequency of the PWM changed.
>
>
> Andy Wander
> Verrex Corporation
>
• ... pick up the true speed of a machine. Can the PWM frequency of a VFD be used for this? If so, can it be scaled in MACH2 to correspnd No. This can only be
Message 2 of 15 , Mar 1, 2004
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Jeff:

>> notice that "index pulse" can be checked on the parallel input to
pick up the true speed of a machine. Can the PWM frequency of a VFD
be used for this? If so, can it be scaled in MACH2 to correspnd

No. This can only be gotten for a once per rotation pulse from the spindle
sensor..

Art
www.artofcnc.ca
• Hi Andy, Are you actually talking about the signals on the wires to the motor or about a PWM signal generated by the VFD to indicate the frequency it is
Message 3 of 15 , Mar 1, 2004
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Hi Andy,

Are you actually talking about the signals on the wires to the motor or
about a PWM signal generated by the VFD to indicate the frequency it is
running at?

Mach2 uses the index pulse to determine RPM and is probably based a number
of pulses per interrupt cycle. It may be based on the speeds you define in
the pulley setup of the spindle. You would have to ask ART specifically.

Best,

Ed Gilbert

Gilbert Engineered Systems
632 Warrenton-Embro Road
Macon, NC 27551
252-257-0539
> -----Original Message-----
> From: Andy Wander [mailto:awander@...]
> Sent: Monday, March 01, 2004 12:05 PM
> To: 'mach1mach2cnc@yahoogroups.com'
> Subject: RE: [mach1mach2cnc] Question on index pulse
>
> Of course, VFD means Variable Frequency, so is this actually a PWM signal?
> I'm so confused...
>
> Andy Wander
> Verrex Corporation
>
> -----Original Message-----
> From: washcomp [mailto:jeff@...]
> Sent: Monday, March 01, 2004 8:42 AM
> To: mach1mach2cnc@yahoogroups.com
> Subject: [mach1mach2cnc] Question on index pulse
>
> I notice that "index pulse" can be checked on the parallel input to
> pick up the true speed of a machine. Can the PWM frequency of a VFD
> be used for this? If so, can it be scaled in MACH2 to correspnd to
> the speed (I believe speed is a linear function to frequency, but the
> numeric value of one will not be the same as the other.
>
> Jeff
>
>
>
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