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
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-----Original Message-----
From: Les Newell [mailto:les@...]
Sent: Monday, March 01, 2004 12:54 PM
To: mach1mach2cnc@...
Subject: Re: [mach1mach2cnc] Re: Question on index pulse
A VFD creates an approximation of a sine wave by pulse width modulating
a high frequency carrier. The carrier frequency is high enough that the
motor effecively filters it out, leaving the sine wave. The frequency of
the sine wave is varied to control the speed of the motor.
This is not really much use to Mach2 as it is only the commanded speed
to the motor. It does not mean the motor will actually turn at that
speed. The best thing to do is fit a sensor on the spindle and have done
with it. That way Mach2 always knows the true speed of the spindle.
Les
Andy Wander wrote:
I can't speak about the intricacies of the VFD, but PWM is Pulse Width
Modulation, where the ONLY thing that is varied is the width of the "ON"
pulse(not both ON and OFF). This means that the frequency of the pulses
does
NOT change, just the ratio of ON-time to OFF time of a (quasi) square
wave.
If the width of both the ON and the OFF pulse changed(and was identical,
you
would NOT have PWM, you would have FM(frequency modulation) of a square
wave.
Andy Wander
Verrex Corporation
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