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Motor[i].PwmSf IDE setup calculation question


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We typically use the system setup utility within the IDE to initially setup motor data structure elements. We use 320VDC servo motors and 230VAC Geo Direct PWM amplifiers, with Gate3 UMAC axis cards.


We recently noticed a discrepancy between the value calculated by the setup utility verses the formula listed in the documentation for the PWM Geo drives.


We use dual-axis GPL102 drives at 230VAC power input, which according to the drive documents yields a nominal DC bus of 325V. Our motor specs list a rated motor voltage of 320VDC. When we use that data per the equations listed in the Geo Drive PWM manual, we get motor.PwmSf = 14518. However, the setup utility is setting PwmSf to 18611 given the same data.


Why the difference?


Is there a risk of motor or amplifier damage if PwmSf is set higher than recommended by the drive docs?


We use the amplifier db that comes with the IDE for setup. We've also verified that the motor voltage is 320VDC in the motor db's we've created for the setup utility.




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Some amplifiers require that the PWM signal actually switch to continue to function properly. If we allow a 100% duty cycle by setting Motor[x].PwmSf to a value of 16,384 or higher, there may be an issue. Delta Tau Amplifers with this issue should fault out, but you might as well be safe.


For amplifiers without this limitation, PwmSf can be set higher and there is actually a reason to do so. For 3-Phase motors we like to use a feature called third harmonic injection. This lowers the voltage of the peak of a sine wave, but keeps it near the peak longer. PwmSf can be set higher to get back to the expected voltage. If it is set higher than 16,384 that means we are allowing voltage saturation for some part of a sine wave (this only really matters at speed).


The GPL 102 shows a maximum of 90% of 16,384 in the hardware specific manual and that should be used. It looks like there is an issue in system setup we will be looking into.

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