Originally Posted by
joeavaerage
Hi,
lets say the incoming supply is 240VAC. When its rectified the peak voltage is:
Vpeak=240 x square root (2)
= 240 x 1.414
=339.4V
This will be smoothed by the DC link capacitors to 339VDC. That's the voltage the IGBTs or MOSFETS have to withstand.
When the motor is decelerating it will generate voltage back into the VFD, and so when decelerating the withstand voltage will be even higher.
If your VFD is equipped with a brake resistor a IGBT/MOSFET switch will connect the resistor direct across the DC bus to 'absorb' the extra voltage due to deceleration.
The IGBT/MOSFET will turn on at somewhere between 385VDC and 400VDC depending on the manufacturer. Thus you see that the worst case withstand
voltage is determined by the manufacturers choice of the brake threshold voltage, not really much to do with the line voltage at all.
In absence of a brake resistor the voltage could go even higher, until its gets to about 20V more than the brake threshold voltage at which time the VFD will
fault 'over voltage'. The VFD will stop trying to decelerate the spindle, but rather just let it coast to a stop. If you want to avoid nuisance trips, and especially
as high voltage excursions can be devastating you are advised to allow the spindle to decelerate SLOWLY. If you want the spindle to accelerate up to speed
fast then be prepared for big current surges, and if you want it to decelerate fast be prepared for big voltage surges.
For production machines where cycle time is critical, that's the price you pay, as a hobbyist you can afford to be a bit more gentle with accel/decel demands.
Craig