The Shop > Electronics & IC Programing

PWM vs VFD

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kvom:
A lathe chuck and work will have a lot more angular momentum than a mill.  So a VFD may well need braking resistors to get it to stop in a reasonable time.  That's because the energy gets retransmitted back to the VFD and must be absorbed.

If you program the ramp-down time shorter than what the VFD can absorb, the VFD will get an overcurrent fault and the motor will freewheel until it stops.  On my mill, I don't have a resistor and the shutdown time is about 5 seconds from max speed.  On a lathe with a chuck, it would probably be twice that.

BillTodd:
Just to second what KVOM has said. Get a VFD with braking resister option if you can, it'll give you greater control over deceleration times.

My HLV-H uses a Siemens VFD without an external braking resister option,  I have to use the machine's own (automatic) mechanical brake to get a reasonable stop time from full speed (i.e. 3000rpm to stop with a 5" chuck takes about 5 seconds) . Without the mechanical brake it would take an uncomfortable amount of time to stop.

Luckily, the VFD has an option to use the separate JOG acc/dec times instead of the normal ones so  I have arranged .the control system to select these quicker stop/start times in the lower speed (i.e. 1000rpm to stop in <2 seconds).

KVOM,

Have you seen any designs for an external voltage limit/brake unit ? I have a couple of 600v IGBTs from a dud VFD and fancy trying to make a add on over-voltage limiter

Bill

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