VFD Speed Search 1

[itsmoked]

I put a VFD into a lathe and during setup I chose to enable "Speed Search" thinking it meant that if the lathe user turned off the lathe but before it came to a stop decided to turn it back ON the speed search would match the current speed and re-accelerate the spindle back to the commanded speed.

It all worked nicely but I'd hadn't actually tested that 'theory'. I never saw any delays.

I get a call from the user, who after a month, had actually put the lathe in place to be used. He was concerned because "there was a delay bordering on dangerous" before the spindle started.

I went over to the place powered it ON and pressed the START button. Nothing. Hit STOP. Hit START again. Nothing. ???
As I stood there contemplating it all for a while the spindle took off. More than thirteen seconds after the START button was pressed! I hit STOP and with a stop watch timed 13.7 seconds after the START button before the spindle started.

I hit START STOP a bunch of times and could see the spindle actually twitch the moment the START was hit.

Going in back and looking at the VFD I could see on pressing start the frequency in a fraction of a second ramped clear up to the set frequency 40Hz then almost instantly it drops to zero. Then within the same first second the frequency jumped without ramping to 6.0Hz and proceeded to sit there for about 13 more seconds before the motor actually starts turning. Once it hit about the speed I'd guess was 6Hz it then ramped to 40Hz at the 2 second acceleration rate I'd set.

Question: What's the logic here that I'm missing?
Question: Why did it start this behavior a month later? (The belts to the spindle were added after the initial test.)

Ultimately I fixed it by just switching from "Speed Search" to "Normal" and all hesitation was gone the customer and I are happy.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[LionelHutz]

Was it a cheap VFD? They tend to not always work quite right.

 

[jraef]

Speed search / "flying restart" is a tricky thing to implement because it almost always has to get a back emf signal to function, and when the motor is standing still, that's problematic. You get one of two responses in my experience; the delay you are experiencing, or the drive occasionally rotates backward for a fraction of a rotation, which freaks people out. It depends on how the sensing circuit is designed, but the result is based largely on residual magnetism in the motor. So you often don't see it happen at first until the motor frame starts to retain magnetism. Some mfrs allow this feature to be tied to a minimum speed so that it is disabled at a full stop, some do not because on things like HVAC fans there can be windmilling taking place at slow speeds, often in the wrong direction and you want the drive to know that. So as a general rule, I recommend to people that unless it's necessary to avoid a slowdown from attempting to reaccelerate, don't enable that feature on machine tools and such.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[itsmoked]

LionelHutz; I'd say it might fit that category but it was their top tier product, so not the cheapest.

Jeff; That totally explains what I was seeing. Thanks.

Keith Cress
kcress -

http://www.flaminsystems.com