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VFD Instant reversing

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Steelcrafted

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#1
Hey everyone I got a question, I have a VFD running my 3 phase motor on my vertical mill, and it's set up bare bones, no braking resistor...the VFD runs the motor no problem....I haven't tried it yet cause I don't want to destroy anything, but can I reverse the motor on the fly without a braking resistor? Like in the instance of power tapping at low speeds...can I just hit reverse and the motor slow down then start reversing.....?? I looked through the custom functions I set up and I couldn't find anything describing this....thanks!

Nate.
 

stupoty

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#2
Hey everyone I got a question, I have a VFD running my 3 phase motor on my vertical mill, and it's set up bare bones, no braking resistor...the VFD runs the motor no problem....I haven't tried it yet cause I don't want to destroy anything, but can I reverse the motor on the fly without a braking resistor? Like in the instance of power tapping at low speeds...can I just hit reverse and the motor slow down then start reversing.....?? I looked through the custom functions I set up and I couldn't find anything describing this....thanks!

Nate.
You should be able to set a de accelerate and accelerate time for the VFD , the shortest time probably depends on the mass of the spindle / chuck , I have my mill set to quite a quick start and stop with my lathe being on a slower start and even slower stop. I made the stop longer as when I was playing with this setting initially I managed to set it so quick it made the 8" 3 jaw unscrew from the nose which was slightly alarming. All with no brake resistor too :)

Stu
 

ttabbal

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#3
On my Bridgeport I have the VFD set for 2 seconds I believe. It does occasionally trip the overload if I'm running high speed with a larger tool. But the e-stop resets it easy enough. As I power tap in low with the back gear, it works well.
 

Steelcrafted

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#4
You should be able to set a de accelerate and accelerate time for the VFD , the shortest time probably depends on the mass of the spindle / chuck , I have my mill set to quite a quick start and stop with my lathe being on a slower start and even slower stop. I made the stop longer as when I was playing with this setting initially I managed to set it so quick it made the 8" 3 jaw unscrew from the nose which was slightly alarming. All with no brake resistor too :)

Stu

Ok cool! I'll have to play with it...so does the decelerate time apply to only when you press stop? Or also if you just hit reverse on the fly?
 

JimDawson

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#5
I have mine set at about 2.5 seconds. Works fine for power tapping. The decel time applies on both stop and reverse.
 

stupoty

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Ok cool! I'll have to play with it...so does the decelerate time apply to only when you press stop? Or also if you just hit reverse on the fly?
the brand I have locks out the reverse unless it has stopped first, although this can probably be changed, it seems to have infinite options for set up :) (maybe a slight exaggeration)

Stu
 

tq60

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#7
Changing speeds and directions on 4 phase are simple frequency and phase changes thus not really breaking as such.

They are all under drive power.

Sent from my SAMSUNG-SM-G930A using Tapatalk
 

mksj

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#8
When changing directions with a VFD, i.e. flipping the direction switch while the lathe is running, the VFD speed will ramp down and then back up per the programmed deceleration acceleration parameters. Some VFDs must complete either speed change before the new command can be executed. but there are lots of parameters that can be adjusted except on some inexpensive VFDs. So say you have 3 seconds for both of these parameters, then it would take 6 seconds to complete the direction change cycle. This is one case where having a VFD that allows stage 1 and stage 2 acceleration/deceleration programming parameters is helpful, this is typically done by a separate programmed input. So on my mill, the default settings acceleration/deceleration are 3/3 seconds, but in tapping mode they are set for 3/1 seconds. There is always some degree of electronic braking occurring with deceleration, but at slower speeds a braking resistor may not be needed.

My Acra LCM-42 control system in the tapping mode has an auto start and auto reverse setup, the trigger point for reversing is set by the depth stop position which use a proximity stop activating a relay that reverses the VFD run inputs. When the cycle is completed the spindle stops and the auto reverse relay is reset. Very convenient for drilling/tapping multiple holes. You need to use a keyed chuck for these operations. My mill also has sensors for back gear so forward is always forward even in back gear, the auto reverse/reset will work with the mill running in either direction. On another build I have put in too small momentary switches for the spindle forward reverse, the spindle only runs when either is pressed and the acceleration/deceleration parameters are automatically set to 1/1 seconds. Very similar to a joystick Jog on a lathe. One could also use a paddle switch MOM-OFF-MOM. If only used in back gear you would reverse the inputs to compensate for the back gear running in the opposite direction. You would need a separate switch to change the acceleration/deceleration rates. This would be very handy for power tapping with a VFD.

Red and Black buttons are momentary forward/reverse, small toggle on the other side of the speed control knob is the for Stage 1/ Stage 2 parameters
David Best 935 system 1.jpg

Auto start/Auto Reverse on an Acra LCM-42 mill
 
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4cyclic

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#9
Convenient tapping method and explanation Mark. Thank you.
 
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