VFD and reversing question

[Kent Jaffrey]

Question for those of you with more VFD and motor experience. I have a PM 3520 and during sanding will often change direction of rotation on the lathe. Other than the risk of the chuck possibly unwinding due to inertia is there any real danger to switching the direction while the lathe is spinning (or more likely switching it while it is still slowing down but not completely stopped)?

 

[Darryl Fective]

There's really no load on the motor, so harm in doing that.

 

[Dean Center]

Kent, I know the feeling you're having. As the lathe slows momentarily to reverse itself, there's a mental opportunity to worry. It may be a hold over from the days of gears and clutches. Good to hear from Darryl that it's not an issue.

 

[John K Jordan]

Other than the risk of the chuck possibly unwinding due to inertia is there any real danger to switching the direction while the lathe is spinning (or more likely switching it while it is still slowing down but not completely stopped)?

Jet/PM say there is no problem with switching the direction, even when the lathe is running at speed. VFDs have a programmable deceleration/acceleration, default setting is 5-seconds. (You can change this if you want.) The motors used are 3-phase which allows a lot of control.

Note that the deceleration is not simple a drift to slowdown, it's a "powered" and controlled deceleration. I've switched directions on both my PM3520b and Jet1642 while it was running a full speed of about 3000 rpm (the speed I often use with thin spindles) with no damage. I've been doing this for years. At higher speeds I think the VFD simply provides more active electrical force to cause the lathe to slow then speed up again - there is no significant stress to the motor, perhaps a bit more heat in the braking resistors.

You might not want to try this with a non-VFD 2-phase electrical DC motor!

Of course, as you mention, there is the danger of a heavy piece loosening the chuck on the spindle threads. With a heavy piece, I first turn the speed down manually then flip the reverse switch and turn the speed back up. Even that works without securing the set screw on the chuck. (No big catches allowed when running in reverse without the chuck secured!)

JKJ

 

[Larry Copas]

I've found with spindle work and most normal size bowls, hollow forms reversing does not cause a problem. I do it all the time and have been doing it for years. The work slows nicely and reverses than the speed picks back up.

With very heavy pieces I've found a different story. Two things can happen. The first is the chuck will unscrew. The second is if the chuck does not unscrew the weight of the piece will over power the deceleration of the VFD and it will trip out. That means it will have to be reset by unplugging for a minute or two. So......with heavy pieces I slow the speed way down before reversing and nothing bad happens.

 

[Michael Brown]

For induction and servo motors, I believe you are safe.

Brushed motors will generate a significant current load as you decelerate the field created by the armature contact. I would worry about blowing fuses. Typically those systems have a direction reverse disconnect, so when you flip the switch, you have to turn the motor on again. (Magnetic switch style).

The reversal unthreading is easily prevented using the grub screws.

I agree with the heavy turnings rule. Off, stop, reverse.

 

[Bill Boehme]

What JKJ says about the programmed ramp-up/down. By the time you are doing the final sanding the mass of the wood doesn't put enough inertial load to unscrew the chuck. However, it's a different story if you start turning a really heavy piece of wood. It would be a good idea to do a free-wheeling coast-to-stop rather than the programmed deceleration to a full stop. I learned this lesson when I suddenly was trying to catch a fifty-pound hunk of mesquite in which the chuck was slowly unscrewing from the spindle.

 

[Bill Blasic]

My first extra little lathe was a Delta 10" lathe and occasionally when I stopped the lathe the chuck would unwind and you know the first thing you try to do is catch it. That only happened a couple times and I got rid of it. But that has led me to always stop the lathe and then flip the switch to reverse even though I have seen pros and others do it on my lathe here.

 

[Kent Reisdorph]

FWIW, on my Rikon, when I hit the reverse/forward switch, the lathe comes to a complete stop. It's basically the same as hitting the off switch. I have to push the On button to start it up after switching directions.

 

[Kent Jaffrey]

Thanks for all the replies. It’s great to get some quick answers and first hand knowledge. Being a physics teacher I have a good handle on inertia and energy but was lacking the knowledge of programming of the VFD. Specifically whether switching direction initiates the braking function (send the energy to the braking circuit) and thus as long as the energy isn’t too big it’s should be fine, or whether it would bypass the normal deceleration mode and work differently, in which case there might be a danger to the electronics.

 

[Don Wattenhofer]

Specifically whether switching direction initiates the braking function (send the energy to the braking circuit) and thus as long as the energy isn’t too big it’s should be fine, or whether it would bypass the normal deceleration mode and work differently, in which case there might be a danger to the electronics.

VFD's usually have settable acel & decel settings and maintain a volts per Hz ratio. The start up of the motor the VFD will ramp up the Hz and voltage until set speed is achieved. The shut down of the motor can and will act like a brake and if the decel rate is to fast it can cause the VFD to trip out on overload (overload is a built in feature), but most VFDs can be equipped with a braking resistor to absorb that excess energy. The woodturning lathe is a pretty simple application and it should not be necessary to stop that quickly. The switching of direction without stopping is not a problem because the vfd is fully capable of ramping down to zero before ramping up in the opposite direction and like some have already said they do it all of the time.

 

[Bill Boehme]

Question for those of you with more VFD and motor experience. I have a PM 3520 and during sanding will often change direction of rotation on the lathe. Other than the risk of the chuck possibly unwinding due to inertia is there any real danger to switching the direction while the lathe is spinning (or more likely switching it while it is still slowing down but not completely stopped)?

The deceleration profile when hitting the reverse switch is normally no different than the deceleration when hitting the stop button. In other words, if the chuck doesn’t unwind when hitting the stop button then it wouldn’t unwind as a result of hitting the reverse switch.

 

[Mark Jundanian]

I'm probably an outlier here, but I prefer to bring the work to a complete stop when reversing. It gives me a chance to examine the surface.

 

[hockenbery]

I'm probably an outlier here, but I prefer to bring the work to a complete stop when reversing. It gives me a chance to examine the surface.

Me too

 

[Darryl Fective]

I'm probably an outlier here, but I prefer to bring the work to a complete stop when reversing. It gives me a chance to examine the surface.

Most of the time, I do too. When sanding, I prefer reverse for outside. When I want to begin sanding, but I forgot to reverse, or when I'm done sanding and I want to make another cut, if it's going the wrong way, I'll just switch direction.

 

[Jamie Straw]

I reverse on a PM or Jet lathe with no concern, they seem designed to do that. Definitely NOT with a servo-motor lathe such as a Harvey. I accidentally switched mine at 400 RPM with a Stronghold chuck and a ~6lb bowl on it. Yikes! Flew about 6 feet off the lathe, and I was lucky to not be in the path.

 

[john lucas]

Personally I just wait until it stops. I'm not in tgat big of a hurry. I have done it once or twice just to see.

 

[Jonathan Silwones]

Jet/PM say there is no problem with switching the direction, even when the lathe is running at speed. VFDs have a programmable deceleration/acceleration, default setting is 5-seconds. (You can change this if you want.) The motors used are 3-phase which allows a lot of control.

Note that the deceleration is not simple a drift to slowdown, it's a "powered" and controlled deceleration. I've switched directions on both my PM3520b and Jet1642 while it was running a full speed of about 3000 rpm (the speed I often use with thin spindles) with no damage. I've been doing this for years. At higher speeds I think the VFD simply provides more active electrical force to cause the lathe to slow then speed up again - there is no significant stress to the motor, perhaps a bit more heat in the braking resistors.

You might not want to try this with a non-VFD 2-phase electrical DC motor!

Of course, as you mention, there is the danger of a heavy piece loosening the chuck on the spindle threads. With a heavy piece, I first turn the speed down manually then flip the reverse switch and turn the speed back up. Even that works without securing the set screw on the chuck. (No big catches allowed when running in reverse without the chuck secured!)

JKJ

Excellent answer John J.