Using a VFD to brake a lathe?

W

WoodBee

Registered
Registered
Joined
Jan 12, 2014
Messages
49
No, I did not mean "break a lathe", I don't need a vfd to do that :)

I am overhauling an old Lathe (AI Hembrug) and am thinking about adding a vfd to run it. It already has a 3 phase motor, and I also have 3 phase power, so that is not an issue. I have used and attached vfd's in the past and think I would like the extra functions the vfd would offer.

One of the things I would really like is a brake. So I am considering the braking function a vfd can offer, since my lathe has no mechanical brake and I am not sure how one can be added. The vfd I have can brake with dc-injection.
My question is: how does a vfd with dc injection braking compare to a mechanical brake?

One of the issues is there is an imperial leadscrew on this lathe, but virtually all threading I do is metric. The usual way to do this on this lathe is simply switching the motor in reverse at the end of the thread which is "instant reverse". Using a vfd means no direction switching between vfd and motor, but having the vfd do that. I know a vfd can be configured for that, but am concerned it is not "instant' enough.

Some technical details of the lathe:
Chuck speeds from 50-4000 rpm
D1-4 camlock
Swing over bed 270 mm (10 5/8")
Three speed 1-1,75-2,5 Hp motor rated for instant reversal

Any experience or opinion welcome.

Peter
 
The problem that you will run into is that the motor becomes a generator on deceleration, that power has to go somewhere. By installing external braking resistors on the VFD you can get it to decel pretty fast, but it is not instant, more like 2 or 3 seconds. If the DC bus in the VFD goes over voltage, the the VFD will trip out, and the motor will coast to a stop. On my 3HP mill, the best stop time I can get is 2 seconds, it all depends on the rotating mass.

Your best option might be to install an electro/mechanical brake on the motor and trigger it with the VFD. Maybe a motorcycle disk brake assembly on the spindle?:think1:
 
I recently installed the Hitachi WJ200-015 on a 13x40 2HP lathe with no mechanical brake, and this was one of my concerns. There are a number of variables that will play into the effectiveness of a VFD electronic brake, the primary one as mentioned is speed/momentum. There is also the quality and adjustments of the VFD (I switched to Hitachi because of poor braking with another brand). With the WJ200 I was able to get down to stopping speeds of about 1 second up to several hundred RPM, and under 2 seconds up to about 800 RPM. Above that, I would get an over voltage error. I then installed a 500W brake resistor http://www.ebay.com/itm/350832569876?_trksid=p2059210.m2749.l2649&ssPageName=STRK:MEBIDX:IT , and do not have any over voltage issues up to 1800 RPM (even with a 1 second brake time it takes 2-3 seconds to stop at these speeds). This type of brake resistor (as opposed to free air types) must be mounted to a large metal surface to achieve the stated wattage (the 1000W version is only slightly more expensive if needed), mine doesn't even get warm after several stops. Another consideration is that some VFD models can connect the braking circuit directly to a resistor, other may require a specific/proprietary braking module/resistor elements (expensive). In one of my machines it was cheaper to replace the VFD rather then buy the braking module for the older VFD. As mentioned, the brake resister value and wattage is VFD specific, other variables include voltage and duty cycle/load type. This is usually stated by the manufacturer (but not always, or may not be in the manual).

Different VFDs have different braking parameters and flexibility, so these all may affect the braking performance. You also may want fast braking for slower speeds/E-stop, and longer (2 step) stopping for higher speeds (4000 RPM, might take a bit longer). So specific to your threading question, you can not achieve an instant stop or motor reversal with a VFD. A 1 second stop/reversal is very doable which equates to 1-3 revolutions at threading speeds, and you have the advantage with the VFD to slow the lathe down to a stop toward the end of your threading. If you use an electronic mechanical braking unit, you cannot use the VFD braking. I had looked at using a go-kart hydraulic disk brake for a mechanical stop, but I doubt I would do much better than the VFD brake, and may be more abusive to the gears.

I am not sure of the electrical connections on your motor with the 3 speeds/HP listed, I am assuming you would use the full 2.5HP adjusted by the VFD. There are also newer VFD/vector 3 phase motors that might perform better, but at a significant price. Maybe others can comment about and differences in up sizing the VFD so one can get slightly higher motor (overload) amperage settings of a Vector motor.
 
Fwiw - I have a 2HP Teco VFD on my 1 HP mill. No braking resistors. E-Stop stops the spindle in .5 seconds. I've only tested instant reverse but that's about what it does. Bang, we're running the other direction.
 
Thanks all for your input.
The main reason for not simply trying out the vfd is that I will need to change all the electrical connections to do that. I am not opposed to doing that, but only if there is a reasonable chance the result is what I want. It would indeed be a good learning experience. I use a vfd with braking on my wood lathe, without resistor. And that works great. But it does not need to stop instantly, and the mass is usually limited. I read somewhere that braking with dc injection does not need a resistor as opposed to braking by "generator"? From your remarks I understand that that only works for relatively low speed/mass combo's (as in my woodlathe)?
When threading usually the mass of the workpiece is limited, and slowing down when getting to the end is indeed a good idea. But the same braking parameters will be used when turning a big workpiece (although those don't turn 4000 rpm either)
As for how to connect the vfd to the multi speed motor: I was planning on leaving the speed selector intact. I know I can't switch speeds when the motor is running with a vfd, but selecting different speeds with a stopped motor might come in handy?
On the other hand, maybe low speeds by vfd with motor switched in high speed may give the same hp as higher speed on vfd and low motor setting?
It is with choices like this it becomes apparent that I only know enough of this stuff to become dangerous ;-)

Maybe I will check if I can put my vfd on the lathe as it is now (before all switchgear), and see if I can do some tests. But I doubt the magnetic switches will allow that :-(
Peter
 
You must log in or register to reply here.
Back
Top