Braking resistor on smaller lathes

[Dean Moldenhauer]

All, I was reading on another forum about the function of a braking resistor on a VFD and comments about how some VFDs have them by default and some are added as an option. In the context of the thread I was reading, it came across as a "no-brainer" to have a braking resistor on your VFD.

So... I have a Oneway 1224 on order and one of the options for their entire line of lathes is to have a braking resistor added. For a lathe the size of the 1224, do you think this is a worthwhile option? I very well could be turning bowls on this lathe, albeit limited to the ~12" diameter. Still, I'm sure some of those blanks could be heavy in relation to the 1 HP motor that is on the 1224.
I will have to check with Oneway, but I think I'm still early enough in the build process to have that option added.

Whaddya' say?

Thanks,
Dean

 

[Bill Boehme]

As I wrote in my post on this topic in the Turners Forum on Sawmill Creek, it generally isn't necessary on woodturning lathes, but it certainly wouldn't hurt. The fact that Oneway offers it as an option shows that they don't see it as being essential, but rather as something that gives users a warm fuzzy. Warm fuzzies help sell more lathes.

 

[Dean Moldenhauer]

I'm not above warm-fuzzies, but I will interpret that as meaning that the resistor is not in the "must-have" column. I was already suckered in on the RFI filter, remote start/stop and the wheel set - the worst kind of suckered in... convinced myself that I needed them .

Thanks for the input, Bill.

Dean

 

[Mark Jundanian]

I have electronic braking on the Nova DVR. I usually leave it set to off. For some reason I like seeing the lathe spin down and the electronic braking seems very unnatural. Admittedly not a very good reason.

The other reason I don't use electronic braking is it tends to unscrew the chuck from the spindle. I've had a Longworth come all the way off. You just need to keep an eye on it.

Now my bandsaw, on that I wish I had a brake.

 

[Dean Moldenhauer]

Thanks, Mark. I wondered a little about that as I think I have seen someone post something similar, except they caught it before it cam off completely. That doesn't sound good - both from a safety standpoint as well as a potential damage to equipment.
I'm right there with you on a bandsaw. I will be shopping for a bandsaw in the not-too-distant future and that is one of the things that I am looking for. In the Lagunas I've looked at, it's what has made me move on from a 1412 to consideration of a 14BX, or other similar models.

Dean

 

[Bill Boehme]

I think that the ramp time on my lathe is six seconds when the motor is running full speed. Whether or not that deceleration is too rapid depends considerably on which speed range you have the belt in ... assuming that your lathe has a multi-step pulley and belt transmission. If I were to have a 100 pound hunk of wood mounted on my lathe with the belt in the high speed range and running at max speed then a six second ramp down time would be too short (don't ask how I know this). If the belt were in the low speed range then the chuck/faceplate might not come unscrewed, but I am not brave/foolish enough to find out.

 

[Davis Stevenson]

I dislike the braking feature on my 18-36 at times, especially when doing large heavy bowl blanks, actually. Without a set screw on a faceplate, I had it slow down so rapidly the momentum unscrewed the faceplate. Another time with a set screw properly torqued, it dragged the set screw around the flat and damaged the last thread on the spindle. The brake just puts too much torque on it for those situations.

I would rather hit a button and have it coast down than have to slowly decrease the speed with those large, heavy blanks.

 

[john lucas]

VFD 's have adjustable braking but we dont have access to the adjustment. Probably due to liability. When I installed my own VFD I played with the adjustments. My Powermatic of course doesnt give you that option.

 

[Bill Boehme]

VFD 's have adjustable braking but we dont have access to the adjustment. Probably due to liability. When I installed my own VFD I played with the adjustments. My Powermatic of course doesnt give you that option.

That depends on the manufacturer and the end user. To be nit picky, the Powermatic doesn't actually have braking, it's just a ramping down of the speed control.

 

[robo hippy]

Hmm, I would consider ramping down and braking to be pretty much the same thing, with the ramping down being the slower braking.... I have never had anything unspin my chucks except when sanding in reverse. When I had my PM3520A, the braking/ramping down would trip the lathe. They helped me adjust to a longer braking time which ended that problem. I do remember some old lathes that had huge hand wheels on the back part of the motor for using your hand as a brake. Not sure if braking systems are on the grinders or not. My 1 hp Rikon spins forever with the heavy D Way wheels, but the Baldor grinders slow down a lot faster...

robo hippy

 

[john lucas]

It's interesting. When I installed an aftermarket VFD there were 2 parameters that I could set that both did the same thing as what I call electronic braking. I never did find out what the difference was so I just played with them a little and set it to what I thought would work and it did. There were probably 20 things you could set and I only knew what 2 or 3 actually did. I'm not an electronics expert. Fortunately the 2 or 3 things I set worked just fine.

 

[Bill Boehme]

Hmm, I would consider ramping down and braking to be pretty much the same thing, with the ramping down being the slower braking....

Would easing up on the gas pedal be the same thing as stepping on the brake? Either one can bring your car to a stop with possible minor inconveniences such as going through a red light before finally coming to a stop. The difference between slowing down and braking is more nuanced with woodturning lathes because we don't really care how or where it stops as long as it stops.

The Delta S1 inverter used by Powermatic is called an "open-loop" controller because it doesn't natively sense the actual motor speed. Other high end lathes such as the Robust use "closed-loop" inverters that sense the actual motor speed to perform better "electronic braking". However, the main advantage of closed-loop inverters is stable motor control at a much lower speed than can be achieved with open-loop inverters.

 

[Doug Freeman]

While I dont consider braking/ramp down as an absolute necessity, I consider it as highly desirable as evs. I love the braking on my Nova Galaxi compared to the long coast down of my previous reeves drive lathe. Much faster to stop the piece for a quick inspection.

Sure things can unwind - thats why I drilled and tapped all my chucks and faceplates for set screws - no problem at all. Dont know about other brands, but Nova specifically points out the need for setscrews due to the braking as well as reverse rotation in the manual and on the control screen. Any reversible lathe should have set screws in place for about any device threaded on. My lone exception are the direct thread wood friction chucks I use to remove tenons/clean up the bottom of bowls and such. Only used with the tailstock and normal rotational direction.

I dont know Robust lathes well at all - if it has a fast ramp down that may suffice. If its a choice between free wheeling or the braking resistor its definitely a no brainer.

 

[Don Wattenhofer]

This is an interesting thread and since I have been applying VFDs to lathes and other machines that I have since about 1990 I am aware that there are features that can be added to the controller to facilitate rapid deceleration (or call it braking if that turns your crank). If the deceleration time can be adjusted and you set it too fast the drive will overload and coast to a stop, but like others have said you don't need it to stop that fast.
Is it really true that the drives supplied with new lathes can't be adjusted or is ti that the programming instructions don't come with the lathe.
An added note: all of the VFDs that I have retrofitted to my machines have a maximum frequency setting so on all of my lathes I have set it to 90Hz which gives me 50% more speed although the torque curve drops off when running beyond 60Hz and I leave the mechanical ratio at it'slowest position. Also note that if you have a 100LB out of balance turning blank and you inadvertently set 90Hz the drive should overload before the lathe walks away.

 

[Bill Boehme]

I dont know Robust lathes well at all - if it has a fast ramp down that may suffice. If its a choice between free wheeling or the braking resistor its definitely a no brainer.

It's actually not as drastic as having to choose between freewheeling coast to stop or the use of a braking resistor to stop in a reasonable length of time (except perhaps in the case of the Nova Galaxi which uses a stepper motor instead of an AC induction motor). It seems like a fairly common misunderstanding among woodturners is that the purpose of a braking resistor is to stop more quickly than otherwise possible. That actually isn't the case unless we want to do a "slam on the brakes" kind of stop. The purpose of a braking resistor is to dissipate the kinetic energy of the spinning mass into heating the resistor rather than into heating the motor windings.

A braking resistor is mostly important where there is a very large amount of energy that needs to be dissipated in a very short amount of time which isn't a typical scenario for a woodturning lathe. A good example of where a braking resistor is essential would be something like a heavily loaded conveyor running downhill (an aiding load on the motor) and needing to stop quickly. However, if you're spinning a very heavy piece of wood at too high a speed and making very frequent stops to inspect things the motor might appreciate being a tad cooler with a braking resistor.

I don't know how many woodturning lathes with variable frequency drives use braking resistors, but I would guess that most do not. My seven year old Robust American Beauty which has a Leeson motor and Toshiba inverter does not use a braking resistor. The ramp down from full speed to stop is about the same as other lathes. I actually did buy a braking resistor a couple years ago (because that's what engineers do), but haven't installed it because I haven't seen a need. Even when I work the heck out of my lathe with frequent starts and stops the motor barely gets warm. Even if I were to install the braking resistor I wouldn't change the ramp down time because it seems to be just right ... just like the porridge that Goldilocks liked when she illegally entered the bears cottage.

BTW, when I got my Robust it included an abridged printed manual for the Toshiba inverter as well as the full manual on a DVD. The inverter was not locked so I am free to adjust whatever I want to. Of course, it hardly needs to be said, but if you don't know what you're doing then don't do what you don't know how to do.

 

[Bill Boehme]

An added note: all of the VFDs that I have retrofitted to my machines have a maximum frequency setting so on all of my lathes I have set it to 90Hz which gives me 50% more speed although the torque curve drops off when running beyond 60Hz and I leave the mechanical ratio at it'slowest position. Also note that if you have a 100LB out of balance turning blank and you inadvertently set 90Hz the drive should overload before the lathe walks away.

It's really the motor specs that determines the maximum frequency. For most motors that aren't specifically designed for inverter duty the maximum frequency without reducing the maximum current is 60 Hz. Also, the minimum frequency for those motors is typically 25% of max frequency (15 Hz) because the low fan speed reduces cooling.