VFD deceleration issue - PLEASE HELP !

My ( Harrison M250 lathe ) I had it on a 2 pole motor for many years and ran off a VFD at 25Hz to get the right speed. It stopped on a dime ! I bought a new 4 pole motor and this improved the finish but now the same VFD needs 0.5secs to stop the machine. WAAAAY longer than before. Big irritation for threading as I am not a machinist and lack practice. I only use the lathe for simple things like making laps and the like. The VFD is an Altivar 2.2Kw single to three phase I bought around 2000 and until now was absolutely trouble free.

Anybody might know why the 4 pole motor takes much longer to stop and what's to be done about it ? Both motors 1.5Kw and same frame size. SAME RPM !

TrueBor said:

My ( Harrison M250 lathe ) I had it on a 2 pole motor for many years and ran off a VFD at 25Hz to get the right speed. It stopped on a dime ! I bought a new 4 pole motor and this improved the finish but now the same VFD needs 0.5secs to stop the machine. WAAAAY longer than before. Big irritation for threading as I am not a machinist and lack practice. I only use the lathe for simple things like making laps and the like. The VFD is an Altivar 2.2Kw single to three phase I bought around 2000 and until now was absolutely trouble free.

Anybody might now why the 4 pole motor takes much longer to stop and what's to be done about it ? Both motor 1.5Kw and same frame size.

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You needed greater inertia for a smoother finish. You GOT greater inertia.

A half-second full-stop, no DC plugging, nor even braking resistor pak and a VFD as can even USE such.. read "not many" .. is actually rather remarkable.

A common default - even on four-quadrant AKA "regenerative braking" Dee Cee drives is around TWO seconds. That can be beaten with "active electronics", and by a LOT!

But why?

If you have an operational NEED to better that figure? You can add a seriously harder stop with a mechanical brake, be it foot, air, hydraulic, or electrically triggered.

Many, if not MOST Machinists have long been comfortable enough doing our work on lathes that had NO formal braking systems .... of any kind.

So.....it makes far more sense at less stress - on the lathe, its systems, and even the operator - to use a method of working that just doesn't NEED anything special as to braking.

Threading? Full sixty years since I was taught... as MOST in the "no brakes" era were taught.. to thread AWAY from a shoulder or other feature as would otherwise have required a repeatable stop-point.. and no longer gave the least damn ...once the "end" ran ..... off.

Into ignorant air. Instead of into.. a shoulder!

It wasn't just clever old greybeards, nor young "superstars".
All-hands needed to know how to do this.

Some of our lathes? The workholder, spindle, drive train, then workpieces massed... in the multiple TONS. Lot of "drama" yah try to call a halt, abruptly, to a matched pair of massive railcar wheels already on on their axle. For any reason.

There are PM threads "reverse threading", "threading in reverse". Also search on "upside down" or "back tooled threading".

You Tube videos exist as well. Grain of salt. Not all those who publish such things are so good that they should publish!



No fear. So long as you grok the concept, the "execution" of any new idea is on you, not they, in any case.

And you do good work. Because you give a damn. Yah?



PS: Altivar... year 2000? ISTR the "71" family wudda been their top-end around 20 years ago? My 10 HP one was made in Indohooliga, later ones in China. Group Schneider tends to "get around".

Welll it is about ten years overdue for a refresh of its capacitor bank with new cans. They die of old age as well as from stress. And phase-conversion, 1-P to 3-P IS "stressful". So "new", not "NOS", ELSE don't bother. Good luck even FINDING "new" that meet the spec and also FIT! I'd plan on having to mount them outboard.

Except that...

A new VFD will generally be cheaper, if not also better, than caps alone, yah but avoid the junque. Might want to start budgetting? A year 2000 VFD is getting rather ripe for failure. Even if it sat on the shelf 19 out of 20 years.

Change your gearing so the motor runs at 15-30Hz for threading, this will decrease the rotational inertia. Newer VFD's can give a bit better motor control and smoothness when you do the autotune, and add a braking resistor for quicker stops. Still, I set up most of smaller lathes with 1 second stopping time, and/or a different braking time for higher speed/momentum. Expecting under 1 second is pushing it. Threading is a skill, just need some practice. That being said I use an electronic stopping system, typically repeats to +/- 0.0002" threading at up to 450 RPM, I do not disengage the half nut.

Not sure that greater inertia is the issue.... The energy stored in the slower motor should be LESS, since the rotor is probably abut the same size (same frame size), but the rotational speed is half, so the mass x velocity squared portion should be 1/4 the original, while the mass is only fractionally different.

The true reason may be that the torque AT THE MOTOR to stop a given mass of workpiece and drive system is going to be more, assuming you changed the pulleys to keep the same work speeds. it is closer to a 1:1 ratio, less reduction.

So the work and drive inertia is the same, and has less mechanical "disadvantage" back to the motor due to speed reduction, because the slow motor needs less reduction.

There is more tendency for the work to "drive the motor" during braking due to the closer ratio of speeds.

You may be able to adjust parameters or even add a braking resistor (energy absorber) to help out. Look at the parameters first.

JST said:

Not sure that greater inertia is the issue.... The energy stored in the slower motor should be LESS, since the rotor is probably abut the same size (same frame size), but the rotational speed is half, so the mass x velocity squared portion should be 1/4 the original, while the mass is only fractionally different.

The true reason may be that the torque AT THE MOTOR to stop a given mass of workpiece and drive system is going to be more, assuming you changed the pulleys to keep the same work speeds. it is closer to a 1:1 ratio, less reduction.

So the work and drive inertia is the same, and has less mechanical "disadvantage" back to the motor due to speed reduction, because the slow motor needs less reduction.

There is more tendency for the work to "drive the motor" during braking due to the closer ratio of speeds.

You may be able to adjust parameters or even add a braking resistor (energy absorber) to help out. Look at the parameters first.

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Well "tiny" goods, so it is relative. My spindle, bare, prolly weighs more than his motor and then the DC motor is a whole nuther animal!

With a "naked" VFD already clocking one HALF second stops, yah, a bit of braking heaters could go awesome.

Also cost more than it is worth?

Fair bet that VFD wasn't built to easily use 'em.

But "tiny" again. A simple friction brake could be quick, easy, and handy.

Guys, I did not change anything. Same pulleys. I replaced the 2 pole motor running at 25Hz ( 1400rpm ) with a 4 pole running at 50Hz ( 1400 rpm ). Motors are same power, same frame size. The VFD could stop the 2 pole motor ( running at 1400 rpm ) in 0.2 secs from 500rpm ( 0.2 sec was the decc setting !) . To me looked like it was stopping on a dime. I went through all the parameters of the VFD and all I can do is to 1) put the old motor back and 2) try another VFD .

Again : the rotational speed of the motors is the same.

As I said, try the 4 pole motor set to 25Hz and adjust the gearing for the same spindle speed.

Are you in scalar or vector (assuming the drive supports vector)?

Have you re-run the motor learning procedure?

What actually happens when you try to stop faster? Does the drive fault out with an error code (which?), or does it just not stop?

TrueBor said:

Guys, I did not change anything. Same pulleys. I replaced the 2 pole motor running at 25Hz ( 1400rpm ) with a 4 pole running at 50Hz ( 1400 rpm ). Motors are same power, same frame size. The VFD could stop the 2 pole motor ( running at 1400 rpm ) in 0.2 secs from 500rpm ( 0.2 sec was the decc setting !) . To me looked like it was stopping on a dime. I went through all the parameters of the VFD and all I can do is to 1) put the old motor back and 2) try another VFD .

Again : the rotational speed of the motors is the same.

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Ah, well, that is different...... So the difference is presumably in the motor itself and how it interacts with the VFD. I assume you have tried cutting braking time down, and either it did not help, or you were already on the minimum settable time..

If you need the stopping time, you might see if the VFD can do DC injection braking. It will heat the motor up, but should give you nearly any stopping time you want, with no overvoltage.

The stopping time is longer because the vfd has a fixed hz per second decelleration rate. Read the manual and change the settings

johansen said:

The stopping time is longer because the vfd has a fixed hz per second decelleration rate. Read the manual and change the settings

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In agree with that.

But it should be settable...... unless there is no more range.

TrueBor said:

Guys, I did not change anything. Same pulleys. I replaced the 2 pole motor running at 25Hz ( 1400rpm ) with a 4 pole running at 50Hz ( 1400 rpm ). Motors are same power, same frame size. The VFD could stop the 2 pole motor ( running at 1400 rpm ) in 0.2 secs from 500rpm ( 0.2 sec was the decc setting !) . To me looked like it was stopping on a dime. I went through all the parameters of the VFD and all I can do is to 1) put the old motor back and 2) try another VFD .

Again : the rotational speed of the motors is the same.

Click to expand...

On many vfd’s the “deceleration time” is implied from full speed, presumably 50Hz. Decelerating from 25Hz would then imply stopping in 1/2 that time. Or in your case it would take twice as long at 50Hz as at 25Hz. Therefore the question is can you and have you tried reducing the deceleration time parameter?

rabler said:

On many vfd’s the “deceleration time” is implied from full speed, presumably 50Hz. Decelerating from 25Hz would then imply stopping in 1/2 that time. Or in your case it would take twice as long at 50Hz as at 25Hz. Therefore the question is can you and have you tried reducing the deceleration time parameter?

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Yes, of course. Give me some credit ! The old 2 pole motor running at 25Hz would stop in 0.2s . New 4 pole running at 50Hz stops in 0.5s . ANy shorter and the drive shuts off and logs an error. I then need to cycle the power.

TrueBor said:

Yes, of course. Give me some credit ! The old 2 pole motor running at 25Hz would stop in 0.2s . New 4 pole running at 50Hz stops in 0.5s . ANy shorter and the drive shuts off and logs an error. I then need to cycle the power.

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Does your drive have a dynamic braking feature and resistor of the appropriate resistance installed?

motion guru said:

Does your drive have a dynamic braking feature and resistor of the appropriate resistance installed?

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+1 this would seem to be the low-hanging fruit of a solution If the drive does not support a braking resistor, maybe time for a new one.

Also MG you probably should learn to pad your posts up to be termite-lenght. They are way too brief and impart way too much good information in a short space.

TrueBor said:

Yes, of course. Give me some credit ! The old 2 pole motor running at 25Hz would stop in 0.2s . New 4 pole running at 50Hz stops in 0.5s . ANy shorter and the drive shuts off and logs an error. I then need to cycle the power.

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Well, shoot..... you had oughta have said that earlier..... different problem, different solution. (Assuming the "error" is an OV (overvoltage) error)

Yes, the resistor if you have the capability for it. That is exactly what they are for.

A vfd typically employs regenerative braking down to say 20% of nominal rpm, below this it will employ dc injection.

This may account for everything observed

motion guru said:

Does your drive have a dynamic braking feature and resistor of the appropriate resistance installed?

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Yes, it does have DC injection and I have a resistor but I never connected it. The weird thing here is that it worked perfectly as it is for almost 20 years. The drive is an Altivar 58 - I'm busy trying to find the programming manual which I must've misplaced a couple of weeks ago.
Bit OT but worth mentioning : changing from 2 to 4 pole motor DID make a distinct improvement in the surface quality in light, finishing cuts.

JST said:

Well, shoot..... you had oughta have said that earlier..... different problem, different solution. (Assuming the "error" is an OV (overvoltage) error)

Yes, the resistor if you have the capability for it. That is exactly what they are for.

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Sorry if I caused confusion ! I have the resistor and will connect it and will also check DC injection once I find the manual. Will report back.