Thread: VFD switching on a Hercus 260

Originally Posted by BobL

The emergency switches fitted to my machines with VFDs just cut power to the VFDs.
On the SAJ unit on the Hercus the spindle stops spinning in less than 1 second
With the Huanyang's on the DP and woodwork lather the spindles take 2.2 s to stop.
I would be interested to hear what others get.

Mills or drills rarely have an inertia problem. Tools and tool holders are usually very light.

Lathes are different, because they spin the workpiece not the tool. Think of a heavy 4-jaw chuck holding a masive heavy workpiece. You easily see times of 10 and more seconds coasting to a halt. That can be a long time, if your tie has been caught and you just managed to stretch to the red mushroom switch. Dynamic braking and DC injection can reduce the braking time from say 10 seconds to 3 seconds. But for this most VFD need be connected to line power (there are very few that can supply their electronics from the motor coasting as a generator).

Lathes with threaded spindle noses (AND no locking clamp!) can indeed unwind their chucks and have them fall on your toe if "plug reversed". Plug reversing a direct mains driven 3 phase motor is very quick, very brutal, very high currents flow. Plug reversing (or braking) with a VFD is a very gentle, well controlled affair, with defined ramp down and ramp up times and a true stop in between. I do not believe that a VFD (without external braking FET/resistor) can at all brake a lathe spindle as fast as to cause a chuck unwinding.

When mounting very heavy workpieces in my lathes (both Hercus260 and EmcoC8 have VFD) I take the time to change the ramp up and down times from 1s to something like 3 or 4 seconds. It is softer on the drive belt, and it prevents the VFD from tripping out due to excessive bus voltage.


Chris

Originally Posted by BobL

Australia is in the blue group ie just about everyone

Interesting to know: Australia is officially a 230V country.

Mains electricity by country - Wikipedia, the free encyclopedia has this to say:

"As of 2000, the mains supply voltage specified in AS 60038 is 230 V with a tolerance of +10% -6%.^[5] This was done for voltage harmonisation – however 240 V (and less commonly 250 V^[6]) is within tolerance and is commonly supplied.^[7] Mains voltage is still popularly referred to as being "two-forty volts". Bathrooms in hotels will often have a type I, C and A socket marked "for shavers only" as pictured on the right. Three phase (415 V) is also used.^[8]

Chris the only problem I've had in that regard is a big 4 jaw chuck I have tripping the protection if I stop it from high speed. I have no external braking resistor so it can't handle the 2 second energy dissipation requirement. It then trips off line and coasts to a stop, exactly the same as if power was simply removed. It's annoying at times as I need to remove mains power to reset the VFD. I could of course fit an external resistor, however I don't use that combination often enough to get too stressed about it and just manually reduce the speed before stopping. Changing the stop ramp time isn't practical as it's mounted within the cabinet.

Pete

Originally Posted by jack620

Any chance you made a circuit diagram at the time you could post here?

Hercus260_Wiring.jpg

That's great thanks Chris.

Originally Posted by Pete F

Chris the only problem I've had in that regard is a big 4 jaw chuck I have tripping the protection if I stop it from high speed. I have no external braking resistor so it can't handle the 2 second energy dissipation requirement. It then trips off line and coasts to a stop, exactly the same as if power was simply removed. It's annoying at times as I need to remove mains power to reset the VFD. I could of course fit an external resistor, however I don't use that combination often enough to get too stressed about it and just manually reduce the speed before stopping. Changing the stop ramp time isn't practical as it's mounted within the cabinet.

Pete

Pete, I have an external braking resistor on the Emco lathe. It does not bring much. You see, the maximum braking current through the external resistor is limited by the same VFD internal FET switch that switches the braking current through the internal braking resistor. The difference is that an external braking resistor has a MUCH greater duty cycle. This helps in an application like a conveyor belt feeding a stone crusher, or a hoist... but on a lathe there are certainly no duty cycle problems. To improve braking on a lathe, you would need an external high current FET switch with braking resistor. Not available for all VFD's, and if available too expensive, and hard to find at a discount. So forget this idea. My experience on both lathes is that a 4-jaw chuck with workpiece needs some 3-4 second ramp time. The Emco chuck is actually lighter, because it is direct mount. Whilst the Hercus 260 has a backplate that adds more weight. Chris

I'm now looking at a Schneider Altivar vector VFD from the supplier in the UK I linked above. They have one with 4 digital inputs and another with 6 digital inputs. The cost difference is about $60. Will I need more than 4 digital inputs?

The more expensive VFD also has 3 analog inputs and the facility to fit an external braking resistor.

Thoughts?

Chris

Originally Posted by jack620

I'm now looking at a Schneider Altivar vector VFD from the supplier in the UK I linked above. They have one with 4 digital inputs and another with 6 digital inputs. The cost difference is about $60. Will I need more than 4 digital inputs?

The more expensive VFD also has 3 analog inputs and the facility to fit an external braking resistor.

Thoughts?

Chris

I have an Altivar 28 for my Sieg X3 mill. Easy to program VFD, with a good set of functions. I like it and can recommend it.

For a lathe you will be fine with 4 inputs. You need 3 inputs for a 3 wire fwd-stop-reverse control. 3-wire means, if any one wire breaks the motor will not start accidentally, I would only use 3 wire control for a machine tool. The 4th input can be programmed for emergency stop.

BTW, the Altivar 28 also has an analog output. Can be used to drive an analog meter instrument, to display things such as instantaneous motor current in %, instantaneus motor torque in %, power supplied to the motor in %, It can be interesting to watch those figures as you perform different tasks......

On the minus points, this VFD has one of these tiny tiny cooling fans, that only have a short life..... it only comes on when the motor is started, but it can be louder than the motor itself.

Chris

PS: before deciding on ANY VFD, download the user manual, make sure you like the way it is programmed. Download the installation manual, make sure you understand the derating criteria. Make sure it can drive your motor at rated power, even at the highest chopper frequency setting, at 40C ambient temperature. In doubt choose the VFD one size larger. NEVER buy a VFD if the manuals are not available as a free download.

Hi Chris,

Originally Posted by cba_melbourne

My experience on both lathes is that a 4-jaw chuck with workpiece needs some 3-4 second ramp time.

By ramp time do you mean(for e.g.) 50hz to 10hz over 3-4 seconds then DC braking?
or
DC braking immediately for 3-4 seconds?

Hi Chris,

I cant see a lathe needing more than fwd, rev, stop and jog but I'm likely forgetten something. And really...... with a speed pot I wonder about needing jog(well not on lathes the size of mine anyway)

Stuart

p.s. " if any one wire breaks the motor will not start accidentally"??? I cant understand that?

Thanks gents. I have downloaded the Altivar manual and will go through it in the next cupla days.

Originally Posted by cba_melbourne

Pete, I have an external braking resistor on the Emco lathe. It does not bring much. You see, the maximum braking current through the external resistor is limited by the same VFD internal FET switch that switches the braking current through the internal braking resistor. The difference is that an external braking resistor has a MUCH greater duty cycle. This helps in an application like a conveyor belt feeding a stone crusher, or a hoist... but on a lathe there are certainly no duty cycle problems. To improve braking on a lathe, you would need an external high current FET switch with braking resistor. Not available for all VFD's, and if available too expensive, and hard to find at a discount. So forget this idea. My experience on both lathes is that a 4-jaw chuck with workpiece needs some 3-4 second ramp time. The Emco chuck is actually lighter, because it is direct mount. Whilst the Hercus 260 has a backplate that adds more weight. Chris

Ok, thanks Chris, good to know it's not worth the bother of the external resistor. I just need to remember to manually slow it down.

Pete

Originally Posted by Pete F

Ok, thanks Chris, good to know it's not worth the bother of the external resistor. I just need to remember to manually slow it down.

Pete

No, you need a lathe with a clutch. Problem then goes away.

PDW

I bought the 0.75kW Altivar ATV12. Delivery to Oz 3-4 days.

I've read the manual and the setup seems fairly straightforward. You enter the motor parameters from the data plate, then perform an auto-tune.

A question to get me going:
What is a sensible max Hz to set? I've read that you can drive most 50Hz motors to 100Hz safely. What about the rest of the lathe components though?

Chris

Originally Posted by jack620

I bought the 0.75kW Altivar ATV12. Delivery to Oz 3-4 days.

I've read the manual and the setup seems fairly straightforward. You enter the motor parameters from the data plate, then perform an auto-tune.

A question to get me going:
What is a sensible max Hz to set? I've read that you can drive most 50Hz motors to 100Hz safely. What about the rest of the lathe components though?

Chris

Congratulations.

If you use a common small HP TEFC 3 phase motor (not an expensive dedicated specialized VFD motor), then:

- if the motor is 2 poles, centrifugal forces acting on the rotor may get too large at 100Hz, I would not go beyond some 70Hz
- if the motor is 4 or more poles, you can get usable power from it up to 100Hz. At about 120Hz, the motor will just produce enough power to overcome its own losses and drive its fan.Centrifugal forces are not a problem because motor makers use the same rotor as for 2 pole motors.


About the rest of the lathe components.... you have to decide if the spindle bearings are up to the task of twice the spindle rpm. If it is a belt drive headstock, everything else is likely to cope fine. If it is a geared headstock, it depends. If it has precision ground hardened gears no problem, especially if it has one or two gears made from tissue reinforced phenolic resin, as such geared head lathes are known to run very quiet even at high rpm (example Emco Emcomat/Maximat). If it is a low cost Chinese geared head lathe...... I would probably not recommend to double its top speed.