Inverters and clutches .

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Whilst talking about motor control systems on the other thread I might as well mention another related problem :

Most of the commercially available inverter systems for driving three phase motors from single phase are very poor in start/stop response . With some the run up / run down time for motor speed is of order of several seconds – sometimes tens of seconds .

When doing any real engineering work the delay is at best irritating and on production work can be costly .

Bear in mind that on many types of work starting and stopping can be an almost continuous activity .

Therefore to be practical many inverter driven machines really need a mechanical clutch / brake as well .

Michael Williams .

.

[nigel jones 5Participant @nigeljones5]

Hi Michael….and?

[jason udallParticipant @jasonudall57142]

What ? no "regen" brakeload?

…most Inverter Drives in CNC use some variant of "toaster"..and or DC injection for braking.. also mechanical spindle brake too for spinde orient…

[John Stevenson 1Participant @johnstevenson1]

Start stop can be changed under programming.

Default is usually 5 or 10 seconds, start and stop which i agree is too long.

I usually program for 1 second each, Reason is if you cut the braking down to below a second without the optional braking module it often throws the VFD into error mode because of the back EMF.

I'm not a big believer of the braking modules, why do you need to stop something dead ? It only causes wear on keyways, splines and other transmission components when not needed.

It's not like most of use are running anything bigger than a couple of horsepower, mainly because Tesco have got all the extra horses in thier meat pies.

[_Paul_Participant @_paul_]

I've noticed stop/start varies quite a bit depending what machine they are running I have two identical Teco 2HP drives one is built into a Varispeed 2J (2HP) and the other powers an old Geo. Taylor mill (1HP TEFC) & Boxford lathe (3/4 HP Gryphon) (and a 1/2 HP Alba shaper occasionally).

The Drive running the old mill & Boxford will (E) stop under a second but the one running the Bridgeport takes some time to run up and quite some time to stop >5 secs I can only guess that is down to the mass of ironwork in the head of it, working the spindle brake produces a faster stop but the drive goes into error with a code of OCD (back EMF?).

I have tried adjusting and mirroring the parameters on both drives but it makes little noticeable difference to the BP.

I would like the BP to stop a little faster might hekp me keep my digts a while longer!

Going back to Michael's original text sadly a clutch wouldnt be practical in my application so I guess my next stop will have to be a braking module never tried one do they actually work?

All this chatting has made me a little Horse….

Paul

[Clive HartlandParticipant @clivehartland94829]

Ramping up and ramping down are /or should be part of the programme of the inverters. Mine can be set to very short times for both, but the slowing down is not without danger as you cannot grab the chuck on my drill until it stops.

Mine is set to about 5secs. ramp up and down and is no problem.

I suppose a good term is. 'Soft start and stop'.

Clive

[Douglas JohnstonParticipant @douglasjohnston98463]

If your not in a hurry the soft start is a great way to operate the lathe/mill. My previous lathe had a rapid start which can't have been good for the drive mechanism of the lathe and it certainly was'nt good for my nerves. The soft start gives you a controlled start which can get you out of trouble if things go wrong at switch on since you can switch off before the motor is up to full speed.

Doug

[KWILParticipant @kwil]

Breaking modules do work, being a resistor to absorb the unwanted power on stoping, however if the ramp times are set sensibly they are not essental. I do not understand Paul's problem, my Bpt responds fully to whatever ramp time i have programmed into the Inverter, having experimented when I first set up the inverter parameters, it remains around 5 seconds (i have not put a stop watch on it). Other than the occasional irritation when drilling out using different sizes of drill in turn, what if it does take a little time, we are not on piece work payments.

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Fizzy :

Therefore to be practical many inverter driven machines really need a mechanical clutch / brake as well .

Crazy text entry system decide to ignore some of my typing and go to sleep mid edit .

Should be a question :

Therefore to be practical many inverter driven machines really need a mechanical clutch / brake as well ?????

General :

The old system of having a generously rated motor running continuously and a mechanical clutch to engage / disengage is still a good one .

Its not just usage convenience that matters when talking about start / stop times etc and inverters . The general idea of stopping and starting larger motors frequently , no matter what the means of doing it , is bad in principle – and especially when the motors are not specially designed for this type of usage .

Michael Williams .

[BazyleParticipant @bazyle]

BIt like the 'fancy' features on my laptop when you hover the mouse over some options it gradually fades them up for an absolute age before you can use them.

As mentioned by Clive 'soft start' was one of the reasons for fitting inverters when first invented to save stress and blowing fuses. The default settings are just a 'starter for ten' which gets the thing working to start with. Perhaps a lot of people are scared of changing the settings in case of messing it up so teh manufacturers should think about sticking a knob on to set it.

However brakes and clutches are extra mechanical bits that cost more than electronics nowadays so we need to learn to live with the new world.

[nigel jones 5Participant @nigeljones5]

thanks

nije

[I.M. OUTAHEREParticipant @i-m-outahere]

Hi all,

I have a question about inverters /vfd units that you may be able to sort for me .

I have a few good 3 phase motors laying around and was contemplating converting my two small lathes to use them and an inverter seem fairly cheap to buy .

All the motors are under 1HP and i have a C2 sieg and a little larger lathe that is a bit bigger than say a C6 sieg and is 230V mains single phase powered (0.55kw motor)

The C2 i got cheap as the controller is stuffed and not worth fixing but at  $280 AU here in Australia for a new one i think i like the reliabillity of AC MAINS powered motors !

How do these inverters deal with emergency cut out switches that one needs to have for safety ?

You need to be able to cut power to the motor fast if there is an emergency so there must be a contactor in there some where with its actuator coil connected in series with the shut of switch.

Now the inverter is isolated from the motor windings once the contacts open, can the inverter sense this ?

Is there any provision for emergency braking on these that can be connected to the emergency cut off switch?

 

Ian

 

Edited By SLOTDRILLER on 20/03/2013 03:21:44

[Anonymous]

You definitely don't want a contactor between the VFD and the motor. That will probably result in a damaged VFD.

Most VFDs should have a 'stop' function, which can be wired as an emergency stop. Depending on the design of the VFD there are several things that can be done once the 'stop' button is activated.

1: The VFD simply stops driving the motor, and the motor coasts to a stop exactly the same as a motor where a contactor is opened.

2: A DC current can be injected into the motor windings to oppose the motor rotation and provide a braking force; known as DC injection braking.

3: Resistors can be used to dissipate the energy in the motor windings and thus stop the motor more quickly than by coasting.

The VFD on my CNC mill uses a braking resistor to stop the spindle within a couple of turns, so that the spindle can be reversed quickly when using compression/tension tapping heads.

In the manual for my Britan repitition lathe it is suggested that one way to stop the motor quickly is to reverse the direction momentarily, thus reversing the currents in the windings. I guess that's a kind of manual method of current injection.

Regards,

Andrew

[BazyleParticipant @bazyle]

Good point about reversing the motor. Some of the bigger mills and lathes (normally outside amateur use in UK) have a speed sensor on the spindle and put it into reverse until it detects it stopped. All good old electromechanical stuff not fancy opto/magnetic gizmos. Probably needs to be disabled when fitting a VFD.

[Mike PooleParticipant @mikepoole82104]

If your lathe has a screw on chuck or faceplate I think a rapid stop could cause the chuck to unscrew. An inverter can stop the machine much quicker than just cutting the power. Robots stop much quicker using a controlled stop rather than just cutting power when the braking will be by mechanical brakes which are only designed to support the arm after it has stopped. If you have a camlock chuck fixing Some savage rates of deceleration can be used provided the regenerated current is dumped into a resistor to stop the DC link voltage rising too high which will fault the drive, this will also cause some attrition to the mechanical parts of the drivetrain as previously mentioned. A clutch has much to recommend it.

Mike

Edited By Michael Poole on 20/03/2013 16:57:03

[Russell EberhardtParticipant @russelleberhardt48058]

The VFD on my lathe can not only vary the slope of the start and stop ramps but can also be set to give an S shape curve to eliminate rapid change of acceleration (jerk). Although it doesn't eliminate the jerk driving the lathe!

The soft start also eliminates the starting stress on the motor and thus the necessity for a clutch.

Russell

[I.M. OUTAHEREParticipant @i-m-outahere]

Thanks for the replies and much needed information .

I had a dig around on the net to see what is available in the way of reasonably priced inverter units and found one made by hounyang (think thats right ) anyhow after a good read of the specifications it seems it has connectors for external emergency stop and braking resistor along with a lot of other featurs i probably will never use .

It has ramp up/down or coast stop features etc and amazingly cost less than the DC controller for the C2 lathe – can't work that one out !

All the components on the DC controller wouldn't cost $20-$30 !

While i'm on here i may as well ask if anyone has a recomendation for motor power rating to use on the C2 ? Original unit was around 400w DC so would a .55KW 3PH unit suffice ?

I have plenty of room behind the machine so physical size is not a problem withing reason anyhow .

I know on the larger lathe it has a .55KW motor single phase and it has been fine as the machine has a 6 speed v belt drive

I will have to make some pulleys for the C2 so i may as well make some 3 groove vee pulleys to give 6 speeds in total using the high and low gearing in the head .

That way i can use the VFD to fine tune my speed settings as needed without slowing the motor speed down too much .

Any thoughts welcome .

 

Ian

Edited By SLOTDRILLER on 20/03/2013 19:28:25

[Ken FoxParticipant @kenfox67095]

One thing which has caught my eye is that no one has mentioned the effect of current limit on stopping. If you call for a rapid slow down via a short time stop ramp the drive will require more current to get this if the load has a high inertia. If then the current limit is set too low the torque will simply not be available and the stop will take longer than expected from the ramp setting. I suggest taking a look at the current limit and setting it to substantially above the motor rating, say 150% to 200% or more. After all, before the days of VFD's an induction motor would draw several time full load current on startup or on stop by plugging or dc injection.

Ken

[Sub MandrelParticipant @submandrel]

Hi Ian,

I converted my CLM300 (which is essentially a C2) to runn of a 1/2 HP motor by replacing the electric motor with a layshaft held in a bracket. The toothed pulley on one end and a three-step pulley where the armature would have been. The motor has a two step pulley. With the back gear this gives me a useful range of 12 speeds with limited overlap. I don't use all the speeds.

I keep toying with upgrading to a VFD, but if I don't I will still be content.

Neil

[I.M. OUTAHEREParticipant @i-m-outahere]

Thanks Niel, If 1/2hp works fine on your lathe then it gives me some confidence to mve forward with the conversion.

[MuzzerParticipant @muzzer]

"If your lathe has a screw on chuck or faceplate I think a rapid stop could cause the chuck to unscrew."

Michael – I believe this is why the "Camlock" spindle was developed. The Americans seemed to like to switch their machines into reverse whilst running at full speed – without anything fancy like an inverter in the way. Time is money I suppose! The Camlock (as used on my Bantam for instance) doesn't care which direction it turns as the chuck is driven by pins and held on by cams. That's quite a bit worse than starting direct on line but they must have been rated for it presumably. Must have taken a nice gollup of current out of the mains, mind!

My lathe inverter has an annoying 1 second delay that can't be avoided. Even pushing the e-stop results in that delay which seems bizarre. Can't imagine what they were thinking of.

Merry

[Anonymous]

Posted by Murray Edington on 22/03/2013 23:15:51:

My lathe inverter has an annoying 1 second delay that can't be avoided. Even pushing the e-stop results in that delay which seems bizarre. Can't imagine what they were thinking of.

Are you still using the late lamented Wavedriver unit? Or did you ditch it in favour of a modern VFD?

Regards,

Andrew

[MuzzerParticipant @muzzer]

No, haven't changed it, it's still the old Xtravert from PDL Electronics who still appear to be in business. Hasn't been a bad unit, 15 years out. Rated at something like 6kW but of course set up for a smaller machine, probably around 2kW, I forget. Apart from the bizarre delay it has all the features you'd want and expect of a modern VFD – plus a noisy fan. Similar unit also on the drilling machine but without the speed control pot etc. It has a 3-phase motor too and this was the simplest way to operate it at home.

Murray

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