‘Hacking’ an inverter

[AjohnwParticipant @ajohnw51620]

The Clark CL430 looks like the worst lathe I ever bought or used in my life Michael. All that could be said about it was that the finish was pretty good. The alignment was terrible. This was shortly after they started appearing in this country and sadly had been "updated" it to 20" between centres by chopping 2" of the tailstock nose which meant it wasn't possible to turn from the start of the work right up a centre. I sold it on bargain pages. As I pointed out the problems 2 people walked away but one just wanted it to shorten bolts in his garage also to look good to the customers.

Interesting cast iron. I reckon it was wearing away and colouring the oil I put on it. Very very dead so good for the finish.

Hope they have improved. Suspect they have.

I sold the Peatol. No one will believe the biggest cut I took with it. One side of a V in a pulley in mild steel and I could only machine it with a form cutter. Circa 3/4 deep cut with microscopically thin curls of swarf coming off – doing this bent the head out of alignment. Also over 0.2" deep cuts in 6" diameter aluminium with decent feed rates. Difficult to see the lathe for swarf. Not sure about the UK but other countries have changed the motor to one similar to the shereline. The one I had used a rather nice GEC 1/4hp that ran rather hot even if doing nothing. I'd say a realistic heavy cut on 1" dia mild steel with that is up to circa 1.5mm with a well sharpened tool but the head will distort over time. These will give an excellent finish. Probably down to the concrete in the bed and nice tight bearings.

I was tempted to get a sherline for a certain type of work but couldn't find one with the kit I would need with it. However the head on these may not be as well aligned as the Taig's are and might also bend if for some reason I had to push it hard. In stead I finished up with a Friday afternoon complete design job to made next Monday from Wabeco. Not all that true really. They are pretty accurate and done correctly – in places.

John

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Edited By Ajohnw on 07/03/2016 16:45:23

[frank brownParticipant @frankbrown22225]

Over running inverters will shorten their life. the question is from what to what? if it is an industrial product it could be rated at full power for 100,000 hours (about 12 years 24/7). For home workshop use, it could be 20 hours a week at 50% output and 2 hours at the 110% rating. This could still work out to be over 5 years. It what happens when its overloaded that is important, I would say that its mainly over heating which is going to kill it, so don't run it too long at 110% with out a cooling off period.

On the subject of the Audi, was the oil pump and oil cooler uprated? similar sort of conundrum.

Frank

[MWParticipant @mw27036]

Ajohnw, i made the mistake of going into model engineering having learned on a colcester master, a completely different sort of animal for turning, but it was really old and kept putting a constant taper onto the work, i still enjoyed the huge range of gears and multiple feed screws it had to offer.

Then i saw the sherline, asked for it, decided they didnt want it and would let me have it. I wont make any bones about it, the DC motor is either a paperweight or a dust collector but not really worth being called a motor for anything other than lego. I have a strict aversion to variable speed DC motors for lathes because of it. However, that said, on soft metals it would produce a finish and tolerancing of a pro, the slides were tight and the movement was silky smooth, very well made, i just dont understand why they go to that trouble only to stick a lawn ornament of a motor on it.

I was sick of it at that point, so i saw the clarke, reckoned it would fit my remit so to speak and bought it. You're right though, its as reliable as a dog that constantly needs to go to the vet and like flogging a dead horse sometimes. I guess i'm just a cheap skate.

Oh well, back to making more saw dust.

Michael W

[AjohnwParticipant @ajohnw51620]

Saab were offering a pretty hefty power increase on the 95 stating that it wouldn't have any effect on the warantee.

Makes me wonder how long they thought the clutch would last as mine died after 25,000 miles at the usual power levels and was replaced free of charge. It started juddering. I recently had an X type one fail at just over 40k, slipping as soon as the engine warmed up. That could be chipped too.

John

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[Chris Evans 6Participant @chrisevans6]

I am happy with my Chinese cheap (sub £95) inverter running my Bridgeport. It is rated to power a bigger motor to allow for start up load. All this was set up by a friend who has 5 of these inverters running his workshop contents.

It is a much smoother not to mention cheaper solution to the static inverter powering my 3hp lathe that cost around £400

[Roland MannParticipant @rolandmann14066]

Just a follow up to what has happened here, which is rather interesting.

My discussions with Drives Direct were fruitless, and so as he was continuing to repeat sell the same 1/2hp inverter as a 3/4hp inverter, with no information in the listings that it was 'hacked', I left negative feedback on ebay.

The next day I noticed that the negative feedback had gone. No idea how this happened, but it seems to undermine the feedback system completely if somehow a seller can remove negative feedback.

I'm trying to find out what happened, but communicating with Ebay is like trying to get an audience with the Queen.

Roland

[John Stevenson 1Participant @johnstevenson1]

He sells more than you on Ebay – Simples

[AjohnwParticipant @ajohnw51620]

It seems some sellers can appeal feed back. I was told that following a serious problem with a seller. He even changed the pictures in the listing when I complained. They told me that he had requested that the feed back was removed. It remained and he made rude comments in several other of his listing for a while and eventually left ebay. forcibly I suspect.

If you can contact ebay and explain exactly why you left it they might re instate it. I've had no problem contacting them and have always had pretty prompt replies once I have found out how – they keep changing the method and of late seem to have automated the usual problems – I had a £99 refund a month or so ago and still have the item. Probably because the seller wouldn't want to fork out the return costs – trouble is I don't want the dam thing and it's big and getting in the way.

John

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[HollowpointParticipant @hollowpoint]

Sounds like its just a way to make more money. Badge up a small inverter to look like a bigger one and pocket the difference. No doubt the smaller inverters cost less to purchase.

[Neil LickfoldParticipant @neillickfold44316]

I would not run that unit on anything other than a .37kw motor or 1/2 hp on single phase.Buying something and then only using it part time is the wrong way to look at the issue. You got sold the wrong item. I have several VFD drives, all run within the power rating of the set up. I like them as it is easy to make the motors run at 60 hz or 20% over 50Hz . It is great. I never have over heating issues if I decide to spend all day in the workshop making my latest thing. It seems that some of these traders on ebay Aliexpress have a system of selling sometimes things that are not what is claimed. I'm in a dispute over some collets. They run out by .15mm when they claim to be 0.015 mm which I was happy to accept. Im not a happy customer with them. I think they should refund your purchase price and they should pay to have the item returned if they want it back. It clearly in your case is not a 1 hp single phase to 3 phase inverter.

Neil

[Michael BriggsParticipant @michaelbriggs82422]

I would not hesitate to run the VSDs in my workshop at 70 or 75 Hz, reduced torque but I have never had my lathe or miller slow down at these frequencies due to insufficient torque.

A point of general interest is I had a conversation with a technical specialist from Rockwell (aka Allen Bradley) this week about reducing the variety of drives OEMs supply to my workplace. They do not recommend that a drive is derated by more that 50% because performance is compromised. This my not be strictly relevant in our relatively simple workshop applications but surely worth a thought. Regards, Michael.

Edited By Michael Briggs on 19/03/2016 22:54:09

[martin perman 1Participant @martinperman1]

Gentlemen,

The company I work for has inverter driven equipment in universities all over the country and at one site we have six units running 24/7 for the last eight years, we had an inverter fail recently because of overheating because the cooling fan in the control box the inverter was in failed, we use Siemens 420's to drive 3 phase semi submersible pumps which have to supply a contant flow rate through filters which means the pumps speed increases as the filters clog.

Martin P

[AjohnwParticipant @ajohnw51620]

Posted by Michael Briggs on 19/03/2016 22:51:46:

I would not hesitate to run the VSDs in my workshop at 70 or 75 Hz, reduced torque but I have never had my lathe or miller slow down at these frequencies due to insufficient torque.

A point of general interest is I had a conversation with a technical specialist from Rockwell (aka Allen Bradley) this week about reducing the variety of drives OEMs supply to my workplace. They do not recommend that a drive is derated by more that 50% because performance is compromised. This my not be strictly relevant in our relatively simple workshop applications but surely worth a thought. Regards, Michael.

Edited By Michael Briggs on 19/03/2016 22:54:09

The inverters provide constant torque what ever the speed which is why the actual HP / Kw drops off when the speed is reduced. It doesn't drop off when the speed is increased. The power output might even increase.

Drop off of course is a loose term as I have used it here but they do aim to provide constant torque.

From memory as it's some time since I bought mine many of the inverter manufacturers suggest that the power output shouldn't be reduced significantly from the rating of the inverter. Figures vary.

John

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[John Stevenson 1Participant @johnstevenson1]

Constant torque is just advertising hype. Just like CFM on compressors when you should be looking at free air displacement and duty cycles on welders.

 

As an example you have a 1Hp inverter running at 50 cycles and it puts 1 Hp out and the torque to match.

Now drop the cycles and revs to 25 hz and the speed goes down because if you put a meter on the cable you are only getting 100 to 125 volts.

It's the nature of the beast, it's how they work.

 

BUT the torque still remains constant IF you base it on 100 to 125 volts.

That's the bit they don't tell you.

 

Why do you think medium sized machining centres like a Haas VF3 have 30 HP spindle motors when they rarely use an end mill over 20mm and facing cutters at 75mm.?

 

It's because they need the 10 or 12 HP available at low revs, remember these don't have a gearbox which is the only way to increase torque.

It's also why the Chinese water and air cooled spindles can bung 3 and 4 Hp out in a frame size that would do credit to a Taig owner.

 

Edited By John Stevenson on 20/03/2016 09:30:07

[Anonymous]

Oeeeer, I hesitate to disagree with John Stevenson MBE, but here goes!

Let's take the same scenario, an induction motor running at 50Hz and producing 1hp. If we now drop the frequency to 25Hz then the speed halves. But the speed halves because the frequency has halved, it's nothing to do with the applied voltage. Nor is the torque to do with the applied voltage. To a first approximation the torque is controlled by the current. So for our motor running at 25Hz, if we keep the current the same, the torque will remain the same. The power output will halve because the speed has halved. As the frequency drops the applied voltage needs to drop, but only because less voltage is needed to drive the same current through the phase windings. Since the motor is turning more slowly there is less back emf opposing the current, so less voltage is needed. The reduced voltage is a consequence, not a cause. Early VFDs used an open loop 1/f characteristic to reduce applied voltage as the frequency dropped in order to keep the current roughly constant. The method works reasonably well, but tends to fall over at low frequencies where winding resistance becomes significant.

It's not for nothing that the professionals allude to Haas horsepower. I'll stick to gear boxes and belts so I get constant power.

Andrew

[John Stevenson 1Participant @johnstevenson1]

Sorry Andrew all down to semantics.

If I get a 1hp motor and run it at 50 hz, 1425 revs for a 4 pole and put it on a brake I will get x lbs/force out of it so that is that the torque is.

Now if I drop this to 25hz and approx 720 revs and put it back on the brake I'll get far less that x lbs/force so how can that be constant ?

It will be constant to 25hz.

Years ago we used to make ink mixers, just like a food mixer on steroids, some had direct drive with inverter control, some had geared motors with inverter control.

Over the years we made literally hundreds of these. That may the inverter suppliers, IMO at the time put us on dealer rate. Their tech guy spent many hours tuning these to get the best out of them.

Printing ink is a funny substance, when it goes in the tub it's a paste or a mixture of pastes depending on type colour etc. These mixers have to start at low revs and as the ink warms up thru friction it gets thinner and you can then increase revs.

Because of this you are starting at the worst point as regards an inverter motor hence some of the larger ones having to have geared motors. It was always touch and go and we were right on the limit with these things as the limiting factor was they had to run off single phase as most were in small mixing rooms with no three phase.

The tech guy is still a personal friend and often calls in for a coffee when in this area. Anyone who knows what he's doing with modern inverters will all the different parameters can do wonders with them.

[Russell EberhardtParticipant @russelleberhardt48058]

Posted by John Stevenson on 20/03/2016 11:14:10:Now if I drop this to 25hz and approx 720 revs and put it back on the brake I'll get far less that x lbs/force so how can that be constant ?

Err – doesn't it depend on how tightly you apply the brake? Tighten the brake until you get the same motor current and you'll get the same torque.

Russell

[John Stevenson 1Participant @johnstevenson1]

Think I'm on to a looser here arguing against theory but why then does an inverter machine struggle at low revs it it has the same torque.

Something that facts can easily be proven.

[MuzzerParticipant @muzzer]

I dunno but I've developed and tested induction (and other) motors and inverters both on the bench and dyno – and in real applications. Fact is, unless you have run out of volts to drive the motor, it will produce the same torque at stall as it will at speed. The torque is proportional to the slip frequency ie the difference between the speed the field rotates at and the speed the rotor rotates at, regardless of direction or absolute value of speed.

The difference is possibly due to the setup of the VFD (limiting torque at low speed to compensate for reduced cooling?) or possibly something bizarre to do with the load characteristic? Generally an industrial VFD will implement constant torque up to base ("rated&quot speed. The motor itself will be capable of somewhat more (up to breakover torque) but you don't want to be experiencing that, as it's not something you can control or predict consistently. The (continuous) rated torque is generally thermally limited although you can often exceed this transiently, limited either by means of an embedded thermistor or a thermal model of the motor.

[jason udallParticipant @jasonudall57142]

I claim to be no expert but don’t induction motors deliver no torque at “sync” and the torque is “generated” by slip…ie that error between output rpm and sync.

For example
50 hz 4 pole..threephase
Thats 150 hz/4 *60 or 2250..so1425/ 2250=63% ie 36% slip.

Any way loss of torque at low rpm…
As said..theory says no..practice seems to say yes.

Might be “losses” in the system that are un accounted for. .fan..bearings…iron losses…?

[Russell EberhardtParticipant @russelleberhardt48058]

Posted by John Stevenson on 20/03/2016 11:57:10:

Think I'm on to a looser here arguing against theory but why then does an inverter machine struggle at low revs it it has the same torque.

Compare with a geared head lathe or belt drive the inverter drive machine gives less torque at low speeds because the gearing used to reduce speed on the non inverter machines increases torque.

Russell.

[AjohnwParticipant @ajohnw51620]

Afraid I can't agree with you JS, They do maintain constant torque but like most things it's a case if just how constant and I believe on the modern ones pretty constant. There was a step improvement in that area a good many years ago now and have probably improved further since then. I'm basing this on some one I interviewed for a job about 15 years ago when the technique was "new", after a fashion. He didn't get it but I had to talk to him about something.

Sadly torque is torque and power is a function of rpm * torque so drop the speed and the kw / hp drops off as it has too because the motor can only take so much current and heating losses are I^2R. This why the volts drop off as the speed is reduced.

The control method used is outlined here

**LINK**

In comparison with cheap dc motor control and also cheap brushless it's also better at maintaining it. Probably why some lathes are slowly being switched to var freq ac drive with more or less normal 3 phase motors.

John

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[John HaineParticipant @johnhaine32865]

As I've said before on another thread. The rotor generates the torque (obviously). It sees a magnetic flux rotating at the slip frequency whatever the actual frequency is. So the rotor currents, for the same stator flux, will be the same for a given slip frequency, and so will the torque (at least give or take). The inverter may be set up to reduce the current at lower frequency. But it could be set up to keep the current and flux the same, which would keep the same torque, but if the motor is running slow it may not get very well cooled.

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