VFD Drives

[JonParticipant @jon]

Posted by Les Jones 1 on 17/04/2011 17:20:11:

How are you measuring the running cost of your inverter ? I suspect that the units being sold to monitor your power consumption may not give accurate results with the current waveform drawn by the inverter. I think the best method is to count revolutions of the wheel in your electricity meter and calculate the cost from that.

Les.

Les i used what my electric provider sent me.

Clamped after meter but before the fuse box and sends wireless to the reader.

Most things seem believable from bulbed lights to kettles.

 

I get no interferance at all not even on a DAB radio a few feet away.

Wired 3 core and earth but outer sheath not earthed.

 

The findings above were with a 5hp digital inverter, incidentally packed up last week £206 repair bill and 7 weeks for max 1 1/2hrs work from new.

Now had to fork out for the 10hp, so far so good but uses an extra 0.1amp on each phase compared with 5hp.

 

Counting the wheel revs totally out the question, it needs to slow at least 4 times then it would be borderline.

I do have a rotary convertor as well 5.5hp.

[John Stevenson 1Participant @johnstevenson1]

I get two electric bills per quarter.

House is on domestic single phase. first 210 units @ 20.05p each

Rest @ 10.05p

 

Workshop on industrial 3 phase, first 78 units @ 15.69p each

Rest @ 13.44p

 

Both these with E-On.

 

One thing I have noticed is that i have recently changed the plasma and Tig sets over from transformer based to inverter based and this has made a noticeable difference.

Noticeable in that I don’t bother checking readings etc but since the change over the amount has dropped a fair bit

 

John S.

[confused.engParticipant @confused-eng]

If your on a 3ph supply then a thing called power factor comes into effect. Electricity supplies get a bit funny as they don’t like low power factors, or unbalanced usage on the supply. If you have a 3ph meter then the meter can measure the phase angle and adjust the usage reading. If you have 3 single phase meters then the supply company adjusts the readings for you. The other thing the suppliers don’t like is heavy short duration loads normally referred to as peak demand and will usually bill you as though you were using this for the whole billing period.

 

If all your workshop sockets are on one phase then this can easily trigger a premium billing period. Put kettle on one phase, compressor on another, etc. Just watch out for wiring regs as sockets on different phases have a minimum separation distance.

 

If you have noise on inverters this is from my experience a very very very difficult mine field to navigate. General problem is bad earthing. Earth to drive must be very low resistance. Earth from drive to motor must be very low resistance.

The cable from the drive to the motor must be a screened cable with an earth core. An ‘SY’ type cable is useful for this. The earthing must be connected to the drives earth connection. The screen must also be connected to this earth connection. The earth core at the motor end should be connected the motor earth terminal and the screen must be connected at the motor end. Best thing to use for this is an EMC gland. Expensive for what they are but worth it in the end. All paint must be removed before connecting the earth or screen. Lots of people don’t bother and wonder why they have problems.

Inverter control terminal, eg run, stop and speed should usually be cabled in a screened cable with the screen connected at one end only – the drive end.

Ferrite cores can help reduce interference, but you need to be careful as these can get very hot if not used correctly. They act as filters and are usually rings or cores that you pass the wire through or clip around the cable, you need to refer to the drive manual for details but with the rings you usually pass the motor phase wires through them twice immediately after the cable leaves the drive.

I have experienced situations where all the above has been done and still had noise problems and the only way to cure it was to disconnect the screen – just doesn’t make sense – brick wall helps.

 

Another thing that can cause excessive noise is the switching frequency the drive is using. On most of the modern drives this can be changed though a parameter but sometimes makes the situation worse.

Don’t fit any switches between the drive and motor as these will be destroyed quickly as the output from the drive is not a pure sin wave and most switch gear is designed to work only with a pure sin wave, especially the cheap far east import stuff.

[Steve GarnettParticipant @stevegarnett62550]

Posted by confused.eng on 17/05/2011 01:01:52:

I have experienced situations where all the above has been done and still had noise problems and the only way to cure it was to disconnect the screen – just doesn’t make sense – brick wall helps.

 

 

Actually it does make sense – and it’s recommended practice! You shouldn’t confuse the safety earthing arrangements with the screening requirements – they aren’t the same thing at all. I have yet to find a situation where having a screen joined at both ends reduced any form of emissions at all – it can potentially increase them, and often it does.

 

With most inverters, you need a common grounding point, which is generally on the inverter itself. All the screened leads you run should have one end of the screen only connected, and generally it should be at that same earthing point that you do this. If you don’t do this, then the screen becomes a conducting path for interference, rather than the screen that it should be, and that means that it can radiate. So your earthing connections for the motor and supply should be solid, and meeting at one point, on the inverter earth tag. The screen on the incoming supply should be connected here too, and so should the screen for the motor cable and control cable, if there is one. But the other ends of these screens should not be connected to anything – they are only screens, and shouldn’t ever be used as conducting paths.

 

The ferrite cores you mention work effectively as a shorted-turn transformer at high frequencies, and if they get hot, then you are generating a considerable amount of interference. If you are getting radiated interference, then almost certainly this will be on the output side of your inverter, and that’s where you will get most advantage from the ferrites. The other crucial thing about reducing interference is to keep the lead lengths as short as possible – this also helps to reduce the wiring impedance – it’s impedance we are concerned about when there’s interference involved, not resistance so much.

 

The other problem that is frequently encountered with inverters is that they trip earth leakage systems. There is only one really effective solution to this problem, and unfortunately it’s rather expensive. That is to purchase a mains isolation transformer, and run the inverter’s input from that. But to play safe with the transformer, it’s going to have to be rated somewhat higher than the total load, and with even a 1HP motor, that means it has to be rated to supply at least 1kVA. The reasons for problems with earth leakage all relate back to what happens on the primary side of an inverter – basically your single (or even three-phase) mains supply is being converted to DC straight from the supply using socking great rectifiers – and with the best will in the world this is likely on some systems to unbalance the load. I should also point out that the whys and wherefores of this vary considerably between systems; what works fine on one mains supply won’t necessarily behave in anything like the same way on another.

 

And whilst you are correct about having no switching in the output, the reason isn’t quite so simple. If you look at the current output waveform from an inverter, you’ll find that it generally is a good sine wave. The voltage output isn’t though, because this is what’s being chopped by pulse width modulation to produce the current sine wave. Where switching really runs into problems though is with the reactive feedback systems that are used to sense and control the motor – switches can screw this up completely, and even leave you with dangerous situations. All the advice I’ve ever seen about this is that you can use plug and socket connections if you must – but make sure they are of good quality.

 

 

Edited By Steve Garnett on 17/05/2011 01:56:15

[John OlsenParticipant @johnolsen79199]

Actually power factor is a factor in single phase systems just as much as in three phase systems, although since the three phase systems are larger the power supply people pay more attention to them. An ordinary (non electronic ballast) flourescent tube ballast will havea power factor correction capacitor in it, even though it is for use on a single phase.

 

OK a bit about screening. For our purposes we can treat interference as being radiated in two ways, capacitively and inductively. Eg a magnetic field and an electrostatic field. Now, the electric field is pretty easy to stop, a simple screen earthed at one end will stop it. A simple screen like that won’t stop a magnetic field. (A good thickness of soft iron would.) The best way to stop a magnetic field is to keep the go and return conductors really close together. This is why wires and cables are usually made up so that the conductors are close together.

 

So by using a screened cable with the screen earthed at one end, preferably the end where the interference is originating, and with all the active conductors close together inside it, we minimise both the magnetic and capacitive effects. Note that if we earth the screen at both ends, interference currents can flow in the screen itself, which means that they are outside the screen, and so will happily radiate.

 

Students of electromagnetic theory will know that I am simplifying things a bit here, but if you know that, you probably don’t need my help anyway.

 

regards

John

[Les Jones 1Participant @lesjones1]

Hi Jon,

If the device you are using to measure the power only senses the current then it cannot give a true power reading. To measure TRUE power the device needs to know the phase relationship between voltage and current. I think the device you are using is just measuring volt amps and even that value is probably only correct if the supply voltage is at its nominal value. (Which is a reasonable assumption.)

 

Les.

[Steve GarnettParticipant @stevegarnett62550]

Posted by John Olsen on 17/05/2011 06:55:59:

 

So by using a screened cable with the screen earthed at one end, preferably the end where the interference is originating, and with all the active conductors close together inside it, we minimise both the magnetic and capacitive effects. Note that if we earth the screen at both ends, interference currents can flow in the screen itself, which means that they are outside the screen, and so will happily radiate.

 

I should have added a bit more about the screening properties, yes – but hey, it was nearly 2am! I did mention the bit about avoiding current flow in them, though.

 

[JonParticipant @jon]

I still have an ongoing problem with the electric supply to inverter.

Trying to run a 3phase 400V 3hp motor non cap start probably equivalent on startup as a 9hp motor.

 

32A MCB type C spur direct to inverter.

 

After blowing up a £650 5hp plug and play after 1 1/2hrs use, tripped in the various contactors in control panel no problem. Upon spindle start it used to trip the MCB and give a ground fault at undefined times.

 

After having to fork out for the 10hp model i am having a similar problem.

This time the RCCD 80A 30Mah trips at undefined times and MCB fine.

Can anyone suggest a way forward?

Thinking either change motor to a cap start 3ph 3hp.

Or is the RCCB a likely suspect Crabtree split load 380/S030 80A 30mA

[KWILParticipant @kwil]

Jon, What you have not told us is what the various Inverter parameters have been set to, soft start ramp up time, line current settings etc.

[JonParticipant @jon]

Thanks for the reply.

No soft start its plug and play.

 

Upon RUN the contactors trip in control panel, so cannot use a soft start to start the motor.

I did run 3 sec soft start on the 5.5hp plug and play, this had the effect of contactors chattering up until near 400V reached.

We assumed the start up loading was more than the 5hp (run equivalent of 7.5hp for 30 secs) could handle.

Running 400V drawing 1.9A no load when running lathe.

 

Worked well yesterday for 4 hrs and an hour last night stopping and starting.

5 mins this morning was fine until second power up.

Lathes a Harrison M300.

[KWILParticipant @kwil]

Jon,

 

My Harrison M300 runs without trouble, original motor as fitted, driven by inverter from single phase mains. The original safety and interlock features are all retained, but are configured to use the inverter.

[JonParticipant @jon]

Same here Kwil and know of a mate with a late Student and Bridgeport running same inverter plug and play.

Have you rewired then?

ie direct to motor with soft start etc Used the two forward/Reverse switches, coolant, brake etc all rewired straight to the inverter input and back out again?

 

[confused.engParticipant @confused-eng]

Hi Jon,

Inverters and RCCD don’t mix very well due to the noise suppression filter in the drive. If you must use an RCCD with the drive you will most likely need one that is designed to work with both AC and DC currents. These tend to be very expensive.

You will also need to check the leakage current of the drive as this is usually quite close to he tripping current of the RCCD. From experience the RCCD usually trips when the drive is delivering a high current or when slowing down.

In the past a badly earthed drive has destroyed the serial port of my laptop (back in the early days) when trying to configure it.

Does your RCCD just power the inverter or the whole workshop? If there is a lot of electronic equipment powered up at the same this can increase the problem due to the filters built into the power supplies. It is possible to test the tripping current of the RCCD if you access to the correct test equipment. If you do the test with everything running in its normal condition safely and perform the test it will give you a tripping current. This current should be 30mA give or take a mA or so. If the tripping current is low then something on the supply is causing a lot of leakage and is a case of disconnecting one item at time and repeating the test until the offending item. It could be a combination of items added together causing. The inverter will be a big source.

Damp can also be a cause of leakage current. If coolant gets where it shouldn’t then this won’t help things.

[JonParticipant @jon]

A very big thank you and confirmed my suspicions. Putting two and two together i am fairly confident its a household item namely a Bosch fridge or other corrupt builders wiring or appliance.

 

RCCD trips when powering the spindle up. When it doesnt trip motor fires up instantly, clunk/bang. Yesterday i mostly spun the chuck whilst powering up and was ok for hours.

 

I dare say i could move the 32A RCB C type on to the house circuit doing away with the split load RCCD,

RCCD powers whole workshop except fluorescent lighting. Normally things are left plugged in but switch off in three pin sockets.

Good earth on inverter, lathe earth going through shielded cable to inverter.

Thanks again massive help.

[KWILParticipant @kwil]

Jon, Read MEW 154 for my complete guide as to how to set up the M300, including the circuit diagram of the minor rewiring. My mains connections are straight in on a special workshop distribution board to keep away from the house RCDs.

[Ramon WilsonParticipant @ramonwilson3]

Posted by Jon on 17/04/2011 15:11:50:

Ray and Ramon, what is your electric bill like now?

Of course this would have to be on a like for like basis.

 

The reason i ask and to be honest cannot believe myself my findings, the power consumption of these digital inverters drawing way more than double a rotary of same size.

 

3HP lathe motor with the Teco MA7200 uses 61p/hr, Transwave rotary 31p/hr + vat of course, both under no load.

Hi guy’s I thought you might be interested in an update to this question.

 

The ‘Chancellor’ – that’s Sue not George – recently announced that we are substantially (and it is substantial) in credit with said electricty supplier over a similar period of last year.

 

This can only be put down to the change in power units – no more surges and dimming lights every time mill and lathe are started. It’s not scientific but definitely supportive even to the extent that a suggestion was forthcoming, from the purse holder, that perhaps I should use some of the rebate to purchase a similar set up for the drilling machine – oh really dear?? how nice

To be fair the same period last year was about when those BR2 heads were tackled and there was an awful lot of stop/start on the mill for best part of a month which probably had a false effect on putting up the monthly DD but someones happy I can tell you

 

The other big saving has been with the compressor. The old one – 1-1/2 hp with a 20 litre tank – would cut in about every two hours for a top up when not in use (never could find the leak) and every few minutes when being used to clean the odd part etc. The new one with much larger (150litre) volume tank is only being charged once a week for the same usage. Obviously it will be different when spraying or bead blasting but that’s not going to happen on a daily basis.

 

In short guy’s it’s kudos all round in this house at the moment

Have a real good Xmas break, may Santa bring you that nice new lathe – with VFD naturally and of course ….

 

All the very best for a productive, peaceful and very happy New Year to you all.

Regards – Ramon

[JonParticipant @jon]

Thanks for the update Ramon.

 

If its any help i use between £2.60 to £3 electric a day, double last and previous years.

 

Single ph 3hp compressor uses 57p/hr whilst tripped in just noticed.

With the 7.5KW inverter the heavier the cut the more the reader registers. So when ploughing 8mm off one pass motor dont slow, it is well above £1.50 p/hr. Havent had the bottle yet to take it up to 1/2″ like me last lathe.

 

The 5.5KW Transwave rotary converter when putting a menial 1/16″ cut on spindle slows and changes note, its there for emergency.

 

Direct Drives have my old 5KW single ph input to 440V 3ph output around £500, not a bad saving for 1 1/2hrs use.

[Gone AwayParticipant @goneaway]

Posted by Ramon Wilson on 21/12/2011 21:23:30:

The ‘Chancellor’ – that’s Sue not George – recently announced that we are substantially (and it is substantial) in credit with said electricty supplier over a similar period of last year.

This can only be put down to the change in power units

The other big saving has been with the compressor

 

Hope you’re right about that, Ramon. I’m in Canada and we use natural gas for heating and hot water. A couple of years back we saw a great improvement in our monthly bill. I put this down to the fact that we’d just had a new, high-efficiency furnace installed but frankly I was uneasy because the saving would have paid for the new furnace in no time flat (it couldn’t be that good surely).

 

At the end of the heating season, the other shoe dropped. The gas company announced that it had discovered that it had been miscalculating the consumption of hundreds of customers and that our bills should have been close to what we were used to in the past. They also sent us a nice big bill to catch up!

 

Still as long as your improved efficiencies look as though they can account for the savings you’re probably OK.

 

(gawd I hate this editor with a vengeance) .

 

Edited By Sid Herbage on 22/12/2011 02:06:41

[Ramon WilsonParticipant @ramonwilson3]

Hi Jon, Sid,

 

I’ve never actually thought about how much it costs to run my little ‘second home’ on a daily basis but it must be more than the rest of the house bar heating (gas)

(Sshh don’t let Sue hear that – I might have to take up stamp collecting ).

 

Seriously though, it probably is our biggest drain on electricity and it would appear that this has made a big difference. I hope it’s not the same case as yourself Sid but our costs per unit appear to have gone up in line with everything else – for once our useage appears to have diminished. Time will tell even more so.

 

What I can say on the VFD front and with absloute certainty is I would never, ever, go back – what a sheer pleasure to use – a real asset for sure, mill or lathe.

 

Regards – Ramon

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