VFD Question

[SillyOldDufferModerator @sillyoldduffer]

Posted by Steviegtr on 13/01/2020 22:58:39:

Out of interest how many of you guys with the inverters use a ten turn pot. I have fitted one & for the life of me I don't know why…

Almost no-one I would guess. I can't think of a good reason for fitting an expensive multi-turn pot to control the speed of an ordinary machine tool. Cutting speeds don't need to be super-accurate and I don't see any benefit in being able to fine-tune to a particular speed. An ordinary cheap pot (often used with a tachometer) allows manual tool speeds to be adjusted quickly and with enough precision to suit the job. And are easily replaced when they fail. The time it takes to go from one end to the other of a 10-turn control is a disadvantage on a home lathe.

Context matters. Two bad mistakes chaps sometimes make on the forum:

1. Assuming what's done in a small workshop should be applied by industry. OR
2. Assuming what's done in Industry should be transferred to a small workshop.

Engineering is about problem solving. The answer depends on the question. Copying what's been done elsewhere is often an excellent solution, but it comes unstuck when the circumstances are different. Always pays to recognise when one's expertise has gone off piste and it's time to engage brain or take advice!

Dave

[Michael GilliganParticipant @michaelgilligan61133]

Posted by SillyOldDuffer on 14/01/2020 09:56:05:

[…]

Always pays to recognise when one's expertise has gone off piste and it's time to engage brain or take advice!

Dave

.

Or preferably perhaps …

take advice, then engage brain and selectively use the bits of advice that make sense to you!

This thread being an exemplar

MichaelG.

[Gerard O’TooleParticipant @gerardotoole60348]

Thanks for all the explanations and advice. I will have to spend some time digesting it all.

My motor, which is the original motor fitted, is marked Siemans-Shuckert , 380V, 2.7/2.8 A, 0.8/1.1kW, cos ó 0.64/0.8, 700/1420 U/min

There is a switch to switch it from low speed to high speed. This switches it from Star to delta ( or Delta to Star – I will have to check)

It is running satisfactorily on VFD with 230V and 29Hz.

I notice that the motor draws almost the same current in low or high speed ( 2.7 or 2.8 A ) and the power rated is 0.8 or 1.1 kW.

Thanks again for all the help.

For now I might just leave it alone as I do not need the very high speeds it is capable of

[old martParticipant @oldmart]

I'm sorry to say that you will be doubly confused after reading through this thread. Some people like to make things so complicated just to show off their knowledge.

[Anonymous]

Posted by Gerard O'Toole on 14/01/2020 13:06:17:

My motor, which is the original motor fitted, is marked Siemans-Shuckert , 380V, 2.7/2.8 A, 0.8/1.1kW, cos ó 0.64/0.8, 700/1420 U/min

There is a switch to switch it from low speed to high speed. This switches it from Star to delta ( or Delta to Star – I will have to check)

It may be switching from star to delta, but is also pole switching from 4 to 8. The key is the last part of the plate:

700/1420 U/min

In other words 700 and 1420 rpm in low and high speed respectively.

Andrew

[john fletcher 1Participant @johnfletcher1]

I nail my colours before the mast and agree with Old mart above.

Hello Gerard from the information you recently posted it would suggest your motor is a two speed one, with POLE changing switching to give the two speeds,and NOT Star/ Delta. John

[SillyOldDufferModerator @sillyoldduffer]

Posted by old mart on 14/01/2020 13:33:52:

I'm sorry to say that you will be doubly confused after reading through this thread…

Don't know about Gerard, but I certainly am!

I think the problem is the various contributions mix several different points about distribution systems as well as 3-phase motors; starting them, the pros and cons of star vs delta, 200 vs 400v, speed as effected by volts, frequency and load, and the special case of bodging a high voltage motor to run at all from a low voltage VFD. Doesn't help that I find the external description of 'why 29Hz' obscure; I think I understand what it means in terms of flux, but not how it alters the price of fish in ordinary circumstances.

Gerard's 13:06 post today reveals he has special circumstances. His motor is 380V, so to get it to spin with an ordinary VFD suggests the 29Hz trick is appropriate. But it's an expedient in that I don't think the motor is working at full potential, power or speed. (However, if the arrangement is good enough to get Gerard's machine working satisfactorily, carry on!)

Gerard says: 'There is a switch to switch it from low speed to high speed. This switches it from Star to delta ( or Delta to Star – I will have to check)' Normally changing from star to delta wouldn't alter a motor's synchronous speed. Isn't it likely Gerard has a switched pole motor like a Dahlander?

Reinforces my view about buying ex-industrial machines: check the electrics! While most have motors that can be driven happily by an inexpensive VFD, others are far more challenging. I'd be nervous buying a machine that mixed 3-phase and single phase functions together on the same feed because that's simple-VFD unfriendly. Is "High voltage and two speed" another combination signalling "buyer beware" because the electrics are likely to need special attention?

Dave

[Anonymous]

Posted by john fletcher 1 on 14/01/2020 14:16:07:

……..with POLE changing switching to give the two speeds,and NOT Star/ Delta.

Clearly the switch is changing the number of poles. But if that's all it's doing why aren't the low/high powers also in the ratio 2:1?

Andrew

[Mark RandParticipant @markrand96270]

Because an eight pole motor will tend to slip more than the same (physically) sized four pole one.

[Gerard O’TooleParticipant @gerardotoole60348]

The Dahlander diagram may be correct. The description I have, translated from the German, is "pole-reversible motor 700/1400 revolutions" – even though the plate clearly shows 700/1420 U/min

i am only using it in the low speed (Delta) configuration .And as I have the base frequency set to 30Hz , I am not even getting the maximum of the low speeds.

What would happen if i switched to the high speed(double star ) connection? Would i achieve higher speed or would I damage the motor?

Sorry , I just realised that I can not connect the neutral so unlikely to be possible.

I have a cheap digital tachometer, like this one, which I could fit to the mill and measure the speed accurately.

 

 

Edited By Gerard O’Toole on 14/01/2020 18:01:47

[Gerard O’TooleParticipant @gerardotoole60348]

When the VFD was fitted, I traced the wiring on the switch and the wiring in the motor

[Anonymous]

Mark: Thanks for the explanation.

Gerald: It should run fine in star. If you look at your diagram in high speed mode wires 4, 5 and 6 are connected together, but they don't go anywhere. You don't need a neutral line to run a motor in star. In theory the current in a neutral wire would be zero.

Andrew

[Gerard O’TooleParticipant @gerardotoole60348]

Posted by Andrew Johnston on 14/01/2020 20:11:05:

Mark: Thanks for the explanation.

Gerald: It should run fine in star. If you look at your diagram in high speed mode wires 4, 5 and 6 are connected together, but they don't go anywhere. You don't need a neutral line to run a motor in star. In theory the current in a neutral wire would be zero.

Andrew

Thanks Andrew. really good to know. I will set up the tachometer and try to see what speeds the motor produces.

Gerard

[SteviegtrParticipant @steviegtr]

I have done a few posts on this thread. I am/was an electrical contractor & built many control panels & installed them. Inverter drives & usually controlled by PLC logic. This is where I eat humble pie on a lot of comments I have made. Yes I have a lot of experience with inverters. Big but is that the clients were always blue chip companies who had there own Electrical engineers. We would build & install the equipment but not actually get involved with the commissioning of them. So forward to today & a bit of yesterday. I fitted my 3ph 1.1kw motor & inverter. Omron 1.1kw. Fit my home made control panel & off I went. The manual is 177 pages long & the inverter has 78 programmable inputs. Off I went clever me. At 1st it seemed fine as it actually turned the motor & looked good. So today I did some more on it & after a couple of minutes of running. POP it tripped out on overcurrent. OL1. Back to computer to read more of the manual. Well I did read a few pages before I switched it on. Not sure about the Chinese ones but on this one it seems you disconnect the drive mechanically by activating the clutch. Then you run the motor slowly & the inverter learns the no load requirements. Then you tell the inverter the Supply voltage… The Supply frequency & the full load current of the motor . In my case in delta connected 240v It is 4.8 Amps. so I programmed all this in & now it is running sweet as a nut. The sorry guys bit is that I stated in earlier posts about the inverter connected in delta having reduced power. Reading further into the manual it stated that the inverter has full power from the get go. One thing I did notice was running at low frequencies the current readout was quite high & actually more than full load stated current. But at 50hz it was taking just 2.0amps. At 65hz my high programmed setting it takes 1.8 amps. So tonight I have ran it for over an hour. Only running with a piece of heavy bar in the chuck. Everything good. Highly recommended for anyone thinking of doing it. With back gear & low speed on the belts the chuck can be slowed to approx. 12 rpm. Not that you would ever want it that slow but it looks good. One point is the Chinese DRO with a single magnet cannot read at the slow end. I think some of the Chinese lathes have 4 magnets so 4 samples per rev. The other point to make is the lathe is almost silent. Unless you engage the back gear then a bit noisier. Regards all & another big sorry for being a pi–lock.

[Paul KempParticipant @paulkemp46892]

You might find 12rpm quite a comfortable speed for cutting say a 4tpi square thread or even a multi start thread

Paul.

[SteviegtrParticipant @steviegtr]

Ah I don't even know how to cut any threads let alone a Acme. Was thinking more of winding my watch up with it. (Joke)

[not done it yetParticipant @notdoneityet]

My lathe states 36rpm in back gear (at 50 Hz). Lathe will run at much less than 12rpm – not that I do that for any length of time or under any appreciable load. Likely lower than 2rpm before it actually stops as I wind down the pot on the pendant! It runs so slow, in direct gear, that I don’t bother with back gear for threading!

[Gerard O’TooleParticipant @gerardotoole60348]

  I eventually fitted the digital tachometer and measured the speeds on the mill in comparison to the speeds given on the gearbox plate for low and high speed . The VFD was set with voltage set at 230V and base frequency at 31.5Hz
Low Speed High Speed measured Speed
95                    190                       128
150                  300                       205
235                  475                       322
375                  750                       389
600                1200                       637
950                1900                     1040
It would seem that the motor is wired for high speed and is achieving over 50% of the expected speed.

Edited By Gerard O’Toole on 16/01/2020 16:35:01

[Mike PooleParticipant @mikepoole82104]

I am wondering if you are muddling the base frequency which should be a motor parameter with the output frequency for the desired speed. Setting the desired output frequency to 50Hz should give the speeds on the plate. The base frequency is information used by the inverter to model the voltage/ frequency curve for the output to the motor.

Mike

[SteviegtrParticipant @steviegtr]

I have worked on some multi speed motors. One such was called a bowl chopper for anyone who has seen one. A huge rotating bowl with a cutting head with large blades. These things can chop up half a cow , with the bones. We took one motor out for rewind. Reason I am saying this is because it was a 4 pole motor. But multispeed, from memory it was 5 speed. The connection box was huge. The windings in most multi speed motors have multiple windings around a pole. So 4 windings on single speed & 8 windings on 2 speed & so on. This does not mean the motor goes from a 4 pole motor to a 8 pole motor as some have suggested. Its just a different winding on the same pole.

[Gerard O’TooleParticipant @gerardotoole60348]

Posted by Mike Poole on 16/01/2020 20:16:24:

I am wondering if you are muddling the base frequency which should be a motor parameter with the output frequency for the desired speed. Setting the desired output frequency to 50Hz should give the speeds on the plate. The base frequency is information used by the inverter to model the voltage/ frequency curve for the output to the motor.

Mike

It is quite possible I am muddling things up.

The actual settings, On the Bosch Rexroth VFD, refer to max. frequency and upper frequency, both set to 31.5 Hz

(= 50 x 240/380) (it should be 30,3 Hz (= 50 x 230/380). This keeps the voltage/frequency ratio constant which I gather from all the answers here is not really necessary.

Thanks

[Carl Wilson 4Participant @carlwilson4]

Old mart should have added "on this site". Lot of clever clogs here.

[SillyOldDufferModerator @sillyoldduffer]

Posted by Steviegtr on 16/01/2020 22:49:14:

…

The windings in most multi speed motors have multiple windings around a pole. So 4 windings on single speed & 8 windings on 2 speed & so on. This does not mean the motor goes from a 4 pole motor to a 8 pole motor as some have suggested. Its just a different winding on the same pole.

Not wrong exactly, but I think this depends on exactly what's meant by a 'pole'. To an electrical engineer a physical pole isn't necessarily the same as a magnetic pole.

If it walks like a duck and quacks like a duck it is a duck. When someone says a 'motor goes from a 4 pole motor to a 8 pole motor', they mean electrically, not literally. I prefer the electrical engineering view because Stevie's definition of 'pole' gives the wrong answer from:

RPM = Frequency * number of poles

I find it easier to think of motor poles in terms of effect rather than their physical layout and like to believe only motor designers have to do both! I could be wrong again…

Dave

[Anonymous]

Posted by Steviegtr on 16/01/2020 22:49:14:

The windings in most multi speed motors have multiple windings around a pole. So 4 windings on single speed & 8 windings on 2 speed & so on. This does not mean the motor goes from a 4 pole motor to a 8 pole motor as some have suggested. Its just a different winding on the same pole.

I don't follow that? The equation for the synchronous speed (in rpm) of an induction motor is 2 times frequency (Hz) times 60, all divided by the number of poles. Assuming that the frequency is fixed then the only way to change the speed is to change the number of poles. Of course windings aren't added or removed, simply connected in a different sequence. But the upshot is to change the number of poles.

Andrew

Edit: Thought better of it

Edited By Andrew Johnston on 17/01/2020 13:18:13

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