Motor speed with inverter.

[Stephen Harris 5Participant @stephenharris5]

Hi, All.

I have a Colchester Student (roundhead) lathe which I want to fit a replacement 3 phase motor and use an appropriate inverter to drive it.

Is there a rule of thumb for how fast maximum the motor should run ??

The motor I want to use is 1400 rpm and for best results would want to go up to 1800/3000 rpm. It is a squirrel cage induction motor.

I will strip the motor and renew bearings etc. if needed .

Just want to get some idea or the potential.

Regards.

[Stephen Harris 5Participant @stephenharris5]

[ZanParticipant @zan]

I converted mine a number of years ago using the original e phase motor. It needed a lot of work including making a sheet metal box to fit inside and behind the control panel to house all the gubbins. There’s a lot of wires if yours is already three phase. I ran the pump with capacitors from single phase. Works ok

regarding speed, my lathe is only used on big stuff, having an s7 as well for the small fast stuff . with regard to its age I chose to use a speed range oon the inverter of 30x7t hz giving a top speed of 1800 but I avoid going that fast as the lathe screams rather a lot

[Tony RayParticipant @tonyray65007]

I usually work on 120% of the stated motor rpm which would be 1680 rpm which is'nt too far away so you can probably run it @ 1800. Assuming your chosen motor is rated for VFD use – old motors don't much like VFD's and can let out magic smoke if the insulation breaks down.

Where is the 3000 rpm coming from ? You won't get that by overspeeding the 1400rpm unit. Do also bear in mind that your roundhead won't have been designed to cope with 3000 rpm and more importantly you'll need to ensure that any chuck you fit is rated for it.

Don't be fooled by the idea that a VFD is the answer to all your speed needs – there is no substitute for using the gearbox and any pulley systems to get the rpm (and torque) you need. I have a VFD on my Harrison M250 I only use the variaible speed @ 6Hz for clocking in otherwise it's just there to drive the 3 ph motor.

Tony

[John HaineParticipant @johnhaine32865]

I've fitted an NT supplied 3ph motor and inverter to my S7, max frequency is 60Hz I think so 1800 rpm. On my mill I fitted a s/h 3 phase motor, 1400 rpm, I actually quite often run it at twice frequency with no ill effects so far. But if you want to do that it's best to fit a properly rated motor that can run off 100Hz.

[Clive FosterParticipant @clivefoster55965]

For all practical purposes a modern vector drive VFD on a industrial type machine motor will give you decent performance over ± 1/3 of nameplate speed variation. Partly because modern vector drives are almost magical in their ability to reduce torque variations over a reasonable speed range and partly because most industrial machines are significantly overpowered for the use we give them so we don't notice the power drop off at lower rpm.

In practice we are generally doing jobs needing much less power so can go even slower before the loss of torque becomes a worry.

3 hp makes scary amounts of swarf!

For older style (less expensive) voltage-frequency drives the "you will never notice" speed range is more like ± 1/4 nameplate variation and low speed losses a little more.

Basic VFD characteristics are constant torque from "very slow" up to nominal nameplate speed and constant power up to a fairly high speed above nameplate until various electromagnetic effects cut in wasting power. Many modern motors have the maximum constant power speed on the name plate. Less common to show the minimum constant torque speed but that usually more of interest for the monster (by usual standard) motors used on CNC mills so the can swing larger cutters with simple VFD control of speed.

When considering engineering in specific motors it all gets very complex very quickly. There are a lot of ways of making a 3 phase motor and significantly wide variation in power curves. Generally, for good electro magnetic, reasons your basic 3 phase motor will self balance to work adequately well between around 40 Hz and 70 Hz so there is already a good deal of inherent flexibility.

Clive

Edited By Clive Foster on 03/01/2023 14:21:52

[Mike PooleParticipant @mikepoole82104]

A modern motor will probably not be harmed even with 100% over speed, consider that a 2 pole and 4 pole motor are mechanically very similar, the bearings are common and operate at a fraction of their rated max speed. The rotor is unlikely to be troubled by the higher rpm but may not have been balanced for that speed. Electrically it will run but may not be operating optimally. The mechanics of the lathe may object more than the motor and the max speed for the chucks should be checked.
Mike

[Martin ConnellyParticipant @martinconnelly55370]

+1 for what Mike Poole said. The natural speed of a 2 pole motor at 50Hz is just under 3000rpm. Change the non-rotating stator windings to 4 pole and you have a motor with a natural speed of just under 1500rpm, 6 pole – 1000rpm. Since the rotating part does not differ in any way 3000rpm should not be a problem for a modern motor. Additionally if the motor is rated for 60Hz as well the maximum should be closer to 3600rpm. I have limited my lathe to what is on the original speed plate because it has plain bearings and chucks usually have a maximum rpm and I don't know what it is for mine so, better safe than sorry. I have done the same for my milling machine, The belt table gives a maximum RPM based on the original motor and pulley system. Not knowing what the limiting factor is I keep it set up so that with the belt option I use for everything I do I can increase the frequency of the motor over 50Hz but it is limited to what the original max rpm was. I don't fancy a big cast aluminium pulley bursting from too high an rpm even if it is in a metal cover.

Just be aware that increasing frequency too much may also reduce the current as the current in the motor coils will decrease as they are inductors. They are often optimised for around the 50/60 Hz frequency.

Martin C

[Robert Atkinson 2Participant @robertatkinson2]

I agree 100% with Mikes comments. Clive is correct for most modern motors assuming they are 4 or 6 pole from family / design that includes two pole versions. This aalso assume a reptuatble make with published data.
The limiting factor is actually likely to be cooling impeller bursting. Our applications tend to have little radial load on the bearings compared to their ratings.
The 6 pole 80 frame size motor I'm using is rated by the maker for up to 180% of the "nameplate" speed.
Just about any moter is fully dual rated for 50/60Hz operation so in the UK/EU we can go up to 120% (60Hz) without even thinking about it.

Robert G8RPI.

[old martParticipant @oldmart]

Be careful with 3000 rpm with a lathe that size, chucks over 6" might be unsafe.

[Ex contributorParticipant @mgnbuk]

I would expect the limiting factor to be the speed rating for the grease lubricated rotor bearings – easily found from bearing manufacturer's data if you can find the bearing numbers. If the bearing numbers are not to hand, go by the nearest bore size up standard deep groove ball bearing to the output shaft diameter (the shafts are usually a bit smaller than the bearing bore).

If the fan were considered an issue (though I doubt that the lightweight plastic fans used on motors would burst), just take it off & replace it with an external blower – something motor manufacturers offer for motors used with variable speed drives as an option. This would be beneficial anyway, as cooling airflow with the standard "stirrers" is much reduced as the motor speed is reduced with a VFD, but heat generation is not.

A modern motor will be a better option for VFD use than an older unit, as modern "high efficiency" motors run with much closer rotor / stator clearances, are better balanced & will have been designed to take inverter operation into account.

Highest speed I experienced syncronous motors operated at was 12000 rpm – "motor spindle" units on an aluminium router hogging wing ribs out of billets. The motor "shaft" was a 40 ISO taper spindle with a drawbar. The bearings were the limiting factor, life being around 9 months in operation. Precison grade bearings lubricated with Klueber high speed grease that required around 2 days to "run in" – but a bit more extreme application than running a Student !

These were unusual motors – most modern CNC machine tool spindle motors have permanent magnet rotors with encoder rotor position feedback, driven by specialist drives – not your average "run-of-the-mill" squirrel cage motor + VFD.

Nigel B.

[Nigel McBurney 1Participant @nigelmcburney1]

If a manufacturer fits a 1450 rpm motor giving a top spindle speed of around 1200 rpm then stick with this speed,running a 55 year old roundhead model would at very high speed be rather foolish.Students of this era had splash lubricated headstock gearboxes and the oil was not filtered when on its way to the very expensive Gammet spindle bearings. Also an old worn gearbox spun at twice its recomended speed has oil which is likely to be contaminated with specs of steel which will ruin the bearings. Much later Colchester students were not much faster,the Master did have for a long time a top speed of 2500 rpm and last ones 3100 though these and the 2500 had 5 hp motors.The Masters even though they had gear boxes similar the students had one advantage, a pump and a large oil reservoir which supplied oil to the top of the gearbox and gave a more positive lubrication. If you try to virtually double a motors speed electronically or double spindle speed mechanically then a lot more powerful motor will be required ie at least 5 hp. Also if the bearings fail with this proposed speed increase the new bearings will probably cost more than the value of the lathe.So be content with what you have got,Another post said that they used a Myford S7 for small high speed work which is what i do. There one big advantage with two lathes,one lathe can very often be used to make spares,modify or repair the other lathe.Faceplates also have a maximum speed which are sometimes marked with the speed.A gap bed student or master will take an 18 inch faceplate which could burst right in front of you.If you had a good apprenticeship and had owned your own lathes for over 60 years you will find that speed is not everything,if very fast speeds are required for producing many thousands of parts then buy the appropriate lathe,

[Clive FosterParticipant @clivefoster55965]

Agree with Nigel about the potential high speed issues.

Hard to see what you'd use such high speeds for on a lathe that size in practice unless trying for book settings with very modern carbide inserts. Unless the job is very small high spindle speeds will give too fast a surface speed.

My S&B 1024 goes up to 2,500 rpm but I rarely find it useful to exceed 1,000.

Clive

[Stephen Harris 5Participant @stephenharris5]

Hi everyone.

First , I must apologize for delay in replying. Our Broadband is VERY poor and I can't always get on-line. It's been very bad for the last few days.

Thanks for all the very useful advise. I will be stripping the motor shortly and overhaul it with new bearings etc. seeing as it is now very old.

This student originally had a two speed motor, 1400/720 ish, so I will really need the slower workings the a VFD can provide and I also take on the comments about running the motor/ machine faster than its original design speed.

Clive. My question had a two fold meaning.!! I also have a Denford Triac miller which I am converting to Smooth stepper and Mach 4 and this will require the motor to run a bit faster then its rating. If I ever get towards a finish on that I will report back, If I live long enough.

Once Again thanks for all your time and advise.

Regards, Steve Harris

[Clive FosterParticipant @clivefoster55965]

Stephen

It seems to be generally agreed that around one and a half time nameplate speed is a suitable maximum for a VFD driven motor before you need to get into engineering things. Hnece a 6 pole motor and VFD makes avery acceptable replacement for a two speed one such as that originally fitted to your Colchester.

It is almost certainly acceptable to run a relatively modern motor up around 90 to 100 Hz effectively doubling the speed. Expect to need some engineering to get it all spinning smoothly. By then it will almost certainly be past the end of the constant power part of the curve so actual power will be falling off. On milling machines high speeds are generally only used for small cutters so power needed is generally low.

Clive

[duncan webster 1Participant @duncanwebster1]

I run mine at up to 75 hz, but not in top belt position so the max spindle speed is unchanged. For bigger jobs I still move the belt.

[Stephen Harris 5Participant @stephenharris5]

Hi, Once again, thanks, for the information.

Didn't realize so much knowledge was out there.

Regards. Steve Harris.

[RobinParticipant @robin]

My mill motor plate states that the 3 phase motor is single phase, it does 2300rpm at 50-100Hz.

At the top it says, "FREQUENCY-CONVERTER MOTOR" which seems to mean, "Numbers randomly selected from each side of the VFD.

Wonder if it really could take 100Hz for 2300rpm? When it arrived the VFD was on its' last knockings, perhaps someone tried

best

Robin

[David JuppParticipant @davidjupp51506]

Robin

Left hand side of data plate is the main motor, right hand side is the cooling fan.

It's common on variable speed drives to have a separate fixed speed motor for the fan.

[Anonymous]

Posted by Robin on 06/01/2023 10:54:17:

My mill motor plate states that the 3 phase motor is single phase,

No it doesn't; It says the fan is single phase and does 2300rpm at 50Hz.

Andrew

[RobinParticipant @robin]

You lot are clever, one little vertical separation missing at the top and I get all confused

best

Robin

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