Oddball inverter

My little Cowells lathe is driven by an 80W 3phase motor. The VFD is Mitsubishi SC A2 400M, which is reckoned to drive up to 400W motors. I've checked that the motor is set up in Delta.

I've just measured the power at the lathe spindle, and I can't get more than 34W, see table below

Speed RPM Torque Nm Power W

1846 0 0

1670 0.148 26

1500 0.217 34

with the speed control set to about half way I get

Speed RPM Torque Nm Power W

880 0 0

793 0.091 7.5

668 0.183 13

I don't think you get to set parameters like current with these devices, anyone got ideas as to why it's not producing the expected power? I wouldn't have thought it was losing that much in the lathe bearings.

Oh and if you're wondering why I've got a three phase motor on a little machine like this, it came as part of a deal.

I don't think you get to set parameters like current with these devices

I have yet to come across an inverter that does not require the input of the basic motor parameters (voltage, FL current, frequency & base speed ) - these seem to be the basic minima to get up and running ? FL current would be particularly important in your case of operating a drive with a motor a long way below the drive capability.

No experience of a Cowells, but are they not plain bearing spindles so maybe higher drag than rolling element bearings ?

Nigel B.

VFDs are all the rage now - but there not a new idea. I have a 1.5Hp unit by mitsubishi from 1990. I have not used it but apart fom speed control there may not be much else. Have you tried contacting newton tesla, some speak highly of them, they may be able to help. Noel.

Duncan

I got a couple of Mitsubishi inverters from Gavin Osman many years ago and have a vague memory of being told to be careful when adjusting parameters as it was quite easy to inadvertently put the beasts into a low power "test mode". Presumably provided so things could be made to go round without risking releasing magic smoke if jam ups occurred and other load related parameters were vastly in error.

Apparently the manual was "less than clear" about this. (Everything else too I reckoned. The manuals in the box were about half the size of the VFD itself.)

Clive

Duncan,

Cannot find that drive on internet - do you have the manual?

From experience on my Lathe's VFD the behaviour may be due to the output voltage to frequency settings being incorrect. A start is max V @ 50Hz and dropping to 80% V @ 20Hz..

Unlikely to be a too-low max current setting as that would probably trip the VFD.

Joe

Presumably power and torque are being measured with a dynamometer? Most excellent!

Do you have a domestic wattmeter or - more accurate - a multimeter reading AC amps. Be interesting to compare the input watts with the output power. If the inverter is drawing less than 80W at full load, then probably a parameter setting problem; if more than 80W, then either the electronics are horribly inefficient, and/or the lathe is wasting power in drive train. Does the power output at the spindle change after the lathe has thoroughly warmed up? (Cold plain bearings are notoriously sticky.)

Measuring input power to my 1.5kW WM280, it's difficult to take a cut heavy enough to fully load the motor. With carbide the resulting spray of red-hot chips takes all the fun out of machining, and I'm not convinced the lathe is rigid enough to work that hard and maintain accuracy. It's a hobby lathe!

More interesting though is the extra load imposed by engaging the banjo in fine feed and taking the lead-screw gearbox out of neutral. This takes 200W extra input with the lathe idling. Roller bearings on the motor and spindle, not sure what's inside the gearbox, but otherwise plain bearings all the way!

Dave

Duncan, I may be able to help if I could find the manual for the VFD. I have searched for it and can't find anything for the SC A2 400M.

Michael

.

Forgive me for asking the blindingly obvious question, but:

How [in detail] is that 80W specified ?

You may find that your 34W measured power at the spindle is entirely reasonable.

Could you rig something to test the power at the motor shaft ?

MichaelG.

.

P.S. ... it is not unusual to specify motor output power ‘at stall’

Edit: some decades ago,  I used one of these:

https://www.magtrol.com/product/dial-weight-dynamometers/

Edited By Michael Gilligan on 12/04/2021 11:05:35

?? Surely when stalled speed = 0, so power=torque x speed = zero too?

The inverter may have a power readout mode. If you can get it to do this, then the information will be helpful. It (obviously) should be less than the total power going in to the inverter and more than the power at the spindle.

If operating under good condition you should see an efficiency approaching about 90% at each stage, so the motor output should ideally be 'approaching' ~80%.

Then there's the losses in the machine, it seems excessive for that much power to disappear in the mechanical drive train, and a few tens of watts should be enough to make a small lathe noticeably warm.

Have you tried disconnecting the drive to spindle and measuring with just the motor running - and does the motor get warm / hot?

.

Yes ... hence the use of quotation marks.

MichaelG.

.

[quote] 

• Magnetically coupled hysteresis brake provides smooth torque application independent of shaft speed. This permits testing motors from no load to locked rotor or armature.

Edited By Michael Gilligan on 12/04/2021 13:55:12

The motor is by Exico exico an industrial type, so I'd expect the power quoted to be output. Part # is 1254689

Calling my set up a dynomometer is being kind, I call it a lash up, but it works, a bit of wood, the kitchen scales and a Chinese tacho with the magnet stuck to the chuck.

The photo of the motor plate is not readable, the details are

400/230V

cosphi 0.74

A 0.31/0.54

I think this means input power is 400*0.31*sqrt(3)*0.74=158W

here's a couple of pictures of the VFD, the only adjustments are for acceleration and thermal cut out

I've got the manual, it says nothing useful. I'll try Newton Tesla tomorrow

That’s strange I’ve never seen a VFD where the input terminals are marked R,S,T usually they are L1,L2,L3 if 3 phase or L1,N if single

Is it a 400v inverter? Rather than a 200v one ..is it expecting 3 phase in? Some will run on 240v on 2 phases

Revel in your moment of fame, Duncan

.

.

MichaelG.

Not that unusual, there are several conventions kicking about including U,V,W; R,Y,B; T1,T2,T3; A,B,C; 1,2,3 and probably others. I've a vague memory RST is a German acronym. R,Y,B refers to Red, Yellow, Blue, which is an obsolete wiring colour code but I believe the others, including L1, L2, L3 are just sequences. We need an expert on the international history of 3-phase labelling systems!

Dave

I have 2 HY inverters both are marked RST for input & UVW for output. I also have a VFD with no makers marks and that also is marked RST & UVW.

regards

.

I am far from being an expert ... but I did find this: **LINK**

https://www.eevblog.com/forum/beginners/r-s-t-l1-l2-l3-u-v-w/

[just to add to the confusion]

MichaelG.

My mistake i meant marked as RST for a single phase vfd