NT VFD control legend meaning

Hi All. Upgraded my old lathe a while back with a Newton Tesla VFD package, and have been very happy with the end result. Just one item needs clarifying though - on the panel is a speed control knob with colour segments making up the legend. Some sections (lower speed ranges) are red.

Now common sense suggests the following

Green segments = OK to use full whack, long as required.

Yellow (highest speed) = Perhaps OK for a bit, not extended use though.

Red = ?. Is this do not use at all or yes OK for a very short bit but not recommended or use at own risk?.

I understand the apsect of motors overheating etc at lower speeds with VFDs, but wondered if anyone had the definitive answer regarding the red segments. There's no mention of these in the NT documentation. I could ask them directly but thought it may be of interest to other members first.

(Apologies for the truly grotty photo)

I would contact Newton Tesla for a "from the horse's mouth" explanation. My instinctive reaction is that it a counter-intuitively poor choice of colours. My similar installations all run from near zero rpm to maximum without restriction.

John

Mike

I think the speed indicated could be misleading, depends on if the calibration suits a 2 or 4 pole motor, the calibration could be indicating twice the speed or half speed.

Emgee

or yes OK for a very short bit but not recommended

Can't read the markings around the pot due to focus, but it rather looks like this is the speed range that is below the motor base speed. Running in this area the motor can draw full current but the cooling airflow will be reduced due to the reduced fan speed - with attendant risk of overheating the motor.

If you want to regularly operate at high loads at below the motor base speed, you could consider removing the shaft mounted fan and fitting an external blower to give the motor continuous cooling air flow regardless of motor speed.

Nigel B.

Better close up pic -

I think I'll drop NT a message about this and will share any info I get back. It seems odd that this hasn't come up before as i'm thinking this sort of package must be in widespread use?

regards Mike

I think this is just an indication that at these speeds the cooling on the motor is compromised and continuous full loads will cause overheating.

regards Martin

I’m with Martin on this, when I fitted my NT set up they said only use in the red for short periods so most of the time now I just select backgear for the short period of work (screwcutting as an example) then the vid comes into its own as it then needs to run the motor at a higher rpm

Martin, Bernard,

Your responses would ring true if the sector between zero and 750 rpm was solid red. As it stands, it indicates it's OK to run in the green bits continuously, but not in the red bands. Likely I'm wrong, again. It will be interesting to see NT's reply to Mike's enquiry, though I doubt they'd admit to my initial explanation!

John

I'm with John on this, I can't understand why the red region is not solid.

Michael

Does the lathe have mechanically changeable speed ranges (belts or gears)? If so those red sectors look as if they could be the bottom end speed that the motor can be run at. So if the speed you want, in the mechanical range that you have set, puts the pointer in the red then select a lower mechanical range and run the motor quicker. Motors don't like being flogged at low speed as the cooling fans aren't working efficiently and the windings can cook. The yellow sector probably shows where the motor is being run at significantly higher frequency than the nominal 50/60Hz and so the torque available will be falling off or the losses could be getting higher leading to overheating

Some aircraft I fly have similar red bands on the rev counter signifying ranges where the engine shouldn't be operated continuously, normally due to propeller/engine resonance. However I don't think that is the case here. I can't think why the VFD/motor combination would have speeds to avoid.

I agree with NigelB, I think it's a warning to be careful at low speeds due to reduced cooling. The increasing width of the red bands indicates that the problem is greater as the speed gets lower. Unfortunately it's not done terribly well, as the graduations from more green to more red are too coarse.

Andrew

Following the variety of opinions offered here, I did raise the question directly with Newton Tesla and have just had a response from George Newton. It is most comprehensive & informative - and something I think will be of interest to many members of the forum as it gives a very clear technical description of VFD operation from a very experienced guy! So have included it in full below (I did ask if he had any objection to that, and he didn't appear to).

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From George Newton at Newton Tesla: