On 4 September 2024 at 18:34 Graham Horne 2 Said:

…
On that basis my my 1400 RPM 4 pole motor can be spun to 2800 RPM with the VFD (100Hz) and the bearings in the motors are the same for both 2 and 4 pole motors. If I am wrong please let me know as I am expecting to be able to hit 2800 RPM and make maybe 3000 RPM on the vertical head (not production pushed but just occasional).

 

…
Graham

 

There’s a limit to the frequency range a 3-phase motor will respond to satisfactorily.    The coils are wound on steel laminations which greatly enhance the magnetic effect but respond rather slowly to reversals.   When a sine wave is input, the current first drives the poles North South, and then reverses to drive them South North.   The changing magnetic field then crosses into the rotor, where it generates the eddy currents that result in the out-of phase magnetic fields that spin the motor.

These magnetic changes take time to push through the laminations, which are usually designed to maximise efficiency in the 50/60Hz range.    Modern motors are more likely to cope with high-speed than old ones, but it’s a gamble – we don’t know what the core is made of.   Old or new, the motor becomes inefficient and loses power and torque when taken too far out of its comfort zone.

A 1400rpm motor spun at 2800rpm won’t have as much oomph as a 2800rpm, but the shortfall may not matter.   If 2800rpm is an occasional convenience, then boosting a 1400rpm with a VFD should work well-enough in short bursts, provided the operator compensates for reduced oomph. But if the machine will mostly operate at high speed, then better to fit a 2800rpm motor.

Try it and see.  Should be acceptable, but no-one can guarantee your particular motor will be ‘good enough’!

Dave

Graham, At one time there was a guy offering new Centec power feed half nuts, I think looking at the Lathes site would be a good start to track this down, or a Centec user group.

Chris Gunn

I know not if there were any significant gearbox changes between the 2A and 2B.

Which exact machine do you have?    The Centec 3 was a very different industrial machine with much larger motor power.  I have been guessing/assuming yours is a 2A? (3/4HP with gearbox).

A 2 HP motor will apply full torque and power at start-up unless programmed for a soft start on the VFD.

If the gearbox was not improved between the 2A and 2B, the gearbox was already taking a 33% increased power for the 2B.  Doubling the power may eventually break something.  Stalling a 2HP motor is unlikely to help matters one little bit.  Your risk, I suppose, but it could happen – even though trying not to work it too hard.

I uprated my motor to ~1.15HP – which I think might well be close to the limit if I stalled it.  I did hope that the 2B gearbox was improved over the 2A but if not, that would represent a 50% increased power. (Maybe a Centec user could comment on the gearbox difference, if any, between 2A and 2B?)

How many drive belts were there on the Boxford?  Did you double the power on that?  The twin belts on the Centec are not likely to be the weakest link.

As I said above, your choice, your risk.

On 4 September 2024 at 19:47 SillyOldDuffer Said:

On 4 September 2024 at 18:34 Graham Horne 2 Said:

…
On that basis my my 1400 RPM 4 pole motor can be spun to 2800 RPM with the VFD (100Hz) and the bearings in the motors are the same for both 2 and 4 pole motors. If I am wrong please let me know as I am expecting to be able to hit 2800 RPM and make maybe 3000 RPM on the vertical head (not production pushed but just occasional).

 

…
Graham

 

There’s a limit to the frequency range a 3-phase motor will respond to satisfactorily.

<SNIP>

A 1400rpm motor spun at 2800rpm won’t have as much oomph as a 2800rpm, but the shortfall may not matter.   If 2800rpm is an occasional convenience, then boosting a 1400rpm with a VFD should work well-enough in short bursts, provided the operator compensates for reduced oomph. But if the machine will mostly operate at high speed, then better to fit a 2800rpm motor.

Try it and see.  Should be acceptable, but no-one can guarantee your particular motor will be ‘good enough’!

Dave

 

 

Not sure what you mean by oomph but assuming the VFD is adequate the motor will have the same torque and thus twice the power at 2800 RPM.
The limiting factor is the VFD having enough voltage capability to overcome the motors back EMF at the higher speed. Even if there is not enough voltage The VFD should still produce enough to maintain half torque and thus full power at higher speeds.
The power loss with VFDs is when running motors at less than their rated speed. You cant increase the current (torque) to maintain the power at lower speed because resistive losses cause heating to increase at four times the current increase. For some reason people seen to only consider using VFDs to reduce speed when they are better at incresing it. Use a 4 or even 8 pole “slower” motor and increase the speed above rated.
This is what I did with my ML7. The 6 pole has the same bearings and fan as the same frame size 2 pole so is quite happy at up to three times rated speed without knowing anything else.

On older motors magnetic losses might be an issue but OP replaced the motor and it’s likely to be fine at 100Hz.

Robert.

On 6 September 2024 at 17:10 SillyOldDuffer Said:

On 6 September 2024 at 12:56 Robert Atkinson 2 Said:

On 4 September 2024 at 19:47 SillyOldDuffer Said:

On 4 September 2024 at 18:34 Graham Horne 2 Said:

…
On that basis my my 1400 RPM 4 pole motor can be spun to 2800 RPM with the VFD (100Hz) and the bearings in the motors are the same for both 2 and 4 pole motors. If I am wrong please let me know as I am expecting to be able to hit 2800 RPM and make maybe 3000 RPM on the vertical head (not production pushed but just occasional).

 

…
Graham

 

There’s a limit to the frequency range a 3-phase motor will respond to satisfactorily.

…
Dave

 

 

Not sure what you mean by oomph but assuming the VFD is adequate the motor will have the same torque and thus twice the power at 2800 RPM.
The limiting factor is the VFD having enough voltage capability to overcome the motors back EMF at the higher speed…

On older motors magnetic losses might be an issue but OP replaced the motor and it’s likely to be fine at 100Hz.

Robert.

Interesting post – I’m convinced I read in a book that magnetic losses were the limiting factor, and found this post on StackExchange Electrical Engineering which I think indirectly supports the idea.   I posit that the design speed determines the nature of the core.

AC Induction Motor design is dependent upon a relationship between applied voltage and frequency, what’s referred to as the “V/Hz ratio”, to produce a given amount of torque at a given speed. So a motor designed for 460V at 60Hz has a V/Hz ratio of 460/60 = 7.67 (to one), motors designed for 380V 50Hz have a V/Hz ratio of 7.6 (the similarity is not a coincidence by the way). Maintaining this V/Hz ratio (+-10%) allows the motor to produce rated torque at any speed, up to the design speed. If you only reduced the voltage without maintaining the ratio, the torque drops at the square of the voltage change, so the motor stalls very easily. What a VFD does is to allow you to attain that rated torque from the motor at any speed below the designed speed without stalling.

And this website offers analysis, including the comforting advice” High-quality, 1,800-rpm and 1,200-rpm motors up to 200 HP should be able to handle up to two times the base speed without problems” and a graph.   Note that torque falls with frequency above design speed.

 

I could be completely wrong about the magnetic reason – need to find and re-read the book!

Wish I was fit  –  I’ve lost several months of hands-on workshop fun this year.   In this example I’d like to attach a brake dynamometer to my lathe and measure how the relationship between torque, power, and efficiency varies with frequency, including driving it briefly at 3x base frequency or higher.

Dave

Hi Dave,

The stack exchange quote is correct with respect to V/Hz ratio when considering a fixed voltage supply or simple inverter drive. For instance if you want to run a 115V 400Hz motor on a 50Hz supply you must drop the voltage to 1/8 or 14.4V The speed and power is also reduced to 1/8.
Increasing speed / frequency means you can increase the voltage to maintain the current and torque. Clearly as speed is increased and torque (current) is maintained by the voltage increase the power is increased. This is a “free” increase in power for a given motor size and weight. This is exactly why aircraft systems use 400Hz.
The graph shows a drop of in power with increased frequency because “The torque reduction occurs because motor impedance increases with increasing frequency. Since a VFD cannot increase the voltage above its supply voltage, the current decreases as frequency increases, decreasing the available torque.” (quote from website you linked to). The incorrect bit is “a VFD cannnot increase the voltage”. Modern VFDs can increase voltage. They also use more complex algorithms to control the current in the motor than simple V/F ratio.
More applicable to our typical use is the fact it considers full load operation. When run at less than full design load there is more headroom from resistive losses etc to allow the motor to draw more current and maintain the power needed.
You can of course get motors intended for VFD use wound for rated voltage at a higher than line frequency. Another option would be to use an 115V / 60Hz motor on a 240V supply with suitable VFD settings. Something to remember if a low cost 115V motor or even machine turns up at low cost.

Robert.