VFD frequencies, what would your max be?

[Hood]

I recently had to remove the servo motor from the spindle of my lathe due to the drive kicking the bucket. I didn't fancy spending £2000 to get it repaired as this isn't the first drive that has died on me.


I now have gone back to the original induction motor fed via the original gearbox which has electromagnetic clutches in it. These clutches can be changed under power so that means I can have 6 speeds from the gears.
The motor is 2 speed so that gave an additional 6 speeds and then in addition there is a back gear which gave a further 12 speeds although quite a lot of them overlapped.

The two speeds of the motor had to be taken out of the equation as I am running from the VFD and to get things up and running quickly I just chose one gear (710rpm) and have been using that with the VFD controlling the speed. I was planning on writing a macro at some point in time to change the clutches depending on the speed required but have not done that yet.
I am considering moving to an SZGH CNC1000T which is a stand alone Chinese controller. It can seemingly control the gears without me having to write a macro and this is where, after such a long intro, I get to my question


There are a choice of 4 gear outputs with the SZGH controller and I am wondering what is thought to be the optimal min/max frequency via a VFD.
Knowing that will help me choose which are the best gears out of the 6 to use.

The motor is a 14HP (11.5KW) 4 POLE 1440 RPM motor, data plate below.

 

[Shox Dr]

Max is usually around 150%. Minium would depend on how much torque the motor can cope with at low hz

 

[eLuSiVeMiTe]

. Minium would depend on how much torque the motor can cope with at low hz

And cooling with reduced fan speed?

 

[Luke]

Low speeds create issues with a lack of self cooling, it's pretty common to fit separate cooling fans if running slow (around 5 - 10 Hz) with significant torque.

As you go deeper in to field weakening, the available torque will drop significantly. Ideally you'd have a torque curve for the motor to check. Spindle motors often have a massive constant-power range as it can be a desirable characteristic.

At least, with a spindle, the dangers of the motor pulling out are less of an issue than on other loads.

 

[Shox Dr]

And cooling with reduced fan speed?

Thats the one

 

[Hood]

Ok thanks for the replies so far.

The spindle motor is basically a bog standard Induction motor and I don't have any torque curves sadly.

The gear ranges with the motor in high speed are
355
500
710
1000
1400
2000

I am very rarely much above 1000rpm so choosing the first 4 gears would be fine as far as above base frequency goes but below base will now be my concern.
I often do parts at low RPMs as they are big diameter and I wonder what torque I would get at lower RPMs?
I suppose I am not going that low on the frequency, maybe 25-30Hz at the normal lowest rpm I use. Would there be a big drop off in torque at those frequencies do you think?

 

[Shox Dr]

The vfd should allow you to set the amount of torque at given hz

 

[eLuSiVeMiTe]

I know little but the homework I did suggested no lower than 30htz and max 70. 60 max does me fine on speeds for my lathe so haven't ventured higher,

 

[Pete.]

I'd check to see if there's a 2-pole version of that motor and if there is I would happily go to 100 or even 120HZ.

 

[Luke]

At 25 - 30 Hz you'll be ok if it's not constant use. You'll have around 80% rated torque.

Below rated speed, it's a balance between torque and thermal management, with a self cooling motor. Performance will suffer once you get too slow, without an encoder, but you probably won't be down at single Hertz?

 

[Hood]

The vfd should allow you to set the amount of torque at given hz

No idea about that, not used to VFDs as I have only ever used them for small motors such as on the oil pump for the headstock. I am more used to servos.

I know little but the homework I did suggested no lower than 30htz and max 70. 60 max does me fine on speeds for my lathe so haven't ventured higher,

That is probably the range I will be in

I'd check to see if there's a 2-pole version of that motor and if there is I would happily go to 100 or even 120HZ.

Na, don't fancy splashing out on the cost of an 11.5Kw motor and don't need the extra speed of a 2 pole.
I have a big Yaskawa spindle motor and drive that I was considering using but it is 6000rpm and it would need a lot of gearing to get it down to usable speeds for my lathe so I decided against it.

At 25 - 30 Hz you'll be ok if it's not constant use. You'll have around 80% rated torque.

Below rated speed, it's a balance between torque and thermal management, with a self cooling motor. Performance will suffer once you get too slow, without an encoder, but you probably won't be down at single Hertz?

Nah won't be constant use, probably 30mins of intermittent cutting at a time and that is fairly light cutting (skimming cast iron hauler plates) so sounds like I should be fine.
Worst comes to the worst I can always programme higher speeds and shove it in back gear, might even manage to write that into a macro but until I get the controller I am not sure what is possible.

Thanks all for the replies

 

[awemawson]

When VFD's first came out years back, there were issues with insulation breaking down at higher frequencies. Apparently there were harmonic components resulting in very fast rising edges that caused this. This resulted in 'inverter rated' motors being offered.

At the other end of the speed range the issue is cooling, with a slow running armature with integral fan not drawing enough air through the stator coils

 

[Luke]

When VFD's first came out years back, there were issues with insulation breaking down at higher frequencies. Apparently there were harmonic components resulting in very fast rising edges that caused this. This resulted in 'inverter rated' motors being offered.

This is due to the PWM pulse/carrier frequency, rather than the output frequency, and yes, dv/dt rates are a problem. The motor will easily see peak voltages twice the dc bus voltage.

An output reactor, or in extreme cases, dv/dt filter, will bring the PWM output back to a sinusoidal form that means the motor insulation is no more stressed than on the mains. This sort of filtering also addresses the issue of eddy current through the motor bearings, on big motors (>100kW ish).

 

[Luke]

If anyone's interested, I've attached four example torque curves, each around 11.5 kW.

• 4 pole induction motor - self cooled
• 4 pole induction motor - forced/externally cooled
• 4 pole spindle motor - forced/externally cooled
• Permanent magnet servo motor, self cooled

You can see that the field weakening range varies significantly, both in terms of the speed the motor can reach, and the torque available.

4 pole self cooled - you can see that the rated torque is determined by the motor's ability to cool itself.


The same motor, with a force vent fan, is capable of delivering rated torque across the speed range, though you'd need an encoder to use this at zero speed.


A spindle motor is designed to have a long field weakening range


And a permanent magnet servo motor has significant overload, as well as field weakening capability.

 

[Hood]

Very interesting, especially that of the forced air induction motor.
I will have to have a look at the Yaskawa spindle motor I have and see what its torque curves look like as I am sure they are in the manual. I should really shove it on eBay as I will never use it, bought it thinking it may do the Chiron until it arrived and I then realised it was rather too big and heavy

Looking at the speed plate on the lathe I could almost get away with using the back gear (green arrow) rather than the normal gear (red arrow). Will have to have a think about this and maybe run it full tilt in back gear to see how it sounds as I would imagine it will be a bit noisier.

 

[Hood]

Here are the curves for the Yaskawa motor and drive I have, it is 2 speed with an automatic switch over so has high and low curves.