Thread: Lathe VFD conversion questions

Originally Posted by LanceC

Good afternoon all,

I'm investigating installing a VFD based variable speed setup for my lathe, and have some questions remaining after a significant amount of reading.

Motor selection
Most AC lathes appear to have a 1440 RPM motor. Would it be bad juju to replace it with a 2800 RPM unit? As I'm hoping to pick a three phase motor up second hand, it would be good to not overlook a two pole motor for no good reason.

If you read the "VFD for Newbies document" in the electronics subform it should explain why the 1440 rpm is superior.
VFDs for Newbies

Frequency range
What is a reasonable frequency range to expect from a motor and VFD that supports vector drive? This will inform what pulley ratios I will need to retain. I know to expect reduced torque at the limits, so have already planned on a 2 hp replacement for my existing 1 hp motor.

It depends on the VFD and the motor and how well they play together.
It's also explained in detail in the "VFD for Newbies document"

Speed range
With working out pulley ratios, what should I be aiming for as my fastest and slowest RPM of the lathe?

It depends what you turn, ie mainly spindles or bowls.
also explained in detail in the "VFD for Newbies document
If you want to make BMHS then 60 RPM is a useful low speed but this is a bit unusual.

Originally Posted by BobL

If you read the "VFD for Newbies document" in the electronics subform it should explain why the 1440 rpm is superior.
VFDs for Newbies

Hmmm, not that I could see.

Originally Posted by BobL

It depends on the VFD and the motor and how well they play together.
It's also explained in detail in the "VFD for Newbies document"

Is how well they play together related to the quality of the motor, or just a matter of get the motor and VFD and it is what it is?

I assume you're referring to the section where you discuss ratios.
Given the three ratios you refer to (3:1, 6:1, 12:1) , what frequency thresholds (lower and upper) are you using to make those calculations? Or am I asking questions that make no sense?

Originally Posted by LanceC

Hmmm, not that I could see.

OK, sorry its not there - it's in a more expended version of that document that I decided not to include on these forums. Looks like I should include it.

A 2880 rpm motor can run from near zero to 3600 rpm (60Hz) with no problems.
In non-vector mode like any motor it won't be making much power below 25Hz so 1440 rpm.
At around 100Hz (~6000 RPM) power starts to drop off slightly BUT this will be starting to be borderline on the bearings especially for a cheap motors.
So I would not advise taking a 2880 motor above this and in most cases keep them below 75Hz (4500 rpm).

A 1440 rpm motor can run from near zero to 3600 rpm (60Hz) with no problems.
In non-vector mode like any motor it won't be making much power below 25Hz so 720 rpm.
At around 100Hz (~~3000 RPM) power starts to drop off slightly BUT the bearings (even on cheap motors) can easily cope with this speed..
Even at 150Hz there may still be usable power and the bearings are only running at 4500 rpm (still OK)
Above that most motors will start to really lose power.

This 1440 RPM motor generates usable power from 720 to 4500 rpm ~6:1 , 2880rpm motor only generates the same usable power from 1440 to 4500 rpm or 3:1.
Vector mode just expands the lower end in proportion.

Is how well they play together related to the quality of the motor, or just a matter of get the motor and VFD and it is what it is?

I assume you're referring to the section where you discuss ratios.
Given the three ratios you refer to (3:1, 6:1, 12:1) , what frequency thresholds (lower and upper) are you using to make those calculations? Or am I asking questions that make no sense?

See above.

The "how they play together depends on something called "inherent motor magnetics" (the magnetic properties of the motor at non-standard frequencies). Some motors have a wider range of motor magnetics which make them more VFD compatible. Old motors tend to have poorer magnetics - one 100 year old motor I played with would "squeak and squawk" and lost a a lot of power at anything over about 60Hz. I can check all this with my HP/dyno but not many folks have one of these, BUT if the motor squeals and starts to get warm, something is not right. Don't force it.

Sorry, for the late reply Bob, I was sure I replied to you this morning, thanking you for your comprehensive answer, but just came back to ask another question and my “thanks” post is missing. So thank-you.

Onto my new question. The difference in cost between a 1.5 and 2.2 kW VFD is inconsequential. As such, is there any reason not to get the larger capacity VFD considering I’m only going to run a 1.5 kW motor.

Originally Posted by LanceC

Sorry, for the late reply Bob, I was sure I replied to you this morning, thanking you for your comprehensive answer, but just came back to ask another question and my “thanks” post is missing. So thank-you.

Onto my new question. The difference in cost between a 1.5 and 2.2 kW VFD is inconsequential. As such, is there any reason not to get the larger capacity VFD considering I’m only going to run a 1.5 kW motor.

Not really as long as you realise the Parameter that limits the max motor current will need to be reduced otherwise it could damage the motor..