Lathe VFD Motor selection

[Rauce]

@Rauce that seems to be my line of thought - can the motor be pushed above the 60Hz frequency to get higher speeds and if so, what would be a "safe" maximum setting? Greatly appreciate the comments

For sure, I’ve run mine up 120hz. Never needed to go higher than that, I’m not sure what a max for a general purpose motor is. Usually the specifications are more concerned with the low end.

I’d be more concerned with the spindle. Say at 120hz you’d be at 1600rpm spindle speed and that chart shows a max of 1100rpm. What can the bearings take?

 

[TorontoBuilder]

@Rauce that seems to be my line of thought - can the motor be pushed above the 60Hz frequency to get higher speeds and if so, what would be a "safe" maximum setting? Greatly appreciate the comments

For all the work involved in making the chnage I can't see why you wouldn't just get the more appropriate motor, They're easy to come by.

But @Rauce is correct, you can do as he says and likely overclock the motor to 120hz without too much worry.

 

[thestelster]

This is an excerpt from a recent article I read:
"Rules of thumb- a 3600 rpm motor can be run safely up to about 75 Hz (or 25% above its rated speed). An 1800 rpm motor can be run safely up to 120 Hz or 200% of its rated speed."
"The real limiting factors for over-speeding a motor are not so much electrical as they are mechanical. Bearing wear and rotational instability resulting from changes in mechanical balancing requirements at speeds above the nominal can lead to motor failure and/or unacceptable vibrations when operating a motor continuously at higher than its rated top speed."

 

[Rauce]

This is an excerpt from a recent article I read:
"Rules of thumb- a 3600 rpm motor can be run safely up to about 75 Hz (or 25% above its rated speed). An 1800 rpm motor can be run safely up to 120 Hz or 200% of its rated speed."
"The real limiting factors for over-speeding a motor are not so much electrical as they are mechanical. Bearing wear and rotational instability resulting from changes in mechanical balancing requirements at speeds above the nominal can lead to motor failure and/or unacceptable vibrations when operating a motor continuously at higher than its rated top speed."

That makes sense, I’ve done 120hz on 1800rpm motors.

So I guess that’s another reason to go to with the slower motor and different belt setting. At 75hz you’d only hit 1000rpm.

 

[TorontoBuilder]

This is an excerpt from a recent article I read:
"Rules of thumb- a 3600 rpm motor can be run safely up to about 75 Hz (or 25% above its rated speed). An 1800 rpm motor can be run safely up to 120 Hz or 200% of its rated speed."
"The real limiting factors for over-speeding a motor are not so much electrical as they are mechanical. Bearing wear and rotational instability resulting from changes in mechanical balancing requirements at speeds above the nominal can lead to motor failure and/or unacceptable vibrations when operating a motor continuously at higher than its rated top speed."

First let me preface this comment with the statement that I was the first person to recommend against a 3600 rpm motor for this application. Secondly I still think it is a poor choice when an alternative can be had for under 100 bucks...

However, I'll add two comments. A hobby machinist's motor is going to be subject to far less wear than a commercial use lathe so assuming starting point is a newish motor it is likely that there willl be no issues with the proposed operation. Most of the bearings in this size motor are rated for much higher speeds than they are operating at. It is typical that the bearings in a decent motor are rated for 9600 rpm, and the lubricant can dissipate the heat from operating at such speeds. I can't say about the loading issues because I dont know what braking the proposed vfd can provide...

My point is this is safe enough to try as a stop gap interim solution, but it is far from best practice

 

[Dabbler]

I agree with Ryan. If you set your lathe to run at 800RPM at 60Hz, it really gives you all the range you need.

Because the 3450 motor has fewer poles, you will see less torque than for a 1725 RPM motor. Really I don't think that will limit you much unless you really need to take really deep cuts. Starting and stopping will heat your motor a bit more than for a 1725 motor. Again not much of a problem, but something to check if you start and stop a *lot*.

 

[PeterT]

Most of the bearings in this size motor are rated for much higher speeds than they are operating at. It is typical that the bearings in a decent motor are rated for 9600 rpm, and the lubricant can dissipate the heat from operating at such speeds.

I think what @Rauce is pointing out is the spindle is rated/recommended 1100 rpm so would be the more limiting factor even if motor bearings capable of much more?
I suspect belt speed or gear train might similarly factor even if the spindle bearings could take it?
Max rpm of chucks decrease as a function of size although typical hobbyist diameter is still pretty high. But some people have found out new sources of vibration & even jaw grip reduction depending on chuck condition & the machine.

 

[lucsimoneau]

Hey guys, thanks for all your replies ... I've been looking around for a 3HP 3PH 1750 RPM motor and can't seem to find anything around 100 bucks ... where would be a good place to find one other then Marketplace?

 

[Susquatch]

Hey guys, thanks for all your replies ... I've been looking around for a 3HP 3PH 1750 RPM motor and can't seem to find anything around 100 bucks ... where would be a good place to find one other then Marketplace?

I found Kijiji to be better than marketplace for used motors. Set up three separate searches for 220 motor, 230 motor, and 240 motor. Sellers call them all kinds of things so you need to cast a wide net. I suppose you could also search on electric motor but that might get a lot of hits to wade through.

Also look at local scrappers and machinery junk yards.

Mainly just find something and then be patient. Most sellers want waaaaay more than they end up selling for. Sooner or later reality sets in. That's the time to make an offer.