60Hz AC motor at 120Hz?

[snaggletto]

Hello,
I've got a small CNC metal lathe that needs a new spindle motor. The application requires basically a 1:1 belt driven ratio to the spindle giving stepless programmable RPM's from nearly 0 - 4000RPM. I need as much torque as possible at lower speeds, less torque at high speeds is fine.

I've been looking at 'inverter duty' motors at automationdirect.com. Specifically this 3HP Marathon Micromax

http://web2.automationdirect.com/static/specs/motorsmicromax.pdf

It claims a 1000:1 constant torque ratio and 2:1 constant HP ratio. It also claims a max constant HP speed of 3540RPM, and max safe RPM of 5400.

As I understand it, I would need to run this 1800RPM motor at 120Hz to achieve ~3540RPM, which I could live with as a max RPM. I've done alot of reading around this forum and others and I've mostly been told that running any 60Hz AC motor over 60Hz can be bad trouble, and that at 120Hz it would be like bomb ready to explode.

Is this motor a good example of what I need for my application? More importantly, can this inverter duty motor, or a common 60Hz motor be operated above 60Hz up to 120Hz without smoke, fire and debris?

As I see it my only other choice is to buy a 2 pole ~3600RPM motor and run it at 60Hz max. However, I understand that 2 pole motors have much less torque than 4 pole 1800RPM motors, and that a non-inverter duty motor will have overheating problems at lower RPM's. Maybe I could install an external powered fan to aid in cooling.

What should I do here. Any advice would be greatly appreciated.

 

[ruggedscot]

This is quite complicated.....

a 60Hz motor being run at 120Hz

Hmmm not a very good idea. These motors will have been designed at performing at a rotational speed of around 1800rpm. If you increase this by 100% you will be increasing the stresses presented to the motor by more than double. You have weight issues vibration isses and the like to take into consideration. Also the amount of energy stored in a mass that is rotating at double the designed speed is scary, if it breaks it may do some serious damage. What are the bearings rated for ? you have to look at all the possibilities here to try and ensure that it is safe and can do what you want it to do. Id go for a gear ratio to step up the speed rather than going the VSD way.

Lower speeds you will require a fan to ensure that the airflow is maintained and that adequte cooling is provided or else you may suffer premature failure through excessive heat.

Rugged

 

[snaggletto]

Hello,
Did you look at the 'inverter duty' motor I linked to in my first post? On the specs sheet, in a graph towards the bottom, it shows the max constant HP RPM of 3540RPM and max 'safe' speed at some 5400RPM. Am I totally ignorant in thinking that this specific motor would safely run at 3540RPM which would be achieved at just under 120Hz? What am I missing here?

 

[DickDV]

Snagletto, you have exactly the right motor there. It is designed to do just what you are looking to do.

A 3hp four-pole motor will develop about 9ft-lbs torque up to 1800. Above that point the torque will drop off in the inverse proportion as the overspeed until, at 3600rpm (double speed), the torque will be one-half of 9ft-lb or 4.5ft-lbs. At 4000rpm the torque will be 9 x 1800/4000 or 4.05ft-lbs.

For machining spindles, this is often just what the machine needs in torque characteristics. The torque increases from 4000rpm down to 1800rpm and is constant at that level from 1800rpm down to zero.

Don't worry about balance or bearings. Any inverter duty motor certified to constant horsepower over a speed range is expected to operate over that same range without significantly shortening its service life. You are a bit out of spec from 3600 up to 4000 but I assume you won't be running up there a huge share of the time.

The motor is rated for maximum safe speed of 5400 so the motor coming apart isn't a consideration either.

Don't make these same assumptions about any old standard duty 1800rpm motor.

 

[snaggletto]

Hello,
Thank you for your reply. I've been all over asking this question.

I'm kinda new to this stuff, so let me just run this by you. For comparison sake, how would a 2 pole 3600 RPM (keeping the frequency at 60Hz) standard motor compare to the 4 pole overspeeded motor above in regards to low end torque and overall torque?

Standard 2 pole 3600RPM motors can be had for half the price of the Marathon Micromax 4 pole 'inverter duty.'

Thanks a million for your time and advice.

 

[ruggedscot]

Sorry Snag....

assumed that it was a normal duty motor didnt realise that it was a drive duty motor....

I should take my own advice sometimes and look at the links provided.

Yes by all means the motor would be safe at the speed quoted.

Ill need to put that book by ralph nadir back on the shelf unsafe at any speed !

 

[DickDV]

A 3hp four pole motor will produce a constant torque level of 4.5ft-lbs from zero to 3600rpm. From 3600 to 4000rpm, the torque will fall off as a function of the inverse of the overspeed, just like the four pole motor does at 1800rpm.

The four pole motor will provide the same torque as the two-pole from 4000 down to 3600rpm. Below 3600 the four-pole gradually increases its available torque until, at 1800rpm, it has double the torque of the two pole.

If you need high torque at slow speeds, (most machining applications do) then the four-pole is the better choice.

You will not damage the four-pole motor by running to 3600 and 4000 rpm.

 

[aolalde]

For this type of small motors, as the Marathon MicroMax designed to operate with variable frequency drive, operating up to 3600 rpm and may be 4000 rpm will be satisfactory.
The rotor diameter and bearings will be into the speed safety parameters. Be cautious when similar conditions are required for larger motors.

 

[DickDV]

Just looking at my previous post, the first line should start out reading "A 3hp TWO-pole motor".

Sorry for the confusion.

 

[Barry1961]

I have always heard the torque dropped off over the base hz as a square function. (60hz/120hz)^2 = .25% of rated torque. But I could very well be wrong.

Barry1961

 

[snaggletto]

Hello,
Yes, below base RPM is called constant torque. This is dependant on the motor, I've found, and can literally be from 0-base RPM with the right motor and drive. Above base RPM is called constant HP, usually up to 2x rated RPM, the torque does drop off quickly. However, in my application, a small metal lathe, this will be perfect. Above 2000RPM your workpiece size is quite small and you take lighter cuts. You don't need as much torque as you would at 800RPM with a larger part and heavier cuts.