VFD and inverter duty or standard motors

[snaggletto]

Hello,
I've got a 5HP 400Hz VFD that I'm going to use on small (4" chuck/5C collets) CNC lathe I'm fixing up with new controls. The stock DC spindle motor ran a belt drive with a minimal 1.25-1 gear reduction. The manual stated 4000RPM as the max RPM at the spindle.

I'm looking to replace the DC motor with an 230V 3phase AC motor and drive it with the VFD. I was thinking I couldn't run a standard motor much over 60hz so I was looking at 3450RPM motors. However, I've been told that lower RPM motors have much more torque. This means I would need to run the slower motors at well over 120Hz to get close to my desired RPM at the spindle.

I've also seen "inverter duty" motors that can run up to 2x the rated RPM (120Hz) at constant HP and have special insulation etc... These are generally more expensive, so are they worth the extra cost/performance over a standard motor?

Will I trash a standard motor running it at or above 120Hz? Any recommendation would be appreciated. Thanks.

[HuFlungDung]

You can still use the proper pulley ratio to get the motor into the optimum operational speed range to run your spindle at whatever speed you are tyring to get. Ideally, with the motor at 120hz, the spindle should be running at top rpm.

I would not recommend running the motor much over the 120hz, but you could always risk it. On a tiny lathe, 5hp is lots of power, so pulley ratio would be the best way to overspeed, IMO. The motor fan may not stand the high rpms, and the whole rotor needs to be well balanced for high rpm operation, too.

Your main concern need not be the high speed, so much as the low. It depends what you will use it for, but a normal VFD without feedback, does not usually "guess" very well at supplying enough current at high load, low speed applications.

[Al_The_Man]

Why are you getting rid of the DC motor? If you do decide to go with a VFD There is a couple of things I would add to HU's comments. One thing I have found is that all VFD's are not created equal, some are just variable frequency drives with very little options, and I have come across people that are disapointed with the results.
It also depends on wether the lathe is CNC or not as you might want/need inputs and outputs for zero speed and up to speed detection etc. Wether remote control, +-10v etc.
I have used Mitsubishi drives in the past and they have a function you might want to look for and that is Magnetic Flux Vector Control, this feature, coupled with auto tuning of your motor has the ability to output high torque down to low speed, high starting torque and prevention of speed fluctuation with load. The tuning runs a series of tests on the motor and plugs in the approriate parameters, saving the job of manual tuning.
The only problem is that if you run the motor for long periods at low rpm, high load you should put an external cooling fan on the motor.
As for motors, I have used standard 4 pole motors up to 120hz. For some years now motors have been wound with high voltage insulation wire, so I don't think the insulation failure is a problem now. As long as the motor is well balanced.
Use a four pole motor and you can run this up to around 3600 with 120hz. Try and get the manual on any VFD you are looking at and see what features it has.
Al

[snaggletto]

Thanks for the responses.

Stupid me, I thought the stock motor was an AC motor, so I bought a VFD. When I went to wire it up, I saw a big + and -. So, I figured I needed to either buy a DC drive or an AC motor and keep the VFD. High voltage DC drives appear to be quite expensive and not very common, where I've looked anyway. I believe the motor should still work, but the controls on the lathe are toasted and I don't have any way to test it. Here is a pic of the nameplate on the stock motor. http://members.cox.net/snaggletto/EM...te%20Right.jpg

Unless I can find a DC drive for the stock motor, I think I'm going to just buy a 2 pole 3600RPM motor and run it at 60Hz.

[Al_The_Man]

The DC motor looks like around 3.4 hp, your right, dc drives can be expensive, if you already have the VFD then you can usually pick up a 3ph motor cheaper, is it a 230 volt VFD, if it is and you intend running it off of 220 single phase in to the VFD you probabally should derate the motor size, so if it is a 5HP VFD then a 3 hp 3ph should be OK.
AL