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Motor De-Rating for Slow Speed VFD


Jun 19, 2020
Hi All,

I have an old Delta-Milwaukee drill press I rebuilt still has the original 3 phase motor, 3/4hp 1750 RPM, the drill press run well and has a cool turret attachment on it. But it goes a little fast for my application, even in the lowest drive ratio.

In the lowest ratio its 4.5:1 so 388 RPM'ish That's a little fast for a larger bit in the materials I use so I'm going to swap the motor.

I found 3 phase 1200 rpm motors 'reasonably' priced and that get's me down to 260'ish, I was considering over-sizing the motor a little and use a VFD to get me down lower when necessary, but I don't know the VFD de-rating rules so I can properly oversize.

Any Info would be nice.

Attached is it and it's glorified drill press friend, that has really good torque, don't need that much, but a little more than what I currently have.


Drill Presses.jpg
Quick and dirty answer is that 3ph motors are constant torque below rated speed (how low depends on the quality of your VFD) and constant HP above rated speed.

I'd not hesitate to use that motor at 20hz, 1/3 speed, for short durations. Keep an eye on the motor temp, but it's also a small drill. I doubt you'll be tapping 1/2-13 in 304 over and over.

Sent via CNC 88HS
Quick'n dirty rule of thumb concerning VFD use on industrial rated machine tools is that you will pretty much not notice torque variation effects between about ± 1/3 speed variation from data plate unless you are overloading past the original design limits. Objectively anything made for serious industrial use will be overpowered.

With modern vector drive VFD boxes odds are you can get down close to half speed before hitting trouble if you sort out the torque boost settings. Which may not be easy if you don't understand whats going on and the manual is a simple do this to do that affair.

Simple explanation of whats going on here HowTo: How to get Maximum Torque at Low Speed from a Simple VxF Inverter should give you the idea but obviously what you actually need to do depends on the inverter.

I think your gonna start hitting the wall on how much torque the standard V belt system can handle.
The torque stays about the same over the range so there is no problem with belts.

Square law applies to things like heaters where when you halve the voltage you also halve the current. It doesn't quite hold there, either, because most heaters' resistance is lower at the lower temperature so it draws more current than you might expect.

The motor draws about the same current at any speed so the hp is a simple speed X torque.

Thank you all,

It's one of those do it right and do it once type thing. I think I'll pop for the 2 hp and see where that goes, and then VFD as necessary, keep an eye on the motor for heat and mount an additional fan as necessary.

Belt slippage is rarely an issue with anything I do, I keep a can of that belt spray around the belt machines just in case, but for the most part I do ok.

Thanks Phil, that's what I was looking for, that little 'this is the rule...' answer.

Thanks all
If it is a TENV motor, you can run all day long at full torque at stall and not worry about over heating. Given the duty cycle of your need and the turn down of your belt ratio I wouldn’t worry about it.

Also, if the motor has a thermal click switch embedded in the windings, wire that into the drive’s overload contact input and drill to your hearts content at as low a speed as you want. The overload will let you know when the motor is getting hot and you can adjust accordingly.
If you configure the correct numbers for the VFD you will know how far you can go. The VFD will tell you where your boundary condition is.

BTW, have never seen a chopped down table for a mill-drill. Very compact.

Once you realize it's just a glorified drill press, with a big ass 10" 4 jaw mounted as a vice cutting the table was a no brainier. I think it has move plus minus 1" center in the last year. Getting ready to add a cross hair laser setup to it, to add to the functionality.

My solution to the same problem was to install a 3 HP, 3 ph motor. I run it with a 20 year old MotoTronic FVD with few bells and whistles. Years ago when I was building architectural iron work, I drilled and tapped 1/2 - 13 holes by the hundreds in 5/8" plate flanges using plain vanilla gun taps out of the drill chuck. Same job with 1 1/2" annular cutters. Hole sawed bird blocks, etc all sorts of high torque fhole making.

When I was in the talking stage of my drill press conversion, the doubters predicted all sorts of trouble overpowering an othewise sturdy but consumer grade drill press. Never has a speck of trouble and I made lots of big holes. Probably the most cost effective project repowering my personal equipment.

Just saying.