3PH VFD Net HP

[ogesII]

I will be replacing a dead 1/2 HP 3PH motor on my mill with a VFD driven motor. What I'm wondering is how much should I oversize the new motor so I can net 1/2HP after 1PH VFD?

I'm guessing a 1HP would be plenty but am wondering if anyone knows for sure.

TIA

[Al_The_Man]

You usually de-rate the VFD not upsize the motor, IOW if your motor is 1/2hp then consult the manufacturer of the VFD you wish to use to see if their VFD's are derated on 1 phase, a 1hp VFD should be ample.
However, for the slight extra cost involved, it may be an opportunity to increase the motor HP if replacing it.
Al.

[cheetahcnc]

Most VFD's do not need the derate under 3HP(eg. Baldor), the 1HP will be more than enough. If your application requires only 1/2HP, the 1hp motor will only draw the amperage needed to overcome the load. VFD's are generally sized to current draw, and the 1HP VFD should be very little difference in cost to the 1/2HP VFD, with the extra power when needed. There are many micro din rail units offered at a reasonable price.

Standard (Baldor) motors can be reduced 4:1 without problems, while their Super E's can go down to 10:1. The inverter duty can go down to 1000:1 (and up to 6000 rpm). The motor choice is important for the speed range desired. Example: a 3450rpm standard (Baldor) motor can run down to about 862 rpm safely, the inverter duty motor can run down to 3.45 rpm while still retaining full torque and being able to take the heat. You definitely want to use a 'constant torque' drive, or one that can be programmed for constant torque. If you run slow for long stretches, an electric fan, or fan kit, can be added to keep the motor cool. As you overdrive the motor, raising the speed over the name plate rating, you reduce your torque. The 1hp will overcome this torque reduction. Example: the 1hp 3450 motor running at 5000 rpm will have about 0.69 hp at that rpm.


Jim

[ogesII]

Originally Posted by cheetahcnc Most VFD's do not need the derate under 3HP(eg. Baldor), the 1HP will be more than enough. If your application requires only 1/2HP, the 1hp motor will only draw the amperage needed to overcome the load. VFD's are generally sized to current draw, and the 1HP VFD should be very little difference in cost to the 1/2HP VFD, with the extra power when needed. There are many micro din rail units offered at a reasonable price. Standard (Baldor) motors can be reduced 4:1 without problems, while their Super E's can go down to 10:1. The inverter duty can go down to 1000:1 (and up to 6000 rpm). The motor choice is important for the speed range desired. Example: a 3450rpm standard (Baldor) motor can run down to about 862 rpm safely, the inverter duty motor can run down to 3.45 rpm while still retaining full torque and being able to take the heat. You definitely want to use a 'constant torque' drive, or one that can be programmed for constant torque. If you run slow for long stretches, an electric fan, or fan kit, can be added to keep the motor cool. As you overdrive the motor, raising the speed over the name plate rating, you reduce your torque. The 1hp will overcome this torque reduction. Example: the 1hp 3450 motor running at 5000 rpm will have about 0.69 hp at that rpm. Jim

The old motor ran at 1200rpm with 275, 425, 700, 1050, 2100 and 4250 at the spindle.

1st 4.37:1
2nd 2.82:1
3rd 1.71:1
4th 1.14:1
5th 0.57:1
6th 0.28:1

Will it be possible to get the same range of RPMs from a VFD? Or are pulley changes still in my future?

[CalG]

In simplification :

Horse Power= Torque X RPM

A motor at Full Load Amps at reduced RPM outputs reduced HP.

Increasing motor HP with a VFD helps to meet HP requirements at reduced motor RPM.

If you needed the full motor HP with the reduction drive, be generous with a HP upgrade for your new motor. Or just continue to use the mechanical speed selection when the conditions warrant. The Variable speed within a range is still there.

Best of worlds!

CalG

[cheetahcnc]

A single pulley setup loses the added mechanical torque multiplication (of the pulleys) for low speed power. I have seen clamp collar pulleys that might make it possible to reverse the pulleys, or swap them for the high speed range set. Probably quicker to have the two pulleys mounted on the motor and the spindle. A ratio around 3:1 will give you 5250 rpm at 1750 motor speed in high range, and 583 rpm in low range with 1750 rpm motor speed. The VFD would allow a generous overlap for both ranges. The extra power will make up more of the difference. You can compromise on a single set of pulleys to get you in the speed ranges you plan to mill the majority of work in, and vary it from there. I only get into the real low speeds, 100rpm +/- ,for rigid tapping, or heavy cutting/drilling in a knee mill with 2-3 three phase hp, they have the low gear range for this. I've been working on something like this for a Lagun knee mill missing the variable pulley and motor. The variable speed units on the knee mills are noisy anyway, and get expensive when piecing one back together with new factory parts. The mill already has the low range which makes it a lot easier.

Jim

[cheetahcnc]

CalG,

Horse Power= Torque X RPM

When you drive a motor over its' designed speed the HP is reduced proportionally. Constant torque drives are designed to keep the torque constant through the speed range of the motor, while the variable torque drives, for fans and pumps, vary the torque in relationship to speed. Otherwise the inverter duty motor designed for 1000:1 reduction would have very little power at that reduction according to the formula.

Jim

[CalG]

Jim

Your notes are in perfect order

"In simplification"

And the use of "amps" as an indication of torque, were my dodges to escape detail. Every situation has it's own quality.

CalG

[ogesII]

Thanks for all the input everyone. Now you are getting me thinking.

The mill came with a 1/2 HP motor. 1/2HP in 1st gear @275 RPM is 9.55 lb-ft of torque. I am guessing the spindle doesn't actually care about HP. It only cares about max torque and max RPM for the bearings. So as long as I stay under ~10 lb-ft and ~4200 RPM I am fine.

Max torque (9.55 lb-ft) at 4250 RPM is 7.7 HP

So what if I fixed the pulleys in 6th gear (77 RPM - 1190 RPM at motor) and powered the mill with a .03in^3 hydraulic bent axis piston motor and limited pressure to around 2500 to keep torque under 10 ft-lbs throughout the RPM range. I'd have a mill with full torque throughout the RPM range and easy full-range variable speed without pulley swaps. I'd also have breaking (which it doesn't have now) and mechanical reverse.

Just thinking out loud...

But liking the idea more and more as I think about it.

[CalG]

Plus you would have all of the acoustic qualities hydraulics can offer ;-)

What?....What did you say?

[Farmers Machine]

I have an old step pully Bridgeport, but my shop is single phase only. I built a 50 horsepower rotary phase converter but on 1/2 horse motor the rotary drew 25 amps just to idle on the floor. Went to Automation Direct .Com and bought GS2-23-PO VFD . 220 volts single phase input, up to 3 HP 3 phase output, you can set the amps accordingly to your motor. It comes with dynamic brake to stop the spindle as fast as .1 second and you can soft start the spindle with out jerking it to full speed instantly. My motor is 1200 RPM. so I set this speed as base speed at 60 hrtz. I set max speed at 3600 RPM to allow fast speed in low gear with out having to jump belts. it cost less than $300 and is very dependable unit, can be remote mounted and run with a forward and reverse lever switch and remote variable speed control. it is a breeze to program and Automation Direct tech support is great.
Good Luck
The Farmer