[How-To] 5hp 3-phase AC motor & VFD control question

[Nightshift]

Rabler, thanks for the calculations and explanation. Right now I'm very close to convincing myself to do the work involved to marry up my gearbox with the 5hp motor. After all, I am retired, have the time, capability and doing so will essentially keep my 10EE operating in the range it was built for. I don't do much thread cutting generally and in the last 10 yrs can only think of once where I used the backgear at all. But it would be nice to keep it for those times when needed. Another bonus to keeping the gearbox is I don't have to change pulleys or belts as long as I make a motor mount to maintain the same motor shaft height as the current DC motor. The VFD provides new top end speed as the most it can do right now is 2,200. Just over-speeding the 5HP/1740RPM motor by 1.57 (94 Hz) gives it a full 2,500 RPM at the spindle. I don't see my use having a need to go over 2,500 RPM as that's the limit of my spindle tachometer and disengaging the drive gear to it seems pointless.

Just writing about this has convinced me to keep the gearbox and pulley arrangement on the lathe now and interface the new motor to it. I really appreciate the help here to sort thru the options and decide on a direction. Its time to start getting dirty and pull it apart while I wait for the new 10hp VFD to arrive. Cheers, Bill

 

[Nightshift]

Good point, I haven't looked inside that reduction gearbox for 10 yrs, as I've only ever engaged it once in that time and I can see by the oil sight window its still full of oil and none has leaked in all that time. I will open it up anyway and inspect. Thanks for the reminder. Bill

 

[mksj]

Either way works, but adding a low speed gearbox is more optimal and also you save on a smaller motor and VFD. There is the cost and time to adapt it to the gearbox and also be careful of over speeding the gears if you were to use an inverter vector motor. The other part to consider is that an inverter vector motor can produce up to 2X it rated torque for short periods (like up to 1 minute) and a much wider operating envelope so one can use a 2.5 to 1 belt ratio which increase the low speed performance and gives a wide full Hp range. Probably not a problem using the motor you have but ideally an inverter rated motor (not vector) will have a wider constant torque and the wiring insulated will withstand the VFD PWM voltage spikes better.Typically not an issue with motors that are dual voltage and you are using them on the lower voltage setting. I also recommend a mid-price VFD like the Hitachi WJ200, Teco E510, Yaskawa V1000, etc. Yaskawa does make a single phase input 5Hp VFD CIMR-VUBA0018FAA which is priced similar to going with an oversized 3 phase input model, but size wise they are about the same.

 

[Nightshift]

Thanks mksj for the added wisdom. I think the reason I ordered a 10hp VFD was because I hadn't decided yet if I was using the 5HP china motor I already have or going with a larger inverter duty 7.5 HP. But this VFD will run the 5HP fine so I'm good with that, especially since I've decided to maintain my 5:1 gearbox for lowend torque. Bill

 

[rabler]

Thanks mksj for the added wisdom. I think the reason I ordered a 10hp VFD was because I hadn't decided yet if I was using the 5HP china motor I already have or going with a larger inverter duty 7.5 HP. But this VFD will run the 5HP fine so I'm good with that, especially since I've decided to maintain my 5:1 gearbox for lowend torque. Bill

When you adapt the reduction gearbox, just think about the option of replacing the current Chinese 5HP with a inverter duty 5HP motor at some point in the future. That might influence the approach for adapting it, and make life a lot easier in the future if you decide you need the inverter duty motor performance.

 

[Nightshift]

Yes another good point Rabler! I'm thinking of a couple ways to tackle this:

1. Do something similar to what Karl_T did with his 1st 10EE, and also Don's Engine where they stripped out the guts of the Reliance DC motor and mounted their 3-phase AC replacement inside the carcass. Cut off the Reliance DC motor shaft with the splines that drive the gearbox and add a lovejoy coupling to connect the two. I imagine the hard part would be centering the new motor in the carcass securely. The beauty of this approach is once the new motor is positioned, everything else just bolts back together and the belt lengths don't change.

2. A little more work would be to build a heavy right angle fixture to allow mounting the gearbox on one side and the motor on the opposite and connect the two shafts with a lovejoy coupling. Once the gearbox is attached it would be pretty easy to determine the motor mount height to align the shafts. This approach most likely will require longer drive belts as the finished assembly would not sit nearly as high off the original motor mount plate as the Reliance DC motor does currently.

I guess either approach would lend itself to a motor swap down the road if this chinese version proves troublesome as a replacement could be mounted the same way. I've read lots of other ways people do this by machining a 1" thick plate to mount both directly, but this would require much more precise work that a lovejoy coupling type connection and I don't really see the benefit for the light use I have for this lathe. Bill

 

[Nightshift]

Rabler, I've been absorbing your calculations and making some adjustments for my actual conditions to compare. Would you look over and confirm if I understand it correctly please?

Current power - Reliance DC 3hp, armature 0-690rpm, field 690-2400rpm plus 5:1 redn gearbox.
With existing 6.5" OD motor/gearbox pulley and 7" OD spindle pulley for a 1.08:1 ratio. So this calculates to:

Motor RPM Spindle RPM Spindle Torq (w/o redn) Spindle RPM (5:1) Spindle Torq (5:1)
690 639 25 128 123
2400 2200 7 n/a n/a

New power - 5hp AC 3-phase, 1760rpm with suggested range of 20-120 hz plus 5:1 redn gearbox.
With existing 6.5" OD motor/gearbox pulley and 7" OD spindle pulley for a 1.08:1 ratio. So this calculates to:

690 (23hz) 639 41 128 190
1760 1630 15 326 75
2700 (92hz) 2500 11 n/a n/a

Same 5hp 3-phase but with 4" OD motor/gearbox pulley and 7" OD spindle pulley for 1.75:1 ratio

690 (23hz) 394 67 79 190
1760 1006 26 201 75
3520 (120hz) 2011 13 n/a n/a

If I did this right, it seems to me I should retain the 6.5" OD drive pulley with this new 5-hp/VFD setup as it provides a little better torque at both low and high ends, while allowing a full 2500 rpm at 92hz.

If I reduced the drive pulley to 4"OD, top end speed will max out at too low at 2000. Am I missing something in all this? Bill

 

[rabler]

Rabler, I've been absorbing your calculations and making some adjustments for my actual conditions to compare. Would you look over and confirm if I understand it correctly please?

Current power - Reliance DC 3hp, armature 0-690rpm, field 690-2400rpm plus 5:1 redn gearbox.
With existing 6.5" OD motor/gearbox pulley and 7" OD spindle pulley for a 1.08:1 ratio. So this calculates to:

Motor RPM Spindle RPM Spindle Torq (w/o redn) Spindle RPM (5:1) Spindle Torq (5:1)
690 639 25 128 123
2400 2200 7 n/a n/a

New power - 5hp AC 3-phase, 1760rpm with suggested range of 20-120 hz plus 5:1 redn gearbox.
With existing 6.5" OD motor/gearbox pulley and 7" OD spindle pulley for a 1.08:1 ratio. So this calculates to:

690 (23hz) 639 41 128 190
1760 1630 15 326 75
2700 (92hz) 2500 11 n/a n/a

Same 5hp 3-phase but with 4" OD motor/gearbox pulley and 7" OD spindle pulley for 1.75:1 ratio

690 (23hz) 394 67 79 190
1760 1006 26 201 75
3520 (120hz) 2011 13 n/a n/a

If I did this right, it seems to me I should retain the 6.5" OD drive pulley with this new 5-hp/VFD setup as it provides a little better torque at both low and high ends, while allowing a full 2500 rpm at 92hz.

If I reduced the drive pulley to 4"OD, top end speed will max out at too low at 2000. Am I missing something in all this? Bill

Your torque numbers are off. You calculate torque at 60Hz, 1760 RPM, based on rated horsepower. For lower RPM on an AC motor w/ VFD, you get roughly the same torque, not more, just like in the graph you posted. (Shunt wound DC motors actually gain some torque at less than full armature RPM).

So adjusting your numbers, I get:

New power - 5hp AC 3-phase, 1760rpm with suggested range of 20-120 hz plus 5:1 redn gearbox.
With existing 6.5" OD motor/gearbox pulley and 7" OD spindle pulley for a 1.08:1 ratio. So this calculates to:

Motor RPM Spindle RPM Spindle Torq (w/o redn) Spindle RPM (5:1) Spindle Torq (5:1)
690 (23hz) 639 15 128 75
1760 1630 15 326 75
2700 (92hz) 2500 11 n/a n/a

Same 5hp 3-phase but with 4" OD motor/gearbox pulley and 7" OD spindle pulley for 1.75:1 ratio

690 (23hz) 394 26 79 130
1760 1006 26 201 130
3520 (120hz) 2011 13 n/a n/a

For an AC motor w/ VFD: Basically whatever spindle torque you get for your gearing/pulley ratio at 60Hz, you will get that torque for any slower speed, or 5x that torque for the reduction gear. Your torque is going to drop off above that. An old motor will generally drop off in torque faster as Hz goes up than will a inverter/vector motor. With an inverter motor, you can probably figure on getting rated horsepower (a linear drop in torque) up to about 90Hz, but exact numbers are something you dig out of the actual motor manufacturers detailed literature. So expecting 13 ft-lbs at 120Hz in the second case is optimistic even for an inverter motor, thus I put it in red.

It's going to be really tough to get equivalent performance out of a non-inverter 5HP motor to match your current setup.

 

[Nightshift]

Rabler, thanks for looking over and correcting my mistakes. I see what you mean now and why you suggested earlier the 1.8:1 ratio to achieve a similar low end torque with the reduction gearbox. I think if I'm gonna try this chinese induction motor I will need to change the drive pulley down to 4". Before I go to much further I will see if I can find a good 5-7.5hp inverter motor to use instead in my area. A little more hp might help fill in the gaps, but I have decided I will keep the redn gearbox in place. Appreciate the help making sense of the options. Bill

 

[Nightshift]

Actually I see another mistake I made in the calcs with the 4" drive pulley. The spindle speeds are correct for the 3 motor speeds listed, but the torque values at 26 and 130 should be 15 and 75 right? So comparing the DC 690rpm armature currently with torques of 25 ft/lb and 123 with the redn gear, this 5hp induction motor is not gonna deliver even close to the same power regardless of the drive pulley size since the most I can get is 75 ft/lb.