Motor RPM clarification

Heychris_1 · Apr 11, 2021

All,

I've spent hours reading and not found exactly what I am looking for. I'm about to buy a motor for my VFD conversion, so I would appreciate confirming the RPM.

My lathe is a 1962 10ee. It's motor spec is 1150RPM by armature control and 3450 RPM with field control.
I would assume this means that the correct motor RPM if I keep the gearbox is 3450. Can someone confirm this?

Does anyone know what the gearbox ratio is?

I've looked at the marathon black max motor, but the website for marathon lists the 1800rpm for it's highest RPM. Am I missing something?

I appreciate the help

Chris

Phil in Montana · Apr 11, 2021

Yes a whole bunch, most use a 1200 rpm motor and drive it with 180 hz (makes 3600 rpm that way ) I dont know what the pulley ratio is or the back gear ratio but the 1200 rpm works out just right...Phil

220swift · Apr 11, 2021

In reading the history of Monarch lathes it says the 10ee gearbox is 5:1.
Monarch made at least 2 size gear boxes. My 1972 has the smaller size and one 1983 hade a larger size.
I don't know if they have the same ratio.
Some people who convert to vfd run a 7 1/2 hp with a gearbox and use a 10hp without a gearbox.

Hal

Don's Engine · Apr 11, 2021

What is your max spindle seed? 2500 or 4000 Rev min? You have a 1:1 direct drive through the gear box and any spindle reduction/multiplication from motor to spindle is handled by the difference in the pulley sizes at the gear box and spindle. Ie... my own is a 2500 spindle max speed. When I installed the 1740 rpm (7.5) motor the spindle runs at aprx; 1600 rpm. So there's a reduction only from the pulley sizes. At 90 HZ I'm running right near the 2500 tach max. So I set my lowest VFD setting is about 7 HZ and my max to 90 HZ to get a full range operation. So I'm only overdriving by about 50%. Its best power range falls right where I do most of my turning from about 300 to 1600 tach speed. Below 300 (aprx 15 hz)it falls off on power pretty quick, so I can always drop to low range through the gear box (5:1) which gives me a range from about dead crawl to around 500 rpm. If you overdrive your motor you'll have to remember the higher you drive it, the greater the drop in torque, the more current eddy issues and the drive back into the motor from your machine when you kick it off will force you into needing a larger capacity braking resistor. Basically its pretty hard on the vfd when you start driving a 60HZ motor above 90HZ or so. A lot of guys will overcomp that issue with a higher rated VFD. A good rule of thumb when I set up a VFD is try to shoot for underdrive by about 50% and OD by 50%. I know on my 10EE I underdive by a lot more, but I have USA made motor that handles it very well. Its a lot of experimentation to figure out what its happy with. So you should try to match your motor rpm closer to what range you want your spindle to run at and figure out your direct drive across the pulleys to do it. The low range (back gear, the 5:1 reduction you are hinting at) will always have your back for very low speed turning. I know that may not answer your question directly, but you do need to look at how it all works together. Don

Briney Eye · Apr 12, 2021

I'm looking at a 5HP Black Max nameplate that says the maximum safe speed is 4500rpm (and a 10hp that says 4200rpm). They spec the torque for 120hz/3555rpm right on the plate, which would seem to indicate that it's safe to run there. I don't think a name-brand VFD would have any problem if you use the appropriate braking resistor. 50% overdrive gives you 2500rpm, and I think they would be very comfortable with that.

beckerkumm · Apr 12, 2021

If running with the gearbox, how much torque at the high end is appropriate for the 10EE ? The motor will be running full up to about 90 hz or close to 2500 rpm unless the pulleys are sized down. I understand the need for the higher hp when running a 1200 rpm motor at 180 hz but how much metal does a 10EE at 2500-3000 rpm need to remove? I'm wondering because my Smart Brown looks to have at least as or even a little heavier spindle and a TENV 5 hp 1800 rpm motor still has a lot of grunt at 3600 rpm for what I need. Dave

beckerkumm · Apr 12, 2021

So the original DC configuration delivered more like 21 lbs torque at the low end and something equal or a little less on the high end? Dave

beckerkumm · Apr 12, 2021

I understand the math and how rpm relates to torque. The modern 10EE uses a 7.5 hp motor ( I assume at 1800 rpm ) which provides constant torque of 21 lbs down to zero rpm. The speed range of the modern 10EE is 40-4000 or 100 times. Considering slip, does this mean monarch runs the motor from 2 hz to 200 hz ( or 1-100 ) or is the top end speeded up higher than 200 hz and the low kept higher than 2 hz? Torque falls quickly after 90 hz but at 120 hz the 7.5 hp motor would still have about half ( 11 ). If the motor needs to go to 200 hz or more to hit 4000 rpm at the spindle, the 7.5 hp motor is providing the additional benefit to compensate for the reduction in torque that is well above the 2500 rpm limit for the old 10EE. If that is the case, the larger motor on an old 2500 rpm machine is less important than when converting a newer 4000 rpm machine. Granted the low end torque is still higher but how much torque is needed for a D1 3 precision lathe ?

I can figure out the numbers if I can find what rpm range the motor of the new 10EE runs. Does anyone know what that hz range is? Dave

220swift · Apr 13, 2021

Chris

There have been a couple guys that posted pictures and results of their repowering 10eeS
You might do a search for them.

Hal

marka12161 · Apr 14, 2021

beckerkumm said:
I understand the math and how rpm relates to torque. The modern 10EE uses a 7.5 hp motor ( I assume at 1800 rpm ) which provides constant torque of 21 lbs down to zero rpm. The speed range of the modern 10EE is 40-4000 or 100 times. Considering slip, does this mean monarch runs the motor from 2 hz to 200 hz ( or 1-100 ) or is the top end speeded up higher than 200 hz and the low kept higher than 2 hz? Torque falls quickly after 90 hz but at 120 hz the 7.5 hp motor would still have about half ( 11 ). If the motor needs to go to 200 hz or more to hit 4000 rpm at the spindle, the 7.5 hp motor is providing the additional benefit to compensate for the reduction in torque that is well above the 2500 rpm limit for the old 10EE. If that is the case, the larger motor on an old 2500 rpm machine is less important than when converting a newer 4000 rpm machine. Granted the low end torque is still higher but how much torque is needed for a D1 3 precision lathe ?

I can figure out the numbers if I can find what rpm range the motor of the new 10EE runs. Does anyone know what that hz range is? Dave

Keep in mind, the power, torque speed calculations for any rotating system become complex in a hurry when analyzing transient load conditions. A machine tool like a lathe is constantly seeing transient load conditions so it is not a simple calculation. To understand quantitatively what is happening inside the machine under transient conditions you need to understand the entire electrical and mechanical system and probably model it. There are some simple relationships that are useful in understanding how some of these factors play together but those relationships are valid in the instantanious (and therefore the steady-state) sense. Throw in a step change in load and it all becomes very complex until a new steady state is reached.

For example, i was considering a new motor for my 16CY. The old motor is a large frame 7.5 job. The motor shop i was talking to recommended a 10 HP replacement. When i asked why the difference, the engineer explained that the old motors had less efficient internal insulation resulting in a more massive frame and armature relative to modern motors. A new 7.5 hp motor would have a much smaller frame size and and therefore much smaller rotating intertia. The engineer recommended the 10 HP motor just for the larger frame size to maintain the ability of the motor to respond to a step change in demand torque. By this logic (which i believe), i could also have gotten away with a new, small frame 7.5 HP motor with an appropriately sized fly-wheel.

The "HP is proportional to the product of speed and torque" relationship would not have given this insight

beckerkumm · Apr 14, 2021

Thanks, Marka. As an old iron machine guy I am familiar and a fan of those inefficient large frame motors that seem to last forever. I have thought about shaft diameter and larger bearings with higher load capability but not about the rotating mass. I would guess a vfd needs to account for that additional mass when braking the motor too. Dave

marka12161 · Apr 14, 2021

thermite said:
Good post. Good thinking!

But there are TWO kinds of inertia at work. The physical rotating mass in view of the Mark One Eyeball.

And the time constant and "quantity" of energy being stored and released in the larger mass and inductance of Iron and Copper.

Weigh an AC motor in the 3 to 10 HP range, 120 to 440 VAC.

Now weigh the shunt-wound, wound-field, not "PM", Dee Cee motor, same service Voltage range and nameplate HP rating.

Compare physical size.

DC usually has rougly double the mass and volume per-HP, if not far more - of Copper coils and Iron (alloy) laminations. Those can store - and release - a pee-bringing amount of energy electro-magnetically, not just kinetically.

Dual-use contactors, for example, are commonly rated at HALF the DC Amperage as AC, and even then have rudely shortened lives unless specifically designed to deal with the arcs involved.

DC environment the interrupted flow is always at full-gallop, no helpful sine-wave probability to disperse some opening spikes at less energy than others, nor any equivalent to the assured-quench of the AC "zero cross" at polarity-reversal that SCR's and solid-state relays make such handy "no drama" use of.

Another reason to adopt "4Q" DC Drives even when the load is "don't-care", is that the primary manner in which they handle reversing or braking spikes .. is to not give-rise to them in the first place! "Zero cross" drop-out is inherent, both directions, even the cheapest and most primitive of SCR's .. then also sensed.. and managed to good advantage in the "Four Quadrant" schema.

Similar logic applies to some VFD installations.

Variable speed may not be needed at all. Nor always even 1-P to 3-P conversion assistance.

So why wuddja pay for a VFD?

Because.. the ability to slow-ramp a motor with a nasty-brutal starting inrush up off it's knees and onto its water-skis with no real drama, first time, any time, all the time, no human intervention required, can be downright golden.

So even the 1-P VFD AKA "inverter" has come to populate household appliances.

My Panasonic "inverter" microwave, LG "linear inverter" fridge-freezer, the new Kitchenaid dishwasher, the big washing machine AND the dryer down in the laundry, small emergency gen-sets, "hybrid", not pure-raw-battery-as-had-DC cordless power tools..

All around us, actually. We don't even know many of those are "in there".

Bill,

My only point was to be careful in over-simplifying something may be very complex. It's been a long time since i studied electrical machines in detail and i know next to nothing about VFDs. I do know to that unless a person has a pretty good idea of the full scope of the physics involved, it's easy to draw conclusions that are simple, elegant and wrong. I should know, i spent a lifetime doing so. I hold a PhD in NE (Naive Engineering).

cyanidekid · Apr 14, 2021

marka12161 said:
Keep in mind, the power, torque speed calculations for any rotating system become complex in a hurry when analyzing transient load conditions. A machine tool like a lathe is constantly seeing transient load conditions so it is not a simple calculation. To understand quantitatively what is happening inside the machine under transient conditions you need to understand the entire electrical and mechanical system and probably model it. There are some simple relationships that are useful in understanding how some of these factors play together but those relationships are valid in the instantanious (and therefore the steady-state) sense. Throw in a step change in load and it all becomes very complex until a new steady state is reached.

For example, i was considering a new motor for my 16CY. The old motor is a large frame 7.5 job. The motor shop i was talking to recommended a 10 HP replacement. When i asked why the difference, the engineer explained that the old motors had less efficient internal insulation resulting in a more massive frame and armature relative to modern motors. A new 7.5 hp motor would have a much smaller frame size and and therefore much smaller rotating intertia. The engineer recommended the 10 HP motor just for the larger frame size to maintain the ability of the motor to respond to a step change in demand torque. By this logic (which i believe), i could also have gotten away with a new, small frame 7.5 HP motor with an appropriately sized fly-wheel.

The "HP is proportional to the product of speed and torque" relationship would not have given this insight

good info marka, that's one crack motor shop you found, care to share where they are?

marka12161 · Apr 15, 2021

cyanidekid said:
good info marka, that's one crack motor shop you found, care to share where they are?

KJ Electric in Syracuse NY. In the end, i kept my old motor. They disassembled it, steam cleaned it, meggered it, changed the bearings, soldered on new leads and painted it. It was pricey, comparable to the cost of a new motor but i didn't want to mess changing sheave sizes and remounting a different frame size motor.

Toms Wheels · Apr 15, 2021

I have done what the OP is considering. A 5HP Black Max motor 1800rpm, with OEM gearbox 6:1 ratio. I changed the original spindle pulley from a 5" to 7". I rarely run above 2K rpm, But the motor tops out a 4500rpm, which would be 4k rpm spindle. Uses a Hitachi vfd, conversion done 14 years ago.

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Motor RPM clarification

Heychris_1

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Phil in Montana

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220swift

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Don's Engine

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Briney Eye

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220swift

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marka12161

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cyanidekid

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