Motor HP question

Quade · Mar 12, 2019

Is there any downside to upsizing the motor HP on a machine like a 11" Logan? I was turning some 3" diameter aluminum and even with relatively thin DOC it was bogging. I know I could use the back gears but I'd rather just be able to power through instead. The lathe came with a .5 hp, I've since installed a 1hp inverter rated motor. Was thinking about upping it to 2 HP. The motor I installed will operate from 0 to 6000 RPM so, I can control the speed from the VFD easily enough.

I'd rather have a powerful motor rather than moving the belts around. My thought is that the only downside might be I exceed the power limits of the drivetrain. It's all cast iron though and seems pretty robust.

Dumpster_diving · Mar 12, 2019

I don't think there'd be a downside. I've got two 11" Logans and I've experimented with everything from .5 hp to 1 hp, AC induction as well as DC PWM controlled.

That said, I don't think the cone drive pulleys will take advantage of a motor much over 1 hp without slipping. Regardless of whether you use leather or serpentine. I think there are other practical limitations to how much cut you're going to be able to take even with the added HP. Also, keep in mind your space between the motor mount and the top jack shaft casting may limit how large you can go physically.

Lastly, you could damage the lathe by exceeding the spindle RPMs. So keep that in mind when tuning/limiting your VFD controls.

johnoder · Mar 12, 2019

you could damage the lathe by exceeding the spindle RPMs

Amen.

Those that speed up OLD machine tools may end up with more unpleasantness than they supposed

A for instance might be what happens to a threaded on chuck that has it's spindle seize and stop in a few micro seconds from a way too fast speed

There it is in mid air. still going like crazy - looking for a place to go

L Vanice · Mar 12, 2019

Quade said:
I... it was bogging. I know I could use the back gears but I'd rather just be able to power through instead. The lathe came with a .5 hp, I've since installed a 1hp inverter rated motor. Was thinking about upping it to 2 HP. The motor I installed will operate from 0 to 6000 RPM so, I can control the speed from the VFD easily enough. ...

If you are running out of power when using the VFD at low frequency, there is a simple explanation. The motor will have constant torque at frequencies below 60. Power is a function of torque and speed. So, if your motor has 1 HP at 60 Hz and runs 1750 RPM, then it will have .25 HP at 15 HZ and run 425 RPM. The motor will have constant power at frequencies above 60, so it will still have 1 HP at 120 HZ.

For that reason, I like to use motors with high HP and low rated RPM when changing to a VFD drive. You get more power at low speeds. For instance, if the motor is rated 2 HP at 1150 RPM, it will have .5 HP at 15 HZ and run 275 RPM.

Low RPM motors usually have a larger frame than 1750 RPM motors, meaning the shaft and the housing will be larger.

Metal removal rate in volume per unit time (by turning, for example) is a function of power, not torque, by the way. And belt slip is an important limitation.

Larry

DMF_TomB · Mar 12, 2019

you can use bigger motor but dont mean belt wont slip under load and you might break stuff
.
was tapping a 1.5" dia thread on lathe and i engage lock pin so chuck wont turn while tapping part in chuck. obviously when i turned 1.5" dia tap with 18" long wrench it damaged pin and pin hole in side of head stock gear. little locking pin not designed for that much force. it just broke. sometimes stuff bends and not breaks. but stuff bending not good either.

Quade · Mar 12, 2019

If you are running out of power when using the VFD at low frequency, there is a simple explanation. The motor will have constant torque at frequencies below 60. Power is a function of torque and speed. So, if your motor has 1 HP at 60 Hz and runs 1750 RPM, then it will have .25 HP at 15 HZ and run 425 RPM. The motor will have constant power at frequencies above 60, so it will still have 1 HP at 120 HZ.

If it's bogging doesn't that mean I just don't have enough torque? I was under the impression that torque was more important than HP for these kinds of loads.

Lastly, you could damage the lathe by exceeding the spindle RPMs. So keep that in mind when tuning/limiting your VFD controls.

The max stock spindle RPM is close to 1500 RPM. The lathe has real bearings. I'll make sure to set the max HZ in the VFD to match the max RPM of the lathe itself. I didn't think of that when I set the max HZ. It depend on what belt I have it on too so, maybe max RPM on the fastest belt? I'd already capped the VFD at 200 hz thinking about your comment, that's clearly too much for the faster belts.

This lathe uses V belts so, I believe it can deliver more power than a flat belt.

JST · Mar 12, 2019

Quade said:
If it's bogging doesn't that mean I just don't have enough torque? I was under the impression that torque was more important than HP for these kinds of loads.

.....

This lathe uses V belts so, I believe it can deliver more power than a flat belt.

Torque IS HP at a given speed.

Yes, the V-belt system will handle more power per belt than the 1" flat belts. My Logan has the 1" flat belts, and will slip them at over about 1/4 HP on the less capable settings, despite having 3 phase and rubber composition belt tightened to almost 50 lb static tension slip point.

Froneck · Mar 12, 2019

Simply put if you increase the HP of the motor and the speed remains that same the increase in HP is due to the increase in torque.

jim rozen · Mar 13, 2019

Wait.

What kind of motor do you have that has a top speed of 6K rpm?

You may have the wrong type of motor in here - suggest you put up a photo of the
nameplate on this motor. Almost sounds like a brush type universal motor.

michiganbuck · Mar 13, 2019

I think .5 hp is low for that machine and 1 Hp should be plenty..2hp overkill but Ok.
Don't think I would run over 2000 on a machine designed for 1500 RPM.

Cutting tool geometry and sharpness can aid the machines ability.

Paolo_MD · Mar 13, 2019

Are we sure that is the motor bogging down (and, if so, at what RPM?) and not a belt slipping.
If the motor is overloaded for the frequency it is operating at, generally the VFD quits and throws an error code.

Paolo

wdTom · Mar 13, 2019

JST,
On my Logan 10" I am using a "rubber" timing belt with the smooth side in, works way better than a leather belt. You just have to take the spindle out to install, not that hard, and keep oil off of it. Grips the smooth steel pulleys great. With very reasonable tension I don't get any slip.

Quade · Mar 13, 2019

What kind of motor do you have that has a top speed of 6K rpm?

It's a Baldor. Nominal RPM is 1740 with a max of 6000.

Are we sure that is the motor bogging down (and, if so, at what RPM?) and not a belt slipping.

The motor stops spinning so, I'm pretty sure the belts aren't slipping. I typically catch it fast enough that the VFD doesn't kick out.

Don't think I would run over 2000 on a machine designed for 1500 RPM.

I agree. I just need to set the max HZ in the VFD to limit it.

Maybe I'm just expecting too much of a relatively small lathe? It'll do .030 DOC on 3" diameter 6061. As a noob, I don't have a good reference for what's considered reasonable.

Paolo_MD · Mar 13, 2019

Quade said:
It's a Baldor. Nominal RPM is 1740 with a max of 6000.

Very nice motor indeed.

Quade said:
...
Maybe I'm just expecting too much of a relatively small lathe? It'll do .030 DOC on 3" diameter 6061. As a noob, I don't have a good reference for what's considered reasonable.

At what frequency are you running when it is doing that?

Paolo

Quade · Mar 13, 2019

60-70 hz.

The Logan has high/low position for speed range on the main belt, then 3 other pulley positions for fine speed control. I'll verify the RPM when I get a chance. I have the range on low and the pulley in the middle I think.

Paolo_MD · Mar 13, 2019

If it was running at 60-70Hz and not at 20-30Hz, there is something wrong with the motor or with the settings on the VFD. Definitely, a 1HP motor at the rated speed should take a .030" cut on a 3" piece of aluminum.

Even a 3/4HP should be able to do it without any problem. ...Unless you have some undue friction/resistance somewhere on the drive-train.

Paolo

JST · Mar 13, 2019

I think the flat belts should do that 0.03 cut, let alone the v belts. At least at the appropriate speed.

BUT, if you happen to be trying to run the whole speed range without changing belts, just using the VFD dial, that may be the issue. Torque does push the tool through metal, but POWER does it at speed. When you slow the motor, you also lose power, so you may not have the power to do it as fast as the rpm demands.

wdTom said:
JST,
On my Logan 10" I am using a "rubber" timing belt with the smooth side in, works way better than a leather belt. You just have to take the spindle out to install, not that hard, and keep oil off of it. Grips the smooth steel pulleys great. With very reasonable tension I don't get any slip.

The belt here is a rubber composition of a similar type, although it has alligator lacing. I do tend to get aggressive on DOC, anywhere up to about 0.2" DOC depending on work size. But the 1" belt is just plain limited in capability.

It also depends on belt stretch, when the belt starts "talking" you know it is getting closer to a slip, that talking is due to belt stretching (and slipping a bit) as the load comes on and off it around the pulleys. When the "on" stretch meets the point at which the belt leaves the pulley, it slips. A belt that does not stretch as much can then presumably transmit more.

Most of the timing belts I see have relatively hard and slippery smooth sides, which do not look as if they will grip well.

Quade · Mar 13, 2019

there is something wrong with the motor or with the settings on the VFD.

This is a good point. I've never seen the VFD show a load higher than 30%. I wonder if the VFD is failing to deliver sufficient current? The VFD is currently set for "general motor". Some of the other motors types might be better. It's a 3HP VFD but you know how Chinese things are. I'll investigate this tonight.

Maybe throw a volt meter on each of the output legs, then stall the engine and make sure it's still sending 220 to the motor.

44dwarf · Mar 13, 2019

Sounds like you bumped into the current limiting setting on the drive.

JST · Mar 13, 2019

Quade said:
This is a good point. I've never seen the VFD show a load higher than 30%. I wonder if the VFD is failing to deliver sufficient current? The VFD is currently set for "general motor". Some of the other motors types might be better. It's a 3HP VFD but you know how Chinese things are. I'll investigate this tonight.

Maybe throw a volt meter on each of the output legs, then stall the engine and make sure it's still sending 220 to the motor.

You normally HAVE TO at least set the motor rated voltage, and motor rated current, motor rated Hz at rated voltage, and maybe a couple other things like max speed, min speed, etc. The important ones of these are available on the motor data plate, the rest (min, max) are to some extent options, although some you can get from a full spec sheet, such as max speed..

If you are using a vector mode, then you may have to enter some things, and the drive should "auto-tune" so that it "knows" the motor characteristics and can get good readings.

It sounds like your "generic" settings are giving "generic" performance...... you may want to correct that.

What have you actually set as far as parameters?

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