Motor HP/torque question

[Mike Cruz]

So, a fellow Creeker and I were discussing this, and while it does seem like common sense to me, I just wanted to hear your thoughts.

My PM90 lathe has a 1 HP motor (torque unknown). It has a manual variable speed (meaning that the pulleys (reeves drive) change the spindle speed. So, at its lowest speed, it has the torque of its capacity. And because the pulleys (reeves drive) change the speed, it still has the same torque at high speed.

Now, the new PM lathes use a 2 HP motor. But they use an electronic variable speed system. So, without a pulley system (reeves drive or other) there is loss of torque at one end or the other (highest or lowest speed). Meaning if it has its full torque at its highest speed (let's say 2000 rpm) when you drop the rpm to 500, then you just decreased the torque by 1/4.

During the discussion, I may have jumbled it up a bit, and misunderstood. So, no jumping on anyone's back if I've got this wrong...

Also, I understand that HP and torque do not go hand in hand. For example, the 350 in a sports car may put out 350 hp, but only 280 ft/lbs of torque. Whereas in pickup truck, a 350 may put out 300 hp, but 400 ft/ilb of torque. And the difference would be in the gearing...or something like that. My point just being that I know they don't necessarily go hand in hand, so, assuming that my 1 HP motor and the new 2 HP motors put out the same torque respectively (the new ones twice mine), was I correct in my understanding of the lathe situation?

[John Coloccia]

When you change speeds on the pulley, the motor may be able to generate the same torque, but the torque at the output shaft will go up and down in proportion to the gearing. As they say, you don't get somethin' for nothin'.

[David Winer]

Oh, boy. I can't wait to see the explanations on this topic.

[John Coloccia]

One more thing to add: A VFD can maintain constant torque, and peak torque at that, up until 60Hz. That's also the speed the motor would turn if there was no VFD. The way this is accomplished is the ratio a Volts to Hz is maintained constant from 0 to 60Hz. As the frequency goes down, the voltage also goes down. If it didn't, you would get more torque but also fry the coils.

So a VFD produces the same torque, throughout the range, as the motor would produce were is simply plugged into the wall and spinning.

Above 60Hz, the motor spins faster than it would otherwise, torque goes down and HP remains constant at peak HP.

Now, in terms of lost Torque at the output shaft of your lathes, if your motor is geared down, then that motor will produce more torque at the output shaft than a non-geared down motor. So in your example, if your gearing were 1:2 for a 1HP motor, and direct drive for the 2HP motor, you would expect to get similar torque at the output shaft because HP goes linearly with torque. Most of your gearing on a lathe, though, are geared up, so in most cases you will get more torque with the 2HP motor than you get with the 1HP motor.

[Dick Strauss]

Mike,
John hit the nail on the head!

VFDs do a good job of maintaining constant torque down to the 5hz range (below that, most motors don't have much torque). Some 3ph motors are built in a way to maintain constant torque down to 1/20 full speed and slower. However, ordinary motors may overheat because the built in fans are turning so slowly and not moving much air at slow motor speeds.

Power = torque x rpm/5252...so if you run your 1hp motor to get a spindle speed of 1000 rpms, you have a torque of about 5...if you change your reeves drive ratios and run the spindle at 500 rpms, you have a torque of about 10 (1/2 the speed gives twice the torque). It is the final driven spindle shaft that really matters in your case.

The PM3520 uses a second set of pulleys to reduce the minimum speeds (125-3200 on high pulleys and 50-1200 on the low pulleys). It has one set of pulleys for small diameter things like spindles (high set) and one for larger items like bowls and platters (low set). The slow pulley set also helps with very slow speed torque dropoff and prevents overheating of the motor at very slow speeds.

Take care,
Dick

[Dan Hintz]

John and Dick have taken care of pretty much every useful point...

[Will Overton]

When you change speeds on the pulley, the motor may be able to generate the same torque, but the torque at the output shaft will go up and down in proportion to the gearing. As they say, you don't get somethin' for nothin'.

Yup! Just like shifting gears on a car.

[Ken Garlock]

Get a Nova DVR lathe. Set any speed you want on the front panel(100-3600), and let the electronics of the variable reluctance motor worry about maintaining that specific RPM.

Watch for year end sales.

[Mike Cruz]

After my head stops spinning like a top, I'll reread it all. Yeah, kinda got it, not sure if it directly answered my question, but maybe it did and the spinning of my eyeballs in my head didn't let me absorb it...

[Mike Cruz]

Not sure, but maybe where I'm getting confused now, and oddly enough, I THOUGHT I understood it before, is that electric motors (like on all our machines) are all rated for HP, but not torque (unlike cars which give you both).

So, let me try to rephrase my original question...

If the new PM lathe had a direct drive with a 2 hp motor, A) would it have full power/torque at its highest or lowest rpm? And B) with only an electronic variable speed (no manual ability for argument's sake) wouldn't it lose power/torque at one or the other extreme?

And on that note, with my lathe only having a 1 HP motor, but manually being able to reduce the speed, does it have an advantage of power/torque over a 2 hp direct drive lathe? Or does the 2 hp one have more power/torque regardless?

I think I just spun my own head.

[John Coloccia]

Not sure, but maybe where I'm getting confused now, and oddly enough, I THOUGHT I understood it before, is that electric motors (like on all our machines) are all rated for HP, but not torque (unlike cars which give you both).

So, let me try to rephrase my original question...

If the new PM lathe had a direct drive with a 2 hp motor, A) would it have full power/torque at its highest or lowest rpm? And B) with only an electronic variable speed (no manual ability for argument's sake) wouldn't it lose power/torque at one or the other extreme?

And on that note, with my lathe only having a 1 HP motor, but manually being able to reduce the speed, does it have an advantage of power/torque over a 2 hp direct drive lathe? Or does the 2 hp one have more power/torque regardless?

I think I just spun my own head.

The core problem understanding this is to understand the relationship between torque and horsepower. HP = torque * RPM. (actually, torque*RPM*scaleFactor, but the scale factor is just there to make the correct units, like multiplying feet*12 to get inches).

So rating a motor in HP is the equivalent of labeling the torque given the RPM the motor is running at. Personally, I don't like the HP rating for calculating the work you get out of a motor, but it's convenient because it tells the electrician what service a motor will need.

Anyhow, a motor on a variable speed control, assuming it's a VFD (variable frequency drive) will develop max torque at ALL RPMs up to the RPM achieved by the service frequency (60Hz, in this case). Most RPMs, actually, as things go funny when you slow way down. Horsepower will rise with RPM since HP = torque*RPM, torque is constant, therefore horsepower goes up as RPM goes up. At 60Hz, you get max torque and HP, i.e. you're dumping as much power into the coils as you possibly can without burning them up, and the motor's turning the fastest. Just to be clear, you could and should get more torque as the motor turns more slowly (to a point), HOWEVER the VFD lowers the voltage to the motor as it lowers the frequency. If it didn't do that, you would burn up the motor because it would draw too much current.

So comparing this with a single speed motor and some sort of gearing, the torque at the shaft will be greater than the torque at the motor when the shaft turns slower than the motor. The torque will be less when the shaft turns faster than the motor.

To answer your question about which one has more torque at the shaft, we have to know the gearing of your drive for the 1HP and the 2HP, and also what RPM you're concerned with. You also have to take into account that these motor HP ratings are not all that accurate and are mostly there to let you know how to wire it up and to make gross comparisons between different offerings within a product line.

Personally, I'd take a variable drive any day of the week moving belts around.

[Mike Cruz]

Thanks, John. I guess the reason I scratch my head as I'm following all the explanations is that I haven't given/can't give you all the variables you need to properly answer my question. At least, that is what I gathered from your last post. It seems to me like, I asked a question about a particular scenario, and I got explanations to how HP and torque work at different rpms. I was wondering why no one just said YES, NO, or WELL...

But I guess you would need to know all the particulars to really answer the question.

Thanks for all your efforts, all.

I guess this whole subject came up because, thought simple thought, we figured that my lathe, being only 1 HP but having a manual variable speed AND a VFD, was actually more powerful at lower speeds (300 RPM for arguement's sake) than the new models with 2 HP with only electronic variable speed and no manual control. Maybe this is how I should have posed my question this way to begin with... Does this make any difference or does it still leave too many variables?

[John Coloccia]

Thanks, John. I guess the reason I scratch my head as I'm following all the explanations is that I haven't given/can't give you all the variables you need to properly answer my question. At least, that is what I gathered from your last post. It seems to me like, I asked a question about a particular scenario, and I got explanations to how HP and torque work at different rpms. I was wondering why no one just said YES, NO, or WELL...

But I guess you would need to know all the particulars to really answer the question.

Thanks for all your efforts, all.

I guess this whole subject came up because, thought simple thought, we figured that my lathe, being only 1 HP but having a manual variable speed AND a VFD, was actually more powerful at lower speeds (300 RPM for arguement's sake) than the new models with 2 HP with only electronic variable speed and no manual control. Maybe this is how I should have posed my question this way to begin with... Does this make any difference or does it still leave too many variables?

Well, they do have a "manual" control. There's still a drive in there. It's just low speed and high speed instead of 6 or 7 different speeds. I would guess, though, that for the lowest speeds your 1HP lathe probably does have more torque at the output shaft.

[Mike Cruz]

That was what common sense said, but I wanted to bounce it off of you guys.

While maybe exaggerated, the quick thinking was that with a manual drive, at 550 rpm, I have 1 HP. But assuming a top rpm of 2200 rom, the 2 HP models, with only electronic VS, would have 1/4 of that...so 1/2 HP. I know that that is oversimplifying and it doesn't take into account that the 2 HP models aren't necessarily direct drive and do have a couple speeds.

Anyway, thank you all for your explanations and input.

BTW, part of my curiosity was aimed at, if ever need be, replacement of my motor, and since the new PM's have 2 HP motors, would I really benefit from it or did they just put 2 HP motors on the new ones to compensate for the new drives to give the new lathes the similar power as the old ones.

[Dan Hintz]

Anyhow, a motor on a variable speed control, assuming it's a VFD (variable frequency drive) will develop max torque at ALL RPMs up to the RPM achieved by the service frequency (60Hz, in this case).

I'll tweak John's comment to add that you won't see max torque going all of the way in the other direction (0 rpm)... there's a limit to what a VFD can do. The better ones will give you a 10:1 reduction in "free spin" speed at max torque (and I can't say I've ever seen one that will give you 20:1 at max torque, as mentioned earlier). That's something to keep in mind, as a free-running motor set for 3000 rpm will not give you a full-torque 100rpm without external gearing.