I was wondering about the horsepower ratings of different electrical motors. I am a student machinist and I was using an older 3HP Harrison lathe that would bog down/stall out at .010 per rev and only .050-.100 depth of cut on 1020 steel. In fact it didn't seem to like me going over .006 per rev on a cut like that. However, I could take the same cut on a 3HP Sharp, going the about the same RPM and it would take it just fine. In fact I couldn't stall out the Sharp as the piece would start chattering wanting to push it out of the chuck at about a .200 depth of cut before the lathe would bog down. This is of course going the same .008-.010 per rev. Why would these be different?

The other thing I was wondering about, and might explain my first problem, is I know electric motors put out about the same amount of torque through out the RPM band. I also know that horsepower is torque over a speed or amount of time. So is the HP of a machine rated for its maximum RPM? How is the HP of electric motors rated?

The first thing I think of is that there is something wrong with that older 3HP motor so you are not getting 3HP out of it.

The HP rating on electric motors mostly depends on heat. A motor can make some amount of power for some period of time before it gets too hot as long as you feed it enough electricity.
A motor that can make 10 HP continuously may well be able to make 100HP for 10 seconds from a cold start, the amps needed may be 15 times as much.

I concur - something is wrong. .010 @ .1 is not a particularly beastly cut, especially for a 3 hp machine.

I still don't understand how they rate horsepower for electric motors. Is it at the maximum RPM given? Would a 5HP 2500RPM machine make say only 3HP at 800RPM?

1 Horsepower is 746 Watts.

Watts = in-oz * RPM / 1351

Mariss

1 Horsepower is 746 Watts.

Watts = in-oz * RPM / 1351

Mariss

I am aware, however does that mean that electric motors have RPM that they're rated at? A 3HP electric motor rated for 1700RPM would be more powerful at 800RPM than one thats 3HP at 2500RPM?

Here is a PDF document for Mitsubishi's spindle motors and drives. (http://www.meau.com/Files/AC_SPINDLE_SYSTEMS_BROCHURE.PDF) See pages 12 and 13 of that brouchure to see the power curves for different motors.

Those ratings are different from the plain old 3hp motor as found on a manual mill or lathe which only achieve rated power when at peak rpm.

There is a big debate regarding this question going on here http://www.practicalmachinist.com/vb/showthread.php?t=155512
Al.

Here is a PDF document for Mitsubishi's spindle motors and drives. (http://www.meau.com/Files/AC_SPINDLE_SYSTEMS_BROCHURE.PDF) See pages 12 and 13 of that brouchure to see the power curves for different motors.

Those ratings are different from the plain old 3hp motor as found on a manual mill or lathe which only achieve rated power when at peak rpm.


So when one of those gets to say 750RPM, from 750RPM->4000RPM it will make the same amount of horsepower. (With the exceptions of the ones that fall off at the end.) However a manual lathe will be more like the HP for an IC engine and make a gradual progression upwards?

What makes the two different? Any links to curves for manual lathes/mills?

Power (that what gets things done) is torque times RPM. I put up the equation, solve the equation for torque, HP, RPM or whatever else you are interested in. It's just simple high school algebra.

Mariss

Power (that what gets things done) is torque times RPM. I put up the equation, solve the equation for torque, HP, RPM or whatever else you are interested in. It's just simple high school algebra.

Mariss

Yea, I know I did quite well in school. (3.8GPA, Full scholarship to both tech school for machining, and college for Mechanical Engineering) I can do the math just fine. My problem is more with the practical understanding. If I run a 3HP manual lathe and it just starts to stall out at 800RPM with a cut we'll call cut X. If I run it at its max of say 1750RPM should it take this cut without much problem? The machines I use at school will chatter and push the part out of the chuck before they'll stall out, but I will be in a machining competition coming up and I was able to stall it out with the cut I mentioned above. Would turning it up some fix that?

When an IC engine is overloaded it stalls and stops running, usually with no harm done. That sets a definite limit to the power the engine can produce.

A good high performance electric motor on the other hand is not self limiting (at least not at a level it can survive) as long as there is volts and amps available it will try to turn any load you put to it. If it is too much it overheats and burns up. Which is why electric motors have control systems, possibly as simple as a circuit breaker to protect the motor. And why I said an electric motors limiting factor is heat.

i think it may have bin mentioned above some where, but what the heck I chip it back in.
There are 3hp peak and then there is 3hp cont. most 3hp peak are raely only 2hp.

Also dont forget about the gear ratio and all the other junk the poor little motor may be turning as well.

Now there is this dang new hp ratings an lawn mowers so a 3.5 hp briggs says it has a peak of 5 hp (its all in the sale) same with air compressors a 5hp compressor will draw at least 15-20 amps on 220v so I guess they have magic motors on a 120v 5hp compressor then... haa lol

How is the speed controlled on the manual lathe? Changing gears or a VFD? If using gears to achieve the 800rpm, then the motor is always turning it's rated speed of (typically) 1725rpm or 3450rpm. That means the motor always turns at the same speed until you start to stall it. As the load increases, the motor draws more amperage trying to maintain rpm which increases the torque and thus the horsepower.

On the other hand, a variable frequency drive will have only half the horsepower when running at half the rated rpm. Huge difference compared the the gears. In fact, one of the biggest advantages of using a gearbox is to get the maximum horsepower over a wide range of rpm.

CNC machines use overwhelmingly fancy VFDs. By using circuitry, they create an artificially flat power curve over a wide range.

AHHHHHHHHHH! Thank you for these last couple of posts its all very clear now. Both the lathes in question were geared head lathes. Still not sure why the power was so different, but I will chalk it up to being an older lathe.