A matter of horse power

[Bruce Schoenleber]

The "I need a new lathe" threads which turn up weekly typically involve an announcement or two about rated horse power and "I can stall this 3hp brand X lathe but my 2hp brand Y keeps on chooglin...". The manufacturers seem to take a lot of license on their HP claims. Why? Or better, how do they get away with it?

A few things I remember from high school physics (a long, long time ago):

1. Electrical power is measured in Watts and my be calculated as voltage X current (volts X amps).
2. There are 746 watts per 1 HP
3. 4th law of thermodynamics (very roughly) "You can't get more energy out of a system than what is put in".
4. There are nuanced differences between electric HP and mechanical HP. I am going to ignore this and just fall back on the 4th law of thermo.

A few things learned from the School of Hard Knocks:

5. Manufacturers lie.
6. You never get 100%
7. Stuff is always more complicated than meets the eye.
8. In the USA we have two options on (residential) voltage, 110 VAC and 220 VAC

Given all this, lets generalize.
If a brand claims 3 HP and is 220 VAC, the current rating must be, at least, (3HP X 746W/HP)/220V = 10.17Amps.
I believe most (all?) service tags list both the voltage and the current ratings, so if your lathe is rated at 3HP and plugs into a 220V outlet the amperage rating must be at least 10.17 amperes. And that would be at 100% efficiency which is never achieved.

Given this, how do the manufactures get away claiming a 3HP unit that is really a 2HP unit when the V and A ratings do not line up?

I know I have simplified this and do not address VFD's, induction motors, ... I am just going for the theoretical maximums and relying on the 4th law of thermodynamics in the "at least" restriction to keep things honest. Kind of the physics equivalent of likening a cow to a homogeneous, frictionless sphere.

 

[Jerry Maske]

If the cow were a GLASS sphere, we could see if she were giving chocolate milk or not!

 

[Ely Walton]

Here's a solution - just get one of these and adapt the 6.5 HP motor to your lathe!!

https://www.sears.com/craftsman-xsp...gclsrc=aw.ds&dclid=CN79ioucq9wCFYW8swodcocMFg

Also note that your formula for electrical power consumption for an inductive load (read motor) should include a power factor (generally about 0.7 to 0.8) - VxAxPF = watts. And, there may be more than one name plate amp rating: FLA - Full Load Amps, and LRA - Locked Rotor Amps, for example. And of course, there is the supply circuit breaker rating!

Perhaps we should return to real horses for measuring power?

Ely

 

[Gary Beasley]

Another thing to consider is how many foot pounds of torque at what rpm. Thats the bottom line. The motor design will have an effect on that and is one reason 3 phase motors are so popular in industry, they produce high torque for the input power. Single phase and dc motors have thier own issues as well as the efficiency of the controllers feeding them. So even if the hp ratings are the same the torque curves may differ for a wide variety of reasons.

 

[robo hippy]

My very old blue no name grinder from Woodcraft was rated 3/4 hp. Can't remember the wattage. My Baldor slow speed grinder was also rated 3/4 hp. Both were 8 inch grinders. The blue one had a higher wattage rating than the Baldor, but about half the horse power. My compressor has a 6 hp motor on it that is about half the size of my Baldor motor on my Laguna 16HD bandsaw.... They must be fishermen....

robo hippy

 

[Owen Lowe]

Given this, how do the manufactures get away claiming a 3HP unit that is really a 2HP unit when the V and A ratings do not line up?

Their rating claims have something to do with “peak” output at the extreme of use. I forget if it’s just before stalling or something like that but while it may be an accurate figure at that specific condition it does not reflect normal use. Many consumer products tout this peak hp - especially the utility vacuums. When comparing the power of one tool with another I always look at amps @ volts rating — which is not a figure they can fudge as I believe it is required by the electric standards organizations to be accurate.

I suspect the top quality lathe makers are not using this marketing tactic...

 

[hughie]

Hmmm, for me its a little different as we have 240v single phase and three phases are 415v and on our locally made lathes it seems that the HP rating is pretty well correct. But have to admit lathes of eastern origin the numbers should be taken with a grain of salt
But for me, I build my own lathes and buy the motors through industrial outlets. So I tend to get what I pay for and I it find darn impossible to stall my 2hp. So much so I keep the belts a bit to allow for slippage in the event of something nasty happening.

 

[Bill Boehme]

The "I need a new lathe" threads which turn up weekly typically involve an announcement or two about rated horse power and "I can stall this 3hp brand X lathe but my 2hp brand Y keeps on chooglin...". The manufacturers seem to take a lot of license on their HP claims. Why? Or better, how do they get away with it?

"Chooglin"? I have brand R. It doesn't choogle, it purrs.

I'll give my take on the other questions further along in this post.

1. Electrical power is measured in Watts and my be calculated as voltage X current (volts X amps).

Yes, but it's not quite that simple. If we were talking about a light bulb or electric heater then yes, but if it is a motor then It is a bit more complicated. The efficiency has to also be accounted for. Efficiency is the ratio of input power to output power. Output power is determined by using a dynamometer to measure torque (Newton•meters since the watt is an SI unit) and then multiplying that by angular velocity in radians per second. Just about everywhere but here motor mechanical power is given in kilowatts. Just don't confuse the electrical input power with the mechanical output power which are both stated in watts. Giving them the benefit of the doubt importers and/or retailers may have missed that important distinction.

2. There are 746 watts per 1 HP

Just be sure to remember not to confuse input power to the motor with the motor output power. If you have an EU motor and want to convert from kilowatts to horsepower use the nameplate mechanical power power output and multiply by 0.746.

3. 4th law of thermodynamics (very roughly) "You can't get more energy out of a system than what is put in".

See my last comment. There are three laws of thermodynamics. There's also a "Zeroth" law which is a mathematical equivalence, the transitive property of equality.

4. There are nuanced differences between electric HP and mechanical HP. I am going to ignore this and just fall back on the 4th law of thermo.

No. Power is power regardless of units of measure whether watts, horsepower, or my favorite -- ounce parsecs per fortnight. As far as I know electrical power is universally expressed in watts, but nothing is preventing you from using ounce parsecs per fortnight (more commonly known as a Billy).

5. Manufacturers lie.

Depends. Small manufacturers of woodturning lathes in the US, Canada, Europe, and Australia are truthful. Some well respected brands manufactured in Asia are truthful. Many Asian built lathes don't have truthful specs or they get corrupted somewhere along the path to the showroom floor.

6. You never get 100%

In sports you give 110% .... Or else you get traded.

7. Stuff is always more complicated than meets the eye.

8. In the USA we have two options on (residential) voltage, 110 VAC and 220 VAC

One of my pet peeves is that almost universally woodturners don't know that for more than fifty years the US standard has been 120 and 240 volts for residential power. And for about fifty more years before that it was 115/230 volts. The original voltage was 110 volts because Edison said that was the optimal voltage for his light bulbs. Motor nameplates typically state a voltage of 115/230 etc. with the assumption of a small voltage drop from the service entrance to the motor.

Given all this, lets generalize. If a brand claims 3 HP and is 220 VAC, the current rating must be, at least, (3HP X 746W/HP)/220V = 10.17Amps.

I presume that you are talking about a three-phase motor that would be used with an inverter. In this case the current in each of the three phases would be divided by the square root of three or 1.7 (which is close enough). Don't forget to include efficiency and use 240 volts in your calculations. I assume that you might be looking at the nameplate on a motor imported from somewhere in Asia. They use kilowatts and take great liberties with round off errors when converting to horsepower.

I believe most (all?) service tags list both the voltage and the current ratings, so if your lathe is rated at 3HP and plugs into a 220V outlet the amperage rating must be at least 10.17 amperes. And that would be at 100% efficiency which is never achieved.

See my previous answer.

Given this, how do the manufactures get away claiming a 3HP unit that is really a 2HP unit when the V and A ratings do not line up?

This has a complicated answer. For US made motors the nameplate numbers are accurate, however, the system integrator or the importer or retailer of Asian made lathes are in many cases lying or are vaguely misleading. Also,there's a loophole in which they could claim that the motor is a unique design that is a dedicated unique component of the machine as a whole and therefore looking at the performance characteristics of the motor as a separate part doesn't apply. The most egregious examples were air compressors before a class action settlement forced some degree of truth in advertising. Horsepower claims on shop vacs are simply astounding.

I know I have simplified this and do not address VFD's, induction motors, ... I am just going for the theoretical maximums and relying on the 4th law of thermodynamics in the "at least" restriction to keep things honest. Kind of the physics equivalent of likening a cow to a homogeneous, frictionless sphere.

I suppose that you're being tongue in cheek given that there is no fourth law of thermodynamics. Here is a humorous discussion about various possibilities of a fourth law of thermodynamics.

 

[Bruce Schoenleber]

Good talk Bill.
The 4 laws of thermo start with zero? Seems odd as only computer programmers start counting at zero and they are a couple of hundred years late to the party. Go figure.

Anyway, beyond relearning how to count, my intention was to as simply as possible point out that the HP rating of the lathe cannot exceed the input power - AC line voltage and current - of the lathe. I did not want to involve all the complications of the system, hence the homogeneous spherical cow reference. If you turn on that 120 volt 6.5 HP shop vac and don't blow your circuit breaker, somebody is fibbin'.

Dang! I can never remember if it is 110, 115 or 120 volts out of the wall, it seems at some point in my life it was all of them. Somewhere in the past I measured my shop receptacles now and then. In reality it varies a fair amount. Brown outs and all...

To, hopefully, sum up, if you are looking at buying a 3 HP lathe that plugs into a 15 amp 120 VAC outlet, I suggest you keep looking.

 

[olaf Vogel]

Hi Bill, since you seem very knowledgeable on the subject.

Slightly different question: in regards to effective HP relative to RPM (and specifically the number of poles on the motor.)
I have a lot of 3PH motors in the shop, mostly 5 hp.

My 2 pole, 3450 rpm motor is (physically) the smallest. easiest to carry and move, light.
The 4 pole 1700 rpm is much bigger, still movable by hand.

I also have a 6 pole which is about 150 lbs
And lastly an 8 pole monster (860 rpm) that is 285 lbs and the same size as my 15 hp converter pony motor.

I've been told that by gearing down, (pulleys, gears etc) to the same rpm, all should have the same effective HP (ignoring any friction losses).
Or is this similar to car engines, where max HP is not the same as torque at lower RPMs.?

Here's the 8 pole Leason I just picked up for $100

 

[Mike Johnson]

Small shops have 120V at the receptacle, extremely large shops have 110V at the receptacle.

Why is that true?

 

[odie]

And lastly an 8 pole monster (860 rpm) that is 285 lbs and the same size as my 15 hp converter pony motor.

Here's the 8 pole Leason I just picked up for $100

View attachment 26107

Man, that is huge, Olaf......out of curiosity, what will you be doing with that motor?

-----odie-----

 

[Bill Boehme]

Good talk Bill.
The 4 laws of thermo start with zero? Seems odd as only computer programmers start counting at zero and they are a couple of hundred years late to the party. Go figure.

I figure that the zeroth law was an afterthought to say that if A equals B and if B equals C then we can also say that A equals C. It wasn't profound enough to have a number greater than nothing.

Hi Bill, since you seem very knowledgeable on the subject.

Slightly different question: in regards to effective HP relative to RPM (and specifically the number of poles on the motor.)
I have a lot of 3PH motors in the shop, mostly 5 hp.

My 2 pole, 3450 rpm motor is (physically) the smallest. easiest to carry and move, light.
The 4 pole 1700 rpm is much bigger, still movable by hand.

I also have a 6 pole which is about 150 lbs
And lastly an 8 pole monster (860 rpm) that is 285 lbs and the same size as my 15 hp converter pony motor.

I've been told that by gearing down, (pulleys, gears etc) to the same rpm, all should have the same effective HP (ignoring any friction losses).
Or is this similar to car engines, where max HP is not the same as torque at lower RPMs.?

In addition to size and weight increasing proportionally to the number of poles, the cost increases exponentially.

Power is the product of speed and torque. Energy can't be created or destroyed, so ignoring small friction losses, if the motor output were to go through a drive train that reduces the speed in half, the power remains the same (to satisfy the conservation of energy) and the torque doubles to maintain the equality.

Older lathes before VFD's came along always had full horsepower available at the spindle for each of the pulley speeds. This was great because a smaller motor could be used and still have adequate power at the lower speeds. When VFD's came along it was great to have variable speed at the touch of a dial, but that convenience meant a substantial low speed penalty in power at the spindle. To overcome the power penalty two things could be done:

• Use a bigger motor
• Use multiple pulley speed ranges

Most manufacturers do both because the cost of bigger motors gets expensive in a hurry. It is advantageous to have a minimal number of pulley ranges because the attraction of variable speed is to get away from changing belt positions.

 

[William Rogers]

Well I'm confused about the motor on my lathe. I am not electrical when it comes to motors. It was made in Taiwan. On the nameplate it states:
2hp output
5.2 amps
230 volts
1.5 Kw
1720 rpm
It has a 2hp inverter.

So if I just look at the 2hp output or the 1.5 Kw (1.5/746) I have a 2 hp motor

If I use amps (5.2 x 230)/746 it is 1.6hp

So is it 1.6 hp or 2 hp?

Then is there efficiency factor applied to both?

 

[Bill Boehme]

Well I'm confused about the motor on my lathe. I am not electrical when it comes to motors. It was made in Taiwan. On the nameplate it states:
2hp output
5.2 amps
230 volts
1.5 Kw
1720 rpm
It has a 2hp inverter.

So if I just look at the 2hp output or the 1.5 Kw (1.5/746) I have a 2 hp motor

If I use amps (5.2 x 230)/746 it is 1.6hp

So is it 1.6 hp or 2 hp?

Then is there efficiency factor applied to both?

It's a three phase motor and the nameplate current is the RMS current per phase. To get the total RMS current you need to multiply 5.2 by √3 by efficiency which typically is somewhere between, 0.7 and 0.8 for imported Asian motors. When I did the math using 0.75 as the efficiency I got a hair over 2 HP so you can quit worrying.

Something else that I might mention that is a common misunderstanding about how motors work. If a motor is rated at 2 HP as an example it doesn't mean that the motor is always putting out full power. If that were the case it would continue to accelerate out of control.

When the motor is running with no external load connected, the only loads are internal factors. When the motor is connected to a lathe so that it is only driving the spindle the the only loads are friction of the belt and bearing which means that the motor only need to provided enough power to maintain speed.The only time that it would need to output full power is when the load is high enough that the motor is close to the point where it can't maintain full speed. Many motors are rated to handle an overload condition for a certain number of minutes with a moderate drop in speed. However, inverters are usually programmed to shut off when a maximum current limit is exceeded for a few seconds.

 

[Emiliano Achaval]

"Chooglin"? I have brand R. It doesn't choogle, it purrs.

I'll give my take on the other questions further along in this post.



Yes, but it's not quite that simple. If we were talking about a light bulb or electric heater then yes, but if it is a motor then It is a bit more complicated. The efficiency has to also be accounted for. Efficiency is the ratio of input power to output power. Output power is determined by using a dynamometer to measure torque (Newton•meters since the watt is an SI unit) and then multiplying that by angular velocity in radians per second. Just about everywhere but here motor mechanical power is given in kilowatts. Just don't confuse the electrical input power with the mechanical output power which are both stated in watts. Giving them the benefit of the doubt importers and/or retailers may have missed that important distinction.



Just be sure to remember not to confuse input power to the motor with the motor output power. If you have an EU motor and want to convert from kilowatts to horsepower use the nameplate mechanical power power output and multiply by 0.746.



See my last comment. There are three laws of thermodynamics. There's also a "Zeroth" law which is a mathematical equivalence, the transitive property of equality.



No. Power is power regardless of units of measure whether watts, horsepower, or my favorite -- ounce parsecs per fortnight. As far as I know electrical power is universally expressed in watts, but nothing is preventing you from using ounce parsecs per fortnight (more commonly known as a Billy).



Depends. Small manufacturers of woodturning lathes in the US, Canada, Europe, and Australia are truthful. Some well respected brands manufactured in Asia are truthful. Many Asian built lathes don't have truthful specs or they get corrupted somewhere along the path to the showroom floor.



In sports you give 110% .... Or else you get traded.







One of my pet peeves is that almost universally woodturners don't know that for more than fifty years the US standard has been 120 and 240 volts for residential power. And for about fifty more years before that it was 115/230 volts. The original voltage was 110 volts because Edison said that was the optimal voltage for his light bulbs. Motor nameplates typically state a voltage of 115/230 etc. with the assumption of a small voltage drop from the service entrance to the motor.



I presume that you are talking about a three-phase motor that would be used with an inverter. In this case the current in each of the three phases would be divided by the square root of three or 1.7 (which is close enough). Don't forget to include efficiency and use 240 volts in your calculations. I assume that you might be looking at the nameplate on a motor imported from somewhere in Asia. They use kilowatts and take great liberties with round off errors when converting to horsepower.



See my previous answer.



This has a complicated answer. For US made motors the nameplate numbers are accurate, however, the system integrator or the importer or retailer of Asian made lathes are in many cases lying or are vaguely misleading. Also,there's a loophole in which they could claim that the motor is a unique design that is a dedicated unique component of the machine as a whole and therefore looking at the performance characteristics of the motor as a separate part doesn't apply. The most egregious examples were air compressors before a class action settlement forced some degree of truth in advertising. Horsepower claims on shop vacs are simply astounding.



I suppose that you're being tongue in cheek given that there is no fourth law of thermodynamics. Here is a humorous discussion about various possibilities of a fourth law of thermodynamics.

@Bill Boehme how in the world do you answer different questions, and can quote on your reply each time? I think I can do it once, but not different times. Do you just copy and paste? Your answer should go on the How do I? section...

 

[Emiliano Achaval]

Hi Bill, since you seem very knowledgeable on the subject.

Slightly different question: in regards to effective HP relative to RPM (and specifically the number of poles on the motor.)
I have a lot of 3PH motors in the shop, mostly 5 hp.

My 2 pole, 3450 rpm motor is (physically) the smallest. easiest to carry and move, light.
The 4 pole 1700 rpm is much bigger, still movable by hand.

I also have a 6 pole which is about 150 lbs
And lastly an 8 pole monster (860 rpm) that is 285 lbs and the same size as my 15 hp converter pony motor.

I've been told that by gearing down, (pulleys, gears etc) to the same rpm, all should have the same effective HP (ignoring any friction losses).
Or is this similar to car engines, where max HP is not the same as torque at lower RPMs.?

Here's the 8 pole Leason I just picked up for $100

View attachment 26107

Is your car really small or the motor is huge? LOL

 

[Bill Boehme]

When I click on reply to your post I see the following in the editor box:



[QUOTE="Emiliano Achaval, post: 137561, member: 36504"][USER=747]@Bill Boehme[/USER] how in the world do you answer different questions, and can quote on your reply each time? I think I can do it once, but not different times. Do you just copy and paste? Your answer should go on the How do I? section...[/QUOTE]

Now, suppose that I want to put each sentence into a separate quote box. I start by putting carriage returns between each sentence and then I copy the QUOTE BB at the beginning of your post and then paste it in front of each sentence. And then I copy the /QUOTE at the end of your post at the end of each sentence as shown below:

[QUOTE="Emiliano Achaval, post: 137561, member: 36504"][USER=747]@Bill Boehme[/USER] how in the world do you answer different questions, and can quote on your reply each time?[/QUOTE]

My Comment #1

[QUOTE="Emiliano Achaval, post: 137561, member: 36504"]JocI think I can do it once, but not different times.[/QUOTE]

My Comment #2

[QUOTE="Emiliano Achaval, post: 137561, member: 36504"]Do you just copy and paste?[/QUOTE]

My Comment #3

[QUOTE="Emiliano Achaval, post: 137561, member: 36504"]Your answer should go on the How do I? section...[/QUOTE]

My Comment #4


******************************************************************************************

After I hit the reply button here is the final result.

@Bill Boehme how in the world do you answer different questions, and can quote on your reply each time?

Comment #1

JocI think I can do it once, but not different times.

Comment #2

Do you just copy and paste?

Comment #3

Your answer should go on the How do I? section...

Comment #4

 

[olaf Vogel]

Man, that is huge, Olaf......out of curiosity, what will you be doing with that motor?
-----odie-----

Its already mounted on the lathe.
Awaiting a new headstock....and me having time to set it up.

Is your car really small or the motor is huge? LOL

Its a mid-sized American (full sized European) sedan
Yup - its huge

 

[Emiliano Achaval]

When I click on reply to your post I see the following in the editor box:



[QUOTE="Emiliano Achaval, post: 137561, member: 36504"][USER=747]@Bill Boehme[/USER] how in the world do you answer different questions, and can quote on your reply each time? I think I can do it once, but not different times. Do you just copy and paste? Your answer should go on the How do I? section...[/QUOTE]

Now, suppose that I want to put each sentence into a separate quote box. I start by putting carriage returns between each sentence and then I copy the QUOTE BB at the beginning of your post and then paste it in front of each sentence. And then I copy the /QUOTE at the end of your post at the end of each sentence as shown below:

[QUOTE="Emiliano Achaval, post: 137561, member: 36504"][USER=747]@Bill Boehme[/USER] how in the world do you answer different questions, and can quote on your reply each time?[/QUOTE]

My Comment #1

[QUOTE="Emiliano Achaval, post: 137561, member: 36504"]JocI think I can do it once, but not different times.[/QUOTE]

My Comment #2

[QUOTE="Emiliano Achaval, post: 137561, member: 36504"]Do you just copy and paste?[/QUOTE]

My Comment #3

[QUOTE="Emiliano Achaval, post: 137561, member: 36504"]Your answer should go on the How do I? section...[/QUOTE]

My Comment #4


******************************************************************************************

After I hit the reply button here is the final result.




Comment #1



Comment #2



Comment #3



Comment #4

Thank you for the great explanation Bill. Now I need to find a post that requires several answers so I can practice, LOL