Lathe Motor Amperage Rating

[MTGreen]

Horsepower ratings for homeowner level equipment are not the same as horsepower ratings for industrial equipment. Homeowner level equipment is rated at peak horsepower which typically is achieved for a small fraction of a second at turn on and not even close while running. Industrial motors are rated as continuous horsepower. The momentary peak horsepower for industrial motors is much higher than the nameplate hp. For 120volt 15amp household AC, a 1 horsepower industrial is about all you can run. 20 amp will let you run about 1.5 horsepower as mentioned in a post above. So the amperage readings you are finding are about right.

A three phase motor draws around 75% of the amperage that a single phase motor will draw with identical power output. That's substantial savings for a big plant which is why they have three phase power almost universally.

I have a belt sander and a large metal cutting bandsaw that are both rated at one horsepower. The belt sander motor is about a quarter of the size and weight and about half the amperage draw of the bandsaw motor. The difference is the belt sander motor is peak rated motor and the bandsaw motor is an industrial continuous rated motor.

I have a shop vac that is supposedly rated at four horsepower which is marketing crap. It's actually a good shop vac, but it ain't nowhere near four horsepower. The thing draws less than 10 amps on 120 volts so clearly not 4 horsepower continuous

 

[LD1]

Horsepower ratings for homeowner level equipment are not the same as horsepower ratings for industrial equipment. Homeowner level equipment is rated at peak horsepower which typically is achieved for a small fraction of a second at turn on and not even close while running. Industrial motors are rated as continuous horsepower. The momentary peak horsepower for industrial motors is much higher than the nameplate hp. For 120volt 15amp household AC, a 1 horsepower industrial is about all you can run. 20 amp will let you run about 1.5 horsepower as mentioned in a post above. So the amperage readings you are finding are about right.

A three phase motor draws around 75% of the amperage that a single phase motor will draw with identical power output. That's substantial savings for a big plant which is why they have three phase power almost universally.

I have a belt sander and a large metal cutting bandsaw that are both rated at one horsepower. The belt sander motor is about a quarter of the size and weight and about half the amperage draw of the bandsaw motor. The difference is the belt sander motor is peak rated motor and the bandsaw motor is an industrial continuous rated motor.

I have a shop vac that is supposedly rated at four horsepower which is marketing crap. It's actually a good shop vac, but it ain't nowhere near four horsepower. The thing draws less than 10 amps on 120 volts so clearly not 4 horsepower continuous

Alot more than a 75% difference between 3phase and single phase amps. Especially in the context you describe of homeowner vs industrial.....because that implies 120/240 vs 480v.

And the "savings" you claim isn't because they draw fewer amps. Because we don't buy power by amperage.

So if you are implying that since a 3amp 1hp motor in a factory at 3phase/460v is gonna be about 1/4 the cost to operate than a 12a/120v motor of the same power.....because it has 1/4 the amperage, you are wrong

 

[MTGreen]

Alot more than a 75% difference between 3phase and single phase amps. Especially in the context you describe of homeowner vs industrial.....because that implies 120/240 vs 480v.

And the "savings" you claim isn't because they draw fewer amps. Because we don't buy power by amperage.

So if you are implying that since a 3amp 1hp motor in a factory at 3phase/460v is gonna be about 1/4 the cost to operate than a 12a/120v motor of the same power.....because it has 1/4 the amperage, you are wrong

I was trying to answer two separate questions and apparently that wasn't clear. The first was the peak power output versus the continuous power output rating system for electric motors which hopefully came across clearly.

The second was in reference to several posts about three phase power.

Power is sold by wattage which is volts times amps. Reference here:

What Is a Watt?

OK, so volts measure the potential for energy to travel and ohms measure the resistance to the electrical flow, but what are amps and watts?

science.howstuffworks.com

A three phase motor draws around 75% of the amperage that a single phase motor will draw with identical power output.

I should have made clear in my statement above identical power output AND identical voltage.

I did not mean to imply that amperage alone is the basis for why three phase is more efficient. It doesn't matter what the voltage OR amperage is, the wattage (which is volts times amps which where the electricity cost comes in) required for a given power output is going to be less with a 3phase motor than a single phase motor.
Here are several references stating that 3 phase is more efficient:

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And here's a real world example from Baldor showing that reality matches theory:

5hp single phase 230v 3600rpm 184t frame general purpose TEFC part #EL3608T full load amps =19.4amps which gives 4462 watts. The 1800rpm model #EL3612T full load amps = 19.1 which gives 4393 watts.

5hp three phase 230/460v 3600rpm 184tc frame general purpose TEFC part #VEM3613T full load amps = 11.8 amps at 230v which gives 2714 watts or 5.9amps at 460v which gives 2714 watts. The 1800rpm model #VEM3615T full load amps = 13.4amps at 230v which gives 3082 watts or 6.7amps at 460v which gives 3082 watts.

In our example taken directly from Baldor for general purpose five horsepower 184t frame TEFC motors:
The three phase 3600 RPM motor draws 61% as much wattage versus the single phase 3600rpm equivalent .
The three phase 1800 RPM motor draws 70% as much wattage versus the single phase 1800rpm equivalent

 

[etpm]

Induction motors can be more or less efficient. So nameplate amperage and voltage are only an approximate indication of shaft horsepower. Your best bet when sizing wire and control electronics is to ignore nameplate horsepower and just look at nameplate amps and voltage ratings. When using the typical VFD it is good to remember that it won't allow the motor to produce more horsepower or more torque than the motor is rated for. It will allow the motor to produce less though. This means that if the motor is run faster than rated RPM the torque will drop. And when run slower than rated RPM the torque will be constant. VFD rated motors will be made to handle the increased heat when the motor is spinning slowly. The motor gets hotter because the fan isn't moving as fast. When using a non-VFD rated motor it is a good idea to add a cooling fan. A muffin fan from a computer is usually sufficient for a 1 hp motor. When using a non-VFD rated motor a good range of frequencies for a 60 hz motor is 30 to 90 hz. This range of course is general as is the info I posted above. But lots of folks use the above info as guidelines and it works very well for them. One last thing to remember: circuit breakers in the load center, AKA the breaker panel, are for protecting the wires connected to the breaker and nothing else. They are not there to protect any equipment. Equipment protecting breakers are separate entities.
Eric

 

[LD1]

I was trying to answer two separate questions and apparently that wasn't clear. The first was the peak power output versus the continuous power output rating system for electric motors which hopefully came across clearly.

The second was in reference to several posts about three phase power.

Power is sold by wattage which is volts times amps. Reference here:

What Is a Watt?

OK, so volts measure the potential for energy to travel and ohms measure the resistance to the electrical flow, but what are amps and watts?

science.howstuffworks.com

I should have made clear in my statement above identical power output AND identical voltage.

I did not mean to imply that amperage alone is the basis for why three phase is more efficient. It doesn't matter what the voltage OR amperage is, the wattage (which is volts times amps which where the electricity cost comes in) required for a given power output is going to be less with a 3phase motor than a single phase motor.
Here are several references stating that 3 phase is more efficient:

Redirect Notice

Redirect Notice

Redirect Notice

And here's a real world example from Baldor showing that reality matches theory:

5hp single phase 230v 3600rpm 184t frame general purpose TEFC part #EL3608T full load amps =19.4amps which gives 4462 watts. The 1800rpm model #EL3612T full load amps = 19.1 which gives 4393 watts.

5hp three phase 230/460v 3600rpm 184tc frame general purpose TEFC part #VEM3613T full load amps = 11.8 amps at 230v which gives 2714 watts or 5.9amps at 460v which gives 2714 watts. The 1800rpm model #VEM3615T full load amps = 13.4amps at 230v which gives 3082 watts or 6.7amps at 460v which gives 3082 watts.

In our example taken directly from Baldor for general purpose five horsepower 184t frame TEFC motors:
The three phase 3600 RPM motor draws 61% as much wattage versus the single phase 3600rpm equivalent .
The three phase 1800 RPM motor draws 70% as much wattage versus the single phase 1800rpm equivalent

Your KWH calculations for comparing a 3PH motor are incorrect and gives a grossly inflated sense of savings that is wrong.

KWH for a single phase is indeed V x Amps. But that is NOT true for a 3-phase.

V x A x PF x 1.732

 

[strantor]

5hp single phase 230v 3600rpm 184t frame general purpose TEFC part #EL3608T full load amps =19.4amps which gives 4462 watts. The 1800rpm model #EL3612T full load amps = 19.1 which gives 4393 watts.

5hp three phase 230/460v 3600rpm 184tc frame general purpose TEFC part #VEM3613T full load amps = 11.8 amps at 230v which gives 2714 watts or 5.9amps at 460v which gives 2714 watts.

This ^ is using single phase equation for 3 phase power. It disregards the power from the additional 2 phases. The problem becomes apparent if you convert the calculated kW back to HP.
2714W ÷ 746W = 3.63HP

The proper formula:
E x I x PF x 1.732
11.8×230×0.91×1.732 = 4277W

4277 ÷ 746 = 5.73HP (exceeds 5HP electrical due to inefficiency).

The 1800rpm model #VEM3615T full load amps = 13.4amps at 230v which gives 3082 watts or 6.7amps at 460v which gives 3082 watts.

13.4×230×0.78×1.732 = 4164w
4164W ÷ 746W = 5.58HP

In our example taken directly from Baldor for general purpose five horsepower 184t frame TEFC motors:
The three phase 3600 RPM motor draws 61% as much wattage versus the single phase 3600rpm equivalent .
The three phase 1800 RPM motor draws 70% as much wattage versus the single phase 1800rpm equivalent

1HP is 1HP regardless of single phase or 3 phase. The advantage of 3 phase is simpler motors due to the rotational EM field(no capacitors required) and lower current in the wires (smaller wires can be used). But electric bill won't change.

 

[MTGreen]

Your KWH calculations for comparing a 3PH motor are incorrect and gives a grossly inflated sense of savings that is wrong.

KWH for a single phase is indeed V x Amps. But that is NOT true for a 3-phase.

V x A x PF x 1.732

This ^ is using single phase equation for 3 phase power. It disregards the power from the additional 2 phases. The problem becomes apparent if you convert the calculated kW back to HP.
2714W ÷ 746W = 3.63HP

The proper formula:
E x I x PF x 1.732
11.8×230×0.91×1.732 = 4277W

4277 ÷ 746 = 5.73HP (exceeds 5HP electrical due to inefficiency).

13.4×230×0.78×1.732 = 4164w
4164W ÷ 746W = 5.58HP


1HP is 1HP regardless of single phase or 3 phase. The advantage of 3 phase is simpler motors due to the rotational EM field(no capacitors required) and lower current in the wires (smaller wires can be used). But electric bill won't change.

Thank you both for correcting me here. I thought I had a handle on wattage calculation from what I've read but it looks like I had just enough information to be dangerous as the three phase calculation is different! I appreciate it! This sort of good info from people willing to share is why I participate in these forums - I learn something new every time!

 

[LD1]

Thank you both for correcting me here. I thought I had a handle on wattage calculation from what I've read but it looks like I had just enough information to be dangerous as the three phase calculation is different! I appreciate it! This sort of good info from people willing to share is why I participate in these forums - I learn something new every time!

Yep, sadly, no free lunch with three phase.

The benefit is just what strantor mentioned....simpler motor (no capacitors)....easy reversing, and smaller gauge wiring. Being able to run 10 and even 15 hp motors on 10ga- 12ga wire and 20-30amp circuits is just unheard of in the single phase realm. We'd be talking about pushing 60amps+ and some heavy wire. Big bucks, bigger conduit, expensive breakers, etc.

 

[etpm]

Another benefit of 3 phase motors is lower starting current. Crummy single phase motors can draw 4 or even 5 times rated running current when starting. Just to be clear I'm talking about induction motors. The motors used in shop vacs, many table saws, hand drills, and Skil saws are "universal" motors. They can be run from AC or DC. Though they are usually optimized for AC when they are meant to be plugged into AC. They do not act like induction motors. And are not made in 3 phase versions.
Eric

 

[strantor]

Thank you both for correcting me here. I thought I had a handle on wattage calculation from what I've read but it looks like I had just enough information to be dangerous as the three phase calculation is different! I appreciate it! This sort of good info from people willing to share is why I participate in these forums - I learn something new every time!

No worries, polyphase power can be a little abstract . I'm not so deluded as to think I'll ever have a complete understanding of it, but an understanding that's "good enough" is, well, good enough. Even with the bar set that low, still every time I think my I've reached it, I discover there's another layer.

 

[Chanceywd]

I read the whole thread looking for the ones using the right formula for 3 ph wattage. Some folks think that they have 2 phase current in their single phase service and the VFD just adds a third one.
Service factor of motors comes into play if you are thinking of loading it near it's rated horsepower. Some of the motors in an effort to be "green" and lower power used have a lower service factor. You will find this on the id plate as S.F.

 

[Dakota_male]

I converted my Craftsman drill press to a 3 phase motor and VFD a couple of years ago, and love it. No more moving belts around, just dial in the speed I want and drill holes. I used a Teco 510 VFD and a Connex 1 HP 3 phase motor I picked up at a local motor repair shop, made a new control panel and added a tach display to it.
I'm looking at doing the same to my Jet B920N lathView attachment 780201e.

View attachment 780198View attachment 780200

I did the same as you with my 2hp milling and drill press and also my 2hp lathe. on the lathe I enjoy the soft start especially when cutting metric threads as I have to reverse leaving the split nut engaged When taking multiple cuts.

 

[strantor]

I read the whole thread looking for the ones using the right formula for 3 ph wattage. Some folks think that they have 2 phase current in their single phase service and the VFD just adds a third one.

Thanks for bringing this up. I think you are being generous in saying "some folks" - I would say "most folks." It is a pervasive misconception perpetuated by many sources online, even in industry whitepapers and in tutorials from respected sources such as:

This stems from the way it is taught in classrooms, especially in electronics curriculum. Students are taught to think of L1 and L2 as two separate sources 180 degrees out of phase, I think because this is most convenient terms in to explain the concept using only the fundamentals which have been previously learned by the time this topic is introduced. Those who don't complete the course of education or don't get a chance to apply it in the real world are deprived of the opportunity to realize that Santa Claus isn't real.

It is much better IMO to teach it as a single source which has been split in the middle. Because that's exactly what it is. You can make a split phase transformer out of an isolation transformer by scratching the enamel off the center winding and soldering a wire to it. This does not magically invert the polarity or shift the phase of half of the secondary. And if you connect that new 3rd wire to earth and call it neutral, still no magic happens. I have made a couple of videos on this topic:

 

[Chanceywd]

Thanks for bringing this up. I think you are being generous in saying "some folks" - I would say "most folks." It is a pervasive misconception perpetuated by many sources online, even in industry whitepapers and in tutorials from respected sources such as:

This stems from the way it is taught in classrooms, especially in electronics curriculum. Students are taught to think of L1 and L2 as two separate sources 180 degrees out of phase, I think because this is most convenient terms in to explain the concept using only the fundamentals which have been previously learned by the time this topic is introduced. Those who don't complete the course of education or don't get a chance to apply it in the real world are deprived of the opportunity to realize that Santa Claus isn't real.

It is much better IMO to teach it as a single source which has been split in the middle. Because that's exactly what it is. You can make a split phase transformer out of an isolation transformer by scratching the enamel off the center winding and soldering a wire to it. This does not magically invert the polarity or shift the phase of half of the secondary. And if you connect that new 3rd wire to earth and call it neutral, still no magic happens. I have made a couple of videos on this topic:

I worked 26 years in maintenance at a monofilament plant in mechanical and electrical. Took a few courses in high school on electric and at Boces. And some electronics and training on Drives ac and dc. No degrees. We had a lot of German equipment and it is often connected in a 220/380 y. That is another thing in how motors are wired. One course sticks out in that the guy stressed each phase is transformer a,b, or c. In a delta a phase is transformer wired from 1 phase to a second phase of the line. In Y it is a phase to neutral.
If you look on the pole feeding your houses you will see 1 transformer. If you see the one feeding a commercial place like Mcdonalds you see 3 for the 3 phase used there. An exception in our area is high leg on a Y line and you will see 1 lg and 1 small.
So all I am saying is there is a lot going on in that little mystery VFD that makes it work on single phase. I have a couple on my rockwell mill and rockwell drill press. And 1.73 figures in the math.

Bill

 

[LD1]

120/240 is still single phase....but the phase is split to two legs at half the voltage

Rarely do I see people screw up the wattage. Cause it's simple amps x voltage.

But yes, screwing up the fundamentals thinking there are 2-phases.

But there were more a few that got the correct formula for 3-phase wattage.

Gotta be careful talking electric on a tractor forum though.... abbreviating 3-phase as 3ph can be misunderstood as 3-point-hitch

 

[Paul_M]

Thank you both for correcting me here. I thought I had a handle on wattage calculation from what I've read but it looks like I had just enough information to be dangerous as the three phase calculation is different! I appreciate it! This sort of good info from people willing to share is why I participate in these forums - I learn something new every time!

I might have missed this point in some other reply, but it's important: All kinds of motors draw more current during start-up. This is because motors generate "back EMF" in proportion to their speed. Back EMF opposes the supply voltage, thereby reducing supply current. If the motor doesn't come up to near its "synchronous speed", this "startup surge" continues. Motors that work against a significant load, and this includes most lathes because of their large inertial mass, can have a long startup surge. Startup surges can last long enough to pop breakers, and this is a consideration in sizing the wiring and the breakers.
Single-phase motors can have a particularly large startup surge. This is because their starting windings are connected to the supply during startup. The startup windings can draw as much or more current as the other stator windings. It is not unusual for single-phase motors to draw 3 or more times their full-load "run" amperage, which is often what is on the nameplate.
Startup surge current is often a limiting factor for VFD operation. Each VFD has an electronically limited ability to supply current which is generally proportional to its horsepower rating. When the motor "tries" to draw more current, the VFD throttles back the supply voltage to keep its output current within its safe output levels. If your VFD/motor combination fails to bring the motor up to the expected speed, this is the usual reason. And, this is the real reason why people say you need to choose a VFD with double or more the HP rating of the motor. In most cases, you can increase the "acceleration time" parameter of the VFD to a value that works, but you may be disappointed in how long it takes the motor to come up to speed.
The fact that the motor generates "back EMF" also accounts for a couple of VFD issues. If you suddenly stop or reverse the motor, the resultant surge of current back into the VFD can trigger its overload circuitry. A common workaround is to set "deceleration time" to a longer time, say 5 to 10 seconds. Again, you may be disappointed if you're used to having the motor immediately stop or change direction. The better way is to install a "braking resistor" (assuming your VFD has this capability" which can absorb the generated power and stop the motor more quickly.
A final note: most electric motors are very efficient, usually upwards of 90%. The same is true of VFDs. In many postings here, people assume that the differences they're seeing are due to efficiency differences. This is usually not the case. Most often, differences are related to either measurement problems or misuse of formulae for calculating power.
I designed VFDs, motors and transformers for many years, so this is not guesswork.

 

[coast40]

I picked up 2 free treadmills for the DC motors and controllers to use in my small milling machine and lathe. Based on Youtube and internet I can make them work. I might need to buy controllers to simplify the wiring.

 

[rScotty]

Thanks for bringing this up. I think you are being generous in saying "some folks" - I would say "most folks." It is a pervasive misconception perpetuated by many sources online, even in industry whitepapers and in tutorials from respected sources such as:

This stems from the way it is taught in classrooms, especially in electronics curriculum. Students are taught to think of L1 and L2 as two separate sources 180 degrees out of phase, I think because this is most convenient terms in to explain the concept using only the fundamentals which have been previously learned by the time this topic is introduced. Those who don't complete the course of education or don't get a chance to apply it in the real world are deprived of the opportunity to realize that Santa Claus isn't real.

It is much better IMO to teach it as a single source which has been split in the middle. Because that's exactly what it is. You can make a split phase transformer out of an isolation transformer by scratching the enamel off the center winding and soldering a wire to it. This does not magically invert the polarity or shift the phase of half of the secondary. And if you connect that new 3rd wire to earth and call it neutral, still no magic happens. I have made a couple of videos on this topic:

Is it just me or are these terrible videos? I sat down to enjoy them, and and quickly realized either they don't understand what they are talking about or can't explain it. Maybe both.
rScotty

 

[MTGreen]

Is it just me or are these terrible videos? I sat down to enjoy them, and and quickly realized either they don't understand what they are talking about or can't explain it. Maybe both.
rScotty

If you can't explain it simply, you don't understand it well enough.

Albert Einstein

 

[rScotty]

If you can't explain it simply, you don't understand it well enough.

Albert Einstein

Thank you, Albert.
rScotty