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 Are Amps, Watts, Volts and Ohms?

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:

Single Phase VS Three Phase: What’s the Difference? | CHINT Blog

The main difference between a single phase and three phase connection is that a three-phase can handle a significant amount of load whereas a single phase cannot.

www.google.com

Advantages of Three Phase Over Single Phase System

Advantages of 3-Φ System over 1-Φ System. Merits of 3-Phase Power over 1-Phase Power. Advantages of Three Phase Over Single Phase System.

www.google.com

Difference Between Single Phase and Three Phase Induction Motor: All you need to know - Industrial Manufacturing Blog | linquip

If you want a detailed description of the single phase and three phase induction motors, here we provide everything you need. Click on it to learn more

www.google.com

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 Are Amps, Watts, Volts and Ohms?

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:

Single Phase VS Three Phase: What’s the Difference? | CHINT Blog

The main difference between a single phase and three phase connection is that a three-phase can handle a significant amount of load whereas a single phase cannot.

www.google.com

Advantages of Three Phase Over Single Phase System

Advantages of 3-Φ System over 1-Φ System. Merits of 3-Phase Power over 1-Phase Power. Advantages of Three Phase Over Single Phase System.

www.google.com

Difference Between Single Phase and Three Phase Induction Motor: All you need to know - Industrial Manufacturing Blog | linquip

If you want a detailed description of the single phase and three phase induction motors, here we provide everything you need. Click on it to learn more

www.google.com

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.