Hello.
If I install a 3 horsepower 120VAC single phase motor and the compressor it has to drive uses/needs only 2 horsepower 120VAC single phase; leaving aside -ignoring- starting current; would the 3 HP motor consume only 15 Amperes from a plain outlet/breaker when running ?

 

would the 3 HP motor consume only 15 Amperes from a plain outlet/breaker when running ?

For a typical single-phase small AC motor with a efficiency of about 80%, the power draw would be about 746/0.8 = 933W per HP, so for 2 HP, the draw would be (933*2) / 120 = 15.5A,

 

I assume that both motors run at the same RMP? If that's the case, then expect the 3HP to draw slightly more current than the 2HP motor. This because it has higher core losses due to its having a larger iron core. Other than that, I'd say that for practical purposes its power draw will be the same. This because power demand is dictated by the mechanical properties of the compressor itself, not the motor.

 

Exactly!
To obtain the optimal efficiency, an electric motor should be loaded as close as possible to its rated shaft power

 

Exactly!
To obtain the optimal efficiency, an electric motor should be loaded as close as possible to its rated shaft power

Not necessarily.
According to Google AI:
A single-phase electric motor reaches its peak efficiency between 50% and 100% load, with optimal performance typically occurring around 75% of its rated load.
EDIT: In a typical small-to-medium single-phase AC motor, resistance (copper) losses account for roughly 50% to 60% of total energy losses, while core (iron) losses make up about 15% to 20%.

So operating a 3HP motor at 2HP should likely be near its peak efficiency.
At above the maximum efficiency point, the I²R wire losses cause a reduction in efficiency.

 

We must remember that 2HP american electric motors do not have exactly the same rating as a 2HP european electric motor ... normally, american motors have a larger safety factor (in my experience, about an extra 20%) than european motors (about only 5%). So their advertised power rating is not an exact spec.

 

We must remember that 2HP american electric motors do not have exactly the same rating as a 2HP european electric motor ... normally, american motors have a larger safety factor (in my experience, about an extra 20%) than european motors (about only 5%). So their advertised power rating is not an exact spec.

This is why I just use horses. No ambiguity. When you hitch to horses to your shaft it's two horsepower whether the horses speak French or English.

 

This is why I just use horses. No ambiguity. When you hitch to horses to your shaft it's two horsepower whether the horses speak French or English.

Sorry, but I beg to differ ... (hey, you started it! )


Comparing French and English horses usually comes down to contrasting notable regional breeds—like the athletic French Selle Français sport horse versus the iconic English Thoroughbred.

Key Differences

• Origin and Development: French horse breeding historically combined native stock with Thoroughbred and trotter bloodlines to produce versatile carriage, military, and sport horses (such as the Selle Français consolidated in 1958). English horse breeding heavily centered on the Thoroughbred, developed for agility, stamina, and supreme racing speed.
• Build and Conformation: French riding horses like the Selle Français often exhibit strong, powerful hindquarters, robust bone structure, and a deep chest suited for jumping. English Thoroughbreds typically feature a leaner, longer-lined, and more angular frame built for maximum galloping efficiency.
• Primary Disciplines: French warmbloods excel heavily in international show jumping, eventing, and dressage. English-associated breeds dominate racing, polo, and high-speed equestrian sports, though both styles overlap significantly in modern global competition.

 

Cruts;
I worked for for a electric motor manufacturer. Back then, late 1990s there was pending legislation, which among other things there was an efficiency overhaul for electrical motors. The curves shown in the internet mostly show ancient performance requirements.
With the newer efficiency ratings in place, specified efficiency should be at the rated nameplate slip. That nameplate slip always occurs between 90 and 95% load.
But the efficiencies gained are not very large, and thus your suggestion is still valid.

 

Sorry, but I beg to differ ... (hey, you started it! )


Comparing French and English horses usually comes down to contrasting notable regional breeds—like the athletic French Selle Français sport horse versus the iconic English Thoroughbred.

Key Differences

• Origin and Development: French horse breeding historically combined native stock with Thoroughbred and trotter bloodlines to produce versatile carriage, military, and sport horses (such as the Selle Français consolidated in 1958). English horse breeding heavily centered on the Thoroughbred, developed for agility, stamina, and supreme racing speed.
• Build and Conformation: French riding horses like the Selle Français often exhibit strong, powerful hindquarters, robust bone structure, and a deep chest suited for jumping. English Thoroughbreds typically feature a leaner, longer-lined, and more angular frame built for maximum galloping efficiency.
• Primary Disciplines: French warmbloods excel heavily in international show jumping, eventing, and dressage. English-associated breeds dominate racing, polo, and high-speed equestrian sports, though both styles overlap significantly in modern global competition.

My horses are Canadian and American.

 

My horses are Canadian and American.

Then both are American.

 

Cruts;
I worked for for a electric motor manufacturer. Back then, late 1990s there was pending legislation, which among other things there was an efficiency overhaul for electrical motors. The curves shown in the internet mostly show ancient performance requirements.
With the newer efficiency ratings in place, specified efficiency should be at the rated nameplate slip. That nameplate slip always occurs between 90 and 95% load.
But the efficiencies gained are not very large, and thus your suggestion is still valid.

I would imagine that there's also a baseline draw that is different. I would expect the no-load draw of a 3 HP motor to be higher than that of a 2 HP motor. But, even if that's the case, I don't know if it translates into a higher current draw at various loads. I'd be interested in your expectations on those points.

 

I would imagine that there's also a baseline draw that is different. I would expect the no-load draw of a 3 HP motor to be higher than that of a 2 HP motor. But, even if that's the case, I don't know if it translates into a higher current draw at various loads. I'd be interested in your expectations on those points.

Inefficiency times total power can vary with sizes (and of course load). More cross-sectional area of copper is used in higher power (for same voltage), this helps efficiency. But the shaft can be also bigger, which increases losses by friction, not to mention the inertia whenever the system's speed change.
I'm no expert in motors, but I think what @cmartinez said about core losses is more than compensated by using only 2/3 of rated power, because of a rise in efficiency. But that's a shot in low light conditions, I have the feeling that it's like that. I could be wrong. I'm interested if anyone want to explain and show up-to-date graphs.

 

Inefficiency times total power can vary with sizes (and of course load). More cross-sectional area of copper is used in higher power (for same voltage), this helps efficiency. But the shaft can be also bigger, which increases losses by friction, not to mention the inertia whenever the system's speed change.
I'm no expert in motors, but I think what @cmartinez said about core losses is more than compensated by using only 2/3 of rated power, because of a rise in efficiency. But that's a shot in low light conditions, I have the feeling that it's like that. I could be wrong. I'm interested if anyone want to explain and show up-to-date graphs.

Here's what I'd expect in a real test when comparing the performance of a 3HP motor vs a 2HP motor for the application originally described:

1. The 3HP motor would be slightly less efficient while running idle due to core losses and increased mechanical friction due to its larger size
2. The 3HP motor would be a little more efficient (but I honestly don't know by how much) while running under maximum load simply because its higher power rating would mean less heat losses, and therefore its windings would run cooler and thus at a lower resistivity than the 2HP motor.
3. I'd expect that one just can't use a larger and larger motor expecting an endless increase in efficiency, and an optimal motor size (and type) exists for this particular application. Simulating this scenario would be an interesting thing to see. But I lack the resources necessary for that endeavor.

 

Here's what I'd expect in a real test when comparing the performance of a 3HP motor vs a 2HP motor for the application originally described:

1. The 3HP motor would be slightly less efficient while running idle due to core losses and increased mechanical friction due to its larger size
2. The 3HP motor would be a little more efficient (but I honestly don't know by how much) while running under maximum load simply because its higher power rating would mean less heat losses, and therefore its windings would run cooler and thus at a lower resistivity than the 2HP motor.
3. I'd expect that one just can't use a larger and larger motor expecting an endless increase in efficiency, and an optimal motor size (and type) exists for this particular application. Simulating this scenario would be an interesting thing to see. But I lack the resources necessary for that endeavor.

I agree in every part of it.

 

2. The 3HP motor would be a little more efficient (but I honestly don't know by how much) while running under maximum load simply because its higher power rating would mean less heat losses, and therefore its windings would run cooler and thus at a lower resistivity than the 2HP motor.

Agrees with that if they are both driving a 2HP load, but not if both are running their rated loads.
For both at their rated full load, I would expect that both would have comparable relative losses and thus efficiency, since that's typically how they are designed.

 

Then both are American.

Both North American, to be sure—but when someone says "American" in Canada they are referring to the neighbor south of them not themselves. The only time someone calls Canadians (or Mexicans) "American" is when they are trying to make whinge about Americans (that is, people in the US).

 

Yes, we here in the US generally call ourselves Americans, since USians, doesn't have a very good ring to it.