kW to HP

[bthielen]

I know this may seem like simple stuff but I?m not a motor expert by any means. Today I ran across a specification sheet for a 3ph, 460vac motor showing its rated power in kW but it did not indicate its hp rating. How do I convert the kW rating to hp? I suspect it isn?t as straight forward as dividing by .745 (multiplying by 1.34) right?

Thanks,

Bob

 

[Jraef]

746 watts = 1HP

So yes, it's as simple as dividing the kW by .746 or multiplying by 1.34

 

[bthielen]

Thank you. Just needed to confirm. I see I fat-fingered the 6.

 

[sgunsel]

That is correct but probably not the right answer.

Motors are generally rated at output. Since all motors are less than 100% efficient, and fractional HP motors may only be 50% or so efficient, the input kW (electrical power to be supplied that you are concerned about) will be more than the rated output in kW. I believe that Europeans only list motor output in kW and HP is not even allowed to be listed on the motor.

 

[steve66]

That is correct but probably not the right answer.

Motors are generally rated at output. Since all motors are less than 100% efficient, and fractional HP motors may only be 50% or so efficient, the input kW (electrical power to be supplied that you are concerned about) will be more than the rated output in kW. I believe that Europeans only list motor output in kW and HP is not even allowed to be listed on the motor.

I agree. Having a KW label makes it confusing - is the KW on the label an input power or an output power?

 

[Jraef]

Semantics.

In NEMA world where we live, the motor's mechanical output is called HP and the energy consumed is called kW. I like that it's nice and clean.

In IEC world, where this motor undoubtedly came from, the motor nameplate kW rating is the mechanical output rating, just the same as HP is for us. When referring to the consumed electrical energy, they call that the "absorbed power" so they can tell the difference because both values would be kW.

Seems confusing to us only because of our frame of reference.

 

[bthielen]

You are correct. This is a Siemens motor. If I understand correctly then, the nameplate rating is the output power rating and not necessarily input power. For some reason I thought it was the other way around but it makes sense since the motor is sized for the load it's driving and not the load it puts on the system.

 

[Jraef]

Yes you're right, we select motors mechanically, not electrically. Just like us, they don't put the "absorbed power" rating on the motor (i.e. we don't put the kW consumption on a NEMA motor). Too many other variables go into that rating.

 

[Open Neutral]

[rant]
I welcome anything that does away with foot-lbs, Smoots, BTU's, and such. Ever study a "Yellow Tag" on an air conditioner?

Every time someone grouses about EU standards, I recall NBS/now NIST was founded 100+ years ago, after the Baltimore Fire, to solve incompatible firehose threads; and they still haven't won the battle. Meanwhile I pull the IEC powercord out of Item 2 on the bench and plug it into Item 3. The same cord!

[/rant]
But dual labels on a motor, KW in/KW out, would get no gripes from me.

 

[Besoeker]

In NEMA world where we live, the motor's mechanical output is called HP and the energy consumed is called kW. I like that it's nice and clean.

No. kW is a measure of power, not energy.
And (yes, I'll get lambasted for this) power is power.
Having two different units (HP and kW) to express it is just plain daft.

 

[Besoeker]

But dual labels on a motor, KW in/KW out, would get no gripes from me.

Most of the cage induction motors I've seen give output power (kW) rated voltage, rated current, and power factor. From that, you can quite easily obtain the input power and efficiency should you want to.

 

[Jraef]

No. kW is a measure of power, not energy.

Oops, guilty of something I often correct others on.

And (yes, I'll get lambasted for this) power is power.
Having two different units (HP and kW) to express it is just plain daft.

Wait, didn't we get that from the Brits, and then THEY changed to match the rest of Europe?

Just sayin'...

 

[K8MHZ]

No. kW is a measure of power, not energy.
And (yes, I'll get lambasted for this) power is power.
Having two different units (HP and kW) to express it is just plain daft.

HP and kW are not the same at all.

Horsepower is a measurement of mechanical power, the amount of power needed to actually move a certain mass a certain distance in a certain amount of time.

kW is a product of the multiplication of volts and non-reactive amperes.

I find it interesting that we measure watt hours, but not horsepower hours.

 

[Besoeker]

Oops, guilty of something I often correct others on.

Wait, didn't we get that from the Brits, and then THEY changed to match the rest of Europe?

Just sayin'...

Hi Jraef

As you know I'm a very old fellow.
At school (secondary education up to age 17) we were taught physics in CGs and MKS units. Centimeter gram second andmMetre kg second. That was the better part of 50 years ago.
Of course I did Imperial before that and I can convert from one to the other.
For as long as I can remember, I have used SI in my professional career.
It's just simpler.

 

[Besoeker]

HP and kW are not the same at all.

Horsepower is a measurement of mechanical power, the amount of power needed to actually move a certain mass a certain distance in a certain amount of time.

kW is a product of the multiplication of volts and non-reactive amperes.

Power is power.
Most are familiar with the Watt as a measure of electrical power.
But it is equally applicable to mechanical power.
A force of one Newton over one metre in one second is one Watt.

 

[Jraef]

Ironically, the term "horsepower" was originally coined (invented?) by none other than James Watt. It was the electrical unit of power that was named after him. I don't know how or when the watt became synonymous with mechanical power, although I accept that it now is.

 

[John120/240]

Horsepower

Horsepower

The amount of energy used by a average horse to raise 33,000 lbs one foot in one minute.
Was it James Watt who figured out that 746 watts = 1 horse power?

 

[tallgirl]

The amount of energy used by a average horse to raise 33,000 lbs one foot in one minute.
Was it James Watt who figured out that 746 watts = 1 horse power?

Only if the horse is 100% efficient. Which it very likely isn't.

 

[Besoeker]

The amount of energy used by a average horse to raise 33,000 lbs one foot in one minute.

That's power, not energy.

Was it James Watt who figured out that 746 watts = 1 horse power?

I should have thought not.
Imperial units were still being used in schools here in UK when was going to school. Although I'm pretty old, that was still over 200 years after Watt would have. It was around 1965 that the UK started to adopt metric measures. There are still many people here opposed to it. The dislike is often stoked by the media, particularly the gutter press.
It's a bit of a shame in a way. Metric or SI (International System of Units) makes many calculations so much simpler.

Anyway, the conversion of HP to Watts is just a matter of the units.
One Watt is a force of one Newton through one metre in one second.
Nice numbers, you see.

Convert feet to metres, pounds to kilograms, throw in the gravitational constant to get force as opposed to mass and there you have it.

Of course, if you used only SI there would be no conversion to be done....

 

[Besoeker]

Only if the horse is 100% efficient. Which it very likely isn't.

The horsepower is based on the output power from a horse, so efficiency really doesn't come into it.