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HP versus Brake Horsepower

HP versus Brake Horsepower

HP versus Brake Horsepower

I seem to be the only one in my company that believes that BHP is the power available to the system before losses due to appurtenances, e.g. a belt drive for a fan. For simplicity sake,(I may not be using an accurate analogy) I suggest that the BHP is the NEMA or nominal horsepower. For example, Greenheck on their cut sheets for fans shows the "Operating Power" then below in the section that tells you the motor size 10 hp or 5 hp . . .
I talked to an EE friend of mine and he told me that he checked a couple of other sources and he was not sure that he agreed with me.
These are a couple of websites that I used in my discussion;
Seems pretty clear to me.
The term BHP comes from the early days of measurement where they actually would apply a brake to the output shaft and calculate HP from the force on the brake.
I could always not be accurate, or the other case is that the term depends on context of usage . . .

RE: HP versus Brake Horsepower

Lead Engineer,

If you want to test the mechanical power something puts out, you use a brake, hence, brake horsepower. Does someone telling you the output of something, have to use the word "brake"?

Quote (Your second source)

Today, you can easily convert HP into different units, like 1 HP that equates to 746 Watts. It can also be converted into British Thermal Units, or BTU, joules and calories.

Watts are okay. Everything else, no.

Back in the day, car engines were rated by gross horsepower and net horsepower. This reflected the amount of stuff attached to the engine.


RE: HP versus Brake Horsepower

It is not about the units of measurement.
Maybe I made the question too complicated. Other engineers in my company are saying that BHP is the power to operate the system, and that HP is the nominal or motor size. And I do not agree. The context is for electrical motors that are driving a mechanical system.
Should I just agree to end the discussion, or just agree to disagree? I have had this discussion with more than a few engineers.

RE: HP versus Brake Horsepower

You are all wrong.

It is about the method of measurement.

Brake horsepower is the horsepower available at a shaft as measured by a brake dynamometer.

The shaft has no consumer attached during the measurement.

RE: HP versus Brake Horsepower

Wellll with electric motors, BHP and net HP are the same. There aren't any accessories between the system and the motor to absorb power. Power delivered by the motor is power delivered into the system.

With a mechanical system being driven by a motor of any kind, I would specify the power required for the system to operate, in HP.

using 'BHP' at all is pretty much reserved for engines in motor vehicles, where there is a need to differentiate between total power output by the engine, and power put into the drivetrain (engine power output - power consumed by engine-driven accessories).

RE: HP versus Brake Horsepower

This conversation sounds like there is no one answer, it sounds like the context of usage defines the meaning. That said, we specify a lot of fans for various systems and Greenheck when you use their sizing program the cut sheet for the given fan has two fields for horsepower; operating horsepower and HP which is the motor horsepower 5, 10, or what have you.

I am hearing that I should just drop the subject because there are too many ways to interpret these two terms; i.e. BHP and HP.

RE: HP versus Brake Horsepower


I am trying to recall all the stuff I did in college thermodynamics. Indicated horsepower is the work done to the pistons inside the cylinder. Friction horsepower is the rate of work required to overcome friction in the engine. Brake horsepower is the power you measured at the output, with a brake. The OP is discussing electric motors.

IHP and BHP are meaningful terms if we are testing an instrumented piston engine on a dynamometer. Is there a standard out there that absolutely requires motor hardware to be attached or not attached when you test for "BHP"?

My experience with electric motors is small ones, rated in Watts. I have never seen the term BW or BHP used for them. You can test power output of an electric motor with a brake. I have never seen the electrical power input rated in horsepower. To me, the terminology is power in(put) and power out(put).

If the electric motor is attached to some sort of inefficient drive like a worm gear, then you have power out of the motor, and power out of the drive. I can test either one with a brake.

HP and BHP probably are the same thing.


RE: HP versus Brake Horsepower

Quote (Lead Engineer)

the cut sheet for the given fan has two fields for horsepower; operating horsepower and HP which is the motor horsepower

'Operating Horsepower' = the power the system requires to operate

'Horsepower' = the maximum amount of power the motor is capable of producing

This isn't really that complicated.

RE: HP versus Brake Horsepower

Quote (Lead Engineer)

Other engineers in my company are saying that BHP is the power to operate the system, and that HP is the nominal or motor size.

You and your other engineers are confusing yourself with this terminology.

Forget the "horse" and Brake Horse" bit and what are you left with - POWER.

Now in a motor driven system you can measure that power in several locations from start to finish and the figures will all be different. There is though no defined location where the term "Brake Horse" is universally accepted.

Hence for something like and electrically driven pump set, you have incoming electrical power (Pe). Then you have outgoing motor or shaft power (Ps). Then finally you have liquid power of the fluid you're moving (Pl) Pe>Ps>Pl.

How much will vary dependant on the inefficiencies of the various parts of the system.

The term BHP came about predominantly from the automotive industry where various fiscal and simple calculations were made on engine size etc to arrive at a fiscal or nominal horsepower. E.g. in France the classic 2CV (2 Cheval Fiscal).

Now anyone unfortunate enough to ever drive or be driven in one of these crates on wheels will know that they are not over endowed with real power, but they have more than 2HP (1.5kW). Hence to act as a term of "real" power from a car engine, people then started using the term Brake horsepower meaning "real" power delivered at the engine shaft output. Put any car on a rolling road and measure power transmitted to the road after going through gearboxes, having power sucked out by A/C, hydraulic pumps, alternators etc and it will be quite a lot less. However it is difficult to measure this so the term BHP has become synonymous with engine power / speed / acceleration.

SO in short forget BHP and HP and just say motor shaft power or shaft power or fan output power or wherever you want to define your power measurement location.
[Added] Electric motor power rating is normally quoted as shaft power coming out of the motor with its fan etc.
Electric power in is higher and is commonly expressed in terms of Full load current and its power factor

Or just use kW and a specific location within the system. There's no such thing as BkW....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: HP versus Brake Horsepower

Quote (drawoh)

Is there a standard out there that absolutely requires motor hardware to be attached or not attached when you test for "BHP"?

Sure are. Standards for whatever you want. That's the great thing about standards, there is always one for whatever.

Wiki does a good job of explaining. https://en.wikipedia.org/wiki/Horsepower#Brake_hor...

Key points:

"Brake Horse Power"

"Horse Power" is the unit.

"Brake" is the method of test, specifying how and where. With a brake dynamometer at the output shaft of whatever you are testing.

There is nothing explicit nor implicit in the definition of "Brake Horse Power" about the unit under test.

Multiple other standards exist that define the required condition of the unit under test.

Stating "345 BHP" without citing the test standard used tells you exactly nothing.

RE: HP versus Brake Horsepower

I believe the HP rating of an electric motor is the power available at the shaft, under conditions defined by NEMA or ASME or somebody, and the kW rating of the motor would be the HP X a factor.
And vise-reversa.

Th electrical power (kW) that must be supplied to the motor is not the motor's power rating, although is of interest to the folks paying the electric Bill.
The equivalent for purchasing an engine would be the Brake specific fuel consumption, which would be multiplied times the out put HP.
BSFC varies a lot on the engines load.

RE: HP versus Brake Horsepower

kW = hp x 0.746

The electrical input is of interest to the electrical engineer to design cables, starters, switchboards etc.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: HP versus Brake Horsepower

I want to thank all of you who replied to my question. I did not expect such a variety of responses.
What I am hearing is that the difference is more or less moot because of the variety of contexts in which these terms are used.
This all started when one of my engineers used BHP on a drawing and the information in the field was operating or operational power. So to be more clear, I think that I will change the column titles to: "MOTOR SIZE" and"OPERATING HORSEPOWER" so that there is no question about the information contained in each field.
Thank you all very much for your help 😊.

RE: HP versus Brake Horsepower

'Motor Size' could very easily be confused with 'Frame Size' which defines the physical dimensions of the motor, usually corresponding to one of a number of standard 'frame sizes'.

A more technically correct pair of descriptions would be "Motor Rating", or even more accurately "Motor Capability", and "Fan Absorbed Power".

Note that 'rating' is the motor capability at a specific set of conditions, typically sea level and at 40°C ambient, where capability is the power output the motor can deliver under a given set of conditions which may not be those at which its rating is stated.

RE: HP versus Brake Horsepower

"Should I just agree to end the discussion, or just agree to disagree? I have had this discussion with more than a few engineers."

I find this statement a little worrisome. Engineers do not agree to disagree. They should develop a common understanding. Your discussion does involve some semantics which can be unclear so an engineer's job is to understand the science behind an issue and use scientific terms to describe it so that everyone can understand unambiguously. Asking your question here was one step in that process.

I see that you are a mechanical engineer. I've noticed that new mechanical engineers often do not understand electric motors very well. There are many types, and it can get complicated. But most industrial electric motors are induction motors which run at almost constant speed regardless of load. A five hp motor can provide up to five hp continuously without overheating, when the ambient temperature is 40C. But if the load only uses two hp then the motor will only provide two hp, and the motor will consume only that much electrical power. This defines what five hp means with regard to an electric motor. You need to define every term you use in similar ways so that you can reach a common understanding with your coworkers, customers and suppliers. And none of this "agree to disagree" unless you are talking politics or religion (or auto mechanics).

RE: HP versus Brake Horsepower

I find your discussion interesting. However, when the other engineers I talk to are unwilling to discuss it, and simply say parenthetically; this is not a discussion. The electrical engineer that I work with has very little understanding of this topic. As for the difference between electric motors and any other motor for the use of Brake horsepower versus Horsepower, is it not just power being supplied to a system? Thus the power input to that system (according to most definitions I have read) is BHP, and the power to operate the system is HP. How does the fact that the power is coming from an electric motor affect the name of the gross power input to the system, i.e. BHP vs. HP?

RE: HP versus Brake Horsepower

Lead Engineer,

I am not sure what you mean by power input. I can put electrical power into an electric motor, almost certainly quoted in Watts. This cannot be measured with a brake. I get power out of motor as torque and speed, which I can work out as Watts or horsepower. I have never seen the word "brake" to describe power out of an electric motor. I can do power into a transmission and power out of a transmission, which is meaningful if the drive efficiency is significantly below 100%. If it were me doing the specifications and/or analysis, it would be electrical power input, motor power, and output power. The word "brake" tells me nothing useful.


RE: HP versus Brake Horsepower

Quote (LeadEngineer)

How does the fact that the power is coming from an electric motor affect the name of the gross power input to the system, i.e. BHP vs. HP?

Because you should never see 'BHP' on the spec sheet of an electric motor. Electric motors have no accessories. BHP is/was a term developed to determine whether a gasoline engine's power rating was measure with or without accessories.

You MIGHT see 'BHP' on the spec sheet of something like fan or pump which is designed to be driven by a motor of any type, in which case the designer is trying to tell you exactly how much power that device needs to consume to operate as designed. But in my estimation this circumstance would be rare, because for such a device, the meaning of a power rating is pretty clear.

As another poster stated- you can measure power at any point in a mechanical system of any complexity, and each point will yield a different value. When stating power values you need specificity.

RE: HP versus Brake Horsepower

Okay, so what do you call the power available to any system? What do you call the power required to operate any system? As mentioned earlier in this thread, Greenheck on their spec sheets have HP and Operating HP. My understanding of this is one term tells me the motor size, and the other term tells me how much power I need to operate the system at the noted conditions.

RE: HP versus Brake Horsepower

Process engineers use the term Absorbed Power to indicate the power absorbed by a fluid (a liquid in a pump or gas in a compressor or the air induced through a fan) as opposed to Driver Rated Power (or driver ISO power in the case of a gas turbine or gas engine).

RE: HP versus Brake Horsepower

Thank you all for all of your suggestions on how to explain my situation to other engineers in the company. I will take what has been posted and think about how to have that discussion.

RE: HP versus Brake Horsepower

Lead Engineer, the screen shot you provided contains some words and numbers taken out of context. I, as an experienced engineer, can't even provide a decent guess as to what the numbers mean. It is possible that some engineer who is familiar with the catalog might give some insight, but those terms and numbers mean nothing out of context and it is pointless to argue about it with anyone. Those terms musts be defined somewhere in the catalog. Realize that catalogs are often put together by non-technical people. You need to know enough to know when you do not have enough information.

RE: HP versus Brake Horsepower

Now you are getting it, you can see the problem I am dealing with. The people that I am speaking with are not experts in this area, one is from power plant support engineering, the other is a "rain-maker" for a company.

RE: HP versus Brake Horsepower

Lead Engineer - yr post of 6 Mar 21:25 I think got it just about right.

As said before forget the Brake bit and just think about where it is and what it is.

So in your thinking HP would be motor rated shaft power in hp or kW
and BHP would be fan or equipment required or absorbed shaft power or maybe inlet shaft power.

however I'm with composite pro - that little snippet you sent makes no sense.
The 263 (BHP) I would have taken as motor rated shaft output power, but what 300/36 is I have no idea. I thought for a moment it was electrical power and amps, but I was way off.

Incidentally you do need about 250A for a motor of this size and voltage so you're going to have cables the size of your arm going into it....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

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