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What is the Difference Between Bkw & kw

03/14/2014 3:09 AM

What is the difference between Bkw & kw

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#1

Re: What is the difference between Bkw & kw

03/14/2014 3:38 AM

Bkw (brake kilowatts) is the power actually required to drive a piece of equipment. In this context, kw would be the power rating of the chosen driver. (Oftentimes the next standard size larger than the bkw.)

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#2
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Re: What is the difference between Bkw & kw

03/14/2014 3:47 AM

Not to be picky but would one not use "kW" as the correct unit for kilowatt?

the Bkw could be something completly different in another context. Or it means Billion kilowatts.

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#3
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Re: What is the difference between Bkw & kw

03/14/2014 4:02 AM

I just followed the OP's usage. In American English, the fully spelled units watt, ampere, and volt are not capitalized; the abbreviations are not always consistent. I doubt that the OP is asking about the difference between a billion kW and one kW.

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#4
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Re: What is the difference between Bkw & kw

03/14/2014 5:28 AM

Given the history of OP I would not be able to confirm this statement. But inconsistent use of units seems to be in fashion with OP so you might be right!

More context in the question would be helpful to give a compentent answer!

And then there is this http://wiki.answers.com/Q/What_is_the_difference_between_brake_kilowatt_and_kilowatt

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#5
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Re: What is the difference between Bkw & kw

03/14/2014 7:26 AM

If it's a billion kW better to say one terawatt, TW = 1012 watt.

Tornado is right in #1 about bkW is power actually required to drive a piece of equipment. As far as I can see, bkW (and bhp) aren't used much nowadays (this side of the pond, anyway), seems to cause confusion, just kW, and it's OK as long as it's clear what's being specified, e.g. absorbed power to drive a piece of equipment.

As in the wiki link, brake kW referred to a brake dynamometer used to measure output power when testing an engine etc. But I'm not sure about the wiki definition of bkW as gross power. If you were buying an engine you would want to be sure about the basis of the quoted power, but on test, whether the auxiliary loads are driven by the engine or independently, the measured shaft output is still bkW (or just kW).

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#19
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Re: What is the difference between Bkw & kw

03/17/2014 2:38 PM

Given these answers, suggestions.....is the OP an ex-wife?

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#9
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Re: What is the difference between Bkw & kw

03/15/2014 1:42 AM

SI units. W=watt

They are consistent if correctly used.

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#10
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Re: What is the difference between Bkw & kw

03/15/2014 1:45 AM

So is birth control.

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#18
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Re: What is the difference between Bkw & kw

03/17/2014 4:21 AM

1 billion watts = 1 Gigawatt, so 1 billionkw = 1000 Gigawatts

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#20
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Re: What is the difference between Bkw & kw

03/17/2014 3:28 PM

= 1 terawatt

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#27
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Re: What is the difference between Bkw & kw

02/10/2021 4:55 AM

What the hell is a Gigawatt?

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#29
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Re: What is the difference between Bkw & kw

02/10/2021 5:33 AM

You're replying to #18 where gigawatt is defined. What part don't you understand?

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#11
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Re: What is the difference between Bkw & kw

03/15/2014 7:54 AM

Assuming B means Brake and applies to an electric motor. Then that is the output of the motor at the shaft. The name comes from the 'brake' - a machine to measure power fed into the 'brake'.

As a small motor is about 80% efficient, you will need a 10kW motor to deliver 8 BkW.

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#12
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Re: What is the difference between Bkw & kw

03/15/2014 8:08 AM

No! The rated power of a motor is the shaft output power. So if it's rated 8 kW (which would be non-standard) it delivers 8 kW shaft power. If efficiency is 80% the electrical power input is 10 kW.

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#13
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Re: What is the difference between Bkw & kw

03/15/2014 10:01 AM

Isn't that what I said?

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#14
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Re: What is the difference between Bkw & kw

03/15/2014 11:10 AM

No, it isn't. A 10 kW motor gives 10 kW mechanical power. In selecting a motor to drive a given load the efficiency doesn't come into it, only into the cabling and the running costs.

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#15
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Re: What is the difference between Bkw & kw

03/15/2014 3:18 PM

I agree in the principle with what you explained. I appreciate you pointing it out. I should have been more careful in the example I used to explain what BkW was.

However, not a lot of people know the finer points of this. If it was common practice or nowledge then there would be no need for the recent 2009 MEP guidance.

In my old days a motor rated 10hp was expected to deliver 10bhp at the shaft. It was only when the rated kW from the label current and voltage was used did it become clear that there was an anomaly in a straight conversion from hp to kW.

It did not matter too much because engineers used the label current ratings in their calculations for cables and costings etc

Changing ratings to kW without reference to efficiency caused caused all sorts of problems because little was known about efficiency ratings - that nowadays appears to be a mandatory requirement on the label, at least it is if you wish to sell motors certified for a global market.

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#16
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Re: What is the difference between Bkw & kw

03/16/2014 2:18 PM

OK thanks. I'm not familiar with 2009 MEP guidance, can you direct me, it might be interesting!

Only other thing I can add is - in the very old days before the design was sharpened up, presumably to keep costs down, motors would give somewhat more than the nameplate power. I remember a time in early 70s when a rep from GEC (the British one) visited my then workplace. One of my colleagues bemoaned the fact that now there was no fat in the design, when previously if your kit needed slightly more power than a standard rating you could get away with the smaller size. The discussion got a bit heated and the rep said "Look, our motors meet the nameplate power, as required by the Trade Descriptions Act, but with little or nothing in hand. If your machine needs slightly more than a standard rating, fit the next bigger one, then you can't go wrong and we all know where we stand".

To complicate things, I believe (it's been a while since I was involved) a motor of a given power rating can be more than one frame size, depending on insulation class, allowable temperature rise etc.

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#17
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Re: What is the difference between Bkw & kw

03/16/2014 9:16 PM

I have never been directly involved in selecting motors.

Here is a link to MEP - but this is one of many.

<ttp://www.motorsystems.org/files/otherfiles/0000/0080/brunner_global_progress_12092011.pdf>

My early experience is largely linked to pioneering compressed air fridge drying. This involved working with small and medium sized compressors, after-coolers, water-cooler pumps, etc, where the motors were part of the original equipment.

And that is where it got a bit interesting. In order to size a dryer it is necessary to know the flow, pressure and temperature of the compressed air, and the ambient temperature range of the working environment that the system was exposed to.

Customers didn't usually know this. So I visited site and went over the system taking down all details from the nameplates on all the equipment, and then checking this off against manufacturers data sheets back in the office. This is where it became clear that there were many anomalies.

I can't vouch for today's practice because I have been retired for 14 years. But back then when hp was given on the nameplate, the power consumption was calculated from the named full-load current and voltage.

When kW started to appear on nameplates (and as you said, that is the output of the motor) it emerged the kW rating in the specs of a compressor was the figure used by managers to evaluate running costs (obviously being quite a lot lower than the true power when efficiency was factored in)

Further, some compressor manufacturers found they could exploit the lack of knowledge of efficiency margins and frame sizes, by 'publishing' lower 'headline' nominal kW ratings, then claiming their motors could deliver (say) 120% 'safe' full-load continuous working. At face value, a very good motor (plenty of fat)

The engineers sized cables and starters etc, on specified current ratings from data sheets, but usually did not bother to calculate power to compare it with the nameplate claimed kW.

As an example: In round figures for easy mental arithmetic:

Motor nameplate rating = 20kW. Continuous Over-load capacity = 24kW. Efficiency = 80%. Power consumption = 30kW. Actual cost = £30kWhr whereas nameplate cost at face value is £20kWhr.

Competitive model nameplate rating = 24kW. Same efficiency same running cost = £30kWhr whereas nameplate cost at face value is £24kWhr.

With a choice of one at £4kWhr cheaper to run, which one would you buy?

Hopefully, the nameplate requirements coming into force will prevent this sort of thing happening. Perhaps it has. I don't know. I am not involved anymo

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#21
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Re: What is the difference between Bkw & kw

03/18/2014 10:47 AM

Interesting that when hp was given on the nameplate, the power consumption was calculated from the named full-load current and voltage (giving electrical power at full load), as they were probably overestimating costs, since motor is unlikely to be operating at rated power, as the compressor power is sure to fall between 2 standard motor ratings, the higher one being used. If they had just converted motor nameplate hp to kW it probably wouldn't be far out, on the assumption that the % load roughly = efficiency, and giving a better figure for sales purposes.

As you say, if they started using compressor input kW that was an underestimate, by ignoring any drive losses (gearbox, V-drive etc) as well as motor efficiency.

Your point about publishing lower 'headline' nominal kW ratings, then claiming their motors could deliver (say) 120% 'safe' full-load continuous working. At face value, a very good motor (plenty of fat) - this seems OK to me. Can make this claim due to the margin between motor rating and compressor power (how much margin depends where compressor power falls between standard motors). It's making a virtue out of necessity, but still a selling point.

Your point about Motor nameplate rating = 20kW. Continuous Over-load capacity = 24kW. Efficiency = 80%. Power consumption = 30kW. Actual cost = £30kWhr whereas nameplate cost at face value is £20kWhr.

Competitive model nameplate rating = 24kW. Same efficiency same running cost = £30kWhr whereas nameplate cost at face value is £24kWhr.

Sounds like you mean compressor power = 24kW in both cases. Clearly first one isn't telling the truth, but I don't think I've heard of e.g. nameplate rating = 20kW. Continuous Over-load capacity = 24kW though I could be out of touch. And they're both under-reporting the actual power costs based on 30 kW electrical. Be good to think the customer made sure he was getting the full facts from each to ensure a fair comparison!

Another point is that on some kit, like power tools, domestic appliances etc, the nameplate power is electrical input, which is fair enough as with an in-built motor the question of matching motor output power to mechanical demand doesn't arise (for the end user) and the important thing is the electrical requirements.

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#22
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Re: What is the difference between Bkw & kw

03/18/2014 8:33 PM

I doubt if you are out of touch. But I could be. Please bear in mind that I used a composite anecdotal situation I came across a few times in my work. I used exaggerated round figures as an hypothetical example to demonstrate how to gain a commercial advantage over competitors by using motor ratings that relied on a weakness in the customers knowledge of power needed to make compressed air.

Yes, you are right, 24kW refers to the load on the motor caused by the compressors that were both the same size (typically 120 scfm at 100 psig). One had a motor with a 24kW nameplate, and the other obtained 24Kw continuously from a motor with a 20kW nameplate. Both consume 30kW when running at full load.

No one is lying. These are verifiable facts in the sales literature and data sheets. Full-load current and voltage is stated but not cross-referenced to motor power. The customer does a face-value load cost comparison based on motor 'size', and a compressor with a 20kW motor is cheaper to run. They place an order.

The fact that they should have based running costs on 30kW is somewhat academic. The compressor is installed and doing it's job. The cables and switchgear are correctly sized. And it would have cost them the same to run them anyway whichever compressor was bought. Except they might have paid a premium for the more efficient compressor with a 'smaller' motor.

If you are in the 'business' perhaps you can tell me if this sort of thing would be possible with MEP labels.

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#24
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Re: What is the difference between Bkw & kw

03/19/2014 6:52 AM

Interesting discussion, but I think we've just about kicked this to death!

Opened your MEP link now (wouldn't open the other day) and it's interesting, but from a quick read I don't think it tightens up things re power figures quoted to customers. Still up to the buyer to ensure he's making a like-for-like comparison.

I see what you mean in your example - no one is lying, it's more a case of the customer deceiving himself, or making a naïve judgement.

24 kW shaft power certainly checks out, I make it that 24 kW at about 63% compressor efficiency. Am I right thinking it's a combined unit, rather than a bare-shaft compressor with an off the shelf motor? The nonstandard powers (20 and 24 kW) suggest it is.

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#25
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Re: What is the difference between Bkw & kw

03/19/2014 8:47 AM

These were the early days of oil-flooded screw compressor packages coming on to the market where hp loads were generally higher than recips for the same flow. They were oil pumps as well. So I guess sales gimmicks were introduced to make the face value of screws more attractive.

Early screws were usually belt-driven by 'standard' motors, but eventually evolved to direct drive and sometimes with frame castings dedicated to mate with the particular compressors.

As you say we've got some mileage out of this aspect. It was a passing phase in my life, and fairly soon the 'losing-sales' compressor boys caught on to these tricks.

Interestingly though, when I visited existing sites and discussed motor sizes in connection with air flows in order to size a dryer, it sometimes emerged that this was the first the customer knew of the power anomalies, and on a couple of occasions, I got a bit of stick for telling them. Folk are funny.

Thank you for taking the trouble to exchange thoughts on this.

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#26
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Re: What is the difference between Bkw & kw

03/19/2014 11:03 AM

No problem! I'm sure we've both learned something.

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#6

Re: What is the Difference Between Bkw & kw

03/14/2014 10:40 AM

Try this, type these elementary questions into a search engine FIRST.

How to Search the Internet - Hannon Library

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#7

Re: What is the Difference Between Bkw & kw

03/14/2014 3:14 PM

The letter "B".

Somebody had to say it.

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#8
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Re: What is the Difference Between Bkw & kw

03/14/2014 4:31 PM

You said it, and very succinctly I might add.

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#23
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Re: What is the Difference Between Bkw & kw

03/19/2014 1:13 AM

The difference is not B....

Bkw-kw

=(B-1)kw

Just saying.

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Re: What is the Difference Between Bkw & kw

02/10/2021 4:58 AM
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