kwh

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Mike01

Senior Member
Location
MidWest
What is the best way to calculate kwh for a motor installation? I was just trying to figure out (roughy) what it would cost to run a pump with a break HP rating of 36, if the HP if a 50HP motor but the break HP is limited to 36 do you calculate the break HP for the kwh?
 

__dan

Senior Member
dimensional analysis

dimensional analysis

dimensional analysis

Not sure what you mean by "break" HP.

If the motor is installed I would measure amps running draw, if not I would use nameplate amps running. Pump loads are usually sized pretty close to nameplate.

Then dimensional analysis, carry the unit dimensions with the numbers.

kWH = runtime (hours) x kW

dollar cost ( $ ) = kWH x $ per kWH, example: 100 kWH x .12 $ / kWH = $12.
 

mivey

Senior Member
break HP has to do with pump and drive inefficiency.

The power added to water = gal/min x dynamic head ft / 3960 = water HP (WHP)

BHP = WHP / pump efficiency / drive efficiency.

If I understand your post, you will be operating a 50 HP motor with a load requirement of 36 HP, correct?
 

mivey

Senior Member
As for cost:
kW = 36 x 0.746 / motor efficiency / VFD efficiency
kWh = kW x # run hours

Cost = kW x $/kW + kWh x $/kWh
 

Mike01

Senior Member
Location
MidWest
ok so...

ok so...

kw=46(bhp) * 0.746 / 0.98 ≈35.0kw
kwh=35.0 * 24 =840kwh per day
Cost = 840.0 * $per kwh
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
Yes, I think you should use the break HP, not the motor HP. Although, you might want to add a little extra to the break HP to account for efficency of the motor and drive. (Maybe 10%, or about 4 HP for a rough guess?)

Motors always have to be sized larger than the load they are connected to (otherwise, we would be overloading the motor.) So a motor doesn't usually draw its full rated load.

And if you simply measure running amps, your estimate will be a little too large, because you aren't taking power factor into account.
 

gar

Senior Member
091002-1043 EST

steve66:

Your statement on sizing a motor is a good general statement. My following comments are not in disagreement with your statement, but to broaden the scope of thinking in special cases.

There are cases where a motor is sized smaller than its continuous rating.

Charles Kettering understood that one could greatly overload a motor for a short time without damaging the motor. Two examples from Kettering's work. The electric cash register, and the automotive starter. Both were the same concept.

I can not quickly find a good reference on the "why" part of the history of Kettering's overloading concept. But following are some references that say he did it.

Essentially the experts said you could not practically use an electric motor to run a cash register because of the torque requirement, and thus the horsepower and size. Kettering realized that the torque was only required momentarily, and that a motor (probably DC initially or series AC) had a very high torque capability, and when run for a short time with a low duty cycle the thermal mass of the motor components would prevent over heating.

See:
http://wiki.gmnext.com/wiki/index.php/Kettering,_Charles_F.#
http://www.absoluteastronomy.com/topics/Charles_Kettering
http://books.google.com/books?id=N4...ge&q=kettering national cash register&f=false

Today most CNC machines are designed such that the spindle motor can be overloaded to about 200% for a short time. I believe a typical value at 150% is 20 to 30 minutes.

.
 

Besoeker

Senior Member
Location
UK
dimensional analysis

Not sure what you mean by "break" HP.
I think he means brake horse power.
The term bhp for some reason is still used for IC engines and a few other applications.
It's what some of us old timers and of those in the colonies would simply term horespower.
:grin:
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
I only see "brake horsepower" so infrequently that I didn't even notice the misspelling. Most cutsheets for fans and air handlers list the actual brake hp along with the motor hp, and that's about the only place I usually see the term.

It's what some of us old timers and of those in the colonies would simply term horespower.
:grin:

I sure hope you meant horsepower. :) The way you spelled it could be mistaken for something completely different. :)
 
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