electric motor heat gains
electric motor heat gains
(OP)
ASHRAE Fundamentals has a chart that says that an electric motor driving a fan generates as much heat as the the total power rating of the motor plus the inefficiency of the motor (assuming the fan and the motor are both in the conditioned space). This seems to negate the energy balance that says if i'm running a fan, most of the energy is moving air, not generating heat? The table states that i can generate 83 HP (212,000 btu/h) from a 75 HP motor?
Am i not understanding the table, or do i not understand the physics?
i.e .. i would think that if i am running at motor full load, my heat genrated would be:
Q = hp(1-motor eff.)
Am i not understanding the table, or do i not understand the physics?
i.e .. i would think that if i am running at motor full load, my heat genrated would be:
Q = hp(1-motor eff.)





RE: electric motor heat gains
Motor efficiency is less than 100%, so electricity in must be greater than work out. This difference is converted into heat by the motor.
As soon as the air leaves the fan, it starts to decellerate due to interaction with the air in the room. This kinetic energy is converted to heat.
Thus, all of the work done to the air, plus the motor inefficiency eventually gets turned to heat.
RE: electric motor heat gains
I don't have access to the ASHRAE standards. Anyway, my assumption would be:
The energy delivered to a fan is totally converted into heat for air conditioning estimation. Thus we should add to a fan with an energy input of 75 hp, the efficiency loss of motor and transmission, which, when taken at 90%, result in
The conventional U.S. conversion being 2544 Btu/h per hp.
Am I on the right track ?
RE: electric motor heat gains
THANKS AGAIN
RE: electric motor heat gains
Btu per (hp)(hr) motor loses
Motor hp (1) (2) (3)
1/8 to 1/2 4,250 2,545 1,700
1/2 to 3 3,700 2,545 1,500
3 to 20 2,950 2,545 400
Load in refr space(1) Outside refr space (2)Load outside(3)
1. For use for when both useful output and motor losses are dissipated within refrigerated space; motors driving fans for forced circulation unit coolers.
2. For use when motor losses are dissipated outside refrigerated space and useful work of motor is expended within refrigerated space; pump on a circulating brine or chilled water system, fan motor outside refrigerated space driving fan circulating air within refrigerated space.
3. For use when motor heat losses are dissipated within refrigerated space and useful work expended outside of refrigerated space; motor in refrigerated space driving pump or fan located
RE: electric motor heat gains
Many air handlers incorporate filters, cooling coils and silencers and therefore require typically a 1000Pa to simply move the air through them plus an additional 300Pa or so to cater for the external ductwork. So a rise of 1.3 degrees is not uncommon.
Friar Tuck of Sherwood
RE: electric motor heat gains
i have a question related to this one... i'm trying to figure out the heat emitted by an engine and a motor in a room that must be air conditioned. tell me if i am on the right track:
the heat energy per unit time emitted by the motor is equal to the electric consumption minus the output hp (since all friction losses etc. generate heat).
the heat energy per unit time emitted by the engine is equal to the input energy from the fuel (taking into account the combustion efficiency) minus the output power (which is the rated output for the engine, right?)
i'm not sure how to account for the energy lost as sound.
i'm also pretty sure that ASHRAE has some standard for industrial equipment loads. can anyone point me towards the relevant standards?
thanks!
RE: electric motor heat gains
Regarding energy as noise, this also becomes heat load as the building muffles the sound. The sound power level will not be significant anyway; consider a 500W car stereo making more noise than a sizeable diesel engine.