Electrical energy converted to heat

[tae]

I am trying to understand how much electrical energy is converted to heat in a building, from motors moving items horizontally and vertically all contained within the building. I had assumed that the only heat released was from the inefficiency in the motor, however further reading suggests that all the electrical energy used by the motors is converted heat through various processes (friction etc.), but this does not sit well with me as work is being done.

Can anyone explain how much of this energy which is used for work (moving items) ends up as heat please?

[dvs_dave]

I imagine you’d be able to look up the efficiency info on the motor spec sheets. You will need to know the power consumption and speed, and there should be a corresponding efficiency rating for that point.

[tae]

dvs_dave said:

I imagine you’d be able to look up the efficiency info on the motor spec sheets. You will need to know the power consumption and speed, and there should be a corresponding efficiency rating for that point.

Yes the efficiency rating of the motors are available. Let's assume 20% inefficient, what happens to the remaining 80% of the electrical energy. Does this also ultimately all end up as heat as the whole system is within the building?

[Mr Pointy]

tae said:

Yes the efficiency rating of the motors are available. Let's assume 20% inefficient, what happens to the remaining 80% of the electrical energy. Does this also ultimately all end up as heat as the whole system is within the building?

Yes, apart from any items that have been lifted vertically as they have greater potential energy.

[tae]

Mr Pointy said:

Yes, apart from any items that have been lifted vertically as they have greater potential energy.

Thanks for this, I was slowly working towards this as a conclusion.

[Mr Pointy]

When building TV studios it was always concerning to calculate how many megawatts of power input were required to output 1V signal into a 75 ohm load.

[spikeyhead]

Once the item moved has stopped, then unless it has gained height, and thus gained potential energy, then all of it will end up as heat.

[ruggedscotty]

There is quite a bit to consider as with all physics its not just so simple.

an electric motor uses electrical energy to generate motion. - a 10kW motor wont take 10kW from the supply unless there it is supplying 10kW of mechanical load and then with motor losses you have to consider that you wont get 10kW mechanical energy out due to those losses.

best explanation I was given, this 10kW motor is being used as a fan. it is a 4 pole motor 3ph 400v being used to spin the fan, this motor has a speed of 1500rpm. the actual speed of the fan will vary on the load that is placed on it. This load will change the speed of the motor and as the motor speed changes it will draw more current from the mains up to its rated power. Most motors have a fan on the end to draw air in and blow it out to cool the motor, in some cases with larger motors there may be an additional fan installed to increase the airflow, and you may have water cooling used in cases where the motor produces a lot of heat and there is little cooling available. If you have connected the motor to an invertor to provide speed control of the motor then this invertor will be producing heat as the electronics operate.

remember power equals volts x amps. P(w) = V x A

the cables that supply the motor ? they will have a resistance and this will produce a volt drop, so your cables will have a power loss and that will be in heat, electricians spend a lot of time making sure that the cables are correctly sized to reduce this loss and to minimise cable heating.

hope your starting to see the amount of heat being produced starts to add up. trying to understand how you are viewing this and where you want to go with your question. the heat being produced will be dissipated and it wont raise considerably as its being taken away. stop that natural convection and bleed away then you could end up with some really hot wiring / motor etc.

Does that help ?

[anonymous-user]

ignore the motor, ask "why does it take power to do things"?


Take a completely horizontal conveyor belt, moving objects along it's length, and immagine that an object starts at zero speed and ends at zero speed.

Where has the energy gone?

It should be obvious, the answer is friction. rolling frictions, aerodynamic friction, sliding friction, all kinds of friction. If there was no friction, then your conveyor you not need any power to work!

Immagine a machine with a big whirling cutter blade that is chopping up potatoes into crisps. Why does this need power? Because, yup, friction. Driving the cutter through the potato is not a zero friction process.

[tae]

Thanks all for your contributions. At first I assumed that the energy ended up as work (movement) and heat. But slowly it dawned that for horizontal motion ALL the energy ultimately becomes heat, I think the only other energy remaining is the potential energy of item moved vertically up.

[NNH]

There are a few other energy conversions, but they are pretty trivial. Some energy will escape from the building as light. I wonder how little of the energy used in a car factory exits stored in compressed springs and fully charged batteries!