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I'm trying to understand how much heat is generated when a given amount of electric power is drawn. And from what I understand, it's pretty much 1 to 1 isn't it?

Example: If I have a 1 HP motor running at 50% load and so pulling about 0.5HP, then isn't the amount of heat coming from the coils equivalent to 0.5HP of heat?

I draw this conclusion based largely on the fact that the only real power used in the motor is supposed to be resistive power. The reactive part produces no heat, nor does it do any mechanical work. Well if I had an Ohmic resistor allowing 1KW of power to flow, then I'm essentially generating 1KW worth of heat aren't I? Granted, not all of the heat is dissipated in the resistor itself. Some of it is dissipated in the various transmission lines and components delivering the power.

Is my understanding correct?

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  • \$\begingroup\$ Not always from the coils. Some comes from friction, and some from phonons. \$\endgroup\$ – Ignacio Vazquez-Abrams Oct 22 '15 at 2:33
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    \$\begingroup\$ More or less. Generally very close to 100 percent of the electric power delivered to any electric device is converted to heat either in the device or in the immediate vicinity of the device through other processes such as mechanical friction heating radiation heating, etc. If you have two wires delivering 300 Watts to a well insulated room, you can be fairly sure that all of that power is contributing to the heating of the room. This will work just as well as a 300 Watt heating coil for the purpose of heating the room. \$\endgroup\$ – mkeith Oct 22 '15 at 2:35
  • \$\begingroup\$ "If I have a 1 HP motor running at 50% load and so pulling about 0.5HP, then isn't the amount of heat coming from the coils equivalent to 0.5HP of heat?" No. If the load is 0.5hp and the motor is pulling 0.5hp then it must be 100% efficient, so it will not be producing any heat. \$\endgroup\$ – Bruce Abbott Oct 22 '15 at 4:05
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    \$\begingroup\$ @mkeith: Doesn't a motor driving, say, a water pump lifting water into a storage pond, convert electrical energy into potential energy, with only a small fraction lost as heat in the motor? \$\endgroup\$ – DJohnM Oct 22 '15 at 7:44
  • \$\begingroup\$ @DJohnM, yes, and it is probably possible to come up with other examples. A battery charger, for example. Anything that converts electrical energy to potential energy which is then consumed outside of the room. \$\endgroup\$ – mkeith Oct 22 '15 at 16:04
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Well if I had an Ohmic resistor allowing 1KW of power to flow, then I'm essentially generating 1KW worth of heat aren't I? Granted, not all of the heat is dissipated in the resistor itself. Some of it is dissipated in the various transmission lines and components delivering the power. Is my understanding correct?

Yes, sir, you are indeed correct.

Unless we account for power loss due to LED illumination or other minor physical details, all energy is dissipated as heat.

That would be the sum of these particular motor loss mechanisms:

1) Your load resistor loss

2) Motor windings (stator copper loss)

3) Friction (drag, viscous) torque

4) Static torque

5) Magnetic core losses caused by the non-zero width of the BH curve.

6) Motor cable resistive loss

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