Motor Rated Watt: Efficiency!?

[Claves]

I was thinking!?

Say you have a 5000w 45H motor you load it with 4500w (72v 62-63A)

Now instead you have a 1500w 30H motor and loaded with the same 4500w. (72v)

The smaller motor will get hot with that load.
The bigger will not.

The heat that is generated on the smaller motor. Will that be gained as efficiency on the bigger 5000w motor?

Am i crazy for asking...

 

[E-HP]

The smaller motor will get hot with that load.
The bigger will not.

Efficiency is determined by the physical characteristics of the motor (windings, stator laminations, etc.).
If the two motors have the same efficiency, say 85%, then they both create the same amount of heat under the same load, but the higher rated motor is able to dissipate more heat continuously. The 5000w motor can dissipate 750w of heat continuously while the 1500w motor can dissipate 225w of heat continuously. There isn’t a gain in efficiency, but there’s a loss of efficiency when a motor heats up, which in turn creates even more heat for the smaller motor.

 

[Claves]

Efficiency is determined by the physical characteristics of the motor (windings, stator laminations, etc.).
If the two motors have the same efficiency, say 85%, then they both create the same amount of heat under the same load, but the higher rated motor is able to dissipate more heat continuously. The 5000w motor can dissipate 750w of heat continuously while the 1500w motor can dissipate 225w of heat continuously. There isn’t a gain in efficiency, but there’s a loss of efficiency when a motor heats up, which in turn creates even more heat for the smaller motor.

Thanks, i see my brainmelt. Same efficiency but the 5000w will be more stable in its delivery the more heat is introduced.

So then the next question to that is:

How much would you lose in efficiency on a overheated motor? How permanent is the damage?

 

[neptronix]

Comparing two motors, one rated at 1.5kw, the other rated 5kw, at 5000w of load:

The smaller motor will make more heat than torque from a standstill, and have significantly lower efficiency once up to speed.
The bigger motor can shed more heat, but won't make as much at this same watt level, and will produce substantially more torque from a standstill on the same power thanks to having an adequate amount of copper and magnet to handle the load.

On the smaller motor you are stressing the copper windings and they do not have as much magnet to produce power against, so instead they make more heat.

Here's a close example that can demonstrate this:

~4kw rated motor climbing 7% hill at 2kw at 82.1% efficiency ( won't overheat )
Motor Simulator - Tools

~1-1.25kw rated motor climbing 7% hill at 2kw at 74.6% efficiency ( will overheat )
Motor Simulator - Tools

Here's both motors doing 38.6mph. The ~5% difference means the small motor uses another 100w
Motor Simulator - Tools

Picture of the above:



These differences get exaggerated the more kilowatts we add.