Efficiency of an electric motor

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Discussion Overview

The discussion revolves around the efficiency of electric motors, particularly in relation to load conditions, work input, and power calculations. Participants explore theoretical and practical aspects of motor efficiency, including its behavior under different loading scenarios and the relationship between work and power.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • One participant suggests that efficiency remains constant as long as voltage and vertical height are constant, while others claim efficiency typically increases with load.
  • Another participant notes that many motor manufacturers provide efficiency vs load specifications, indicating a common understanding that efficiency can vary with load.
  • A participant presents a formula for efficiency using work input and output, questioning why efficiency appears constant when considering mass and time, yet efficiency seems to increase with torque.
  • Some participants argue that work input and output are largely independent, driven by different factors, and suggest using power instead of work for efficiency calculations.
  • Questions arise regarding the efficiency of the motor at zero load and at stall conditions, with participants expressing uncertainty about how to calculate power using RPM and radius.
  • One participant summarizes that when the motor is not doing work, such as at zero load or when stalled, the output power is effectively zero, leading to zero efficiency.

Areas of Agreement / Disagreement

Participants express differing views on how efficiency behaves with varying loads, with some asserting that it increases with load while others question this notion. The discussion remains unresolved regarding the exact relationship between work, power, and efficiency in electric motors.

Contextual Notes

Participants highlight the complexity of defining efficiency in terms of work and power, noting that different factors influence mechanical and electrical inefficiencies. There is also uncertainty about the appropriate equations to use for calculating power input in specific scenarios.

JayCAquino#4
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The motor runs on dc current. I understand efficiency is useful work/work input. It is attached to the pulley. I think efficiency will remain constant as long as voltage of mains and vertical height remain constant. Am I wrong? Other people claim efficiency increases with load
 
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Usually, efficiency increases with load in an electric motor. Many motor manufacturers will provide spec sheets showing the efficiency vs load.
 
Using the formula useful work/total work input. I use mg(delta)h/ivt and this suggests efficiency is constant because when mass increases so does time. But how come when you take into account torque efficiency increases with load?
 
The formula for work input doesn't tell you anything about efficiency. Work input and output are mostly independent of each other, being driven by different factors.
 
russ_watters said:
The formula for work input doesn't tell you anything about efficiency. Work input and output are mostly independent of each other, being driven by different factors.
Oh okay so I should use power instead of work. Thanks for the help. Much appreciated
 
Right: work input is a combination of mechanical and electrical inefficiencies, which are functions of different things.
 
JayCAquino#4 said:
Other people claim efficiency increases with load
What is efficiency when the load is zero? I.e. the motor is spinning freely at it's no load rpm? What about when the load is very large and the motor is stalled so not rotating?
 
russ_watters said:
Right: work input is a combination of mechanical and electrical inefficiencies, which are functions of different things.

What equation do I use to calculate my power input if the motor is connected to a shaft which is lifting a load off the ground. I know the rpm and the radius I just don't know how to put it all together
 
billy_joule said:
What is efficiency when the load is zero? I.e. the motor is spinning freely at it's no load rpm? What about when the load is very large and the motor is stalled so not rotating?
I don't know how to use rpm to calculate power
 
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JayCAquino#4 said:
What equation do I use to calculate my power input if the motor is connected to a shaft which is lifting a load off the ground. I know the rpm and the radius I just don't know how to put it all together

P = IV (electrical power)
P = Fv (mechanical power)

Efficiency:
η = P out / P in
JayCAquino#4 said:
I don't know how to use rpm to calculate power

It's P = Tω

My questions were somewhat rhetorical. The answer to both is 0% as there is no work being done so output power is 0 W.
 
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