EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: florian_1034 on June 22, 2016, 07:49:27 am
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Let's say I have a motor connected to a load. Motor efficiency is 100%. And we have two formula: P=U*I and P=F*v. So my question is: If I measure the power consumed from electric grid by the motor and then I measure the power output by the motor, this two powers will be equal? Is electric power :-DMM the same as mechanical power :-/O?
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Yes to the first question, no to the second.
An electric motor is a converter of electric to mechanical power. If the motor is 100% efficient then the conversion is perfect and by definition all electrical power is converted to mechanical. That takes care of Q1.
Only no motor really is 100% efficient so there will always be various losses dependent on the nature of the particular motor. At least there will always be thermal losses in the windings and magnetic circuits. Sometimes noise is loud enough to figure in the efficiency and there might be other loss mechanisms that don't immediately come to mind.
Electric and mechanical power are 2 distinct and separate things so they are not "the same". There might be a near 1:1 correspondence between one and the other as when converted by a motor but the same they are not. Definitely not in nature but in practice neither in magnitude.
A minor comment regarding the formulas. They are correct as stated but the mechanical one applies to linear motion whereas motors are usually (not always!) rotary in nature. So a more appropriate form would be:
P = T*w, where
T = torque,
w (omega) = angular velocity.
In the SI system (the only correct one and those who disagree can exit stage left) torque is given in newtonmeters [Nm] and angular velocity in radians per second [1/s] and then you get power directly in watts [W].
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Electric power is the rate at which electrical energy is transferred by an electric circuit.
Mechanical power. In physics, power is the rate of doing work. It is the amount of energy consumed per unit time.
They are essentially the same, rate of energy transfer/consumption per unit time. Both nominally in Joules/sec,
but can be expressed in various ways.
Regards, Dana.
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Thanks a lot Kremmen ! Your response is well explained and very helpful to me. :-+
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Let's say I have a motor connected to a load. Motor efficiency is 100%. And we have two formula: P=U*I and P=F*v. So my question is: If I measure the power consumed from electric grid by the motor and then I measure the power output by the motor, this two powers will be equal? Is electric power :-DMM the same as mechanical power :-/O?
With your 100% assumption: yes. P is universal, but always define the "box"
If you then add an ideal gearbox and an ideal generator, you have an 100% efficient frequency converter.
A not-100% version existed, to convert 60Hz to 50Hz on boats going from EU to USA.