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Michael31091
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Homework Statement
Hi, well my task is to construct a DC motor. With 25w rotational mechanical output power. It cannot draw more then 5A current, and has to have 3 or more poles in rotation. The motor must be brushed/commutated. There is a max of 30v D.C. input. The motor needs to be built and tested within a few days, so ill need calculations done by tomorrow (brutally tough i know)
Homework Equations
I was not given relevant equations for the question, this is half my problem, id like some advise on which equations i should be using to caluclate the necessary parts to build the motor which will acomplish the goals stated above.
The Attempt at a Solution
My attempt came up short when i came across the term 'back emf' or 'induced emf'. i found an equation:
Vo = IR + Ve
Where:
Vo = power supply
I = current
R = terminal resistance (is this calculatable for my question or do i need to build and measure this, and what exactly is this resistance? where is it found.. i thought it was the overall resistance of the motor)
Ve = Induced EMF (Back EMF)
Further continuation of this formual suggest substituing Ve = w Ke
Where:
w = angular velocity
Ke = Induced EMF constant (how do i determine this?)
In other attempts at solving this i came up with the following:
Pi = VI
Where:
Pi = power input (i assumed motor efficiency to be 50% as someone suggested this was a good idea, how does this affect other calculations, it should have more effect then just doubling the desired output getting an input power of 50w)
V = voltage
I = Current (as no more then 5A is allowed, i set this to 3A so I am well within the boundaries)
therefore i get voltage:
V = Pi/I
V = 50/3 = 16.67 Volts
As a DC power supply is used to power the motor, this should not be a problem?
I then attempted to find the resistance of the motor, thinking i could calculate the number of coils i needed.. evidently this led me no where as i used 'V=IR'. Apparently this resistance value is the resistance of the motor when not moving, resistance changes? I know a section of my research said that the current drawn by the motor is reduced the faster it spins. I don't know how or what I am calulating from here, can someone please help?
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