1. The problem statement, all variables and given/known data 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) 2. Relevant 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. 3. 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 im 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 dont know how or what im calulating from here, can someone please help?