DC motor design calculations, help?

[Michael31091]

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?

 

[Mindscrape]

Can you reformulate your question to be a bit more clear and concise?

 

[Michael31091]

I would like to work out the number of coils i need to use in a 3 armature DC brushed motor, using AWG 24 wire, 3A ( can be up to 5 if needed), the magnets being used have B=1.25 T ( id also like to know if a stack of 4 would make 5T? ) The output of the motor must be 25W of mechanical rotational power (Power = torque x angular velocity)

 

[Michael31091]

Just updating, I've worked out how to calculate the Torque, and the Angular velocity. But to find the angular velocity i need the Back EMF, and I am still unsure as to how i can find this theoretically before i find the angular velocity? Also to find torque i need a value called 'Field density at end of pole (or armature)' and i assume this is just the field density caused by the input current in the coil of the armature, but i haven't been able to verify this yet.

so i ask, how do i work out the back emf, if i don't know angular velocity? or what other method can i use to find the angular velocity?

and when finding torque using 'T = Fr' where 'F = BIl', what is this B value and how is it calculated?

thankyou for helping!

 

[paranormal]

I would first suggest starting with a basic understanding of DC motors and their components. This will help you determine which equations and calculations are relevant for your design. Additionally, it may be helpful to consult with a mentor or colleague who has experience in motor design.

That being said, here are some suggestions for your design:

1. Determine the torque and speed requirements for your motor. This will help you determine the necessary power output and the type of motor (e.g. brushed, brushless, etc.) that you will need.

2. Use the power equation (P = VI) to determine the necessary input voltage for your motor. Keep in mind that the efficiency of a motor is not always 50%, and it can vary depending on the design and operating conditions.

3. Use the torque equation (T = Kt*I) to determine the necessary torque constant (Kt) for your motor. This will help you determine the number of turns and wire gauge needed for your motor's coils.

4. Use the speed equation (w = (V - IR)*Kv) to determine the necessary speed constant (Kv) for your motor. This will help you determine the number of poles in your motor, as well as the size and shape of your magnets.

5. Consider other factors such as mechanical design, cooling, and efficiency when designing your motor. These can have a significant impact on the overall performance and reliability of your motor.

Overall, designing a motor is a complex task that requires a deep understanding of electromechanical principles and careful consideration of various factors. It is important to thoroughly research and plan your design before attempting to build and test it. Good luck with your project!