Designing a DC motor.

Mutant_Fruit

Anyone know any tips/hints/examples on the web/general info on how to get the most efficient DC motor.

Our power supply is a 1.5v 'C' type battery, and misc equipment such as paperclips, 100's of metres of insulated copper wire and a load of other crap. This is only a small "motor", not a car engine

I basically need info on how to work out the optimum current flowing, optimum number of coils to spin through the magnetic field etc. All advise welcome.

Capt'n Midnight

As you probably know AC induction motors are more efficient than the small DC ones. Not sure but at a guess it would be that an inductance would tend to stop AC flow where as with a DC motor there would be no back emf until you were up and running.

Would it be cheating to use an electronic circuit to generate the AC ?

Aldi have a motor kit - no theory but might be userful for testing empiracilly in typical Blue Peter style.

Mutant_Fruit

No, we can only use the equipment supplied, so we have to make a DC motor. I was just wondering if it was possible to "optimise" it using some formulae, so we could work out how many coils we need to get the fastest speed.

THe reason is that the more coils there are, the more induced current there will be once the coil starts spinning, this limiting max speed.

But if there aren't enough coils, we'll be limiting our max speed.

Capt'n Midnight

Have a look for "principles of electric machines" - it's a book

You have three options for the magnetic field which affect the speed/power

a/ permenant magnet - at a guess your only choice
b/ field coil in parallel with rotor - you can vary the field strenght with a resistor
c/ field coil in series with rotor

I can't remember which is faster, or has more torque or has more startup torque but they are all different

as an aside considering how small and cheap diodes are you'd think they would use permemant magnets on mains driven motors by adding a bridge rectifier - but it don't happen - there must be some reason why...

Mutant_Fruit

as an aside considering how small and cheap diodes are you'd think they would use permemant magnets on mains driven motors by adding a bridge rectifier - but it don't happen - there must be some reason why...

As far as i know, its because big magnets can lose their magnet field over time. Also, they become hopelessly unwieldy as you increase the size of your motor. Therefore not a great choice for large motors.

I'll investigate that book, and those three things you mentioned, hopefully one of em will prove better than a permanent magnet, but we shall see. With only DC current (max of 1.5V/5AMPS i think), i'm not sure what results i can realistically expect from the alternative options, but we shall see.

Stoner

I've never built a motor for scratch, I always ment to but never got arround to it, there must be lots of stuff on the net about it. However I would imagine that the quality of the turns/windings would be the best place to start and improve on your quality issues. The direction of turns and your ability to produce the flux lines in a uniform direction will be very important. There are the simple formula relating to force and number of turns and current, direct relationships between them etc that i am sure you know.
I have seen motors being rewound and the machines that do it are very carefull to keep the direction and tightness of the winding consistant.Some large motors do have a seperatly excited rotor fields, this gives greater control of speed etc, I agree that you'd expect to see more sep excited fields as the cost of rectifiers etc has dropped, i guess that so much of this stuff is made and bought in Asia and whatever is the cheapest goes, I'd imaging that another set of slip rings to get power to the rotor current would be expensive and something else to break.