Calling machines experts!
I am brushing up on my electromagnetic theory etc. in preparation for teaching some undergrads. I've been going through the standard "DC Motor" threory and come across a conundrum I can't resolve. It has been causing some good debates in the coffee area today but so far, I didn't find any convincing arguments.
The question is, on a DC motor armature (rotor), is the force (torque) produced on the WIRES, or on the STEEL (core), or a mixture of them both? According to standard texts, there are two ways to work out the force.
1) You can work out the B field due to the field winding, and then do F=BIl on the armature turns, ignoring the MMF that THEY produce which slightly adjusts the B field due to the field winding. You then convert F to a torque by using the armature radius.
2) You can work out the change in field energy using the changing mutual inductance between the armature coil and the field winding, and then do T=dW/dtheta where W is the stored field energy. I'm ignoring the reluctance torques since I'm assuming a nice round armature core.
If I do the above methods carefully, they actually give the SAME torque result which is reassuring (both numerically and also algebraically if you collapse all the equations together right back to equivalent symbols (I, u0, dimensions etc)).
My conundrum is that method 1) clearly suggests that the force (torque) is on the wires, and that they would need to be glued to the rotor very well or they will rip themselves off.
However, method 2) suggests that it is much more a magetic field interaction between the steelwork parts, and that if I put a permanent magnet in, instead of a wound rotor, it would still have a force and torque, and there would be no armature wires - the force/torque would have to be between the steel/magnet structures.
So, which is it, how are the forces actually distributed in a DC machine armature? Is there some force on both, or all on the wires, or all on the steel? The textbooks all seem to gloss over this ....
Dr. Andrew Roscoehttp://personal.strath.ac.uk/andrew.j.roscoe