DC Motor Problem - Magnetic Field Calculation

[Shahil]

[o)]

I really need help in calculating the magnetic field generated by a horse-shoe shaped solenoid which is being used as a permanent magnet in a DC motor.

The calculation is VITAL (well, I think so) as I need it to be able to predict the angular speed (in revs per minute) of my motor.

If there is sa way to do the prediction without this, I would gratly appreciate it.

FYI - We are being source a 10 volt, 2 amp current to make the motor spin. I need help ASAP as if my motor doesn't work, I fail this subject at University.

[arcnets]

The magnetic field in a long solenoid is
H = IN/l, where
I is current,
N is number of turns,
l is length of solenoid.
I think you can use this, even though the solenoid is not straight.

The B field is
B = [mu]0[mu]rH,
where [mu]0 = 1.2566 * 10-6 Vs/Am.

[mu]r depends on your core material. Furthermore, it depends on how strongly the material is magnetized. OK, if you use a DC of 2 Amps, I should suspect that the core is always fully magnetized. At least in the stator. For iron, you can assume a maximum value of 5500. But be careful here, since other materials can have very different values of [mu]r.

I think the least risky method is, to measure B directly with a Hall probe.

[Shahil]

thanks for the help on this problem. Your calculations made sense BUT (ironically and I think I got what I deserved 'coz I started researching so late) our prof gave us the exact formaula to use in our calculations!!!!

w = (V - Ir) / k * magnetic flux

w = angular speed
V = voltage
I = current
r = resistance
k = constant (taking account friction etc.)
(v - Ir) = back EMF induced by motor

Wen enquired about the magnetic field (B), he said it's not needed!!

NOOOOOOOOOOOOOOO!!!!!!!! [g)]

Anyway, FYI my motor spun at 900rpm, which was 200rpm slower than the winner.

thanks anyway[:)]