Question: A 250-V, 4-pole, wave-wound d.c. series motor has 782 conductors on itsarmature. It has armature and series field resistance of 0.75 ohm. The motor takesa current of 40 A. Estimate its speed and gross torque developed if it has a flux per pole of 25 mWb Answer: Step 1 Mechanical Engineering homework question answer, step 1, image 1 Mechanical Engineering homework question answer, step 1, image 2 Step 2 Mechanical Engineering homework question answer, step 2, image 1 Step 3 Mechanical Engineering homework question answer, step 3, image 1

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Answer:

The magnetic force,
                FB = IdlxB

The wire jumps towards the observer, means the force on thewire is out of the plane of the paper or along the Zaxis(horizontally) taking current along the X axis and B along the Y axis.

The vector cross product dLxB is along +ve z-axis only if the B field is vertically upward,along the Y axis. For this the upper pole must be a South pole.

(a)  Given Data L = 10 cm = 0.1 m is the length of the wire in the B field.

The max force, Fm = BIL

          I = Fm/B.L

            = 7.5x10-2 / 0.22x0.10

           I = 3.41 A

(b) The wire is tipped at 10o with thehorizontal means,

the angle betwen the wire (dL) and B field is 90 -10 = 80o

The, Force, F = BIL sin 80

                      = Fm.sin 80

                      = 7.5x10-2 x0.9848

                      = 7.386 x10-2 N

One can also take the effective length of the wire inthefield,

                 L' = L cos 10o

Then,          F= BIL cos 10o

                      = Fm.cos 10o

                      = 7.5x10-2x0.9848

                      = 7.386x10-2 N