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TYPES OF ARMATURES

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Neets Module 05-Introduction to Generators and

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To check the direction of rotation of this armature, you should use the right-hand rule for motors.Hold your thumb, forefinger, and middle finger at right angles. Point your forefinger in the direction offield flux; in this case, from left to right. Now turn your wrist so that your middle finger points in thedirection that the current flows in the winding on the outside of the ring. Note that current flows into thepage (away from you) in the left-hand windings and out of the page (toward you) in the right-handwindings. Your thumb now points in the direction that the winding will move.

The Gramme-ring armature is seldom used in modem dc motors. The windings on the inside of thering are shielded from magnetic flux, which causes this type of armature to be inefficient. The Gramme-ring armature is discussed primarily to help you better understand the drum-wound armature.

DRUM-WOUND ARMATURE

The drum-wound armature is generally used in ac motors. It is identical to the drum windingdiscussed in the chapter on dc generators.

If the drum-wound armature were cut in half, an end view at the cut would resemble the drawing in

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If the drum-wound armature were cut in half, an end view at the cut would resemble the drawing infigure 2-7, (view A), Figure 2-7, (view B) is a side view of the armature and pole pieces. Notice that thelength of each conductor is positioned parallel to the faces of the pole pieces. Therefore, each conductorof the armature can cut the maximum flux of the motor field. The inefficiency of the Gramme-ringarmature is overcome by this positioning.

Figure 2-7.—Drum-type armature.

The direction of current flow is marked in each conductor in figure 2-7, (view A) as though thearmature were turning in a magnetic field. The dots show that current is flowing toward you on the leftside, and the crosses show that the current is flowing away from you on the right side.

Strips of insulation are inserted in the slots to keep windings in place when the armature spins. Theseare shown as wedges in figure 2-7, (view A).

Q10. Why is the Gramme-ring armature not more widely used?

Q11. How is the disadvantage of the Gramme-ring armature overcome in the drum-wound armature?

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