Interpole Motors.—An interpole motor has in addition to the main poles, a series of interpoles, placed between the main poles. The object of these poles is to provide an auxiliary flux or "commutating" field at the point where the armature coils are short circuited by the brush.
Figs. 433 to 437.—Parts of the type S interpole motor built by Electro Dynamic Co. They are as follows: 1. yoke—commutator view; 2. interpole coil; 3. top R. H. main coil; 4. bottom R. H. main coil; 5. main pole; 6. interpole; 7. armature shaft, R. H. bearing; 8. commutator; 9. armature wedge; 10. armature coil; 11. brush ring; 12. brush carrier insulation; 13. brush carrier; 14. brush guard; 15. carbon brush; 16. brush holder; 17. cross connecting cable; 18. oil ring; 19. commutator end bearing bushing; 20. pulley end bearing bushing.
Ques. What is the object of the commutating field produced by the interpoles?
Ans. Its object is to assist commutation, that is, to help reverse the current in each coil while short circuited by the brush, and thus reduce sparking.
Fig. 438.—Interpole motor as built by the Electro Dynamic Co. This type of motor is devised to prevent sparking at all loads by the use of interpole magnets, that is, small magnets placed between the field magnets. The interpoles set up a field in a direction to stop and reverse the current in the armature coils while they are short circuited by the brushes.
Ques. What is the nature of the commutating field?
Ans. The excitation of the interpoles being produced by series turns, the field will vary with the load, and will, if once adjusted to give good commutation at any one load, keep the same proportion for any other load, provided the iron parts of the circuit be not too highly saturated.
Ques. State briefly how sparking is reduced or prevented by the action of the interpoles.