Electric motors

There is a wide variety of d. c. and a. c. motors. There are shunt motors, series motors, synchronous motors, induction motors, single-, two-, and three-phase motors. They are used to drive various machines. A ball-bearing fully-enclosed fan-cooled direct-current motor is shown in Fig. 1.

Fig. 1 Ball-Bearing Fully-Enclosed Fan-Cooled Direct-Current Motor

1. field coil; 2. armature spider; 3. commutator key; 4. commutator sleeve; 5. commutator mica V rings; 6. commutator bars; 7. commutator metal V ring; 8. front inner bearing cap; 9. bearing lock washer; 10. bearing lock nut; 11. front outer bearing cap; 12. bearing assembly screw; 13. brush yoke; 14. brush stud insulation; 15. brush holder stud; 16. brush holder; 17. eye bolt; IS. armature laminations; 19. frame; 20. armature coils; 21. armature end plate; 22. back inner bearing cap; 23. vellumoid gaskets; 24. ball bearing; 25. back outer bearing cap; 26. armature shaft; 27. grease seal; 28. armature key; 29. back bearing bracket; 30. front bearing bracket

Direct-current motors are of three principal kinds, and are named according to the manner in which their field coils are connected to the armature. They are named respectively: series, shunt, and compound.

In the series motors the field windings and armature are connected in series with each other. All the current which passes through the armature passes through the field coils. The field windings are therefore composed of a few turns of thick wire. Starting under heavy load, a series motor will take a large current to provide the huge torque required.

The field coils of shunt motors are connected direct across the brushes, hence they have the full voltage of the mains applied to them. The shunt motor may be called a constant speed motor, and is suitable for driving machine tools, lathes, wood-working machines and any machines requiring a steady speed.

A compound motor has both shunt and series field windings and therefore partakes of the nature of both types of motors.