- - Smooth acceleration, retardation, or deceleration.
- - A carefully controlled rate of acceleration or retardation.
- - Controlled speed changes, over small or wide ranges.
- - Accurate and positive speed matching.
- - Control or limit of the torque or tension.
|Figure 1: Direct Current (DC) Motor|
- Field Structure - necessary to provide a magnetic circuit for the flux and to hold the field coils. It consists of a yoke, pole bodies, pole faces and sometimes interpole to assist commutation.
- Field Coils - may consist of : a) shunt coils wound with many turns of small wire, connected in series with each other and across the line or in parallel with the armature. b.) series coils with fewer turns and larger wire, which are connected in series with the armature line. c.) both shunt and series coils for a compound motor. d.) interpole.
- Armature - the rotating element, consisting of slotted punchings pressed onto the shaft. These punchings perform the dual purpose of providing a path for the magnetic flux and of carrying the armature winding.
- Commutator and Brushes - the commutator consists of a large number of copper segments which are insulated from each other and from the armature shaft. These copper segments are connected to the armature windings. The commutator and brushes combination is necessary to feed the current to the armature.
- Commutating Poles (Interpoles) - occasionally used in fractional horsepower DC motors to improve commutation. These are small auxiliary poles placed halfway between the main poles and connected in series with the armature. There maybe as many interpoles as main poles, or only as half.
Figure 2: Parts of a DC Motor. (a) field Structure (b) armature
(c) front end showing commutator, brushes and brush riggings (d) complete motor