Module 3.12 DC motor and generator theory

Knowledge levels

Sub-module	A	B1 / B2 / B2L	B3
3.12 DC motor/generator theory	—	2	1

Content: basic theory; construction and purpose of components; factors affecting output and current direction in generators; factors affecting power, torque, speed and rotation of motors; series, shunt and compound motors; starter-generator construction.

Basic principle

• Generator: mechanical energy → electrical energy. A conductor rotated in a magnetic field has an EMF induced in it (Faraday). Direction by Fleming's right-hand rule.
• Motor: electrical energy → mechanical energy. A current-carrying conductor in a magnetic field experiences a force F = B I l. Direction by Fleming's left-hand rule (First finger Field, seCond finger Current, thuMb Motion).
• The same machine can act as either - the basis of the starter-generator.

Construction

Part	Purpose
Yoke (frame)	carries the magnetic flux, supports the poles
Field poles and field windings	produce the magnetic field
Armature	rotating laminated core carrying the conductors in which EMF is induced (generator) or current produces force (motor)
Commutator	copper segments insulated with mica; acts as a mechanical rectifier, reversing the coil connections every half turn so output (or torque) stays in one direction
Brushes	carbon blocks pressed on the commutator by springs, connecting the rotating armature to the external circuit
Interpoles	small poles between main poles, in series with the armature, to reduce sparking at the brushes by neutralising armature reaction
Compensating windings	in pole faces of large machines; also counter armature reaction

Armature reaction: the armature's own magnetic field distorts the main field and shifts the magnetic neutral plane, causing sparking. Corrected by interpoles, compensating windings or brush shift.