1
Basic Principles of Motors and Generators
Prof. George Shoane
March 12, 2010
Introduction
The relationship between electricity and magnetism are provided by two fundamental
laws:
Ampere’s and Faraday’s laws.
Ampere’s law states that electric charge in motion
(current) gives rise to a magnetic field.
Conversely, Faraday’s law states that a changing
magnetic field gives rise to a voltage (and thus a current). These laws provide the linkage
between electricity and magnetism, and serve as the foundation for understanding electro-
magnetic devices.
When applied to certain classes of devices, these laws become
embedded in other laws or specific equations, as in the case for moving coil transducers
and motors.
Moving-Coil Transducers
An important class of electromagnetic devices is moving coil transducers, in which the
conductor is free to move in a magnetic field. These include microphones, loudspeakers,
electric motors and generators.
In these transducers, the relationships between magnetic
field, current, and the induced force is described by the Lorentz equation:
f
= q
u
x
B
(1)
where force,
f
, is the force on charge q,
u
is the movement of q, and
B
is the magnetic
field flux density, with vector direction emanating out of the north pole towards the south
pole.
For simplicity but without a loss of generality, we assume the three vectors,
f
,
u
and
B
,
are mutually orthogonal. The directions of the vectors are often described by Fleming’s
left hand rule for motors and right hand rule for generators.
Fleming Left Hand Rule (LHR) for Motors
The basic principle for the Fleming LHR is that current flow causes movement of a
freely-moving conductor in space.
Thus, if a current is applied through the conductor in
a magnetic field
B
, where
u
is in the direction of current, then
f
would be in the direction
of the induced deflection of the conductor.
Fig. 1.
Fleming’s Left Hand Rule.