HYSTERESIS MOTOR 171
of the exciting coils, by the angle of hysteretic lead, a, which is
constant, and so gives, at constant magnetic flux, that is, con-
stant impressed e.m.f., a constant torque and a power propor-
tional to the speed.
Above synchronism, the iron disk revolves faster than the
rotating field, and the line of resulting magnetization in the disk
being behind the line of m.m.f. with regard to the direction of
rotation of the magnetism in the disk, therefore is ahead of it in
space, that is, the torque and therefore the power reverses at
synchronism, and above synchronism the apparatus is an
hysteresis generator, that is, changes at synchronism from motor
to generator. At synchronism such a disk thus can give me-
chanical power as motor, with the line of induction lagging, or
give electric power as generator, with the line of induction
leading the line of rotation m.m.f.
Electrically, the power transferred between the electric cir-
cuit and the rotating disk is represented by the hysteresis loop.
Below synchronism the hysteresis loop of the electric circuit
has the normal shape, and of its constant power a part, propor-
tional to the slip, is consumed in the iron, the other part, pro-
portional to the speed, appears as mechanical power. At syn-
chronism the hysteresis loop collapses and reverses, and above
synchronism the electric supply current so traverses the normal
hysteresis loop in reverse direction, representing generation of
electric power. The mechanical power consumed by the
hysteresis generator then is proportional to the speed, and of
this power a part, proportional to the slip above synchronism,
is consumed in the iron, the other part is constant and appears
as electric power generated by the apparatus in the inverted
hysteresis loop.
This apparatus is of interest especially as illustrating the
difference between hysteresis and molecular magnetic friction:
the hysteresis is the power represented by the loop between
magnetic induction and m.m.f. or the electric power in the
circuit, and so may be positive or negative, or change from the
one to the other, as in the above instance, while molecular mag-
netic friction is the power consumed in the magnetic circuit by
the reversals of magnetism. Hysteresis, therefore, is an electrical
phenomenon, and is a measure of the molecular magnetic fric-
tion only if there is no other source or consumption of power in
the magnetic circuit.