50

ELECTRICAL APPARATUS

this winding becomes at the same time a p-polar short-circuited
winding. On the rotor, in some slots, the secondary current of
the n-polar and the primary current of the Ti'-polar winding flow
in the same direction, in other slots flow in opposite direction,
thus neutralize in the latter, and the turns can be omitted in
concatenation — but would be put in for use of the structure as
single motor of ?i, or of nr poles, where such is desired. Thus,
on the rotor one single winding also is sufficient, and this arrange-
ment of internal concatenation with single stator and single rotor
winding thus is more efficient than the use of two separate motors,
and gives somewhat better constants, as the self-inductive im-
pedance of the rotor is less, due to the omission of one-third of
the turns in which the currents neutralize (Hunt motor).

The "disadvantage of this arrangement of internal concatenation
with single stator and rotor winding is the limitation of the avail-
able speeds, as it is adapted only to 4 -=- 8 -r- 12 poles and
multiples thereof, thus to speed ratios of 1 -r- J^ -f- M? the last
being the concatenated speed.

Such internally concatenated motors may be used advantage-
ously sometime as constant-speed motors, that is, always run-
ning in concatenation, for very slow-speed motors of very large
number of poles.

37. Theoretically, any number of motors may be concatenated.
It is rarely economical, however, to go beyond two motors in
concatenation, as with the increasing number of motors, the
constants of the concatenated system rapidly become poorer.

If:

YŤ = g -jb,

ZQ = r0 + J$Q,
Zi = ri + jxlt

are the constants of a motor, and we denote :

Z = Z0 + Zi = (r0 + rO + j (a?0 + 3j)

= r + jx

then the characteristic constant of this motor — which char-
acterizes its performance — is k.

if now two such motors are concatenated, the exciting admittance
of the concatenated couple is (approximately) :