352

ELECTRICAL APPARATUS

of the short-circuit current usually is far greater than the waste
of power in the resistance leads. To have any appreciable effect,
the resistance of the commutator lead must be far higher than
that of the armature coil .to which it connects. Of the e.m.f.
of rotation, that is, the useful generated e.m.f., the armature re-
sistance consumes only a very small part, a few per cent. only.
The e.m.f. of alternation is of the same magnitude as the e.m.f.
of rotation—higher below, lower above synchronism. With a
short-circuit current equal to full-load current, the resistance of

FIG. 169.—Commutation with resistance leads.
the short-circuit coil would consume only a small part of the
e.m.f. of alternation, and to consume the total e.m.f. the short- •
circuit current therefore would have to be about as many times
larger than the normal armature current as the useful generated
e.m.f. of the motor is larger than the resistance drop in the arma-
ture. Long before this value of short-circuit current is reached
the magnetic field would have disappeared by the demagnetizing
force of the short-circuit current, that is, the motor would have
lost its torque.
To limit the short-circuit current under the brush to a value
not very greatly exceeding full-load current, thus requires a re-
sistance of the lead, many times greater than that of the armature
coil. The i2r in the lead, and thus the heat produced in it, then,
is many times greater than that in the armature coil. The space
available for the resistance lead is, however, less than that avail-
able for the armature coil.
It is obvious herefrom that it is not feasible to build these
resistance leads so that each lead can dissipate continuously, or
even for any appreciable time, without rapid self-destruction,
the heat produced in it while in circuit.
When the motor is revolving, even very slowly, this is not nec-
essary, since each resistance lead is only a very short time in