350

ELECTRICAL APPARATUS

brush. But even with a moderate short-circuit current, the
sparking at the commutator may be destructive and the motor
therefore inoperative, if the phase of the short-circuit current
greatly differs from that of the current in the armature coil after
it leaves the brush, and so a considerable and sudden change of

^,**"
^ —

VC
LTS 0.25.
0.20 0.15 0.7

X
^

/

/

/

0.05

/
0 4
0 6
0 8
0 1C
10 L
AMP.PEriSQ. !0 1^0 160 1
N. JO 2
)0 2
10 2
10 2
30 2
JO 3
)0

FIG. 167.—E.m.f. consumed at contact of copper brush.

current must take place at the moment when the armature coil
leaves the brush. That is, perfect commutation occurs, if the
short-circuit current in the armature coil under the commutator
brush at the moment when the coil leaves the brush has the
same value and the same phase as the main-armature current in

.- —
«.. —
VC
- —
H.TS l.fi

___ -"
+** — •
^— — —

-1.4.

/
X^

1 2

/
/^

1 0

/

0 8

/
/

0 6

/
/

_a.4

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CL2.

A
0 J
0 3
0 4
0 E
0 6
AMP. PER SQ. 0 70 ^O *
IN. 0 1
)0 1
10 1
20 1
10 1
^0 1
>0

FIG. 168.—E.m.f. consumed at contact of high-resistance carbon brush.
the coil after leaving the brush. The commutation of such a
motor therefore is essentially characterized by the difference
between the main-armature current after, and the short-circuit
current before leaving the brush. The investigation of the short-
circuit current under the commutator brushes therefore is of