SINGLE-PHASE COMMUTATOR MOTORS 359

Since (B' lags 90° behind its supply voltage, e, and also lags 90°
behind <B, by equation (2), and so behind the supply current
and, approximately, the supply e.m.f. of the motor, the voltage,
e, required for the excitation of the cornmutating poles is approxi-
mately in phase with the supply voltage of tlie motor; that is,
a part thereof can be used, and is varied with the speed of the
motor.

Perfect commutation, however, requires not merely the elimi-
nation of the short-circuit current under the brush, but requires
a reversal of the load current in the armature coil during its
passage under the commutator brush. To reverse the current,
an e.m.f. is required proportional but opposite to the current and
therefore with the main field; hence, to produce a reversing e.m.f.
in the armature coil under the commutator brush a second com-
mutating field is required, in phase with the main field and ap-
proximately proportional thereto.

The cornmutating field required by a single-phase commutator
motor to give perfect commutation thus consists of a component
in quadrature with the main field, or the neutralizing component,
which eliminates the short-circuit current under the brush, and
a component in phase with the main field, or the reversing com-
ponent, which reverses the main current in the armature coil
under the brush; and the resultant cornmutating field thus must
lag behind the main field, and so approximately behind the sup-
ply voltage, by somewhat less than 90°, and have an intensity
varying approximately inversely proportional to the square of
the speed of the motor.

Of the different motor types discussed under IV, the series
motors, 1 and 2, have no quadrature field, and therefore can be
made to commutate satisfactorily only by the use of commutator
leads, or by the addition of separate commutating poles. The
inverted repulsion motor, 3, has a quadrature field, which de-
creases with increase of speed, and therefore gives a better com-
mutation than the series motors, though not perfect, as the quad-
rature field does not have quite the right intensity.

The repulsion motors, 4 and 5, have a quadrature field, lag-
ging nearly 90° behind the main field, and thus give good com-
mutation at those speeds at which the quadrature field has the
right intensity for commutation. However, in the repulsion
motor with secondary excitation, 5, the quadrature field is con-
stant and independent of the speed, as constant supply voltage

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