I agree with Robbie in the motor choices of solution which he has
outlined in MMD 010629.  I must add that I have seen replacement motors
from the 1920's which replace the Welte-Mignon DC motor of circa 1910
with an 1150 rpm one/eighth horsepower AC motor and a pulley having
several diameter choices, the suggestion being that the top motor speed
should be about 1150 rpm.  Also, in an upright circa 1914-5, I saw
an original DC motor whose specification plate also was rated for
1150 rpm at full voltage.

While it is certainly possible to operate these instruments with an AC
motor, the quiet operation of a two speed system is quite remarkable.
I always suggest to owners of either T-100 (red) or T-98 (green)
systems that they consider a DC motor to benefit from the quiet
operation.  In my own piano I use a vintage DC motor with a bridge
rectifier and no filtering.  I experience no audible hum because I have
new rubber motor mounts supporting the motor.  I feel that a DC motor
yields a superior performance.

Mark Reinhart
Charles Town, West Virginia

 [ The modern permanent magnet DC motor (2 wires) needs well-filtered
 [ DC to run without hum, but a motor with an electromagnetic field
 [ (4 wires) can be powered by unfiltered rectified full-wave DC,
 [ and run without hum, if the time constants of the field and armature
 [ circuits are matched.  The reason is that the motor torque (assuming
 [ constant speed and a constant load) is the roughly the ratio of the
 [ field current and the armature current.  When the time constants are
 [ matched, the current ratio stays constant in spite of the severe
 [ ripple of the unfiltered DC supply voltage.  An inductor in series
 [ with the armature might be needed.   -- Robbie