When running, a DC motor acts as a generator, producing a "back EMF"
(voltage) which limits the current passing through the motor.  It is
therefore essential that the motor run when connected to the power
source to prevent excessive current from burning out the windings in
short order.

Some things to look for:

1. An open circuit in one of the armature windings connected to the
commutator.  In this condition the motor will start some of the time
but not all of the time.  For example, if the armature has twelve
windings, there is a 1-in-12 chance it will not start, meaning the dead
spot is engaged when power is applied.

2. Dirty commutator, a common problem in motors that have not been run
for many years.

3. Scored commutator.  It needs to be re-surfaced to eliminate
inconsistent wear patterns.  This must be done on a special lathe.
There are many motor repair shops still able to do this.

4. Pot-metal brush holders that have swollen, preventing brushes from
moving inward to compensate for wear.

5. Brushes frozen in place by a mixture of carbon dust and dried-out
gummy oil.

This of course is not a complete list of possible troubles, but you
have to start somewhere.

For any piano motor, AC or DC, a time-delay fuse or circuit breaker of
the appropriate ampere rating is practically mandatory for safety
reasons, in case the motor fails to start or freezes up while running
unattended.  AC motors may produce excessive heat affecting the
lubricant, if nameplate voltage is around 110, since line voltage today
is typically 120-125 VAC.  AC motors in pianos normally contain a
centrifugal switch which disengages the starting winding when a certain
speed is reached, and when running they also produce a current-limiting
back EMF.  A typical AC piano motor may draw around 10 amps to start,
which drops to maybe a third of that, once up to speed.

All that said, it seems possible -- even likely -- that the rectifier
used with the DC motor in question has enough output for running it,
but the current demand for starting it exceeds the rectifier's
capacity.  You might expect in that situation that the rectifier would
either blow a fuse or start smoking.  But some of the modern ones will
limit the current before that point is reached -- possibly bad for the
motor.

Even if the motor will start, once given enough juice, it should be
checked for any of the above-mentioned conditions.  If healthy, there
should be minimal sparking at the commutator.  In case you are not able
to see the commutator, at least the brushes should come out easily for
inspection and replacement if necessary.  Wear patterns will reveal the
condition of the commutator.  If in doubt, take the motor apart.  Also
check the bearings for wear and clogged channels, preventing proper
lubrication.  Oil or grease reservoirs should have all dried, caked, or
gummy lubricant cleaned out, and then be replenished.  All these things
will do much to prevent problems later on.

Jeffrey R. Wood