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 4.3.12 Driving-Machine Motor
4.3.12.1 Inspection Made with Power On. Close the mainline switch and proceed. Have the
dumbwaiter operated in each direction making frequent stops, and observe the operation of the
motor and brake. Commutators or slip rings, where provided, should be observed for excessive
sparking or brush chatter. Inspect bearings as previously described. Inspect all motor fastening
bolts to determine whether they are in place and tight.
4.3.12.2 Inspection Made with Power Off. Open the mainline switch and examine brush
holders, commutators or slip rings. Determine the condition of the brush holders and brushes
and whether any brush is stuck in its holder or is worn to the extent that either the brush holder or
the metal connector on the brush could touch the commutator or slip ring. If sparking or
chattering was noticed when the machine was running, check the brush spring pressure.
Examine the commutator (or slip rings) to determine if it is burned, pitted, grooved, or scored,
and is clean and free from oil. Also check the commutators for high mica conditions. (See Figure
2-13) Become very familiar with these signs and be constantly on the look out for them.
Dumbwaiter performance is directly related to good electrical continuity through the commutator
or slip rings. Check the slots of the commutator for any accumulation of carbon, copper dust, oil,
or other substances. Check exposed armature and field terminal connections to see that they are
tight. Special attention should be given to the shunt field connections of DC motors. Loose
connections here could result in a runaway condition. connections not properly made may result
in poor electrical conduction and points of resistance. This condition creates heat. Examine the
leads to be sure they are not broken or their insulation is not cracked or broken. Improper
adjustments can force motor winding to carry more current, which produces heat, which will
cause insulation to deteriorate. Check to see that the motor windings are free of oil, dust, or lint
deposits. Accumulations restrict cooling air circulation causing heat build-ups, which result in
poor performance, insulation damage, and possibly fire.
4.3.13 Driving-Machine Brake
4.3.13.1 Inspection Made with the Power On. Close the mainline switch and proceed. Run the
car and observe the operation of the brake. The brake should not chatter. It may apply on or
before the completion of the slowdown and leveling operation. Check to see that the brake is
automatically applied on normal stops. The clearance between the brake shoe and the brake
drum when the car is running should not be greater than necessary to permit free running.
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