CHECKING SMALL DC MOTORS

A quick check for small DC brush type motors, such as those used for the
spindle in a CD player, is as follows: 
     If you can scrounge an old analog ohmmeter, put it on the lowest
scale and connect it to the motor, after removing at least one leg from
the circuit. Ideally, the motor should not have any mechanical loading on
it (such as a belt, if you're checking a VCR loading motor). Cassette
basket motors with a worm-gear arrangement may have to be removed to test
them.
     A small motor should turn slowly, powered by the 1.5 volt battery in
the meter. A good motor will show a nice, steady indication on the meter,
and the indicated resistance will be fairly high... 30 to 50 ohms or so.
If the needle vibrates back and forth, the motor brushes and/or windings
may be bad, or the commutator may be clogged with metal chips or debris
from the worn brushes. It may be necessary to give the motor a twist to
get it going. If it shows a very low resistance, or it hits a spot that
makes it stop, it's bad. Try reversing the ohmmeter leads... the motor
should run backwards easily.  Some motors like CD spindles will be noisy
run backward as they have worn in one direction only.
     If the motor is out of the unit, try twisting it sideways and upside
down while it's running. Any fluctuation on the meter generally means the
brushes are close to being worn out. Intermittant motors (a dead spot in
rotation) will usually show up that way. Slow the motor by finger pressure
on the shaft. The meter should show less resistance, but still not
fluctuate wildly.  Stop the motor by holding the shaft. The meter now
shows the resistance of the windings. Turn it slowly with your fingers in
both directions. Abnormalities can be seen on the meter and felt with your
fingers. 
     If you don't have an analog ohmmeter, or you need to check a larger
motor, the test can be done with a variable power supply, a dropping
resistor of about 10 ohms and a scope. Hang the scope across the motor and
power it through the series resistor. Run the supply up until the motor
starts turning and observe the scope. You should see a fairly constant DC
voltage across the motor with spikes on it due to the action of the
brushes wiping the commutator. If the voltage across the motor fluctuates
wildly, the motor has a problem. 
     To get an idea of what you should see and feel, try checking several
new and old (preferably bad) motors. Replacing a spindle motor is a real
pain, so if you can check it without removing it, that saves a lot of time
and trouble.

Ray Carlsen
CARLSEN ELECTRONICS... A leader in trailing-edge technology.