Publication LZ-UM001A-EN-P - January 2008
Troubleshooting
25
5.
Repeat step 4 comparing V-W to U-W. In this case U-W should lead
V-W by 60
o
. The shapes and peak voltages should be approximately the
same. Note that probe common = W. Be sure to use the same phasing
direction as in step 4.
Check Measured Back EMF to Specification
By comparing your measured and calculated Back EMF constant to the
motor’s specified back EMF constant, you can verify the correct installation
and general health of the magnets and coil. The force constant has a direct
relationship to the back EMF constant, so this test also checks the force
constant. The calculation is based on the analysis of one motor
electromechanical cycle. Problems can occur at any point along the motor
travel, so check that the Back EMF waveshape is consistent throughout the
whole travel.
1.
Turn the drive power OFF.
2.
Disconnect the motor leads from the drive.
3.
Using a storage oscilloscope, connect one channel across any two phase
leads.
4.
Move the motor at a very steady and constant speed in either direction
by hand. This is the motor’s phase-phase back EMF.
5.
Capture and analyze one electrical cycle.
ATTENTION
Even with motor power disabled and leads disconnected,
permanent magnet motors can generate high back EMF voltage
when moving due to external forces.
time (s)
V
(p
K-
p
K
)
one cycle
