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TB155 Rev2 Quick Checks for Servo Motors
Overview
The following quick and easy checks can often detec t a bad servomotor and prevent damage to the servo
drive electronics
Windings Check
1 With the motor cables/wires and shaft completely disconnected spin the motor shaft with your fingers it
should turn fairly easily
*If the motor shaft is very difficult to turn you may have an internally shorted winding or something is
physically resisting the shaftΓ’s motion such as dir t and debris or worn bearings
2 Short the windings together On DC brush servos t here are two wires On AC brushless servos there ar e
three wires
*There should be a noticeable increase in drag but you should still be able to spin the shaft with your
fingers
*If there is no additional drag you may have an op en winding or bad magnets
*If the resistance is not smooth but has skip spots the motor is bad
Note If all three windings of AC servos are not sh orted together the motor will appear to cog when
turned
Resistance Check
1 With a good quality ohm meter set on the lowest possible range the following are typical motor winding
resistances measured at the motor minus the resista nce of the meter used
DC Brush Motors Resistance AC Brushless Motors
Resistance
Centroid 17in lb DC* 11 ohm +/ 5 ohm Centroid 400W AC** 35 4 ohm +/ 0 1 ohm
Centroid 29in lb DC* 1 4 ohm +/ 0 3 ohm Centroid 750W AC** 2 4 ohm +/ 0 1 ohm
Centroid 40in lb DC* 1 1 ohm +/ 0 2 ohm Centroid 1kW AC** 0 9 ohm +/ 0 1 ohm
Centroid 2kW AC** 0 6 ohm +/ 0 1 ohm
Centroid 3kW AC** 0 8 ohm +/ 0 1 ohm
* Given resistances for DC servos refer to the lowe st measurable value at any point in the shafts rotation
** Given resistances for AC servos refer to the res istance across any Γ’twoΓ’ windings and are independe nt
of shaft position
If a motor winding is shorted the measured resista nce will typically be lower than that of a Γ’goodΓ’ motor
Here a comparison of similar motors can be very use ful in diagnosing a problem For motors with a
damaged commutator or an open winding there will b e a lack of continuity In the case of a damaged
commutator or open winding on a DC brush servo continuity will be lost when the brush passes over
those areas For an AC brushless servo with an open winding there will be no continuity at any position
2 Measure the resistance between the motor chassis and each motor power terminal The measurement
should be greater than 10M ohms
*A lot of motors will have painted or anodized surf aces which will not give an accurate reading To
minimize the chances of getting an inaccurate readi ng measure the resistance between 2 points on the
motorΓ’s frame It should be exactly 0 ohms Then co nnect 1 of the meter leads to the motor power
terminals
*Measure this resistance both at the motor and with the cables installed prior to connecting motor to drive
WARNING Never remove the brushes from a DC motor They do not wear out and more costly damage
may result by removing them unnecessarily
Document History
Rev2 Created on 2017 11 17 by #270
Rev1 Created on 2003 07 24 by #000