| Test stator windings and feedback
device
1)
Our first step in troubleshooting a Servo Motor is
to eliminate the major components first. The two major components
of a Servo Motor are;
(I) the
feedback device used, such as a resolver, encoder tachogenerator
or hall sensor,
(II) stator windings.
- Feedback
Devices: Tennessee Armature and Electric
Resolver –
check sine & cosine output, counts per rev., check resistance
through excitation, sine and cosine windings, check alignment.
Encoder – check counts per rev., output
voltage levels, output angle symmetry and check alignment.
Hall Sensor – check output voltage levels,
output angle symmetry and alignment. Tachogenerator – check output voltage. Check
output waveform for spikes or ripples.
- Stator
Windings: Tennessee Armature and Electric
Check winding resistance
phase to ground and phase-to-phase and compare with factory
ratings.
Test
run servo motor and document results.
 2) If the windings are good and the feedback
device is functional we will proceed by ‘test running' the
servo motor. Running the servo motor during inspection is
a very important step. If there are any mechanical problems
(rough/faulty bearings, bearing cap loose, rotor loose on
shaft or rubbing the stator core, etc.) they will usually
be detected while running the servo motor. We will also document
the RPM, Voltage and Current for comparison after the motor
has been repaired.
Check
BEMF.
3) The next step is to reverse drive the motor
and check the Counter EMF also known as Back EMF or BEMF.
This is used to check the rotor magnet strength. Depending
on the magnet strength and the winding configuration, the
motor will produce a certain voltage when it is rotated at
a certain RPM. This voltage output is very important because
the strength of the magnets has a direct affect on the torque
the motor will produce. The generated voltage and the waveform
are both documented. If the magnets are determined to be weak
they will be remagnetized. Tennessee Armature and Electric.
Test
holding brake. Tennessee Armature and Electric
4) If the servo motor has a holding brake
it will be tested at this stage. The holding torque will be
checked in three different positions and these readings will
be documented.
Disassemble the motor.
5) After we make a visual inspection of the
outside of the servo motor for obviously damaged components
we will disassemble it.
Clean
and/or sandblast all parts and inspect.
6) After the servo motor is disassembled Tennessee
Armature will clean and/or sandblast and inspect all components
individually.
‘Wash
and Bake' stator or rewind stator.
7) If the stator windings are shorted, but
we are unable to visually detect any damage to the windings
after disassembly, we will ‘wash and bake' the stator as follows;
If any liquid (oil, coolant, water, etc..) has entered the
motor and soaked into the windings it will generally cause
the winding ‘resistance to ground' to be very low but not
completely shorted. We then bathe the windings with a special
solvent, pressure wash the solvent out and bake the windings
dry again to be re-tested. If the test fails again the stator
will be sent to the winding department to be repaired and/or
rewound.
Test stator windings again.
8) After the stator has been
repaired or rewound, the windings will be tested.
- High-Potential Test – 1000 VDC, up to 50,000
Mega ohms, tests winding resistance to ground.
- Surge Test – 1000 Volts, phase-to-phase
winding waveform comparison.
- Phase-to-Phase resistance test – Tests to
find if windings are shorted coil to coil.
Check
end frames thoroughly and machine if needed.
9) Once
the motor is disassembled and the windings and feedback device
are taken care of, the end frames are thoroughly inspected.
The end frames aka end plates or end brackets are the heads
at each end of the motor. These end plates seat the bearings.
Tennessee Armature will use new bearings to check for a snug
fit in each end frame, making sure there is no wobble, spin
or play in the bearing seat. We also use micrometers to measure
the bearing diameter and compare to measurements of the end
frame bearing seat diameter. If the bearing seats are loose
or out of tolerance we will send the end frames to our machine
shop where one of our machinists will correct the problem
accordingly.
Replace
Bearings.
10) Anytime we disassemble a motor, we replace
the bearings. Bearings are generally one of the most inexpensive
parts of a motor and when they fail the motor has to be completely
disassembled to change them.
Check
rotor security to shaft and balance if needed.
11) The rotor security to the shaft is checked.
We will make sure the rotor is pressed on tightly and the
keyway is not worn.
Inspect
O-rings, gaskets and seals. Replace if needed.
12) Any and all O rings, gaskets, or seals
will be checked to make sure they are not damaged and are
well within tolerance. If needed they will be replaced.
Assemble
the motor.
13) Tennessee Armature, at this stage, will
re-assemble the motor and install & align the feedback
devices.
Test
run servo motor, document, check end play, radial play and
run out.
14) After all the motor components have been
assembled we will test run the motor, listening for noise
and observing all aspects. Once again we will document the
RPM, Voltage and Current readings for comparison with the
initial readings. These before and after readings will show
how certain failures effect the RPM, Voltage and Current characteristics
of each motor.
Paint.
15) The last step of the repair is paint and
cosmetic.
Before any
repair work is performed the repair cost will be quoted to
the customer. The only time the repair price is not quoted
prior to the actual repair will be in an emergency situation.
Rotor balancing
is also available at the customer's request but is not done
on every motor. Otherwise, while we are test running the motor,
we will check to make sure it runs smoothly and without vibration.
The shaft
run out, endplay, and radial play are also checked with a
dial indicator and documented. |
