Main Control Board Test Points - Allen-Bradley 1395 Troubleshooting Manual

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3-16 Malfunctions Not Indicated by a Fault
Armature Current Control Malfunctions
Malfunction
Drive faults on an overcurrent
fault as soon as the contactor
closes.
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Symptom
A step in torque command results in a
excessive pulse of armature current,
faulting the drive.

Main Control Board Test Points

The following paragraphs describe test points on the Main Control Board
that can be used to diagnose armature bridge malfunctions:
ATTENTION: Extreme care must be used when probing the
!
test points on any board. Improper techniques may result in
damage to the circuit and personal injury. Do Not attempt probe
tests that are not listed here. Damage to extremely sensitive
circuits may cause unpredictable drive operation and unintended
motor rotation.
TP2
Armature Current Feedback can be used to monitor the armature current as
sensed by the current transformers in the AC lines. These signals are
unipolar representations of the armature current. Conduction in either
bridge results in a positive waveform in respect to AGND. Either an
oscilloscope (preferred) or a DVM can be used to monitor this signal with
respect to AGND (TP57). Scaling of the signal is 2V = bridge rated current
(drive nameplate rating).
The monitored waveform should have equally spaced pulses at a period of
2.7ms. Waveform 5 represents continuous current conduction. Waveform 6
is discontinuous current conduction.
TP21
Armature Voltage Feedback is sensed at the output side of the DC contactor.
It is scaled down to a +/-10V signal and accurately reflects the voltage
waveform across the motor armature. The waveform measured at TP21 is
inverted in respect to the actual voltage across A1 to A2. Either an
oscilloscope (preferred) or a DVM can be used to monitor this signal with
respect to AGND (TP57). It is scaled on the low voltage drive (150-300V
DC) as 1V signal = 65V armature voltage. The high voltage drive
(300-500V DC) is scaled as 1V signal = 125V armature voltage. The
waveforms shown in Figures 3-5 through 3-11 represent the typical signal
that occurs during the four quadrants of bridge operation.
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Solution
• Current loop may be improperly tuned. If the gains are too high
the initial current command will cause the current regulator to
over react and command too much current. Tune the drive to a
lower current loop gain using the Autotune procedure.
• Motor armature winding may be shorted. Check armature
resistance from A1 to A2, A1 to PE, and A2 to PE. If necessary,
disconnect the motor armature leads from the 1395 and use a
Megger to apply high voltage from A1 to PE and A2 to PE. This
will indicate if motor insulation breakdown has occurred.
• If the motor is not shorted, a malfunction has occurred in the
armature bridge. Refer to Magnetics/Power Structure section
for further details on correcting armature bridge failures.
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