Run the pre-call check in a fixed order that rules out the causes the operator can see
Say what each step rules out and which section of this chapter it comes from
Name the five things that go in the first sentence of the service call
Name the four things never done while waiting for the call to be returned
Why the Order Matters
The amplifier has already told you most of what it knows ({sec:03.4})
In order the check takes ten minutes; out of order it takes a day
Rule one: nothing is reset until the panel has been written down; the latched fault, the one the amplifier holds until someone resets it, is the only witness
Rule two: no cabinet is opened; every step is done from outside and states the system's state
This is the operator's check on a system that was running
The Check, As Found and With the System Down
One minute, the panel: state, latched fault, fault history, module count, output meters
Two minutes, the interlock chain, walked at the machine: cooling, field, doors, stops, stroke, the controller's abort
One minute, the facility: three phases, room temperature, water or air supply, the amplifier's filters and fans
Two minutes, the load, drive off, field off, system at on or below: cables, armature connection, fixture bolts, the gap
The Check, With the System Brought Up
Bringing the system up is the one diagnostic reset, and the low-level pass counts as it
One minute, the field: current, voltage and their ratio against the log for the mode, cold or warm
One minute, the gain and the controller: logged gain, drive at zero, control channel traced, sigma as allowed
Two minutes, the low-level run: drive fraction, output voltage and current, and their ratio, against the log
If the trip repeats on the same condition, the check ends there and the call is made
Reading the Comparison
Same setup, same numbers: the machine is the machine it was; look at the test and the article
More drive for the same response at the table: a lost or reduced field
More voltage for the same current: a coil or connection with more resistance
More current for less response at the table: a mechanical change
Voltage with no current: an open circuit
A section out of service is found at step one, on the panel and in the log, not reliably in the ratio
The Pre-Call Check, as a Card
Minute
Step
System state
Read or do
Rules out
Section
Reading
1
1. The panel, written down
as found; nothing reset
state; latched fault and history; module count where shown; output meters at zero drive
Figure 3.26. The pre-call check, as a one-page card: each step in order with its minute budget and the state the system is in for it, what to read or do, what it rules out, the section that explains it, and a box for the reading. Meant to be printed and kept on the machine.
Symptom to Step
Figure 3.27. Symptom to step: six entry symptoms, tripped and latched, will not go ready, lost force, drive-limit abort, noisy or harsh output, a peak in the response data nobody can explain (a harmonic, or a line at the amplifier's switching frequency, 4.3), each pointing to the step of the check that resolves it first and from there to the section of the chapter.
The First Sentence of the Call
The fault name as latched, with its earlier entries in the fault history
The test condition: frequency, level, elapsed time
The meter readings, output and field, before and at the trip
What the log comparison showed
What the check has already ruled out
Not said: a diagnosis, or "we reset it and it ran" unless both are on the log
The Call, Two Ways
The way a service engineer wants to hear it
The way it usually arrives
1. The fault
Thermal trip, output module B, latched; fault history shows the same trip three times this month.
It tripped.
2. The condition
Full-level random, 20 to 2,000 Hz, 12 grms, at minute twenty, third run of the afternoon.
During a test.
3. The meters
Output 240 A rms at 85 V before the trip; field 410 A at 118 V, log says 420 A at 112 V cold.
They looked normal.
4. The comparison
Low-level run reads the same drive fraction and current as the log, voltage up twelve percent.
We did not compare.
5. Already ruled out
Interlock chain closed at the machine; field in the right mode; filters clean; three phases present; cable terminations tight.
We tried another fixture.
Not said
A diagnosis. 'We reset it and it ran,' unless both are on the log.
It is probably the amplifier. We reset it and it ran.
Figure 3.28. The call: the five facts in order, each written the way a service engineer wants to hear it, against the same five the way they usually arrive.
Four Things Never Done While Waiting
The cabinet is not opened: the bank, and a field whose discharge path failed, can be live after the breaker
No interlock is jumpered and no limit is raised
The fault is not reset again: the one diagnostic reset was the check
No module is pulled or swapped on a guess
Decide
Keep the pre-call check printed and on the machine, with the log beside it
Run it in order every time the system does something it did not do yesterday
Arrive at the phone with the five facts; everything after that is the service engineer's
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