3.5 Before You Call

After this section you can: 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, and name the four things never done while waiting for the call to be returned.

The amplifier has already told you most of what it knows (3.4). The check exists to collect that, to walk the things upstream of the amplifier that the panel cannot see, and to compare today against the log, before anyone opens a door or picks up a phone. Done in order it takes ten minutes. Done out of order it takes a day, because the step that would have found the cause is the one that got skipped on the way to the cabinet. This is the pre-call check, the operator's check on a system that was running; a service engineer meeting a machine cold puts a meter on the coils, the connections and the incoming power before anything is switched on (8.2). The steps hold for any manufacturer's system: where a panel lacks a reading, the step names what to use instead, and where the machine's own manual differs, the manual governs.

Two rules before the first step. Nothing is reset until what the panel says has been written down, because the latched fault (the fault the amplifier holds on its panel until someone resets it) is the only witness and reset erases it. And no cabinet is opened: every step is done from outside the cabinets, and each says what state the system is in. A check that needs a hood lifted or a panel removed is not this check's; it goes in the call.

The steps run in order. The first four are done as found or with the system down. Before step 5 the system is brought up, and bringing it up is the one diagnostic reset of 3.4, with the low-level pass counting as it: if the trip repeats on the same condition, the check ends there and the call is made with what it found.

1. The panel, as found (one minute). Written down, nothing reset. The state, in this book's names or the panel's own. The latched fault by name, and the fault history if the panel has one. The module count against the number the machine has, where the panel shows one, and any open log entry for a section out of service; otherwise the fault history. The output meters, which should be at zero with the drive at zero, and the last readings if the panel holds them. This step collects 3.4, and a fault name here is the first word of the service call.
2. The interlock chain, as found, walked at the machine (two minutes). Cooling: blower running or water flowing, confirmed at the machine, not at the panel; filter, duct, flow and temperature indications; conductivity on a water loop (2.6). Field: on, in the mode the log assumes (3.3). Doors, covers, emergency stops. Stroke interlock not latched. The controller's abort output not asserted. Rules out: the external interlock, a common trip and the one most often misread as an amplifier fault.
3. The facility, as found (one minute). Three phases present at the building's supply to the room, by the phase indicator where one is fitted; a new or changed hum from the cabinet is a sign of a lost phase (4.4). Room temperature against what the cooling was sized for. Water or air supply to the heat exchanger and the blower. The amplifier's own filters and fans. Rules out: the supply fault and the over-temperature trip that are the building's, not the amplifier's, before the system is brought up on them.
4. The load, with the drive off, the field off and the system at on or below (two minutes). That is the rule of 3.3 for the gap. Drive cable connections that can be reached without opening a cabinet, tight and dry. The armature connection, where it can be seen without lifting the hood. Fixture bolts and the moving mass as logged (2.3). The gap looked into from above the table: nothing in it and nothing steel near it (3.3). Rules out: the open circuit (voltage with no current, 3.4), the ground fault at a termination (a cable's end connection; 4.4), and the resonance that moved because the fixture did (2.5).
5. The field, against the log (one minute, system up). Field current and voltage against the logged values for the mode in use, cold if the machine is cold and warm if it is warm, and the ratio against the logged ratio (3.3). Mode switch in the position the test was equalized in. The field cooling and the armature cooling both reaching the shaker, now that they are running. On a tapped supply, a low warm current on a hot machine is read as a hot coil, not a fault. Rules out: the lost-force call that is a mode switch, and it finds the hot coil before it trips.
6. The gain and the controller (one minute). Gain at, or restored to, the logged setting for this fixture and profile (3.2). Controller output connected, drive at zero, control channel on the transducer the procedure names and the cable traced (2.1). Sigma clipping as the requirement allows. Rules out: the drive-limit abort that is a gain change, and the test that is controlling on the wrong wire.
7. The low-level run and the comparison (two minutes). Whether the article stays on the table for this pass is the test engineer's decision, made before the pass and written on the log; on a one-of-a-kind article the default is the fixture with a mass in the article's place. Run the low-level pass and read the four numbers that matter against the log for this fixture and profile: drive fraction, amplifier output voltage and current at the same frequency, and the ratio between them (3.1, 3.2).

Same setup and the same numbers means the machine is the machine it was, and the problem is elsewhere, in the test or the article. Same setup and different numbers means something changed in the chain, and the direction of the change says where:

• 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.
• An output section out of service: found at step 1, on the module count and in the log, and in the current available at full level; at low level it may not show in the ratio at all.

Five things go in the first sentence of the call, in this order:

1. The fault name as the panel latched it, with any earlier entries of it in the fault history.
2. The test condition when it happened: frequency, level, elapsed time.
3. The meter readings, output and field, before and at the trip.
4. What the log comparison showed.
5. What the check above has already ruled out.

A service engineer who hears those five can often name the module on the phone and arrive with the part. A call that opens with "the shaker lost force" buys a visit to run this check for you.

Two things are not said: a diagnosis, because the symptom belongs to the operator and the cause to the service engineer; and "we reset it and it ran," unless the trip and the reset are both on the log with their conditions, because a trip that cleared on reset is data about a fault that is still there.

While waiting for the call to be returned, four things are never done. The cabinet is not opened; the bank, and the field if its discharge path has failed, can still be live behind the door after the breaker (4.4). No interlock is jumpered and no limit is raised (2.6, 3.4). The fault is not reset again: the one diagnostic reset of 3.4 was the check, and every reset after it is the habit that ends in the story at the end of 3.4. And no module is pulled or swapped on a guess, because a module that has been handled is a module that has to be checked before it is trusted.

The story that opened this chapter ends here. The lab that called about lost force had the five facts on its own panel and in its own log the whole time, and the check would have found the isolated section in its first minute. The visit that was scheduled and paid for ran the check on arrival, found the section and the log entry in three steps, and spent the rest of the day on the things the lab had been meaning to ask about. That is the best outcome a service visit can have, and it was available for the price of a phone call with the right first sentence.