Five Readings to Take Before You Call Anyone
The five-meter sequence our technicians run before condemning any part: supply, control voltage, per-leg current, contact voltage drop, and insulation - in that order.
Half the service calls we take end with a fault that the plant's own meter could have named first. Not because anyone lacks skill — because nobody agreed on which readings come first. Here is the sequence our technicians run before anything gets condemned, on almost any machine that runs on three-phase power.
1. Supply, at the source
Line-to-line at the disconnect or the top of the starter: on a 480 V system you want roughly 456–528 V, with the three legs within a couple of percent of each other. A missing or sagging leg here ends the diagnosis — everything downstream is a victim, not a suspect.
2. Control voltage, at the coil
Whatever runs your control circuit — 24 V, 120 V — read it across the contactor coil while someone presses START. Full voltage and no pull-in condemns the contactor. No voltage sends you upstream through the stops, interlocks and fuses: the full supply appears across whichever device is open. That one rule finds more control faults than any other sentence in this trade. Practice it on our free contactor trainer.
3. Current, per leg, under load
Clamp each motor lead with the machine working. Compare against nameplate FLA — or estimate it with the FLA calculator if the plate is painted over. All three high says mechanical overload. One leg wildly different says winding or connection trouble. All three at nameplate while the overload keeps tripping says check the overload's sizing, not the motor.
4. Voltage drop, across every closed contact
Under load, a closed contactor pole or breaker pole should drop essentially nothing — millivolts when new, under half a volt in service. Whole volts across one pole is a burnt contact quietly single-phasing your motor, and it shows up here months before it shows up as smoke.
5. Insulation, before re-energizing
If a motor tripped hard and you are about to hit reset: winding-to-ground with a megger first, locked out. A motor that trips, resets, runs, and trips again is telling you its insulation story one chapter at a time — the megger reads the last page early. Our motor trainer walks the full test set.
The habit that makes the list work
Write the numbers down — every time, same order. Five readings with last month's values beside them turn "it seems worse" into "leg two is 9 % low and it wasn't in June." That page is the cheapest condition monitoring there is.
Run the sequence and you will fix most faults yourself. When a reading doesn't make sense — or points somewhere you'd rather not open alone — that is the part we do, on site within 24 hours: (562) 805-3250.