press_mechanical — count_cycles (ct-clamp)
Physics
Section titled “Physics”A mechanical (crank/eccentric) press has a flywheel that stores energy between strokes and releases it through the clutch during the stamping stroke. The main drive motor therefore draws a repeating current peak once per stroke: torque demand spikes as the flywheel reloads after each hit, then falls off as the ram idles. A split-core current transformer (CT) clamped around one phase conductor of the main motor leads senses this AC current non-invasively (no conductor is cut or disconnected) and outputs a proportional AC signal. Because the flywheel’s stored energy smooths the mechanical shock of the stroke, it also smooths the current waveform: the peak is a broad hump, not a sharp spike, so the detector works on peak amplitude over a short window rather than a single sample threshold. Sample rate must satisfy Nyquist for the highest stroke rate the press can run: a 60 V-notch press can exceed 100 strokes/min (1.7 Hz), so sampling at 2000 Hz gives >1000x margin and lets the detector resolve the peak shape (rise and decay of the current hump) rather than just detecting a crossing, which is what makes per-press amplitude/duration tuning possible.
Sensor
Section titled “Sensor”- Spec: split-core AC current transformer, clamp-on, 0-100 A range, 1 phase
- Signal: scalar (A), range n/a
Example parts
Section titled “Example parts”- Magnelab SCT-0750-100 split-core CT (100 A, 0-333 mV output) — clamp ID sized to the motor lead; confirm with the press’s nameplate FLA
- YHDC SCT-013-100 split-core CT (100 A, 0-1 V output)
- CR Magnetics CR3110-3100 split-core CT (100 A, 0-5 VAC output)
Mounting
Section titled “Mounting”Clamp the split-core CT around a single phase conductor feeding the press’s main drive motor (inside the motor starter enclosure or at the motor junction box), with the conductor centered in the core and the core fully latched. Route the CT leads away from high-voltage power conductors and strain-relieve them; do not clamp around more than one conductor (that measures the vector sum, not the phase current).
Wiring
Section titled “Wiring”CT output (mV or V AC, per the chosen part’s burden resistor) goes to a 2-channel isolated analog input on the edge box’s DAQ module; shield the twisted pair and ground the shield at the DAQ end only, to avoid a ground loop with the motor starter enclosure.
Validation
Section titled “Validation”run the press for 50 manually-counted strokes at its normal production speed and compare the detector’s counter delta over the same window
Expected: counted strokes within +/-1 of the manual count (CT clamps run 98-100% accurate once thresholds are tuned to the press’s flywheel-smoothed current signature)
Safety notes
Section titled “Safety notes”- de-energize and lock out the motor starter circuit (LOTO) before opening the enclosure to install the clamp; a split-core CT does not require breaking the conductor, but the enclosure itself is live
- verify the CT’s voltage rating and insulation class match the motor circuit before installing; never install a CT rated below the circuit’s line voltage
- a CT with its secondary open-circuited while current is flowing can develop a dangerous high voltage; keep the burden resistor/output load connected at all times once the clamp is closed around an energized conductor
45–180 USD, difficulty 2/5, ~20 min install downtime.