Contactor Drop-out Voltage
The voltage level at which a pulled-in contactor releases its contacts as the coil voltage falls. This level is markedly lower than the pull-in voltage; once a contactor has pulled in, it keeps holding at much lower voltage, and this behaviour becomes a source of trouble on long control lines.
The pull-in force of an electromagnet is inversely related to the air gap. When the core is far away the force is weak, so a high voltage is needed to pull in. Once the core has seated, the gap closes, the force rises and the contactor holds at a much lower voltage. The result: there is a wide hysteresis between the pull-in and drop-out thresholds.
Its two typical consequences in the field:
- Failure to pull in: if the cable cross-section is insufficient on a long control line, the high current at the moment of pull-in creates a voltage drop along the line and the coil cannot cross the threshold. The contactor hums and does not pull in.
- Late drop-out: as the supply falls, the contactor may stay pulled in longer than expected; this increases the risk of two contactors overlapping during a direction change.
The measures are clear: choose the control line cross-section from a voltage drop calculation, place the contactor as close to the receiver as possible, and leave a short dead time on direction changes. If you use a freewheeling diode, allow for the drop-out time getting somewhat longer; a small resistor in series with the diode reduces this delay.
The context in which this term is used: Wireless remote control installation: pairing, relays, safety