Contactor Coil Economizer - Battery Design

A contactor coil needs considerably more current to pull the armature in and close the contacts than it needs afterwards just to hold them shut. If a coil is driven at full pull-in current for as long as it stays closed, most of that current only ends up as heat — in the coil itself, in the driver electronics, and in whatever else has to dissipate it. An economizer circuit is the general name for anything that reduces coil current once the contactor has closed, without letting it drop so far that the contacts re-open. This page expands on the pickup and hold phases mentioned in the main Contactors overview.

Why Pull-In and Hold Current Differ

Moving the armature across an air gap against spring pressure takes a comparatively large magnetic force, so the coil needs a correspondingly large current while the contacts are still separating. Once the armature has seated and the air gap has closed, though, far less force is needed to keep it there — magnetic reluctance drops sharply once the gap closes, so the same holding force is available at a fraction of the current. Sizing the coil drive for pull-in current the whole time it is closed wastes that difference as continuous I²R heating, which matters more on a DC coil energised for the entire time a vehicle is in Drive than it would on a coil only pulsed briefly.

Economizer Approaches

Dual-wound coil with an auxiliary contact: the contactor carries two coil windings in series — a low-resistance winding that supplies most of the pull-in ampere-turns, and a higher-resistance winding. An auxiliary contact, mechanically linked to the main contacts, short-circuits the high-resistance winding while the contactor is open so the full pull-in current flows. Once the contactor closes, that auxiliary contact opens, routing current through the high-resistance winding and dropping the steady-state current to the hold level automatically, with no external control electronics needed.
Series resistor switched in after pull-in: a single coil is driven at full voltage to pull in, then a resistor is switched into series with the coil — either by a timer, an auxiliary contact, or the control electronics — to drop the holding current to a fraction of the pull-in value. This is mechanically simpler than a dual-wound coil but dissipates the “saved” power in the resistor rather than avoiding it, so it trades coil heating for resistor heating rather than eliminating the loss outright.
PWM (electronic) economizer: the coil driver applies full voltage continuously until a timer or a sensed pull-in event (such as a current spike settling, or an auxiliary contact changing state) confirms the armature has seated, then switches to a pulse-width-modulated drive at a duty cycle low enough to hold the contactor closed but well below the current needed to pull it in. This is the approach used in most modern electronic contactor drivers, since the duty cycle can be tuned in software and adjusted for coil temperature or supply voltage without any extra mechanical parts in the contactor itself.

Design Considerations

Timing the transition: switching to hold current too early, before the armature has fully seated, risks the contactor failing to close or bouncing back open; switching too late wastes the power the economizer exists to save. Designs typically build in a fixed delay with margin, sense the coil current profile, or use an auxiliary contact to confirm the armature position before dropping to hold current.
Supply voltage and temperature variation: hold current has to stay above the minimum needed to keep the contactor closed across the full range of supply voltage sag and coil temperature the system will see in service, since coil resistance rises with temperature and reduces the current available at a given hold voltage or duty cycle.
PWM ripple and audible noise: a PWM hold drive that switches too slowly can let the armature vibrate slightly at the switching frequency, which can show up as audible buzzing or, over enough cycles, as additional mechanical wear at the contacts; drive frequency and filtering are chosen partly to keep this ripple small enough that the armature stays effectively stationary.