Electronic protection at a bimetallic price

Rockwell Automation has introduced a compact electronic motor protection relay, which has a similar price to a traditional bimetal overload relay, but is said to offer several benefits. The self-powered device can replace a thermal overload relay directly but generates only about 2.5% of the heat, minimising the need for cooling in enclosures.

While traditional overload relays pass motor current through heating elements to simulate an increase in motor temperature, the Allen-Bradley E1 Plus (shown below) uses solid-state components to create accurate thermal models. A custom chip processes the motor current data constantly to determine the time-current motor-loading status.

A thermal memory circuit allows the relay to monitor the heating and cooling of motors while both while they are operating, and afterwards, helping to extend their lives.

Trip repeat accuracy is claimed to be ±1% compared to around ±5% for conventional thermal overload relays.

The new relay also has a phase-loss detection circuit which responds in just 3s, compared to up to 40s for traditional overloads.

The electronic relay uses standard wiring with direct contactor mounting and provides normally open or closed, isolated contacts. Its 5:1 FLA (full load Amperes) range means that one electronic device can span the same range as four bimetallic devices. Ratings from 0.1-90A are covered by just eight models.

An enhanced version of the relay includes a reset mode and four trip classes chosen using DIP switches. An optional DeviceNet module can transmit information including overload warnings and device status, and to allow trips to be reset via a network.