Overload Relay Protection

Overload relays are a cost-effective solution to protect expensive electric motors from failure due to overload conditions, phase-loss or current unbalances.  We carry a full line of Weg overload relays to complement the Weg contactor line.

Protected Conditions:

Overload  can occur when the motor is asked to do more work than it was designed for, or from mechanical failures in the machinery.  Attempting to run too much material too quickly through a machine is one common cause of failure.  Pump motors can also be overloaded by attempting to pump more water over a given amount of time than the motor was designed for.  Other mechanical faults can also cause overloading, including motor and machinery bearing failures.

Phase-Loss is a common cause of motor winding failures and occurs when one of the fuses in a three-phase circuit blows. Weg thermal overload relays detect phase-loss and shut down the motor by opening up the contactor.

Current Imbalance can occur when one there is a voltage drop or surge in one of the phases in incoming power.  The higher or lower voltage causes more or less current on one of the power leads to the motor, which can eventually burn up the winding. The Weg thermal overload relays sense current imbalance and shut down the motor.

Standard Features

  • Class 10 Trip characteristics (read more below about Class trip ratings)
  • Selectable Auto/Man reset button (read more below)
  • Settable current overload range
  • Phase-loss and current imbalance protection
  • Isolated 1 NO & 1NC contact
  • 2 and 3-pole versions available (single and three-phase)

Additional Features on Electronic Overload Relays

  • Selectable trip classes 10, 20, and 30
  • Wide current set range
  • Red flag trip indicator
  • Much lower energy consumption compared to standard thermal overloads
  • Optional remote electric reset

What is Trip Class?

Class 10, 20, and 30 ratings on overloads refer to how quickly the overload will respond to an overload event.  Similar to a fuse, the larger the overload, the more quickly the relay will trip.  The graphic below shows the response times vs overload for the 3 classes:

 thermal overload class trip graph

Multifunction Reset / Test Button

The button has four positions:

  • A  - Auto Reset Only: The test feature is disabled, and the relay will attempt to reset itself after the recovery time period.
  • Auto = Auto Reset / Test: The test featuer is enabled, and the relay will attempt to reset itself after the recovery time period.
  • Hand - Manual Reset / Test: The test feature is enabled and the relay will only reset after the button has been pressed and the recovery time period has elapsed.
  • H - Manual Reset Only: The test feature is disabled and the relay will only reset after the button has been pressed and the recovery time period has elapsed.

 Thermal Overload vs Solid State Electronic Overload

Weg makes both thermal and electronic versions of overload relays.  Thermal overloads use bimetalic metal (heaters) to detect when an overcurrent condition exists.  Electronic overloads use a self-powered electronic circuit to detect current.  The electronic relays are a little more exepensive, but have a much wider current setting range, and can detect phase unbalance and phase loss conditions much faster than the thermal type, offering greater protection to motors.