Thermal Overload Relays

Thermal overload relays prevent an electric motor from drawing too much current and overheating.  Thermal overload conditions are the most likely faults to be encountered in industrial motor applications. They result in a rise in the motor running current, which produces an increase in the motor's thermal dissipation and temperature. Overload protection prevents an electric motor from drawing too much current, overheating, and literally burning out.

Thermal overload relays can be bimetallic relays, eutectic alloy relays, temperature control or probe relays, and solid-state relays.  A bimetallic device is made up of two strips of different metals. The dissimilar metals are permanently joined. Heating the bimetallic strip causes it to bend because the dissimilar metals expand and contract at different rates. The bimetallic strip applies tension to a spring on a contact. If heat begins to rise, the strip bends and the spring pulls the contacts apart, breaking the circuit.  A melting alloy (or eutectic) overload relay consists of a heater coil, a eutectic alloy, and a mechanical mechanism to activate a tripping device when an overload occurs. The relay measures the temperature of the motor by monitoring the amount of current being drawn. This is done indirectly through a heater coil.  Temperature control relays are used to protect the motor by directly sensing the temperature of the windings using thermistor or RTD probes. The motor must have one or more positive temperature coefficient (PTC) thermistor probes embedded in its windings. When the nominal operating temperature of the probe is reached, its resistance increases rapidly. This increase is detected by a threshold circuit, which controls a set of relay contacts.  Solid-state relays have no moving or mechanical parts. The relay calculates the average temperature within the motor by monitoring its starting and running currents. A solid-state relay is a type of overcurrent relay.

Important performance characteristics to consider when searching for thermal overload relays include full current load range, trip class, and temperature trip range.  The adjustable current value allows the relay to be set to the full-load current shown on the motor rating plate. This is the current range of the thermal component that can be adjusted to the desired trip point.  The trip class is the maximum time in seconds at which the overload relay will trip when the carrying current is at 600% of its current rating. Bimetallic overload relays can be rated as Class 10, meaning that they can be counted on to break the circuit no more than 10 seconds after a locked rotor condition begins. Melting alloy overload relays are generally Class 20. Although thermal overload relays are designed to protect motors against overload currents, they must be capable of handling large currents without tripping for short periods during motor starting (run-up). They should, however, trip quickly if the starting currents last too long.  The temperature trip range is the trip point setting range for relays designed to monitor the temperature of the motor stator windings.

Other important specifications to consider when searching for thermal overload relays include motor load phase, motor voltage, control circuit voltage, contact or output ratings, pole specifications, features, and environmental operating parameters.  The motor load phase can be single-phase protection or three-phase protection.  The maximum motor voltage is the applicable maximum motor voltage.  The control voltage, if different from the motor voltage. This is called "separate control" and means that the control circuit gets its power form a separate source; usually lower in voltage form the motor's power source.  Contact output ratings include the contact current rating and contact maximum rated voltage.  Poles can be single pole, double pole, triple pole, four pole, or greater than four pole.  Common features for thermal overload relays include ambient temperature compensation, automatic reset, built-in emergency override, built-in trip indicator, hermetically sealed, programmable or adjustable trip time, ground fault detection, phase loss detection, phase reversal detection, unbalance protection, and underload protection.  An important environmental parameter to consider is operating temperature.

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Part Numbers for Thermal Overload Relays