Relay protects 3-phase motors from thermal overload.

Press Release Summary:

Model SEL-749M protects against nuisance tripping while enabling release of maximum horsepower. True Temperature-Based Thermal Overload Model tracks heating effects of load and current imbalance while motor is accelerating and running, thereby handling all motor operation scenarios. Adaptive Overcurrent Element technology enables unit to protect against high fault-current conditions when Current Transformer saturation is present.

Original Press Release:

New SEL Relay Technology Prevents Nuisance Tripping from Cyclic Overloads-Releases Extra Horsepower from Motors

(Pullman, WA) Three-phase motor applications protected by thermal overload relays can now avoid nuisance tripping, due to new technologies from Schweitzer Engineering Laboratories (SEL). Incorporating SEL's exclusive True Temperature-Based Thermal Overload Model thermal overload protection and patent-pending Adaptive Overcurrent Element, the new SEL-749M Motor Protection Relay offers comprehensive protective functions for all industrial and commercial three-phase motors.

Unlike traditional overcurrent-based motor relays, which can misdiagnose motor health during cyclic overloads and incorrectly trip just when you need a motor the most, the compact SEL-749M Relay protects against nuisance trips while enabling the release of maximum horsepower. Applications for this affordable new relay include two-speed and reduced-voltage-start motor applications.

The SEL-749M Relay protects against locked rotor, running overload, and negative-sequence current imbalance through a patented thermal overload model. This technology accurately tracks the heating effects of load and current imbalance while the motor is accelerating and running, thereby effectively handling all motor operation scenarios, including cyclic overloads, without nuisance tripping.

The integration of SEL's innovative Adaptive Overcurrent Element technology enables the SEL-749M Relay to protect against high fault-current conditions when Current Transformer (CT) saturation is present. This CT-saturation condition frequently occurs when using undersized CTs, and can expose a power system to catastrophic damage.

The SEL-749M protects "soft start," reduced-voltage motors with wye-delta starters from high voltage and current spikes, as well as high starting torque and mechanical stress. Thus, customers can enjoy the benefits of reduced-voltage starting, including decreased system maintenance and downtime, without the associated electrical and mechanical wear and tear.

For applications that require two-speed motor protection, the SEL-749M offers settings for a second value for the rated FLA (full load amps) motor current, CT input ratio, and thermal overload protection, selected via a digital control input. Such two-speed applications include designed two-speed processes (e.g. blowers), briefly increased motor loading, or achieving maximum loading for ambient temperature excursions (e.g. day/night loading on exposed water pumps or conveyor belts).

Option cards, which can be field-installed into the base relay or configured with the initial order, enable customers to obtain voltage inputs, including undervoltage, overvoltage, underpower, power factor, and reactive power elements. Base relay inputs/outputs are expandable from 2 inputs/3 outputs to 5 inputs/7 outputs, plus a 4-20 mA analog output using an optional I/O expansion card.

The SEL-749M provides simple, yet powerful communications capabilities. Front and rear serial ports are available for communicating with ASCII terminals, local HMI, SCADA, or DCS systems. The new SEL relay offers Modbus. RTU and DeviceNet communications options, as well. Other powerful SEL-749M features include oscillographic event reports (as many as 23 nonvolatile 15-cycle filtered or raw reports) and metering functions eliminate or reduce external recorder and metering requirements. The Sequential Events Recorder (SER) stores the latest 512 time-stamped relay element operation entries in nonvolatile memory.

Exclusive SEL-5030 ACSELERATOR. software (included with the relay purchase) provides a convenient method for creating, storing, and deploying relay settings through a user-friendly Windows-based environment. This software also automatically range-checks all settings. In addition the ACSELERATOR software provides easy access to relay event reports and event oscillograms.

About Schweitzer Engineering Laboratories (SEL) Industrial Solutions Group: For more information about this and other SEL products or the location of the SEL representative nearest to you please visit our website at, or contact the SEL Industrial Solutions Group by phone (615) 507-2184; fax (615) 507-2188; e-mail:; or mail to 130 Seaboard Lane, Suite A7, Franklin, TN 37067.

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