You're right. Using standard overload relays with VFD outputs can be problematic due to the PWM waveform. The harmonic content and high-frequency switching can lead to inaccurate readings, nuisance tripping, and even damage to the overload relay itself. Here's a breakdown of things to consider and some options

Why Standard Overloads Often Fail with VFDs:
PWM Waveform Distortions

VFDs produce a PWM (Pulse Width Modulated) waveform. This waveform doesn't directly provide a clean sinusoidal voltage like the utility grid. While the average voltage and frequency are controlled, the instantaneous voltage and current have significant harmonic distortion.

Harmonic Heating

Harmonics cause additional heating in the motor windings. Standard overloads are designed to respond to the fundamental frequency current, not the total current (including harmonics). This means the motor can overheat before the overload relay trips.

Inaccurate Current Sensing

Many overload relays rely on thermal elements or current transformers that are sensitive to the fundamental frequency. The PWM signal can cause these elements to react erratically, leading to premature tripping or failure to trip when necessary.

dV/dt (Voltage Rise Time) Issues

The rapid voltage rise times (dV/dt) produced by VFDs can create capacitive currents that interfere with the overload relay's sensing circuitry.

What to Look For in a VFD-Compatible Overload Relay



True RMS Sensing

This is crucial. A True RMS (Root Mean Square) sensing overload relay measures the effective current, accounting for the harmonic content of the PWM waveform. This gives a more accurate representation of the actual motor heating.

Filtering

Look for overload relays that have built-in filters to attenuate the high-frequency switching noise from the VFD. This helps improve the accuracy of the current measurement.

Compatibility Certification

Some manufacturers specifically certify their overload relays for use with VFDs. This gives you added assurance that the device will perform reliably in this application.

Adjustable Trip Class

Having an adjustable trip class (e.g., Class 10, 20, 30) allows you to tailor the overload protection to the specific motor and application. Slower trip classes (e.g., Class 30) are often recommended for motors that experience longer acceleration times or frequent starts.

Wide Current Range

Choose an overload relay with a current range that covers the full load ampacity (FLA) of the motor(s) you are protecting.

Separate Mounting Options

As you requested, look for models specifically designed for DIN rail or panel mounting, independent of a contactor.

Brands and Models (Suggestions and Where to Look)



Important Note

Always verify the manufacturer's specifications and application guidelines to ensure the overload relay is suitable for your specific VFD and motor combination.

Eaton (Cutler-Hammer)



XT Series Electronic Overload Relays

These are often recommended for VFD applications. They offer True RMS sensing, adjustable trip classes, and communication options. Some models are DIN rail mountable. Specific models within the XT series that might be suitable include the XTOM

series. Check Eaton's website and documentation for VFD compatibility.

SmartWire-DT Compatible Overloads

Some Eaton overloads are compatible with their SmartWire-DT communication system, allowing for easy integration with PLCs and HMIs. This can provide advanced monitoring and diagnostics.

Siemens



3RU21 Thermal Overload Relays

These are suitable with proper current derating. Check Siemens's documentation and application guidelines for VFD compatibility and derating factors.

3RB3 Electronic Overload Relays

Designed for more sophisticated protection, they offer features like True RMS sensing and communication capabilities. Some models are available for separate mounting. Check Siemens's website and documentation for VFD compatibility.

Simocode Pro

Siemens also has motor management systems like Simocode Pro that can provide advanced protection and control. These are designed to work with VFDs.

Rockwell Automation (Allen-Bradley)



E3 Electronic Overload Relays

These offer advanced protection features, including True RMS sensing and communication options. Some are DIN rail mountable. Check Rockwell Automation's documentation for VFD compatibility.

193-ECM Solid-State Overload Relays

Specifically designed for motor protection with VFDs. Offers true RMS sensing, adjustable trip class, and other advanced features.

Schneider Electric



TeSys LR9D Electronic Overload Relays

These are designed for motor protection and are often recommended for use with VFDs. Look for models with True RMS sensing and adjustable trip classes. Check Schneider Electric's website and documentation for VFD compatibility.

Tesys GV4 and GV5 Motor Circuit Breakers

Some of these have advanced features and may be suitable for VFD applications. Consult the documentation carefully.

ABB



EF Electronic Overload Relays

These offer True RMS sensing, adjustable trip classes, and communication options. Check ABB's website and documentation for VFD compatibility.

Ekip UP

A digital motor protection and control unit, offers true RMS measurements and is suitable for VFD applications.

Carlo Gavazzi

Carlo Gavazzi offers a range of solid-state overload relays that are designed to work with VFDs. Their relays provide true RMS sensing and adjustable trip parameters.

Where to Look for More Information



Manufacturer Websites

The websites of the manufacturers listed above are your best resource for detailed product information, specifications, application notes, and VFD compatibility charts.

Distributor Catalogs

Major electrical distributors (e.g., Graybar, Rexel, Wesco, Platt Electric) carry a wide range of overload relays and can provide technical assistance.

Application Notes

Search for application notes from VFD and overload relay manufacturers that specifically address the topic of overload protection in VFD applications. These notes often provide valuable guidance on selecting and configuring overload relays.

IEEE and IEC Standards

Refer to relevant IEEE and IEC standards for motor protection and VFD applications.

Important Considerations



Derating

In some cases, even with a VFD-compatible overload relay, you may need to derate the FLA setting slightly. This accounts for the harmonic heating in the motor. Consult the manufacturer's recommendations.

Motor Temperature Monitoring

For critical applications, consider using motor temperature sensors (e.g., PTC thermistors or RTDs) in addition to the overload relay. These sensors provide direct temperature measurement of the motor windings and can provide early warning of overheating. These sensors can be connected to the VFD or a separate protection relay.

Coordination

Ensure that the overload relay is properly coordinated with the upstream circuit protection devices (e.g., fuses or circuit breakers) to prevent nuisance tripping or damage to equipment.

Steps to Take


Identify the Motor(s)

Determine the full load ampacity (FLA) of each motor and any specific application requirements (e.g., frequent starts, heavy loads).2.

Check VFD Documentation

Consult the VFD manual for recommendations on overload protection. Some VFDs have built-in overload protection, but you may still need an external relay for better performance or coordination.3.

Research VFD-Compatible Overloads

Use the information above to identify potential overload relay models from reputable manufacturers.4.

Consult with a Supplier or Engineer

Discuss your application with a knowledgeable supplier or electrical engineer to ensure you select the appropriate overload relay and configure it correctly.5.

Thoroughly Test:* After installation, thoroughly test the overload protection system to verify that it functions as intended.By carefully selecting and configuring a VFD-compatible overload relay, you can provide reliable motor protection and prevent costly downtime. Flag for review