You are correct; properly connecting the shield in VFD cable is crucial for its effectiveness and is a strong suspect in your motor overheating issues. Let's break down why and what you should do

Why VFD Cable Shielding is Important
Mitigation of Electrical Noise (EMI)

VFDs (Variable Frequency Drives) create a lot of electrical noise (Electromagnetic Interference, EMI) due to the rapid switching of power. This noise can:

Disrupt sensitive electronic equipment

This includes your PLC, control circuits, communication networks, and even other VFDs.

Cause motor heating

Reflected waves from the VFD back to the motor due to impedance mismatches caused by the VFD can lead to increased circulating currents in the motor windings, resulting in overheating and premature failure.

Damage motor insulation

The high-frequency components of the VFD output voltage can stress the motor insulation, leading to breakdown.

Grounding Path for Leakage Current

The shield provides a low-impedance path to ground for high-frequency leakage currents produced by the VFD.

Reduces Common Mode Voltage

The shield can help reduce common mode voltage, which can cause bearing currents in the motor.

Shield Termination: The Correct Way

The best practice is to terminate the shield at both the VFD end and the motor end. Here's why: Complete Shielding

Terminating both ends creates a fully enclosed Faraday cage around the conductors, providing maximum protection against EMI.

Lower Impedance Path

Terminating both ends provides a lower impedance path for high-frequency currents to return to the source (the VFD).

Reduced Common Mode Noise

Helps reduce common mode noise by providing a path to ground for common mode currents.

How to Terminate the Shield1. VFD End

Use a dedicated grounding lug or clamp designed for VFD cable shields. These are often integral to the VFD's enclosure. Ensure the shield is firmly connected to the lug, making good contact over a large surface area. Avoid simply twisting the shield wires together. The lug should be connected directly to the VFD's ground bar.2.

Motor End

Use a similar grounding lug or clamp system that mounts to the motor frame. Many motor junction boxes have provisions for this. Ensure good contact between the shield and the lug, and between the lug and the motor frame. Consider using a 360-degree termination clamp to achieve a low impedance path. These provide the most surface contact with the shield and provide the best EMI protection. If your motor junction box does not have a provision for shield grounding, you may need to install one.3.

Inspect for proper connection Use a multi meter to verify that there is good continuity between the VFD cabinet ground and the motor chassis ground. A low impedance reading in the cable shield indicates a proper connection.

Your Motor Overheating Problem: Root Causes and Solutions

Since your shield is not connected, it's highly likely the VFD noise is contributing to your motor overheating. However, not being an inverter duty motor is a huge part of the problem.1. Non-Inverter Duty Motor

Standard motors are not designed to withstand the voltage stresses and high-frequency components produced by VFDs. This is likely your primary issue. Inverter duty motors have:

Improved insulation

Designed to withstand the voltage spikes.

Bearing insulation

To prevent circulating currents (bearing currents) that can destroy the bearings.

Better cooling

To handle the increased heat from VFD operation.2.

Reflected Waves and Motor Impedance Mismatch

Long cable runs, particularly without proper shielding, can lead to reflected waves from the motor back to the VFD. This increases voltage stress on the motor insulation and can contribute to overheating.3.

Harmonic Distortion

VFDs introduce harmonic distortion into the power system. While the cable shield helps, additional filtering may be required.

Recommendations1. Connect the Shields Properly

As outlined above, ensure the VFD cable shields are terminated at both the VFD and motor ends using appropriate grounding lugs. This is your first step.2.

Replace with Inverter Duty Motor

This is critical. You must use an inverter duty motor with a VFD. A 2 HP motor is relatively inexpensive compared to the cost of downtime and repeated motor replacements. Consider a motor with shaft grounding to reduce bearing currents.3.

Motor Protection

Install a motor overload relay or other motor protection device to protect the motor from overheating.4.

Line Reactor/Load Reactor

Depending on the cable length and VFD type, a line reactor (on the VFD input) or a load reactor (on the VFD output) can help reduce voltage spikes and harmonic distortion. Consult with a VFD expert to determine if this is necessary. Load reactors are more effective in this case.5.

Consider Motor Chokes/Filters

In some cases, a motor choke or filter on the output of the VFD may be necessary to further reduce voltage spikes and harmonic distortion.6.

Cable Length

If the cable run is very long (over 100 feet), consider using a VFD with a built-in output filter or using a load reactor at the VFD output.7.

Check Motor Cooling

Ensure the motor's cooling fan is working properly and that the motor is not mounted in an enclosed space that restricts airflow.8.

Motor Bearings

If the bearings are failing prematurely, this could be a sign of bearing currents. Insulated bearings or a shaft grounding ring may be necessary.9.

Consider dV/dt Filters

dV/dt filters reduce the rate of voltage change (dV/dt) at the motor terminals.10.

Check Motor Nameplate

Confirm the motor is rated for the voltage and frequency of the VFD output.11.

Consult an Expert

If you continue to have problems after implementing these recommendations, consult with a qualified electrician or motor specialist who has experience with VFD systems.

Important Cautions Safety First

Always disconnect power to the VFD before working on any electrical connections.

Proper Grounding

Ensure that the VFD, motor, and cable shields are properly grounded to a common ground point.

Cable Routing

Keep VFD cables separated from other signal cables to minimize interference.*

Tight Connections: Ensure all electrical connections are tight and secure.By addressing the shielding issue and, most importantly, switching to an inverter-duty motor, you should significantly improve motor life and reduce your maintenance costs. Good luck! Flag for review