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VFD cable

If your motor application with speed range between 0-Ns in constant torque mode, then use normal 3 core XLPE cable with shielding by terminating the shielding at only one end. In case, your application is for Constant Power with speed range beyond Ns and upto 2Ns, then instead of normal cable other alternative possibilities shall be considered.

It is absolutely paramount that you terminate the shield at both ends and you must do this by using a 360 degree clamping method such as an EMC gland, saddle clamp or similar.

If you terminate one end only, or use pigtails to make the connection, you are increasing the high frequency impedance between the VFD frame and the motor frame. This will result in a major increase in the conducted noise through the installation.

The reason the VFD manufacturer requires you to use a variable speed drive type motor feeder cable is to reduce the possibility of EMC current caused by the VFD pulsing to escape and not return to the VFD via the cable shield. These currents are at high frequencies in the 500kHz or higher range (radio frequency) and high frequency current only flows in the outer skin of the conductor. If the shield does not have sufficient skin effect, then the impedance will be too high and the EMC current will look for a lower impedance path to return to the VFD through the rest of the plant.

The motor and variable frequency drive must be bonded together for a low impedance return path back to the VFD (source). The bonding must be with braided straps and concentric bonding clamps to ensure low high frequency impedance connection.

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