Without having the opportunity to see the installation I can only guess what is the cause. A few pics would be useful, but I do not think we can handle these on the Forum.

So Here Goes:

Reduce the switching frequency as low as possible, this will reduce emissions, radiated & conducted, and hence the interference. If this 'hack' improves the situation it is a good indicator of the cause of the fault, but not the cure, efficient screening is necessary.

Leakage from the motor will be significant, and will have to find the Low Impedance route back to the noise generator, which is the inverter drive. If this route is obstructed with high a impedance circuit the interference will interfere with local devices, and the drive internal communications.

Ensure the motor cable is screened, throughout its length, and each end is terminated by clamping at the motor end and the chassis of the inverter drive. Cable screen can only be effective if the screen can present a low impedance to high frequency circulating currents, these currents will be in the region of MHz, top end of the VHF band.

If the motor cable is fitted with an isolator, the screen must be routed around this break using a clamping method, possible using copper braid, and make this as wide as possible. The aim here is to conduct very high frequency currents, travelling from the motor to the inverter drive.

Tractor

In order to support the source of 'high frequency circulating currents'.

The necessary rotating field applied to the motor is produced by 'Pulse Width Modulation' (PWM) from the inverter drive, where power transistors switch the DC Link supply in a pattern. Switching ON or OFF, to create a pulse, the Voltage rising to the DC Link Voltage level, and then overshooting, before settling, this action produces 'ringing', add this noise generated by the PWM, producing a very high frequency, within the motor coils.

Motor coils supplied by a sinusoidal supply for a standard network, 400V 3 phase, show very low levels of leakage; but they do have electrical capacitance at low levels, Pico Farads (pF), benign at 50/Hz sinusoidal supplies, but at high frequencies become very active, and readily pass currents between motor coils and the motor body.

Consider these currents as 'noise', generated by the Inverter Drive manufactured PWM, leak from the motor and must have a free 'low impedance path' return to the drive chassis. A poor low impedance path will cause these currents to route via local instruments for example, in order to locate the 'noise generator'.

For the low Impedance path use flat electrical braid as wide as practicable, steel conduit, the armour of SWA cable is effective if terminated by recognised clamping technique. Round electrical cables cannot conduct these currents due to the round cross section.

The installation of inverter drive devices, requires some planning in order to protect local systems which can be vulnerable to this interference.

Here, I have tried to provide an explanation and understanding for drive installations. If I can elaborate further please come back.

Tractor