I wanted to know what benefits if any, there would be to partially installing screened motor cables from a VSD. I have an existing unscreened motor cable run of about 45 metres from VSD to motor, with a junction box 5 metres from the motor. Would I get an noticeable noise screening if I was to replace the last 5 metres from the junction box to the motor, with screened cable?

I have sensitive electronic equipment mounted next to the motor and cable from junction box, but can not relocate or separate the motor cable or electronic equipment. Replacing the entire motor cable run with screened cable is not an option.

 

This a complicated question that is dependent on equipment and wiring geometry, frequency, voltage and currents, susceptibility of electronics to interference, whether the motor power is grounded, enclosures, cabling, etc. If the cable is in open air, the further it is away from the electronics the better (distance and orientation are your friends). A grounded at one end screened cable typically only shields against electrostatic coupled noise (capacitive coupled - voltage based). The higher the voltage of your motor power, the more the potential for coupling noise capacitively and placing a grounded screen will help, but how much is an unknown quantity. Five meters away from your sensitive electronics should be a sufficient distance. A grounded screen will not provide protection against magnetic coupled noise (inductive coupled - current based) as opposed to steel conduit which will provide protection. The more motor current; the more potential for coupling magnetic noise. There is also the question of the enclosures that contain your sensitive electronics and their susceptibility to coupled noise. Grounded steel enclosures for your sensitive electronic equipment will provide protection against electrostatic and magnetically coupled noise. Your cabling to and from your sensitive electronic may be exposed and should be orientated 90 degrees to the motor power cable if possible.

Are you currently experience interference based noise problems?

William (Bill) L. Mostia, Jr. PE
ISA Fellow, FS Eng. (TUV Rheinland)
WLM Engineering Co.