When running an AC motor with a VFD (Variable Frequency Drive) it is important to use the VFD motor cables. AC VFD’s use transistors to simulate an AC waveform using PWM (Pulse Width Modulation) at a high frequency, generally 2-16Khz. This high frequency is broadcast down the motor output cables and is emitted as noise to the environment along the run to the motor. This noise plays havoc on all sensitive equipment near the motor cables. Some types of sensitive equipment include Ethernet communications cables, computers, and even the feedback coming back from the motor the VFD is controlling. Any low voltage signal like TTL feedback (5V encoder), sin/cos feedback (1V p-p), analog signals (0-10V), and communications can be affected.
Real World Applications
In a real world application we have seen a motor (in this case a Nidec servo motor) being wired to a Nidec Unidrive M700 with Sin/Cos feedback fail to autotune even when using VFD cable due to the shield wiring not being grounded at both the motor and the VFD. Once the proper wiring was done by landing both the ground and shield wires to the grounds on both the motor and the VFD the autotune was able to be completed and the motor ran the application as expected. When connecting VFD motor cables, both the ground and shield/drain wires should be connected on both ends. An ounce of prevention is worth a pound of cure is an old saying that is very true in this situation. Preventing the noise from entering the environment is the best way to mitigate noise issues in applications that use AC VFD’s.
If you are having intermittent issues with any equipment near an AC VFD that does not use four conductor VFD motor cables
with an overall shield you should take a look at changing out the motor cable with VFD cable.
We have heard some say they have never had to use VFD motor cables and have never seen an issue with noise or the VFD cable is too expensive. Because you haven’t seen the problem yet doesn’t mean you shouldn’t be using VFD cable. If you never want to experience issues with unexplained “ghosts in the machine” then using VFD cable in an AC VFD application is a must! There are many applications where you can get by without VFD, but knowing when and where to use it is what a MES Engineer can help you determine. We have a dedicated staff here ready to answer your questions. We offer field service and phone support in a pre-sale to post-sale environment. Call us today! 317-844-7328