Editor’s Note: This article was created with expertise from Peter Cox, the director of industrial projects for our industrial cable group.
VFDs, or Variable Frequency Drives, are used by manufacturers to control motors at variable speeds, thus matching the speed of the motor-driven equipment to the load requirement. For example, to operate a long conveyor system, of let’s say beer production, a motor is used to turn the drive pulley and move the conveyor belt forward.
Nowadays, such a motor would typically be connected to a VFD so that if the motor does not need to run at full speed, it can be operated at a lower speed. This provides multiple benefits, including longer motor life, energy savings, improved process control and higher reliability.
VFDs sound great, and they are, but there is one drawback to them. They generate electrical noise that radiates out and can disrupt electronic equipment, network systems and even instrumentation. In some cases this can disrupt manufacturing, leading to downtime and quality control issues.
There is a simple way, however, to maximize the benefits of VFDs without decreasing the performance of nearby devices and systems. That way is to make sure that the cables connecting the VFDs to motors are high-performance ones. Furthermore, if the motor is high power (200 horsepower or greater), then parallel VFD cables offer a significant performance benefit.
Let’s take a look at how parallel VFD cables improve the safety and performance of high-power motors.
The motor that turns the pulleys for this conveyor belt system is connected to a VFD for improved control and energy savings.
1. Select High-Performance VFD Cable Rather than Construction-Grade Cable
A challenge in purchasing VFD cables is that there are no standards for them. Thus, it can be difficult to differentiate between minimum construction-grade cable sold as VFD cable and high-performing VFD cable that protects motors and ensures the maximum benefits from using a VFD system.
Below is a quick guide to help you differentiate between the two.
Table 1: Comparison of the characteristics of construction-grade and high-performance VFD cable.
In summary, the benefits of high-performance VFD cables are:
- Better shielding and larger grounds for less electrical noise (common mode current)
- Flexible, tinned stranding for easier installation/termination and tighter bending radius
When the application involves motors of 200 horsepower (149 KW) or more, the conductors must grow out in proportion to the power, in order to ensure the required ampacity. Ampacity is the maximum amount of electrical current a device can carry before overheating, or as limited by statutory requirements.
When multiple smaller cables are paralleled, as permitted under the National Electrical Code, more efficient conduction is achieved. In fact, high-performance parallel VFD cables with full-sized grounds provide more effective conduction and significantly larger grounds, not only in total, but in each cable, as compared to construction-grade VFD cable.
Thus, a high-performance solution with multiple parallel VFD cables has the following advantages:
- Overall copper use is reduced
- More ground copper is installed, leading to reduced noise emissions
- Cables are more flexible and easier to terminate
- Lower pulling force is required during installation
A 600 HP (447 KW) motor is serviced with multiple high-performance parallel VFD cables.
(Quadruple 4/0 AWG Belden 100% ground VFD cable)
VFD Cable Specification Assistance
To help you select or specify the right industrial VFD cable, available below is an application note on parallel VFD cabling that:
- Looks at NEC code provisions and explains how they apply
- Works through specifying the right VFD cable for a particular usage
- Provides a handy table showing buildups for motors of different power
Have you experienced VFD noise issues or had trouble installing thick VFD cables? Let me know your challenges and how you are dealing with them.
- Blog: Why High Performance VFD Cable is Important
- Webpage: VFD Cable
- Document: Unarmored Variable Frequency Drive (VFD) Cable Termination Guide
- Blog: VFD Cable Line Extension with MCM VFD to Support Large Horsepower Motors
- DesignNews.com blog: Engineering Directives Not Followed... Again
- Webpage: VFDS.com
- Online Presentation: Variable Frequency Drives and Mining Applications
Peter Cox is the Director of Global Industrial Projects with Belden. He has a Bachelor in degree in Engineering from Carleton University in Ottawa Ontario and is a licensed Professional Engineer. He has worked in the Cable, Drives, or Automation Industry or as a consultant to those industries and markets for his entire 34 year career. Peter has extensive experience in the application of Systems Drives, as well as extensive first hand exposure to many if the common application issues.