Many industrial manufacturers are looking for ways to reduce their power consumption both to reduce costs and greenhouse gas emissions. Industry accounts for over 40% of worldwide energy consumption and 65% of its power demand comes from electric motor-driven systems. One way to greatly reduce the energy required to run motor systems is to use what are called “VFD”s.
Variable Frequency Drives, or VFDs, are a silver bullet for industrial energy conservation. They reduce the energy required to run motor systems by precisely controlling their speed. Depending on load characteristics, a motor running at half speed may require as little as 1/8 of the power it would require at full speed.
Although VFDs have been available for 25 years, they are more compelling than ever thanks to the push for cost and energy savings. Today I am going to explain a little bit about VFD technology and in particular look at why choosing the right VFD cable is important to the long-term success of the installation.
A simple representation of a VFD system
Above is a diagram of a VFD and motor system, which includes 3 key parts:
• The VFD itself with an operator interface
• The cable that connects the VFD to the motor
• The electric motor that is controlled by the VFD
The magic of the VFD is that it is able to control the rotational speed of the AC motor by controlling the electrical power supplied to the motor. Older motor controls weren’t precise enough to support variable speeds but today’s VFDs can very precisely and almost instantaneously alter the speed of a motor whenever its load or process demands change.
Besides energy savings, there are a number of other benefits of using the precision control provided by VFDs to run motors. These include:
• Improved process control
• Reduced manufacturing waste
• Longer useful life of motors
• Reduced maintenance of motors and mechanical components due to less wear and tear on them
• Higher reliability
Everything about VFDs sounds great, however, there are “side effects” from using them that need mitigation—and this is where selecting right cable comes in.
The single most significant problem with VFDs is that they generate disruptive electrical noise in the environment around them that can create other problems in the manufacturing process. This noise radiates out to other devices such as electronic equipment, commercial-grade Ethernet systems and even simple instrumentation wire, decreasing their performance.
For example, one manufacturer spent a lot of effort trying to determine why a piece of equipment was not operating properly. When the cable attached to a VFD on the floor below the troublesome device was replaced with high-performing VFD cable, the problem went away. In other cases solid state safety relays have triggered emergency shut downs because of disruption from the noise of construction-grade cable between VFDs and motors.
Particularly if your facility includes noise-sensitive equipment or if you have long cable runs, you want to make sure that you install high-performing VFD cable.
There are 6 important considerations when selecting the appropriate cable for a VFD system.
1. Ample Grounding Configuration and Termination
An improperly grounded VFD cable creates noise-related issues within the system and improperly terminated cables can release captured noise current. The more copper there is at ground potential, the more effective the cable will be.
2. Proper Shielding to Contain Noise
To avoid noise problems, VFD cable needs to be effectively shielded. Our research suggests that shielding systems that include dual copper tape or combination foil/braid types are the most appropriate for VFD applications. With such shields the cable returns excess noise to the drive, keeping the motor and nearby equipment running properly.
3. Sturdy Insulation for Superior Electrical Performance
Cross-linked polyethylene (XLPE) insulation far surpasses PVC/nylon as an insulator for VFD cables because it can withstand voltages as much as 3x higher. This allows for longer cable runs, protects the motor and increases the efficiency of power transfer from the motor to the drive.
4. Appropriate Stranding
VFD cables with tinned copper strands rather than just copper strands provide good corrosion resistance and thermal stability. Tinned cooper connections are much less likely to oxidize and degrade at hot spots.
In addition, a high strand count enhances cable flexibility and significantly reduces harmful noise.
5. Industrial Hardening
VDF cables need to be reliable and rugged enough to handle the harsh industrial environments in which they are placed. It is important to choose industrial-grade cabling that can withstand humidity, grit, sunlight, oil and other conditions that can break down less-robust materials.
6. Use Manufacturer Recommended Cable
It almost goes without saying it’s best to use VFD cable that is recommended by the manufacturer of the VFD.
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.
The above guidelines will help you weed out construction-grade cable from your consideration. The final step is to carefully match your application and VFD drive to the range of high-performing cables provided by a vendor such as Belden. To help you do that, consult the white paper and guides available at the end of this article.
It’s not often that you learn about a straight forward way to significantly reduce energy consumption and save money with existing technology. That’s the great news about VFD systems.
Once you decide to implement VFD for motor control, don’t risk losing production time, damaging machines or causing a system failure by using the wrong cable. High-performing VFD cable ensures motor uptime and reliability, protects sensitive instrumentation adjacent to control systems and allows for long cable runs. In addition, it maximizes your investment in VFD.
How have VFDs helped you reduce costs? Have you had any noise problems after commissioning them? Let me know your thoughts and feedback.
• 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