Why use variable frequency drives?
The full load efficiency of standard AC electric motors ranges from around 80% for the smallest motors to over 95% for larger ones, but the efficiency of all electric motors drops significantly as the load is reduced below 40%. As a result, good practice dictates that motors should be sized so that full load operation corresponds to at least 75% of the rated power of the motor.
In many installations, a VFD is the best way to ensure that a motor is sized correctly and runs efficiently. A typical VFD-motor installation will exhibit system efficiencies of up to 90% or more at full load. At reduced loads, the efficiency drops, but not as significantly as with single-speed motor installations. For example, VFDs coupled with motors may have 90% efficiency for all loads greater than 25% of full load.
Uncontained CMC noise can wreak havoc
Many existing VFD installations use interlocked armor TECK or continuously welded and corrugated aluminum armor (CCA) cable to connect from the VFD to the motor. These cable types will release excessive electrical noise in many installations, which can wreak havoc with the sensitive electronic components, network cabling and other sensitive wiring found in most modern industrial facilities.
The solution is to use specially-designed VFD cable with a foil braid or copper tape shield and suitable ground wires. For larger cables it becomes important to use a symmetric ground design to balance the induced ground currents. Compared to TECK or CCA, this type of VFD cable provides superior performance in terms of common mode current (CMC) containment and in other areas.
With VFD cable, CMC is effectively contained within the cable and returned back to the ground point. With both TECK and CCA cable, CMC may be returned back to the VFD enclosure metal causing electrical noise to radiate from the enclosure into surrounding areas. This radiated electrical noise, commonly referred to as electromagnetic interference/radio frequency interference (EMI/RFI), can often cause certain electronic components, cabling and wiring to malfunction.
VFD cable specifics
VFD cable features grounding systems which are specifically designed for the application. These systems utilise much more copper in the grounds to reduce induced and coupled ground currents. For higher power applications VFD cables utilise symmetric grounds to reduce induced currents. TECK has neither the grounds size, nor symmetry required to reduce ground currents and potential difference. The lack of symmetric grounds in TECK cable results in the introduction of excessive current noise into the grounding system, particularly with high power VFD-motor installations.
So TECK cable not only generates more current noise due to the lack of symmetric grounds, but also radiates more of this noise into the environment due to intermediate termination of shields at the VFD enclosure instead of the VFD itself.
High performance VFD cables use either a braided shield or a double copper tape with contra-helical application to maximise surface area for high frequency conduction and to minimise the inductive effects of a single tape.
Conclusion
Selecting and installing the right cable for VFD-motor connections mitigates or eliminates many of the issues that can arise from the inherent characteristics of VFD installations. A relatively small investment in properly designing, selecting and installing the right VFD-motor cabling increases reliability, improves safety and reduces maintenance.
For more information contact Greg Pokroy, Jaycor International, +27 (0)21 447 4247, greg@jaycor.co.za, www.jaycor.co.za
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