Variable frequency drives (VFDs) are firmly established in manufacturing settings. However, sometimes engineers forget that power cables affect overall VFD reliability and performance. Here we review four cable design elements that help keep VFDs up and r
For all their energy savings and control benefits, when VFDs fail, they bring motor-driven industrial operations to a dead stop. To avoid such situations, savvy engineers ensure drive-system reliability both at its power electronics and its most vulnerable component — the power cable that connects the VFD to its motor.
In recent years, power electronics have become very reliable, deftly handling the voltage spikes, inrush currents, harmonics, and other power distortions that arise during VFD operation. Their controls can also prevent damaging electrical conditions or shut down the drive altogether if power distortions rise to unsafe levels.
In contrast, cables have no such protection, and can fail if subjected to electrically induced heat or voltage exceeding levels that their insulating layers can tolerate. Cables in industrial settings can also be subject to mechanical loads and chemical exposures that lead to damage and premature failure.
Fortunately, it is possible to avoid VFD-related cable failures and associated downtime by carefully considering a few cable-construction details.
Insulation and jacketing materials deliver different levels of electrical performance — so cable materials must be matched to VFD requirements.
Even medium-power VFD applications run the risk of damage from voltage spikes or other power distortions. Here, consider VFD cables that make use of semiconductive layers between the conductors and primary insulation: For decades, semiconductive insulation systems have been employed in high-voltage power-distribution cables. Now, they're increasingly applied in VFD cables to protect against electrical damage.
Semiconductive insulation works by alleviating discharge and high points of electrical stress in primary insulation — for greatly improved cable reliability, life, and dielectric strength. Testing under ICEA T-24-380 demonstrates that corona inception and extinction by semiconductive compounds outperform those of other insulative systems.
Another VFD application challenge is the long cable runs increasingly common in modern manufacturing facilities. Capacitance loads increase with length, so long cable runs have an increased risk of overload conditions that can trigger the VFD's protective systems. Cables with cross-linked polyethylene (XLPE) insulation can minimize this risk.
XLPE insulation has a relatively lower dielectric constant that reduces the capacitive effect in long cable runs. XLPE also features excellent thermo-mechanical properties that allow its insulation to withstand the heat generated by overcurrent conditions.
VFD applications requiring high-precision control impose other requirements on the cable as well. Though it may not be obvious, the cable's insulation can actually influence the control response of the drive. Appropriate insulation systems minimize transfer impedance and improve propagation velocity to produce a more efficient control response.
With VFDs typically installed in factory environments, cables must be engineered with key mechanical attributes to withstand mechanical abuse and environmental extremes.
During installation, flexibility makes handling and routing easier. In use, flexible cables are less susceptible to damage from bending. Specifying stress-free cable, which is created by removing back twist, is one way to improve flexibility and is especially useful in tight bend radius installations.
- Oil Resistance
For applications that expose cable runs to oil, select cables with a PVC jacket formulated for oil resistance. Cables that comply with Underwriters Laboratories' (UL) stringent oil-resistance standards are usually surface marked as Oil Resistant I or Oil Resistant II.
- Crush Tested
Cables certified as TC-ER (for both tray and exposed run installation) must pass rigorous mechanical tests for cable crush and impact resistance, including UL Standard 1569. Because these rugged cables need no conduit, they can significantly drive down installation time and cost.
Because VFDs can be susceptible to electrical noise, shielding in both the cable and its connectors is paramount.
One highly effective shielding type combines a triple-laminate foil tape and 85% braid coverage to eliminate two noise problems:
It prevents externally generated noise from entering and causing internal signal disruptions.
It prevents noise generated from within the cable itself from exiting — noise that can otherwise cause unintended disruptions of nearby electronic equipment.
Connector shielding should provide full shield grounding, low transfer impedance from the shield to conduit entry plates, and 360° of termination. (Connectors with these features are referenced in Section 4-5 of Rockwell Automation's Wiring and Grounding Guidelines for Pulse Width Modulated AC Drives.)
Cable stranding designs, which vary substantially, also affect drive system efficiency. One often-neglected consideration involves the cable's circular mil area, or CMA. Cable conductors with a large CMA have lower dc resistance, which in turn translates to significantly lower voltage drop across a given length of wire.
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|Cable||Dc resistance (Ohms/1,000 ft)||Impedance (Ohms)||Flexibility (Durometer)||Physical resiliency||Corona (Voltage)|
|Typical VFD-acceptable cable||K More voltage drop||K||K||L||L Dielectric strength reduced|
|VFD-specific cable||J Accomodates long runs||J||J||J||K Dielectric strength reduced|
|Surge-guard VFD cable||J Accomodates long runs||K||J||J||J High inception/extinction|
At this article's open: VFD cable incorporates semiconductive insulation (as a functional surge guard, the first layer over the tinned copper wires), foil tape, and a tinned-copper wire braid shield. This chart shows the benefits of VFD-designed cable (which among other things, sports increaded wire copper) plus those of such cable with a surge-guard layer.