For all their energy savings and process control
benefits, variable frequency drive (VFD) systems have a downside too. When these
drives fail, they can bring motor-driven industrial processes to a dead stop. To
avoid this costly downtime, smart engineers carefully evaluate reliability when
configuring a drive system.
All too often, however, the reliability analysis
focuses solely on the VFD power electronics and neglects the most vulnerable
component in the drive system–the power cable that connects the VFD to the
motor.
The truth is, however, that
power electronics have already become very reliable over the years. By design,
they can handle the typical 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 if power
distortions rise to unsafe levels.
Cables do not have any such protection and can fail
if subjected to electrical conditions that generate more heat or voltage levels
than the cable’s insulating layers can tolerate. Cables in industrial settings
may also experience mechanical loads and chemical exposures that lead to damage
and premature failure.
Fortunately, it is possible to avoid VFD-related
cable failures and the associated downtime by paying attention to a few cable
construction details.
Materials. Not all insulation and
jacketing materials are created equal when it comes to electrical performance.
So it’s important to match cable materials to specific VFD application
requirements. To take three examples:
-
High Potential For Electrical Damage. As the likelihood of damage from voltage spikes or other power distortions increases, consider VFD cables that make use of semiconductive layers between the conductors and the primary insulation. These semiconductive insulation systems have for decades been employed in high voltage cables. More recently, they have been applied to VFD cables to protect against electrical damage.
-
Overload Conditions In Long Cable Runs. When the installations require long cable runs, the risk of capacitance loads triggering the VFD overcurrent protection system rises too. The use of cables with the right insulation system can minimize this risk. Cross-linked polyethylene (XLPE) insulation, for example, has a relatively low dielectric constant that reduces the capacitive effect in long cables. XLPE also has excellent thermo-mechanical properties that allow the XLPE insulation with withstand the heat generated by overcurrent conditions.
-
Precision Control. It may not be obvious, but insulation choices can influence the control response of VFD systems. For applications requiring precision control, an approprieate insulation system will minimize transfer impedance and improve the velocity of propagation to produce a more efficient control response.
Mechanical Properties. With VFDs typically
installed in factory environments, cables should be engineered to withstand
mechanical abuse and environmental exposures. Some of the key mechanical
attributes in VFD installations include:
-
Flexibility. Enhanced flexibility pays off throughout the cable’s life cycle. During installation, flexibility makes handling and routing easier. In use, flexible cables are less susceptible to damage from bending.
-
Oil Resistance. For industrial applications where oil exposure is a concern, make sure that prospective VFD cables comply with UL Oil Resistance requirements.
-
Crush Tested. Consider whether cables have the crush resistance needed for the installation. Cables certified as TC-ER, for both tray and exposed run installation, have to pass rigorous mechanical tests for cable crush and impact resistance, including UL Standard 1569. Because these rugged cables need no conduit, they can drive down installation cost and time significantly.
Stranding. To
minimize voltage drop and maximize efficiency, consider the cable’s circular mil
area (CMA). Cables whose conductors have a large CMA have lower DC resistance
than cables with a smaller CMA. Low DC resistance, in turn, translates to
significantly lowers voltage drop across a given length of wire.
Visit our website
Like us on Facebook
Join our Linkedin Network
Watch us on Youtube
No comments:
Post a Comment