Are Your Wires Crossed?
By Rick Hoffman and Joe Rammage
We recently evaluated a 15kV, 5000 hp, induction motor in a WPII enclosure manufactured by a foreign company. During an on-site inspection of the main terminal box it was discovered that the high voltage leads were touching. This raised significant concerns about the long-term reliability of the motor. Becht Engineering investigated the issue to determine the industry standards and best practices concerning routing of the motor high voltage leads.
Shielded vs Non-Shielded Cables
The National Electric Code requires all cables rated above 2.4kV to be shielded to prevent concentrated electromagnetic stresses from forming between the cable conductor and ground when the cable is in close proximity to a grounded surface. The addition of the metallic shield smooths out the electromagnetic field eliminating high stress points. Without it, the increased density of electromagnetic stresses where the cable is in close proximity to ground, deteriorates the cable insulation over time and may result in a line to ground fault. As a general rule, cable manufacturers do not have a non-shielded cable offering for cables rated above 2.4kV. However, the NEC does allow an exception to the cable shield requirement to manufacturers of electrical equipment where those cables are completely enclosed within the manufacturer’s enclosures such as motors and switchgear. As a result, medium voltage motor manufacturers have standardized on non-shielded cable for the motor leads between the stator windings and the main motor terminal box. This practice requires that they take precautions to ensure that the cables are securely anchored and routed away from all grounded surfaces.
What About Proximity of Phase Leads?
In the case of the motor that was being evaluated there were concerns about the close proximity of the leads of each phase to the other phases. We discussed this issue with several senior technicians at nationally recognized motor repair shops. During these discussions it became clear that it has been this vendor’s standard practice over many years to route their motor leads from the stator connection to the main motor junction box with all phases bundled together. While this practice raises concerns, we could not find a standard or a published recommended practice that restricted it. Best practice in the industry is to use solid copper bus for the motor leads and route them separately from the stator winding connection to the terminal box. While most manufacturers offer this option, it is rarely chosen by the customer due to its high cost. The standard practice for most motor manufacturers is route the phase conductors separately or route the conductors of each phase together but separately from the other phases. While this vendor’s standard of bundling all phases together may not be universally used in the industry, it does not appear to be a significant cause of failure. Interviews with several senior technicians at nationally recognized motor repair shops revealed no failures of motor leads in their shops over the years due to the bundling of the phase conductors together. In addition, we could not find an industry standard that specifically prohibited this practice.
In conclusion we could not find evidence the practice of bundling leads and phases together caused reliability problems. However, if this is a concern for your organization, you should specify in your motor procurement standard that only the same phases can be bundled together or leads must be routed separately.
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