Understanding motor rewind advantages

A documented best practices rewind performed by a highly qualified service provider can lead to significant improvements in a motor's power density and reliability

By Rick Payton, Integrated Power Services -- Plant Engineering, 11/18/2009 11:28:42 AM

Given the substantial capital investment electric motors represent and their annual cost of operation, which can exceed 10 times the purchase price, efficiency is of prime concern to plant operators.
The belief that motor rewinds generally lose efficiency is widely held but outdated. Advances in materials, process discipline and stringent quality assurance can dependably match - and often improve - electric motor efficiency. A documented best practices rewind performed by a highly qualified service provider can lead to significant improvements in a motor's power density and reliability. In many cases, having a motor rewound can help users lower operating costs, improve motor reliability and uptime and an improve return on investment.

The value of motor rewinding
According to a recent study by the Green Motor Practices Group, properly planned and performed rewinds cause no efficiency loss in electric motors. Performance improvements, increased power density, reliability and efficiency come from advances in materials and craftsmanship. For example, better resins and insulating tapes improve thermal dissipation. Automated coil forming technology and precise application of insulating tapes ensure consistent coil duplication for improved installation and operation.
A qualified service provider can ensure that the rewound motor meets or exceeds original OEM efficiency. A good repair facility will include key best practices in a detailed, documented specification. All repairs and rewinds should begin with a thorough assessment. This assessment should include recording nameplate data, a visual inspection prior to disassembly and winding details.
The nameplate should reveal whether the motor is special or Premium Efficient. The visual inspection should be performed before the motor is taken apart to record primary frame components; shaft rotation; orientation of the end bracket and bearing caps; bearing sizes, types and clearances; orientation of the shaft to the main terminal box; degree of contamination such as moisture, acid/base degradation or lubricants; the axial position of the rotor relative to the stator; and signs of excessive heat or bearing wear.
Wiring details are investigated to determine if the motor has the original factory winding. Also at this time, measurements are taken to replicate the winding if the information is not readily available.

The importance of core integrity
Core integrity is fundamental to motor efficiency and reliability. Damage to the core is among the first things a good service provider will check - both visually and through testing. For example, a winding failure can create enough heat to melt an area of the core. A bearing or shaft failure can cause the rotor to strike the stator.
Extreme care must be taken in the repair process to use proper burnout temperatures (around 680 F) to retain the quality of the steel lamination core plate. Proper handling procedures should be followed for removing the old winding, cleaning the core and installing the new winding.
On Premium Efficient motors up to 300 hp (NEMA sizes), damage to the core means that repair is not feasible. It's more economical to replace the failed motor with a new Premium Efficient motor in this size range. However, if availability of a new motor is an issue, the motor can be repaired and returned to service in many cases. It should be noted that it is more difficult to maintain the NEMA Premium OEM rating due to the grade of electrical steel used in these motors.
On larger machines, repairs and rewinds can offer good value, cost-efficiency and return on investment. On these machines, cores can be repaired by replacing damaged laminations with like components without efficiency degradation. Using like components means using steel with equivalent electrical properties, equivalent insulation dimension and thickness of material. The number of laminations removed must be equal to the number of laminations replaced, which will restore the core to its original dimensions.

Improved winding
The winding offers key opportunities to improve motor efficiency. The service provider can increase wire diameter, pack more copper into each slot and reduce the length of the coil loop as it exits one slot of the stator core and enters another. The shorter the turn area of that loop - coupled with increased circular mil area of magnet wire - the more power density and efficiency the motor has.
When hand winding, more copper can be inserted into a stator slot. Typically, the length of the end turn can be accurately controlled when done by hand. According to IPS data, a 10% increase in conductive copper area is feasible in 90% of all motors.

Efficiency, loading
Motors operate at maximum efficiency when they are fully loaded. Oversizing motors is a significant installed base issue. Efficiency drops dramatically below 70% loading. Oversizing is often done as a means to ensure greater reliability.
Premium Efficient motors run cooler, use higher grade materials and have tighter manufacturing tolerances. Oversized motors are expensive to operate and typically do not necessarily offer a longer service life.
As with any maintenance decision, the end user has the ultimate responsibility for plant reliability and uptime. A root-cause investigation is necessary to ensure that the cause for the primary mode of failure is properly addressed. For large motors, repairing or rewinding can help users lower operating costs, improve motor reliability and uptime and an improve return on investment.
Rick Payton is senior vice president, Western Region, Integrated Power Services (IPS). Prior to joining IPS, Payton worked for Baldor and Reliance Electric for 28 years, most recently as vice president of Integration and Motor Sales.