Chuck Yung
EASA Senior Technical Support Specialist 

When rewinding a motor, the service center often feels restricted to the original design. Sometimes, we encounter a motor design we wish had never been developed. The random-wound, 2300-volt motor design falls into that category. Most of us would prefer to see medium voltage (2300-4160 volt) machines built exclusively using form coils. The form coil winding (Figure 1) assures uniform volts/turn stresses and reliably seals the windings against hostile environments.

From the manufacturer’s perspective, a random-wound, 2300 volt motor represents a substantial reduction in manufacturing cost. For the service center, the challenge is to successfully rewind them while providing a reliable repair.

Procedures for Enhancing Success
Here are some techniques which will enhance your success rate. First, the use of inverter-duty wire reduces the possibility of failure between turns. Turn insulation is more critical for a random winding than a form-wound motor because the voltage between turns of a random coil could be as high as the voltage per coil. Inverter-duty wire has more stringent manufacturing tolerances, resulting in more uniform coating thickness and fewer voids per unit length of wire. Recent Hipot testing of twisted pairs of wires has found that two tightly intertwined wires can consistently withstand 20,000 volts.

Doubling the slot liner helps protect against ground failures, and the use of Nomex®-Mylar®-Nomex® laminates combines the mechanical strength of Mylar with the temperature resistance of Nomex. Voltage creep distance increases in proportion to applied voltage, so slot liners should protrude one full inch (25 mm) beyond the end of the slot (Figure 2). 

Reduce Potential Voltage Stresses
When practical, there are a couple of other tips to reduce the potential voltage stress within each coil. First, if the connection is not already a 1-wye, adjust the turns and wire area in hand to make it a 1-wye. 

Insulation between phase groups should be doubled, and lacing should keep the wires tightly bundled without displacing phase insulation. Nomex phase insulation has an advantage in that it absorbs resin, while the varnished cambric conforms better to the winding geometry. Be careful not to trim phase insulation too close to the conductors; it should protrude at least ½” (13 mm) to reliably separate conductors of adjacent groups. 

Windings with anything other than a 1-wye connection should have additional phase insulation placed mid-group to manage the volts/coil stresses. For example, a 72 slot, 4 pole with 12 groups of 6 coils should have additional insulation between the third and fourth coil of every group. 

Another concern is partial discharge (PD), which occurs when air adjacent to a conductor exceeds its dielectric strength. A 0.040” (1 mm) void is large enough to permit partial discharge to occur. To minimize voids, multiple varnish treatments are needed and/or the use of high-build resins. The goal is to create a void-free winding to reduce the risk of partial discharge. With round wires randomly oriented in the slots, voids are inevitable.

Here is the method used by one manufacturer to successfully produce 2300 volt random windings: 

• Preheat the stator to evaporate the wire lubricants remaining from when the wire was manufactured. 
• Dip the stator with the bore vertical until air no longer bubbles from the slots. 
• Bake the stator with the bore horizontal until the resin is tacky. 
• Remove from the oven and allow it to cool to 130-150°F (55-65°C). 
• Vacuum pressure impregnation (VPI) the stator, using a short cycle [draw a 2-Torr (2mmHg) dry vacuum, transfer the resin in and run a one-hour pressure cycle]. 
• After curing the VPI resin, apply a two-part epoxy to top-coat the winding extensions. This can be done using the conventional trickle method or by using a siphon gun to spray the mixed epoxy directly onto the coil extensions. 

Determine Partial Discharge Inception Voltage (PDIV) of Winding
To determine the PDIV (partial discharge inception voltage; the voltage at which PD occurs) of a winding, perform this simple “lights out” test. Drape the stator with black plastic or a thick tarp, and use a surge tester to slowly increase the test voltage to 1.5 x line voltage. Watch for visible sparking (evidence of partial discharge). You may also be able to hear the arcing. Infrared (IR) cameras cameras allow this inspection to be done without the blackout; ultrasonic listening devices may also be useful. If the PDIV is at or below line voltage, partial discharge is likely to reduce the winding life. Additional resin treatments should be applied to reduce the voids and raise the PDIV. Note that this test is not needed for every rewound stator. Use it to establish an expected PDIV for your winding methods and resin build. 

Reducing Coil Movement
Some service centers spray the coil extensions using a two-part epoxy to reduce voids in the end turns. Others hand tape the coil extensions to improve resin retention. That also increases the mechanical bond between coils, which should reduce coil movement. Coil movement can also be reduced by securely lacing a surge rope to each coil extension, as is routinely done with form coil windings. 

It is important to realize that these tips will help increase the chance of success; they do not guarantee success. These are difficult motors to rewind, and the chance of failure is significantly higher than for a random wound motor rated under 600 volts. Warranty considerations should be discussed thoroughly with the customer. They probably bought that random wound motor because it was less expensive than a comparable form-wound motor. We can help them understand that the trade-off could be reliability. 

When a random wound 2300 volt winding fails, that should be viewed as an opportunity to encourage the customer to allow you to replace the core with one designed for form coils. For more information on this process, see “Considerations for random to form winding conversions” in the December 2019 issue of Currents. 

EASA’s engineers can do the redesign, including the slot geometry, to change a random wound stator to form coil design. When making this conversion, plan to use magnetic wedges in the form wound stator to minimize zig-zag losses. We can almost always improve the current density when making this conversion.

Categories: Technical topics, AC motors, Stators/windings, Lacing/bracing, Random wound, Winding insulation, Repair procedures & tips, Wire

Tags: Motor rewind/repair Insulation Coils Voltage stresses

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