Chuck Yung
EASA Technical Support Specialist
Note: This article was originally published October 2001 and was updated September 2021.
When rewinding a motor, the service center is restricted by 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 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. And competitive pricing is important if they want to sell motors. The great challenge to the service center is in successfully rewinding this design while maintaining profit.
Procedures for Enhancing Success
Here are some tricks learned from service centers with good track records in random-wound, 2300-volt motors, which should enhance the success rate with these challenging rewinds. First the use of inverter-duty wire reduces the possibility of failure between turns. Turn insulation is more critical with a random-wound motor, where the voltage between turns could be as high as the voltage per coil. Inverter-duty wire has been found to be 100 times as sike resistant as quad-build film wire.
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 at least 1 full inch (25 mm) beyond the end of the slot.
Phase insulation should also be doubled, with additional insulation between adjacent coils. An alternative to inserting phase insulation after every coil is to tape every coil, or every second coil. An advantage of taping coils is that resin retention is improved.
Reduce Potential Voltage Stresses
When practical, there are a couple of other tips to reduce the potential voltage stresses 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 to not trim phase insulation too close to the conductors – it should protrude at least ½” (12 mm).
Another concern is PD (partial discharge, or corona), which occurs when air adjacent to a conductor exceeds its dielectric strength. A 0.040” void is large enough to permit partial discharge to occur. For this reason, multiple varnish treatments are strongly recommended, 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 the only manufacturer to successfully produce 2300-volt random windings:
- Preheat the stator to “Gas off” all wire lubricants remaining from when the wire was produced.
- Dip the stator with the bore vertical, until air stops escaping from the immersed stator.
- Bake the stator, and allow it to cool to 130°-150°F.
- VPI the stator, using a short cycle (1 hour dry vacuum, transfer resin in and 2-3 hour pressure cycle.)
- Depending on the resultant resin build, consider top-coat the winding extensions using a 2-part trickle epoxy.
Determine 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 heavy tarp, and use a surge tester to slowly increase the test voltage to 1.5x line voltage. Watch for visible sparking (evidence of partial discharge). Sometimes it is possible to hear the discharge, too. If the PDIV is at or below line voltage, partial discharge may cause the winding to fail prematurely. Additional resin treatments should 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 2-part epoxy to reduce voids in the end turns. Some hand-tape the coil extensions to improve resin retention. That also increases the mechanical bond strength between coils, which should reduce coil movement. Coil movement can also be reduced by securely lacing a surge rope to each coil extension.
It is important to realize that these tips will help increase the chance of success; they do not ensure success. These are difficult motors to rewind, and the chance of failure is significantly higher than for a random wound motor rated 480 volts. Warranty considerations should be discussed fully 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 tradeoff 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. 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 usually also improve the current density when making this conversion.
ANSI/EASA AR100
More information on this topic can be found in ANSI/EASA AR100
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