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. Here are some techniques which will enhance your success rate.

Desde la perspectiva del fabricante, un motor de 2300 voltios con bobinado aleatorio representa una reducción sustancial en el costo de fabricación. Para el centro de servicio, el reto es rebobinarlos con éxito y ofrecer una reparación confiable. Aquí hay algunas técnicas que mejorarán su tasa de éxito.

This webinar recording shares some of the “best practice” rewind methods used by (and learned from) EASA service centers around the world: connection recognition, best insulating materials, wire choices and tips to save time and effort.

This webinar recording looks at several aspects of winding design to prevent increased temperature rise and decreased efficiency.

There are benefits and drawbacks to the use of multiple circuits in a 3-phase winding. Whether discussing a random winding or form coil winding, some of the considerations are shared. Topics covered include:

• Increasing circuits, turns
• Rotor pullover during starting
• Increased risks
• Factors beyond our control

Manufacturers almost always utilize machine-inserted concentric windings for random-wound, three-phase stators when their processes can facilitate it due to lower manufacturing costs. Many service centers can produce concentric windings too, but the most common practice is to utilize the two-layer lap winding. For form-wound stators, the two-layer lap winding is almost always used by manufacturers and service centers alike. The purpose of this article is to provide some tips for working with odd-turn (unequal-turn) windings, or two-layer windings where the total number of turns per slot is an odd number (e.g., 3,5,7,9…n). In such cases, the top and bottom coil sides must have a different number of turns.

Most plant engineers and maintenance staff can attest to the reliability of standard-efficiency motors that have been repaired or rewound using industry best practices. They also know repair can cost far less than replacement, especially when the motor has special features. Despite this, some of them hesitate to have failed energy-efficient motors (NEMA Premium models, in particular) repaired because they’ve heard it degrades efficiency.

So, what’s the right answer? Is the decision to repair, rewind or replace a failed energy-efficient motor as simple and straightforward as you may have heard?

One of the pleasures of helping EASA members is in discovering challenges or specific areas where we can improve on the original design of the motor manufacturer. The most recent of these, for me, has been a noticeable cluster of calls about motors with multiple windings. The call usually starts with something like this: "We wound this motor, and one speed was terribly burned." Another one I often hear is: "We rewound both speeds, and the surge test pattern for one speed indicates a winding problem." These are but a couple of examples of a design issue we are seeing with motors having more than one winding. While the use of variable-frequency drives (VFDs) is increasingly common, there are still applications using 2-speed, 2-winding motors. Cranes are a good example of one such application. When a core has more than one winding, the two windings behave as a transformer. Applying voltage to either winding induces voltage in the other winding because the two windings are inductively coupled. As long as both windings are connected 1 wye, and the leads of the second winding are left open, no magnetizing current is drawn by the second winding. When both windings are conventional and symmetrical, this arrangement works just fine. The problems start when either winding deviates from the symmetry that is so important to 3-phase motor performance.

Topics covered include:

• Transformer effect
• Visualizing the current flow
• Parallel circuits

Time is a precious commodity, so making the best use of it can help you provide the best possible service to your customers. Following are just a few topics covered:

• Tearing down of small metric motors
• Avoiding chuck jaw marks when shaft is in the lathe
• Avoiding over-shoot when mounting shrink fit couplings
• Making superior brazed winding connections
• Forming copper tubing without crimping
• Better babbitt pouring results
• "Hot dam"
• Maintaining a centered hole in end of shaft when shaft is shortened
• Removing stuck outer bearing races from end bells
• Removing an inner race stuck on a shaft

One of the most significant changes to occur in our industry over the past several decades has been the shift from simply repairing equipment to solving the customer's problem. By that, we mean determining why it failed, and improving the suitability and reliability of the equipment for the application. Doing so requires knowledge about what the motor is doing, where it is operating, and under what conditions. Our customers appreciate results and are increasingly aware of the value a good service center adds above and beyond a simple repair.