Chuck Yung 
EASA Technical Support Specialist 

One of the many interesting things we get to do as repairers of rotating electrical equipment is to rewind the rotating poles of synchronous ma­chines. Handling large poles can present special challenges, but the actual winding process itself can be downright enjoyable—if we are ad­equately prepared. This article will outline procedures that take the mystery out of successful pole winding. The procedures are also helpful for stationary field coils and interpoles. 

One of the first important steps is to draw the connection. Most synchronous rotors are con­nected in series; however, you don’t want to miss a series-parallel connected rotor (Figure 1). 

Mark the poles and put them back where you found them 
Indelibly mark the poles to identify the cor­rect position—and placement—of each pole. Winders are sometimes so eager to get right to the winding part that they have been known to over­look this important step. Use number or letter stamps to label each pole and the hub in line with that pole. The reason: If the poles are not exactly the same height and weight, arbitrary replacement will result in an irregular airgap and excessive dynamic unbal­ance. If the poles for a self-excited generator are not reinstalled in the correct positions, the polarity of the poles’ residual magnetism will be random and then the generator may not put out voltage. 

Next, remove the poles. Bolted poles are usually not a problem, but the wedged style can be difficult to remove. Tip: Use a press to push down on the pole (towards the shaft) to relieve pressure on the wedges and then drive the wedges out. If that doesn’t work, tack weld the thick end of each wedge—one to the hub, the other to the pole—and use a press to push the pole axially. Once it starts moving, the wedges loosen and the pole will slide right off. The tough part is getting it started. 

Save a sample coil
Most of the time, the pole iron can be removed from the coil without burning out the coils. Just support the coil ends on blocks and use a mallet to tap the pole out. That saves both time and oven fuel. It also preserves a coil for dimensioning and comparison to the new coils. Use a bandsaw or reciprocating saw to cut through the cross-section of one coil, and document the turns per layer, number of layers and total turns per coil (Figure 2). Mark the lead position on the drawing, too. 

Manufacturing collars 
Save an upper and lower collar as samples for the new ones. You can use a drill, saw and router to make new collars, but why would you want to do that? Send the samples to a company such as The Gund Co. (MM&M Electrical Supply) or Port City Cabinet Works, Inc. Both are EASA Associ­ate members. They use CNC equipment to manufacture identical collars(Figure 4). The job will look better and it will probably cost less than the labor you’d spend making them yourself. 

Vintage synchronous poles often have collars made from maple or phenolic board. Consult your supplier when deciding on the best replacement material. Radius and rotating speed are at least as important as insulation class. We should be a lot more concerned about mechanical strength (com­pressive and shear) than temperature rating. Table 1 lists the insulation class of common materials; there is a lot of general confusion about the insulation class of micarta, melamine and similar products. 

Wire size 
Find out whether the original wire size is avail­able before ordering the collars. It may be necessary to adjust the collar thickness if you have to substitute wire size (Figure 3). The collars should be firmly clamped when the poles are at­tached to the hub and the poles must seat solidly in place. These are DC coils, so resistance is im­portant. A change in resistance will change the current, which changes the ampere-turns. In other words, if the wire size changes, so does the field strength. The customer might have to adjust the excitation voltage to maintain the correct power factor (if a motor) or output (if a generator). Any wire size change should match the original wire area; avoid changes that are not within 5% of the original circular mil area. 

Remember that (thickness x width) – radius factor = square mils 
Square mils x 1.2732 = circular mils 

If a wire change is necessary, select one that will layer neatly within the same cross-section as the original coil. If the layer width will be differ­ent, change the collar thickness to compensate. 
Insulate the pole iron, if winding directly onto the pole (that is usually the best method) - or build a form to the correct dimensions. If the coil has lead tabs, weld the starting tab onto the wire, back from the end. Clamp the wire end to the pole, start the wrap, and then braze the lead tab so that it will be positioned correctly. Normally, the lead tab should be positioned right in the middle of one end of the pole. Be sure to start on the correct end. 

Maintain wire tension 
The key to effortless winding of rotating poles is to maintain adequate wire tension and to re­verse-bend the wire (see Benny Darsey’s article on page 5 in this issue.). This makes it easier to keep the turns together without gaps and to keep the collars against the edges of each layer. Make the crossovers on the end of the coil (not the sides) and insulate every crossover to prevent shorting. A strip of Nomex works well, and has sufficient shear strength to resist “cut-through.” 

If each pole is marked on the connection end, be sure to start winding each pole at that end (Fig­ure 5). If the design uses half-turns, be sure to alternate the starting end correctly. For example, the even numbered poles might all start on one end and the odd numbered poles on the opposite end. 

For poles with an amortisseur winding, tape or otherwise pad the sharp corners to prevent the wire from being scratched. Use slight hand pres­sure to deflect the wire sideways around sharp corners that could damage the wire. 

Once a turn or two have been wound onto the pole, increase the tension and continue to wind the first layer. Use an epoxy suitable for wet-winding of rotating poles to coat the first layer. When making the step up to the second layer, place a strip of insulation under the pressure point between the crossover and the last turn of the pre­vious layer. When rectangular or square wires are stacked, that corner-to-corner contact is the most likely place for shorts to occur. If the corner bends are sharp, place a strip of insulation on each cor­ner before winding on the next layer. As more layers are added, the radius increases. 

VPI or dip varnish work well for most station­ary windings, but the centrifugal force acting on rotating fields exceeds the bond strength of most resins. Use only resins that are specially devel­oped for use on rotating windings. The bond strength is higher, especially at operating tem­peratures. Generally, you want a thixotrophic resin. The most popular products for wet-winding rotating poles are thick (high viscosity) and are easier to work when thinned by warming a con­tainer on a hot plate. 

When the next to last layer is in place, check the total turn count and verify that the last turn of the final layer will be positioned where you planned. If the last layer will be laid on from the outside (towards the shaft), wind it as the previ­ous layers. If the last layer runs towards the outside, it is important to start it so that the last turn fits against the outer collar. That prevents the layer from slipping due to centrifugal force, when in service. 

Securing the last turns 
Tip: To secure the last two turns, take a minute before starting the next to last layer and figure out where the last turn will be located. Place a doubled piece of flat tie cord under the turns that will be directly beneath the last turns, so that when the last two turns are installed, the tie can be used to secure them. Some winders use this tie on both ends of the coil and even on the sides of long coils. It really makes a differ­ence in the appearance. 

Categories: AC motors, Synchronous motors

Tags: Synchronous motors

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