Chuck Yung
EASA Technical Support Specialist

Have you ever wondered how a lightning rod works? That small rod atop a large building extends less than a yard (meter) above the building. Yet the lightning is attracted to that sacri­ficial rod rather than to the building. If you have not wondered about lightning rods, I'll bet you are curious about what lightning rods could possibly have to do with electric motors. 

The common denominator between electrical windings and lightning rods is geometry. Lightning strikes the light­ning rod because the voltage stresses are higher between the pointed rod and the cloud than between that same cloud and the building. The pointed shape raises the potential voltage stress at the end of the lightning rod. For the same reason, we often see winding failures affected by the geometry of a coil, a connection, or a sharp corner within a stator frame. 

When the subject of voltage stress comes up, many people assume that the voltage stress does not exceed the line voltage of the winding. Our early as­sumptions were that voltage stresses followed a linear pattern and could simply be calculated by the relation­ship of circuits, coils and turns per coil. 

Static voltage stress 
The traditional formula used to de­termine the volts per coil, for example, is based on the number of coils, circuits and phases, and whether the connection used is a wye or delta: 

Volts per coil = Phase voltage x Number of circuits x 3 phases /Number of coils 
Where phase voltage = line voltage for delta, or 58% of line voltage for a wye. 

We based our voltage stress limits on the static voltage stress calculation above. For random windings we at one time used a limit of 40 volts per coil; then 70 volts per coil. Today, we rou­tinely see manufacturers use designs at 115 volts per coil. There are even 2300v random windings in production today, with higher voltage stresses. 

What do those manufacturers do differently and how do they successfully wind motors with such high volts per turn? For one thing, they use phase in­sulation, and add .phase paper. midway through the groups. This reduces the voltage potential within the group, but the real beneÞt is in reducing the voltage potential where wires cross at anything approaching right angles. That under­scores the value in the old technique of taping each coil at the end-turns. 

A closer look 
Let's take a closer look at how ge­ometry affects voltage stresses, and how that inßuences some of the methods that have been used by manufacturers and repairers over the past century. One of the Þrst things we need to understand is the inßuence of the geometry of the conductors themselves. That goes for intended conductors such as magnet wire and form coils, as well as unin­tended conductors like a stator frame. 

For the various combinations of shapes in Figure 1, the voltage stress between two parallel ßat plates is the lowest. The voltage stress between a rounded surface and a ßat surface is higher, while the voltage stress be­tween a sharp point and a ßat plate can be signiÞcantly higher. We will get to the math behind this in a moment. 

Here are some of the areas that repairers deal with every day, where geometry can affect the voltage stress: 

1. The voltage stress is highest at the corners of form coils closest to the bore. 
2. Gradient tapes must be used with conductive tapes to control par­tial discharge (PD) in high-volt­age machines rated above 6 kV. 
3. The way that connections are formed, especially loop series for form coil windings. 
4. Conductors crossing each other in a perpendicular direction have considerably higher voltage stress than if the same two conductors are parallel or crossing at a slight angle. 
5. A chamfer at the end of a com­mutator reduces the potential for flashovers. 

To demonstrate the physics behind this, go to your local science center, and experiment using the Van de Graaf generator (Figure 2). The large sphere can store high voltage static charges across its surface. Approach the sphere with your palm out, .at towards the sphere, and measure the distance before the static charge discharges to your palm. Now try it with your .nger extended pointing towards the sphere. The same magni­tude voltage will cross a considerably greater distance to reach the tip of your finger. The reason the voltage jumps further to your fingertip is that the .ngertip has a much smaller radius than your palm. (If you have teenagers, you can use them for this demonstration.) 

You have just demonstrated a simple principle that influences the design and construction of electric motors. 

Loop series 
In the early days, when our prede­cessors were .guring all this electricity out, they used loop series (Figure 3) even on 440-volt stators. Some of our understanding of the influence of the conductor geometry was trial-and-error, but since a “stub” series (Figure 3) required less time and insulation to form, the loop-se­ries was soon relegated to windings rated 2300v and higher, then for windings rated above 4000v, and .nally (for most manufacturers) the loop series was used only for windings rated above 6000v. 

Older winders may remember the admonishment against using a “sore thumb” or stub-series lead connection. You may even remember, as I do, an old-timer winder cautioning you to use a loop series because “all the voltage can run out the end” of a straight stub con­nection. As it turns out, the description was not as inaccurate as you might think. 

Relationship between voltage stress and radius 
There is an inverse relationship between voltage stress and the radius of a conductor. That applies to sharp versus gradual bends, to corners of rectangular wire on form coil machines, and to the voltage stress that is present at the end of conductive coatings on high-voltage machines. 

We will use the following formula which describes the voltage stress be­tween a .at surface and a conductor with a radius (corner / point / round). The re­ality is that we are often concerned with voltage stresses between two conductors, neither of which is a .at plate. So the voltage stresses involved may be even higher than determined by this formula. 
E = (2V) / (r ln (4d/r)); 

where E is the voltage stress in volts per mm, V is the voltage poten­tial between the two conductors being evaluated, r is the radius distance (in mm) of the conductor, and d is the distance (also in mm) between the two conductors. 

It is no surprise that the voltage stress increases with an increase in the voltage potential between the two conductors, or that the distance between them is an important factor in whether or not discharge occurs between them. What this formula demonstrates is that the radius has a significant impact on voltage stress between two conductors. 

The voltage stress increases in proportion to the voltage, but the radius has the greater in.uence. To il­lustrate, the cells in Table 1 show the result of calculations done using the same voltage and distance, but with varying radii. 

Chance of arcing 
If the voltage stress exceeds 3000 v/mm, partial discharge is likely to occur. So the closer a live conductor is to ground, or the closer together two conductors are, the more chance of arcing. But the presence of a sharp radius such as a corner plays a much larger role in raising the voltage stress which can initiate an arc. 

Two parallel strands of magnet wire have moderately low voltage stress. The same two strands, crossing at a right angle, have voltage stresses magni­tudes of order greater. That can be seen considering the two conductor paths in 2-dimensions. While not a ßat plate, the rounded edges are less likely to initiate an arc than two knife-edges. And the closer the intercept is to a right angle, the smaller the effective radius presented. 

Phase insulation is critical 
Consider the top coil sides crossing bottom coil sides. They form some­thing close to a right angle. While the voltage stresses are certainly higher for the phase coils, the sharp crossing angle is critical because it increases the voltage potential between the conductors. The presence of phase insulation is critical. When a winding has maximum circuits, and a delta connection (with its higher phase voltage), the voltage potential is increased. 

When dismantling motors, we sometimes observe a place where the winding arced to ground. Invariably, this occurs at a bolt or a protruding corner on the frame or end bracket. Once again, the smaller radius raises the potential voltage stresses. 

With DC machines, how many times have you seen flashover damage to the corner of a brush box or to the square corner at the end of the commutator? Many repairers use a Þle to lightly break that corner, pro­viding a small chamfer. They do so because it reduces the voltage stress at the very place where many flashovers initiate. 

Winders avoid sharp points when brazing a connection joint, not just because that point might puncture the in­sulation, but also because that sharp point (small radius) can dramatically increase the po­tential voltage stress between that point and the nearest potential conductor. whether that is to ground or another conductor. 

Potential for PD damage 
Finally, those of you who repair machines rated over 6 kV are accustomed to using conductive material on the straight section of the coil, with gradient (semi-conduc­tive) material overlapping the ends of the conductive mate­rial and extending for several inches. Absent that semi-con­ductive gradient tape, the abrupt end of the conductive material becomes a sharp radius, greatly increasing the potential for PD damage to the coil insulation at the edge of the conductive tape. 

Plug in the 7-mil (1.8 mm) thick­ness of the conductive tape, and 6900v phase voltage for a wye-con­nected 13kV winding, into the above formula. The resulting voltage stress of 19200v with d= 0.5 mm (.020.) is extremely high - no wonder that we find PD damaging the groundwall insulation of the coil sides. 

Categories: Technical topics

Tags: Voltage stresses

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