Jim Bryan
EASA Technical Support Specialist

When designing or redesigning an electric motor, there are many trade-offs. For instance, opening up the air gap to improve power factor might diminish the efficiency. For every action taken in adjusting the motor’s design, there is a reaction at some other point that affects the motor’s performance.

One of the critical considerations involves the interaction of the turns per coil and pitch. As you shorten the pitch, it requires more turns to generate the same torque. But, as you increase the number of turns, the wire takes up more space in the slot. The way to overcome this problem is to decrease the wire size. But decreasing the wire size decreases the current density, in­creasing the I²R losses and operating temperature. 

Factors to consider
To optimize the motor design, all of these factors must be considered. This article will discuss just two of those factors: the pitch and turns per coil. How they interact and how they affect the performance of the mo­tor will be addressed. See Figure 1. Our goal is to match the original design as closely as pos­sible, usually within 2% of magnetic flux density and current density. As a very broad and gen­eral guide, the minimum current density for open motors is 330 circular mils per amp (6.0 A/mm²) and for enclosed motors, 450 circular mils per amp (4.4 A/mm²). Note:  In North America, current density is expressed inversely as wire area per amp or cir­cular mils per amp (CM/A). In the rest of the world, it is expressed as amps per wire area or amps per millimeter squared (A/mm²).

Many designs, especially premium efficient motors, will have current densities lower than these. We should never increase the current density by more than 5% (2% is better). Often the cooling circuits of premium efficient motors are optimized for the design; relying on these minimums will cause extreme temperatures that will shorten the motor’s life. Similar consideration should be made of the magnetic flux densities due to the possibility of saturation. It should be noted here that in an AC machine, decreasing the cur­rent density (in­creasing CM/A or decreasing A/mm²) is generally a good thing. As long as the coils can be inserted without damage, more wire cross section will help the motor run cooler and be more ef­ficient.

Shortcuts: Good ones and bad ones
Shortcuts are used by winders who have years of experience. Some short­cuts are valuable and timesaving while others might create problems. One of the shortcuts assumes that when con­verting a concentric winding to lap, all you need to do is use half the turns of the full slot coil and take the middle span of the concentric family of spans. This is not a safe assumption!

The example we will use is a 250 hp, 4-pole, 72-slot, 48-coil, with a 1-12,14,16,18 pitch and 7,7,14,14 turns. (See Table 1.) One winder decided to convert this to a lap winding. The shortcut suggested he use 7 turns with 1-15 pitch. This new design results in a 5% increase in magnetic flux densities. If the original motor was near the limits of flux density, the motor could now saturate, resulting in high operating temperatures. Increased inrush current could also cause the motor to trip the overloads when returned to service. A better redesign would be 7 turns, 1-17 pitch. The flux and current densities of this design nearly match the original de­sign so we can expect the performance to be comparable as well.

As the pitch is decreased or short­ened, the flux produced is increased. Generally for 4-pole and slower mo­tors, the longer the pitch, the fewer the number of turns, the larger the wire size and the better for the motor. There are limits of course. The chord factor should not exceed 0.991 as shown in Table 2. Also on 2-pole motors with a stator bore of less than six inches (150 mm), the winder will have a very difficult time inserting the coils if the pitch is too long. The January 2004 Currents article titled “Coil Pitch and the Search for the Perfect (Sine) Wave” is a valuable resource in redesigning 2-pole motors which have particular considerations that must be addressed.  

The number of turns per coil is the other factor in the equation.  Increas­ing the number of turns per coil will “weaken” the motor. In other words, the lines of flux developed in the core and airgap will decrease. 

Horsepower available varies as the square root of the flux density. Also as the flux density increases, so does the locked rotor current. As noted previ­ously, shortening the pitch increases the flux.  To compensate for this and maintain the original flux densities, we will need to add turns.  

Number of turns in coil
The adjustments we are able to make depend on the total number of turns in the coil. If, as in our example above, we have 7 turns per coil, drop­ping or adding one turn will change the flux 1/7 or 14%.  This could lead to problems with performance or in­rush current. If the coil had 35 turns, the change would be 1/35 or 3%, so a much finer adjustment can be made. This is also why it is sometimes neces­sary to use unequal turn windings. See the February 1999 Currents article titled “Rewinding Motor with Odd Turns Doesn’t Have to be Frustrating.” Changing our 7-turn winding by ½ or even ¼ turn gives us opportunity for finer adjustments.  

When changing speed or horse­power, these conditions become even more important. The higher the speed, the more flux is developed in the back­iron; the lower the speed, the more flux in the tooth. Changing from 4-pole to 2-pole, for instance, will not be possible if there is not sufficient backiron to handle the increased flux. Of course, increasing the horsepower increases the flux by the square root as previ­ously discussed. For example, if the horsepower is increased 20%, the magnetic flux will increase by the square root of 1.2 or 9.5%.  The limit of this increase depends on the amount of stator core stack and the size of the stator slot for wire fill.

Conclusion
The re-designer must consider the interactions of the design param­eters being changed. All of the core measurements and winding data are necessary to properly evaluate the ef­fect of these changes.  Where possible, use a longer pitch with fewer turns while maintaining the original flux densities as closely as possible. This will provide more slot space to main­tain or improve the current density by using the largest wire that will fit. 

Categories: Stators/windings, Design/theory/application

Tags: Windings Motor design

Rate this article:

5.0

Print