Chuck Yung
EASA Technical Support Specialist

This started as an article to explain those cases where a 2-pole winding concentric-to-lap conversion will not run. The cause has to do with the coil pitch selected and slot spatial harmonics. These harmonics have a harmful effect on motor performance. The key is avoiding certain coil pitches, and the “problem” 2-pole pitch depends on the number of slots and the coil pitch. To make the article more useful, it includes tables to identify “preferred” coil pitches for 2-pole, 3-phase windings, as well as those that should be avoided.

One of the greatest pleasures of— and biggest challenge to—our industry is that we get to work on equipment of all vintages. Where else can you work on a machine designed and built a century ago and a state-of- the-art machine on the same day?

Electric motors have been produced for more than 120 years by literally hundreds of manufacturers worldwide. We get to repair many of them: some good designs and some not so good.

While manufacturers have learned from decades of experience, some of the motors we repair date from early on their learning curve. And not all new design engineers start at the same point on that learning curve.

Coil pitch and 2-pole machines
One area where this is especially true is coil pitch. Most readers are aware of the potential danger when a motor has a full-pitch winding (1.0 chord factor). One reason designers use a chorded (short-pitched) winding is to reduce those harmon- ics. A full-pitch winding is more likely to have severe cusps and higher noise levels than a chorded winding. We occasionally see full- pitch designs on new motors. For designs with 4 or more poles, the coil pitch should have a chord factor between .900 and .996 to avoid those slot harmonics.

But 2-pole machines with such a wide pitch are difficult to wind; so much so that manufacturers use a chord factor between .707 and .866 for most 2-pole lap windings. Since the optimum pitch is impractical for 2-pole machines, knowledgeable designers avoid a coil pitch that results in significant 5th or 7th harmonics. Those are the harmonics most disruptive to the line frequency sine wave.

What do we mean by “disrup- tive?” Well, in extreme cases, the waveform is so distorted that the motor simply cannot accelerate. The motor fails to accelerate past a severe dip (a cusp) in the torque curve, also called a “saddle torque.”

Even within the suggested range for 2-pole chord factor, there are certain coil pitches that may result in undesirable performance. If the coil pitch selected results in harmonics, the designer must increase the flux in order to produce the required torque.

Problems can occur when we do any of the following:

• Copy a poor original design
• Change the coil pitch without understanding the consequences
• Select the wrong pitch when redesigning a concentric 2-pole to a lap winding
• Leave off a special feature such as an intersperse

Repairers, rewinding motors produced over the past century by many different manufacturers, do encounter 2-pole motors with poor coil pitches.

Knowing which coil pitches can cause excessive slot harmonics can alert the repairer to potential problems. Reducing slot harmonics does not always require a longer pitch; sometimes a shorter pitch is the better fix.

A good guideline is to avoid any coil pitch that results in a harmonic with a magnitude 5% (or higher) of the fundamental frequency. The percent slot spatial harmonic is different for each combination of coil pitch and slots per pole. So you won’t have to calculate them, they are identified in the tables on the previous page. The coil pitch and correspond- ing chord factor (k_p) are given, with the percent slot spatial harmonic below. Don’t worry about lower magnitude harmonics—the 5%  limit is reasonable.

For example, consider the 1-6 pitch in the 18-slot, 2-pole table. Even though the k_p of .831 is well within the suggested range  for chord factor, it results in an unacceptably high 6% slot harmonic. If the 1-6 pitch was selected for a concentric to lap conversion, the motor might not even accelerate to full speed.

A 6% negative-sequence harmonic robs the motor of torque. Higher flux densities are  required to compensate. By selecting a more favorable coil pitch, with the same flux density as original, we are increasing net torque.

Side benefit of increasing coil pitch
Increasing the coil pitch has the advantage of requiring fewer turns per coil to produce the same flux, so the copper area can be increased in inverse proportion to the turn change.
As part of any redesign involving a 2-pole (e.g., a concentric to lap conversion), coil pitch selection should consider the calculated turns per coil, the flux density and the desirability of the coil pitch options.

Be sure to calculate the turns required to maintain the same flux. To determine the new turns per coil, divide the old k_p by the new k_p , and multiply by the old turns per coil.
For example, a 100 hp, 2-pole motor has 36 slots, and the follow- ing data:
13 turns per coil, Pitch 1-11
2-Delta connection

Checking Table 3, we find that the original coil pitch has a 5.1% 5th harmonic. One option is to increase the coil pitch to 1-14, which has less than 3% harmonic content. The new turns per coil are determined by: 13 x (.766/.904) = 11

In this case, increasing the coil pitch by 3 slots will reduce the required turns per coil from 13 to11. The increased slot room permits a wire area increase of 18% (13/11 = 1.18) without increasing the slot fill. That increase in wire area reduces the winding (I²R) losses,  meaning the motor should operate cooler.

We improved the performance without making the windings tighter in the slots, so the winder should be pleased.

Reduced I²R losses improve reliability and in this case could increase the efficiency by close to 1/2 a percentage point.

There are many 2-pole motors with less-than-optimum coil pitches in service, offering a possible opportunity to improve performance. This highlights one more opportunity to better serve our customers.

When a 2-pole motor requires rewind, coil pitch is one variable we can control. But we need to avoid certain coil pitches, when possible, to prevent reduction in performance. Increasing the coil pitch permits the use of fewer turns, with an increase in copper area.

For any design change, coil pitch is one of several variables. Use these tables to avoid selecting the wrong coil pitch. Don’t forget to look at the bore diameter when considering an increase in coil pitch. Someone has to put the coils in!