Mike Howell, PE
EASA Technical Support Specialist 

Concentric to lap winding conversions are common practice in many EASA service centers. For most stators, this change is feasible and can be made with little difficulty. In some cases, however, it can be quite challenging and result in undesired outcomes due to reduction in copper or insulation. One such challenging rewind is that of the two-speed two-winding hoist or crane motor.

The most common configuration we see from technical support inquiries and the example explored in this article is the 2/12 pole 36 slot stator rated with a 6:1 power ratio (constant torque). Though there are exceptions, the most common as-found data is consistent with Table 1. The windings described are both single-layer windings, and they are generally insulated from one another in the slot.

Table 1: 2/12 pole 36 slot stator — typical
High Speed (2 pole)	Low Speed (12 pole)
6 groups of 3	18 groups of 1
1-14, 16, 18 pitch	1-4 pitch
1Y	1Y consequent pole

 

 Many winders convert these windings to two-layer lap windings because they do not have concentric winding heads, they are not comfortable with the connections, or both. 

Concentric winding heads
Concentric winding heads are commercially available from several reputable manufacturers for reasonable prices, especially when considering potential savings due to reduced winding time. Additionally, some service centers have manufactured their own concentric winding heads, with at least one taking advantage of affordable 3D modeling software and a 3D printing additive manufacturing process (See Figure 1).

If a service center’s winders are currently unfamiliar with the use of concentric windings, management might be apprehensive to make a capital investment in concentric winding heads. Several EASA members have commented that after making this transition, especially on smaller stators, they save time and materials. From a winding process perspective, one of the most challenging aspects reported has been uncertainty with the internal connections. 

Internal connections
One of many benefits of EASA membership is access to EASA’s publication, Internal Connection Diagrams for Three- Phase Electric Motors. Many winders use this resource daily, and it’s especially helpful when some unusual connections come along. While most winders seem to grasp the connection format quickly when doing two-layer lap windings, applying the same connections to concentric windings seems to be more challenging. EASA Technical Support periodically receives requests for concentric connection diagrams when the connection diagrams in the book can be applied to lap or concentric windings. Figure 2 includes a simplified circle diagram and a detailed winding layout using group end numbering consistent with EASA’s Internal Connection Diagrams for Three-Phase Electric Motors for both the 2 and 12 pole windings referenced in Table 1.

Concentric to lap conversion
Let’s explore the high speed winding - 6 groups of 3 coils using a 1-14,16,18 pitch. First, changing from a single layer winding to a two-layer winding with a separator between the bottom and top coil side will result in approximately a 3-5% increase in slot fill. Figure 3 shows a stator slot as it transitions from single-layer to two-layer to four-layer. Aside from the space required by the additional separators, strands will tend not to nest as well when additional insulation is introduced in the slot. To stay within 2% of the original magnetic flux density and current density without increasing the number of turns per slot, a 1 to 17 coil pitch (89%, 160°) would be required, and this is very difficult to wind. For a 1 to 14 pitch (72%, 130°), the winding requires about 10% more turns and an additional 15% of slot space is rarely available to accommodate this change. The high-speed windings in these motors are generally designed with back iron magnetic flux densities near saturation, so they are sensitive to increased magnetic fields.

For the low-speed winding, the same slot fill challenges of going from a single-layer winding to two-layer winding are present. And, working with 18 groups of 1 (consequent pole) is less cumbersome than the 36 groups of 1 required for the conventional 12 pole two-layer lap winding. The low-speed windings in these motors are generally designed with tooth magnetic flux densities near saturation, so they are also sensitive to increased magnetic fields should the turn count be reduced because of slot fill issues. In most cases, the high-speed winding is inserted first, and if slot fill becomes an issue, changes tend to be made to the low-speed winding in an effort to save the high-speed winding. 

Both windings in these machines are also usually designed with a relatively high current density because of their intermittent duty cycle. They can be very thermally sensitive to reduction in copper area per turn. 

Lap to concentric conversions
Converting as-found lap windings to the windings shown in Table 1 can be easily done by hand or by using EASA’s AC Motor Verification and Redesign Software (ACR). 

Section 2 of the EASA Technical Manual includes conversion factors for lap and concentric windings along with instructions for their use. If using ACR, input the as-found lap winding and convert to lap using a 1-19 pitch for high speed (36 slot 2 pole), and a 1-4 pitch for low speed (36 slot 12 pole). The number of turns per coil in both windings will be twice the number of turns calculated for the lap winding. An example of this type of conversion is given in Table 2. With few exceptions, we recommend using a series wye (1Y) connection for both windings. Also, when doing these conversions in ACR, always choose “Half” in the “Round Turns” option since you are going to multiply the answer by two.

Table 2: ACR Lap to Concentric Example
High Speed (2 pole)		Low Speed (12 pole)
As found	As left	As found	As left
6 groups of 6 8 turns 1-15 pitch 1Y ACR Calculated 6 groups of 6 7.5 turns 1-19 pitch 1Y	6 groups of 3 15 turns 1-14, 16, 18 pitch 1Y	36 groups of 1 23 turns 1-4 pitch 1Y ACR Calculated 36 groups of 1 23 turns 1-4 pitch 1Y	18 groups of 1 46 turns 1-4 pitch 1Y consequent
This change results in approximately a 12 to 15% reduction in slot fill due to turn reduction in the high-speed winding and separator elimination for both windings. Depending on the situation, margin may be used to increase copper area per turn, improve ground insulation or to achieve a reasonable fit if the as-found fill was extremely tight.

In many cases, this conversion will allow you to increase the copper area per turn while still producing a proper snug slot fill that is comfortable to wind and avoids undue mechanical stresses on the coils and insulation. 

Other speed ratios and slot counts
While the 2/12 pole 36 slot stator is most common for these motors, there certainly are other speed ratios and slot counts used. Some manufacturers build 2/12 pole motors using 48 slot stators. The low-speed winding in this configuration is inherently unbalanced and should be designed with a series wye (1Y) connection. Other combinations such as 4/12 pole or 4/16 pole are also sometimes seen and can present their own challenges depending on the number of slots and poles. However, the 2/12 pole 36 slot design addressed here is the most common and these tips can make those rewinds much easier.

Categories: Technical topics, AC motors, Miscellaneous, Cranes & hoists, Repair procedures & tips

Tags: Cranes/hoists Concentric windings Winding redesign

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