Mike Howell
EASA Technical Support Specialist

When rewinding the shunt fields of a DC machine, it is important to avoid making changes that could negatively impact performance. The recommended practice is to maintain the manufacturer’s winding configuration during the repair. That is, the field circuit connection, turns per coil, mean or average length of turn (MLT) and wire size should not be changed. However, service centers do sometimes encounter issues around wire size availability. The purpose of this article is to provide some guidance for making wire size substitutions when the original size is unavailable.

For guidance in changing conductor material, see the December 2011 Currents article “Aluminum-to-copper magnet wire winding conversions: Considerations for deciding whether wire area should be reduced.”

Wire size
Our goal is to use a replacement wire with the same diameter and therefore same cross sectional area as the original. When working with round magnet wire, it is very likely that the original size was manufactured according to some sizing standard. However, due to existence of multiple standards, you should never use a wire gauge to determine the original size. Always measure with a micrometer and then determine the size from the measured diameter. The two most common round magnet wire sizing conventions are American Wire Gage (AWG) and metric.

The AWG size can be calculated once the diameter is known. However, the calculation is complex, so it is recommended to simply cross reference the measured diameter with its corresponding size on a reference table such as EASA’s Solid Round Magnet Wire Data chart (see Table 1).

Metric sizes are much easier; the size is simply the diameter in millimeters. However, if you measure the diameter in inches, you can cross reference the metric size using EASA’s Solid Round Magnet Wire Data chart tables as well. For example, if the diameter is measured to be 0.0394 inches, you can see in Table 1 that the original wire is metric size 1.0 mm.

Service centers that usually work with AWG sizes may not have access to metric sizes or even some AWG half sizes and vice versa. In order to facilitate a timely and cost-effective repair for the customer, it is sometimes necessary to make a substitution and it is desirable to do this using simple and practical calculations. The approach here will be to essentially wind two coils in series per pole using two different wire sizes such that the finished coil has the same number of total turns and same total resistance as the original.

Calculations
The objective when making this change is to maintain the shunt field ampere-turns per pole (see “Importance of Wire Size” at the end of the article for more information). We will be considering the cross sectional areas of three wire sizes: (a) the area of a size larger than the original, (b) the area of a size smaller than the original and (c) the area of the original size. The ratio of area (c) to area (b) should not exceed 1.15. The total number of turns must not change.

T_c = T_a + T_b

We want to calculate a number of turns T_a of wire area (a) and a number of turns T_b of wire area (b) such that the average cross sectional area over all the turns is very close to the original wire area (c). If change to the coil length is insignificant, then the coil resistance will be sufficiently close to that of the original. The calculations are as follows:

Although the new coil will have approximately the same I2R heat loss as the original, the portion of the coil with the larger wire area will have a lower current density than the original while the portion of the coil with the smaller wire area will have a higher current density. Therefore, it is prudent to check the current density in the portion of the coil with the smaller wire area. Standard units of current density are amperes per unit of area (e.g., A/in², A/mm²) but it is customary when working with areas in circular mils to use area per unit of current or circular mils per amp (CMA).

When calculating current density, assume an even current division between parallel paths. That is, assume that series connected coils carry rated field current, series-parallel coils carry half of rated field current, etc. If the rated field current (If), the conductor area (Af) and the number of circuits (cf) are known, then the current density (J) in standard units (e.g., A/in², A/mm²) is calculated as follows:

If calculating current density in circular mils per amp, then the calculation is modified as follows:

It is recommended that EASA’s Technical Support Department be consulted if the current density in the smaller conductor is less than 500 CMA (or greater than 3.9 A/mm²).

If the current density in the smaller wire size is acceptable, the shunt field is then rewound by first winding T_a turns of wire with area (a) and then splicing to it the wire with area (b) and winding an additional T_b turns. The larger wire portion should be wound first to minimize any increase in field coil temperature rise. Additionally, it may be beneficial to pot the field coil to the pole piece (Figure 1) to improve heat transfer. See the January 2000 Currents article “Follow These Procedures To Reduce Problems When Rewinding Field Coils” for guidance with this process.

Example 1
A set of four shunt field coils to be rewound each has 700 turns. The wire diameter is measured to be 0.0394 inches and cross referenced with Table 1 to identify the wire size to be 1.0 mm. The service center performing the repair cannot obtain that size. The rated field current (If) is 2.7 amps and the fields are connected in series (cf = 1).

1.  Choose new wire sizes.

From Table 1, they choose the closest size above (#18 AWG) and below (#18.5 AWG) the 1.0 mm that they have. Based on these three sizes, they have:

a = 1620 CM (0.823 mm²)
b = 1440 CM (0.732 mm²)
c = 1550 CM (0.785 mm²)

c/b = 1550/1440 = 1.08 < 1.15

Note: It does not matter which units of area you use for your calculations providing you use the same units throughout.

2.  Check the current density.

Since the current density is acceptable (greater than 500 CMA or less than 3.9 A/mm2), proceed with the turn calculations.

3.  Calculate the new turns.

4.  Check the total turns calculated.

Example 2 – special case
If the original wire area (c) is very close (e.g., within 5%) to the area of one available size (a), it may be practical to rewind the coil with the one size. An example of this would be if the original wire size was #18.5 AWG (1440 CM, 0.732 mm2) and the service center had 0.95 mm (1400 CM, 0.709 mm2) available. This change would eliminate the need for a splice in the coil and would constitute a change in cross sectional area of less than 5% as shown below.

In this case, the current density will be sufficiently close to the original and while it is always good to verify (can identify errors in connection data, etc.) it is not necessary to evaluate it for this specific change.

Example 3 – special case
If the original wire area (c) is very close (e.g. 5%) to the average of new wire areas (a) and (b), it may be practical to wind both coils at the same time (2 in-hand) and connect the finish of coil (a) to the start of coil (b). Since the two coils to be connected in series would be wound simultaneously, only half as many turns by the winding machine would be required. In this case, the ratio of area (c) to area (b) still should not exceed 1.15. An example of this would be if the original wire size was #18.5 AWG and the service center had #18 AWG and #19 AWG available. They would have:

a = 1620 CM (0.823 mm²)
b = 1290 CM (0.653 mm²)
c = 1440 CM (0.732 mm²)

c/b = 1440/1290 = 1.12 < 1.15

This change still requires a splice in the coil, but it would constitute a change in average cross sectional area of less than 5% as shown below.

In this case, it would be important to check the current density in the smaller conductor as previously demonstrated. The benefit of this method is two-fold: First, heat dissipation in the smaller wire is improved by intermingling the two sizes; second, winding time is halved.

Appendix 1: Splicing guidance
Remove the magnet wire insulation before beginning. Some magnet wire insulation can leave a residue that inhibits brazing. Braze using a silver solder (never use soft solder) and alloys containing 3% silver or just phos-copper are typically suitable for this type of braze joint.

Practical tips

1. Secure the wire to be brazed.
2. Tin (deposit) a bit of the brazing alloy onto the end of wire 1.
3. Slip a 2-3 inch (50-75 mm) length of snug acrylic sleeving over wire 2, at least 3 inches (75 mm) from the end.
4. Touch the second wire to the tip of the first, play the torch over the joint, and the brazing alloy on wire 1 should flow to wire 2 when the temperature of both is correct.
5. Cool the joint quickly with a damp cotton rag.
6. Inspect visually and by feel to make sure the joint is smooth with no burs that could penetrate the insulation.
7. Slide the sleeving up over the cooled joint.

Appendix 2: Importance of wire size
The torque produced by a DC motor is proportional to the product of the armature current (Ia) and the field magnetic flux density (B).

So, for instance, if the field magnetic flux density was reduced, more current would be required to flow through the armature to produce the same torque. Now, let’s look at how the shunt field wire size can affect the field magnetic flux density.

The field magnetic flux density is proportional (although not directly) to the field coil ampere-turns (AT).

As the name implies, the field coil ampere-turns are calculated by multiplying the current in field coil by the number of turns. So, to produce the same torque without changing the armature current, the field magnetic flux density and thus the field coil ampere-turns must remain constant. To maintain the same field current at rated field voltage and a given temperature, Ohm’s Law applies and the resistance must be held constant: V = IR.

The resistance of a coil (R) is directly proportional to the length of the coil and inversely proportional to the cross sectional area of the conductor.

If the number of turns and general coil geometry are maintained, there will not be a significant difference in length (L). This leaves the conductor area (A) as the remaining variable. Increases in the conductor area will cause a decreased resistance and therefore an increase in field current at a given voltage. Increasing the field ampere-turns will reduce armature speed requiring a corresponding increase in armature voltage to return to rated speed. However, most modern drives are current-set, so using a slightly larger wire would reduce the voltage required to obtain the current and the net field strength would be the same. Decreasing the conductor area will do the opposite and for a given field current will result in increased heating of the field coil as well.

Categories: DC motors, Field coils, Design/theory/application, Repair procedures & tips, Wire

Tags: Motor design Shunt fields

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