In electrical engineering, coil winding is the manufacture of electromagnetic coils. Coils are used as components of circuits, and to provide the magnetic field of motors, transformers, and generators, and in the manufacture of loudspeakers and microphones. The shape and dimensions of a winding are designed to fulfill the particular purpose. Parameters such as inductance, Q factor, insulation strength, and strength of the desired magnetic field greatly influence the design of coil windings. Coil winding can be structured into several groups regarding the type and geometry of the wound coil. Mass production of electromagnetic coils relies on automated machinery.
Terminology related to electric machines This article is about coil winding technology and much of the article is specific to electric machines. This section provides definitions of terms used later in the article.
Rotor and stator
An electric motor or generator consists of a cylinderical rotating part called the rotor and a stationary part called the stator. For maximum efficiency, a gap between the rotor and stator is kept as small as possible, typically 1–2 mm.
For most AC generators, the stator acts as the armature, and the rotor acts as the field magnet.
Laminations The rotor and stator are both assembled from a stack of laminations stamped from sheets of electrical steel, typically 1 mm thick. The laminations are designed to minimize eddy currents, which would otherwise waste energy and create heat.
Slots and teeth Windings consist of insulated conductors, which pass through slots cut along the axis of the stator. The spaces between the slots are called teeth. The shape of the slots and teeth depends upon the design of the electric machine. Slots may be rectangular, they may taper, they may be circular, etc.
The wider part at the end of the tooth is called a tooth shoe.
Some authors use the term "groove" to mean the same thing as slot. In this article, the term groove refers to the indentations between parallel conductors in a layer of a winding.
Poles Pole means magnetic pole, and poles always come in pairs, usually referred to as "North" and "south." Poles may be physical objects in fixed places, or poles may be virtual objects moving as the machine operates. A pole may be a single piece of magnetic material, or a pole may be distributed over multiple teeth.
Electrical degrees A rotor makes a complete rotation in 360 degrees. That amounts to 180 degrees to pass by each pole of a two pole machine. The current in conductors also goes through a complete cycle in 360 degrees.
For a four pole machine, each pole spans only 90 degrees. During the 90 degrees the current in a conductor goes through a half cycle, or 180 degrees. Because there are four poles, and 4 × 180 = 720, the conductors go through two complete cycles for each complete turn of the rotor.
To distinguish between the mechanical movement of the rotor, and electrical activity in conductors, the terms mechanical degrees and electrical degrees are used.
mechanical degrees indicate the amount the rotor has turned about its shaft. electrical degrees indicate the amount the currents in conductors has changed.
In general, electrical degrees equals mechanical degrees times 1/2 the number of poles.
No matter how many poles, each pole always spans exactly 180 electrical degrees, and it is called pole pitch.
Electrical degrees and mechanical degrees are related as follows:
where P is the number of poles.
Principles Efficient coils minimise the materials and volume required for a given purpose. The ratio of the area of electrical conductors, to the provided winding space is called "fill factor". Since round wires will always have some gap, and wires also have some space required for insulation between turns and between layers, the fill factor is always smaller than one. To achieve higher fill factors, rectangular or flat wire can be used.
The fill factor can be calculated from:
- Wire gauge including the varnish insulation - Number of windings - Cross section of the coil body
Dense packing of wires reduces air space, and is said to have a high fill factor. This increases the efficiency of the electrical device and an improved heat conductivity of the winding. For best packing of round wires on a multi-layer winding, the wires in the upper layer are in the grooves of the lower layer for at least 300 degrees of the coil circumference. The wires occupy a dense package which is called “orthocyclic winding”. The opposite of this would be a random wire structure within the winding space, which is called “wild winding.”
Wild winding Also known as jumble winding, with this type of winding structure only poor fill factors can be achieved. The random wire placement leads to a wider distribution of resulting wire length on the coil body and consequently a wider range of electric coil resistances. Despite its disadvantages, it is common in mass production. It is characterized by low demands for machinery and operator and can be wound with very high speeds. Wild windings are mostly applied in contactor- and relay coils, small transformers, Ignition coils, small electrical motors, and generally devices with relatively small wire gauges up to 0.05 mm. Achieved fill factors with the use of round wires are about 73% to 80% and are lower compared to orthocyclic windings with 90%.
The winding height can be estimated using the formula:
- Wire gauge including the insulation - Number of windings - Width of winding
Helical winding
The wires are placed helically in every layer. Owing to the direction of movement from layer to layer changing between right-hand and left-hand, the wires cross and locate themselves within the gap of the layer underneath. A wire guiding of the lower layer is not existent. If the number of layers exceeds a certain limit the structuredness cannot be maintained and a wild winding is created. This can be prevented with the use of a separate layer insulation, which is needed anyway when the voltage difference between the layers exceeds the breakdown voltage of the wire insulation.
Orthocyclic winding
This type of winding structure creates an optimal fill factor (90.7%) for round wires. The windings of the upper layer need to be placed into the grooves provided by the lower layer.
The best volume use is found when the winding is parallel to the coil flange for most of its circumference. When the winding has been placed around the coil body it will meet with the previous positioned wire and needs to make a step with the size of the wire gauge. This movement is called winding step. The winding step can occupy an area of up to 60 degree of the coil circumference for round coil bobbins and takes one side of rectangular coil bobbins. The area of the winding step is dependent on the wire gauge and coil bobbin geometry.
If the winding step cannot be executed properly then the self-guiding ability of the wire is lost and a wild winding is produced. Overall, the first intruding wire mainly determines the location and quality of the winding step. It should be recognized that the wire needs to enter in a possibly flat angle into the winding space. That way an unnecessary bending of the wire is being avoided and the needs space for the second winding step is minimised. For orthocyclic wound coils, the winding step areas is always located at the area of wire entering the winding space and is being continued in helical form against the winding direction. As a consequence a bigger winding width of the coil, leads to a bigger winding step area along the circumference of the coil. The created offset leads to a different position of the layer step, from the first to the second layer, in comparison to the wire entry. This behavior repeats itself with every layer which leads to a spiral shaped crossover section at the side of the winding. As wires are crossing within the crossover section the resulting winding height is increased. As a result, orthocyclic wound coils with a round coil ground are never circular in the cross over section, but the radial moving winding and layer step creates a hump shape. Experience has shown that, depending on the winding width, coil and wire diameter, the crossover section is about 5 to 10 percent higher than the regular winding height.
Location and size of the crossover section Because the winding should be mostly positioned parallel to the winding flange, meeting an orthogonality condition, it is necessary to adjust the winding width to the number of turns per layer of the winding. In particular for non-circular shaped coil cross-sectional areas, it is desired to locate the crossover area to the small side of the coil body, also called winding head. This is due to the fact, that non-circular coils are being mounted on a sheet metal package or in a circular arrangement. The coils should be rather small, in order to avoid a contact with the neighboring coil or the sheet metal package. For orthocyclic round coils three winding geometries can be defined:
a) Equal number of turns per layer
b) Unequal number of turns per layer, starting with shortened layer
c) Unequal number of turns per layer, starting with longer layer
The choice of the winding structure to be used basically depends on the design of the coil or the coil body. Among others, the available space conditions for winding width and winding height must be considered. Moreover, it is possible to influence the location and the end of the last winding by selecting a clever winding pattern. The winding height of an orthocyclic coil winding results from the following equation:
-Winding height - Number of layers - max. wire gauge above the varnish (CuL)
Since an orthocyclically wound coil with at least 300° of the circumference of winding layers has the tightest circle package of the wire cross-sections. This winding method reaches the highest fill factor and is the best way to fill the available winding cross-section with round wires. Square coils are seen as orthocyclically wound when the winding and layer jump occur only on one of the sides of the winding cross-section. In theory, a geometric fill factor of 0.91 will be reached. In practice, however, the value cannot be reached because there exists a winding jump and layer jump area and the wire insulation is not taken into account.
fill factor = = =
= 0,907
In general, it can be stated that the requirements that need to be fulfilled for orthocyclic windings are very high. The summation of all tolerances has to be kept at a very low value. Consequently, the following values can be named as indicatory values: a) Tolerance of the winding space width
- tolerance of winding space - max. wire gauge including the varnish coat (CuL)
b) Tolerance of the wire Max. wire diameter tolerance should not be larger than
- gauge – wire tolerance - number of turns per layer - nominal wire gauge including the varnish coat (CuL) This corresponds to about half the wire tolerance according to the specification of the German standard DIN46435.
Manufacturing orthocyclic windings