Coil cost drivers

The cost stack of a wound part

A coil quote is built from material, conversion, test, packing and overhead. The proportions change with size, volume and specification, but the same few drivers appear on most parts.

On a small relay or solenoid coil, the magnet wire and the winding time usually dominate. On a larger transformer, the core and the copper dominate, and the labour is a smaller share. On a potted coil, the resin and the process time become significant.

A buyer who asks for a single piece price cannot see which driver is high. A should-cost breakdown, even an approximate one, makes the conversation specific. It also shows which design change would actually reduce the price.

Magnet wire

The wire cost depends on the conductor weight, the insulation build and the copper price. The conductor weight follows the turns, the mean turn length and the wire diameter. A long end turn or a generous bobbin adds copper that does nothing for the magnetic design.

Copper is a commodity, and the wire price moves with it. A quote should state the copper basis and the review mechanism. A fixed price with no index is either hedged into the number or exposed to a sharp movement.

The insulation grade also costs money. A polyimide or a high temperature enamel costs more than a standard grade, and the difference grows with the thermal class. Specifying a higher class than the thermal calculation needs is a common and avoidable cost.

The wire diameter has a double effect. A larger wire costs more per metre but lowers the resistance and may allow fewer turns. The optimum is where the material, the loss and the window balance, and it is worth checking rather than assuming.

Bobbin, insulation and resin

The bobbin is a moulded or machined part with its own tooling, material and tolerance. A standard bobbin is cheap and available. A custom bobbin with a moulded pin block, a complex wall and a tight tolerance costs more and takes longer to tool.

The insulation materials add up: slot liners, layer insulation, tape, sleeving and phase separators. Each is a purchased item with its own minimum order and lead time. A design with five insulation items has more cost and more supply risk than one with two.

Impregnation and encapsulation add resin, process time and equipment. A vacuum pressure impregnation cycle costs more than a dip, and a potted coil costs more again. The specification should say which function the resin is providing, so the process can be matched to it.

The thermal class ties these together. A class H coil needs a class H bobbin, class H tape and a compatible varnish. Upgrading the wire alone does not upgrade the part, and it adds cost without adding performance.

Winding time

Winding time is the machine cycle plus the load and unload. It rises with the number of turns, the number of layers and the pattern. A layer wind is slower than a random wind, and a bank wind is slower again.

The turns count is set by the electrical design, but the pattern is often a choice. A random wind on a coil that does not need a controlled build can cut the cycle time significantly. A layer wind on a coil that needs a predictable diameter may be worth the extra time.

Changeover matters at low volume. A machine that needs a new guide, a new program and a new setup for every batch carries that cost into the piece price. A family of coils that shares a bobbin and a wire can reduce the changeover.

The windinghouse labour rate and the machine hour rate are commercial facts, but the cycle time is a design outcome. A drawing that adds ten turns for no functional reason adds cost on every part.

Termination

Termination cost depends on the method and the number of joints. A pin moulded into the bobbin with a welded wire is fast and repeatable at volume. A flying lead with a hand soldered splice is slower and more variable.

The number of terminations matters as much as the method. A coil with two leads is simpler than one with four, and a coil with a moulded connector is simpler to assemble than one with loose leads that have to be dressed.

The reject rate at termination can be high if the process is not controlled. A weak weld, a cold solder joint or a damaged enamel may pass a visual check and fail at test or in the field. The cost of that reject belongs in the quote, even if the supplier does not show it.

Strain relief and lead dressing add labour. A design that anchors the lead in the bobbin and specifies a short, direct route is cheaper to build than one that relies on a hand tied loom.

Test and yield

Every coil is usually tested for resistance, and many are tested for inductance, insulation resistance and high potential. The test time and the equipment cost are part of the piece price.

A tight resistance band or a 100 per cent high potential test adds cost. So does a functional test that requires a fixture and a trained operator. The test plan should be proportional to the risk, not copied from a larger part.

Yield is the hidden driver. A process that produces a 2 per cent reject rate carries the cost of the scrapped material and the test time. A process with a 10 per cent reject rate carries much more. The buyer sees the piece price, not the yield, unless the quote is broken down.

The cost of a field failure is larger than the piece price. A coil that fails in an assembly can stop a line, and the containment cost can exceed the value of the whole batch. That is why the test plan should be reviewed with the quality team, not only the buyer.

Volume, tooling and batch size

Tooling for a coil is usually modest compared with a stamping die, but it is not zero. Bobbin tools, winding programs, termination fixtures and test fixtures all have a setup cost. At low volume, that cost is spread over few parts.

Batch size affects the material buy and the changeover. A small batch may not reach the wire supplier's minimum, and the winding machine may spend more time setting up than running. A larger batch lowers the piece price but ties up inventory.

The volume forecast should be honest. A quote based on an annual volume that never arrives leaves the supplier with a setup cost it cannot recover, and the price rises at the next order. A realistic volume band is more useful than an optimistic peak.

A family of parts with a common bobbin, wire and termination can share tooling and reduce the effective setup. That is a design decision as much as a sourcing one.

Building a should-cost model

A simple model adds the wire weight times the wire price, the bobbin and insulation cost, the winding time times the machine rate, the termination time, the test time and a scrap allowance. Add overhead and margin to reach a price.

The table below lists the drivers to test first. The percentages are illustrative and will differ by part, but the order is a useful starting point for a small coil.