What is it?
A stator in a BLDC (brushless DC) motor is the stationary part that goes in the rotor (or surrounds it for inrunner motors). It is made up of stacked steel or iron sheets called laminations and has coils of wire wound in slots around its outer slots. When electric current flows through these coils, they create a magnetic field. This magnetic field interacts with the permanent magnets on the rotor (the rotating part), making the rotor spin.
The thickness of these laminations and the quality of the electric steel used affect the efficiency of the motor. The stator is also the structural core of a motor, so imperfections in its production can create an unstable and perhaps unusable motor.
These stators come in various sizes with the important dimensions being the slot length, depth, and gap. The inner gap of a stator is where the housing goes to keep the rotor concentric with the coils. Integrated actuators also use this inner gap to place the reducer to make a compact, high torque density actuator.
This is a very important component in a motor, and one with perhaps the highest "idiot factor", i.e. the premium on the raw material used, of any subcomponent in a motor. Getting this production done right at scale is essential for a robotics stack.
How is it made?
To make these you basically need to punch the stator shape on a sheet of electric steel (steel with ~3% silicon mixed in for superior magnetic properties), stack the punched sheets, fuse them together, and power coat for protection.
For a minimum viable production of stators you need a list of basic SKUs and highly optimize the production for those. There are a few common sizes around which a lot of the actuators currently used are made, getting the cost of those stators down to single digits will go a long way in bringing down the final cost of the motors.
Electric Steel Spools
You can buy a 20 tonne spool of high efficiency electric steel from manufacturers in China for $1583/ton. These sheets are 100mm in width and 0.2mm thick, which implies 131,579 meter long spool, which will yield 1.31M stamps at $0.024 per stamp. A stator of height 10mm will require 50 stamps giving us ~26,200 stators, 20mm height 100 stamps 13,100 stators, etc.
100mm stators
| Height | Stator Count | Raw Material Cost |
|---|---|---|
| 10mm | 26200 | $1.17 |
| 20mm | 13100 | $2.34 |
| 40mm | 6550 | $4.68 |
60mm (219298 meters, 3.65M stamps, $0.009/stamp)
| Height | Stator Count | Raw Material Cost |
|---|---|---|
| 20mm | 36549 | $0.9 |
| 40mm | 18274 | $1.8 |
| 60mm | 12183 | $2.7 |
200mm (13158 meters, 65,789 stamps, $0.048/stamp)
| Height | Stator Count | Raw Material Cost |
|---|---|---|
| 10mm | 36549 | $0.9 |
| 20mm | 18274 | $1.8 |
| 40mm | 12183 | $2.7 |
[Side Note: Industrial metals business is a $200B/yr industry in the US, and electric steel is a small part of it. I will have someone a lot more experienced with that part of the stack to write about the economics of making these sheets shortly. Getting the raw material at such prices is not a trivial task at all. At the time of writing this article tariffs on steel imports from China is 75%.]
Tooling
Making the progressive stamping die that punches the lamination is a highly skilled and expensive task. Production of these precision dies with proper coatings and maintenance requires skill and careful attention.
Making a smaller die like those for a 60mm stator is approximately $12,000 which can have a lifespan of 5M to 6M stamps. Being a little conservative you can estimate 1 die for every 25 tonnes of steel processed at 60mm width, which is adds a cost of $0.0024 per stamp.
Larger dies for 200mm stators can be $22,000 to $25,000 and have a lower lifespan due to higher stress per stamp. At a 1M stamp lifespan you add $0.025 per stamp of tooling cost.
This is if you contract out this production to experienced shops that make dozens of dies every month. Setting up a scaled factory for tooling is out of the scope for this article.
Powder Coating
A 100 x 10 stator has a surface area of approximately 150 cm^2, applying a 2mm powder coating implies ~45g of powder coating. You can buy this powder for $4/kg in bulk. This adds up to $0.18 of powder cost per stator. Doing this math for other sizes is shown below
| Stator Size | Height | Powder Coating Raw Cost |
|---|---|---|
| 100mm | 10mm | $0.18 |
| 100mm | 20mm | $0.30 |
| 100mm | 40mm | $0.54 |
| 200mm | 10mm | $3.07 |
| 200mm | 20mm | $5.05 |
| 200mm | 40mm | $9.24 |
| 60mm | 20mm | $0.62 |
| 60mm | 40mm | $1.25 |
| 60mm | 60mm | $1.70 |
Industrial Machines
You need a spool feeder ($16,000), a 45T servo stamping press ($64,000), a 5T hydraulic stator core forming machine ($3,400), an automated powder coating and curing setup ($42,380), and 2 staff per production line to run this operation.
You need one of each of these machines for a single stator production line, so 3 sets for this minimum viable operation. Total cost ~$450k. Assuming a 5 year life span for the machines, and $120k/yr/staff in pay uand benefits.
You'll also need a forklift ($10,000), some transformers ($12,000), and other ancillary machinery ($10,000), for factory operations.
Total $482k + $240k/yr recurring
Factory Space and Energy
A single production line will take about 2000 sqft, 6000 for 3 lines, and another 4000 sqft for manoeuvring space, storage, and an office space. 10,000 sqft total. In an exurb of a mid size city you can get a space like this for $40,000/yr. Give or take $20,000/yr in electrical, heating, and cooling.
$60k/yr recurring cost
Expected Throughput
This number only makes sense if you constraint yourself in some ways. By using single production lines that operate 16/7 with staggered shifts with weekly time for maintainance, loading, operations, purchasing, minimizing downtime due to lack of raw material inputs, having backup tooling, keeping maintainance staff on call, etc. you can get a bext case scenario for this minimum viable operation, and then multiply it some efficiency factor of 90% or so.
For 100x10 stator which is used in the Saturn Motor and 24Nm actuator the throughput is 1 stator/min * 60 min/hr * 16 hr/day * 6 days/week * 52 weeks/yr = 299,520 stators/yr. At that scale the cost per stator is $1.17 for the enameled electric steel + $0.18 for powder coating + $1.04 welding + $2.5 amortized tooling cost + $0.09 amortized machine cost + $0.8 for labor + $0.2 factory rent and electrical = $5.98. Let's double this and round up to $12 to get a neat profit. The cheapest Saturn has been able to find such stators is $8.42 from China. Shipping is $177.22.
The amortized costs include machinery and factory space that can do a ton more productive work, meaning the $5.98 price we arrived at is a very conservative estimate. There are further efficiencies to be made in this operation.