A few weeks ago I got bored at work and started delving into the dark world of axial flux motors. I posted all of my findings in a stream of consciousness blog to Facebook-land, but people screamed at me and told me it was the wrong place to post this. I disagree, as I got a lot of great input from John Reid, Adam Bercu, and James Cooper on the subject, all of which have more experience and actual knowledge than me. Come with me now as I recreate this blog about a funny man named Cedric Lynch, international patent disputes, world records, and axial flux.
DISCLAIMER: The information presented in this blog is mostly without reference and is downright fiction at places. Please do not take anything as assumed or legal fact.
No not linear induction we’re done talking about that.
What is an E-Tek?
It is a permanent-magnet brushed DC motor built and sold by Briggs and Stratton. It is nominally rated for around 10hp at 48V, however, we have noticed that they can take far more volts and power
Where can I buy one?
You can’t! Production ceased in the early 2000’s
There are about 5 different motors sold right now as E-Teks that are not E Teks. We’ll come to those in a bit
IN THE BEGINNING
Somewhere around 1983, autodidact and garage tinker Cedric Lynch started looking into making his own electric bicycle as a way to avoid paying for gas or a car. He quickly realized that all electric motors on the market were induction type used for high voltage AC or glorified low-wattage hobby parts. With there being no particularly good DC brushed motors, and micro controllers required for brushless drive still in their infancy, he did the natural thing: made his own.
Using A BUNCH OF SCRAPS IN A CAVE (flattened and cut tin cans in his garage) he assembled his first ever AXIAL FLUX motor and put it on his full-fairing electric bicycle, which he would then put about 50,000 miles on over the course of the next few years.
But what is an axial flux motor? As a contrast to your average brushed/brushless motor which has magnetic flux moving radially away and around a rotating shaft, an axial flux motor has the magnetic forces moving in the same axis as the shaft.
The magnetic path in an axial flux motor is simpler, and allows for the use of linear grain-oriented laminations, which are far cheaper and simpler to produce than radial grain oriented ones. Additionally, the simpler high-flux density rotor requires less copper leading to lower motor inductance. This decreases the amount of stored inductive energy in the motor which tends to spike as the brushes pass across the phases, decreasing brush arcing and improving brush life. All in all, the axial flux design grants relatively high efficiency (93%!) at low RPMs and a higher torque constant than you’d normally see on a large brushed motor. (Thanks to John Reid for help here!)
Following the smashing success of his electric bike, his motor came to the attention of several electric boat makers who purchased several hand-made copies from him, all shed-built. These motors would soon go into world record attempts, scoring several records through the 80’s and all the way into the present day.
As the world records for electric bike and boat efficiency piled up, so did the demands for more and more of his motors. Cedric partnered with with a London investment firm who helped him secure patents on his motors and acquire professional manufacturing facilities, in exchange for rights to his designs. And with that, in the late 1980’s he was off to the races making electric motors.
But there was a big speedbump almost right out of the gate. Somewhere shortly after forming the Lynch Electric Motor (LEM) company, a German supplier of his parts connected the dots on his manufacturing process and ran away with the design. I’m not naming names, but I am saying that currently the only continental European company that currently supplies brushed axial-flux motors is PERM.
These PERM motors tend to not like being over-ridden from their specs, and are prone to centrifugal explosion if spun to fast, housing cracking and breaking, and shaft shearing from generating too much torque. (As reported by Ray Billings and Gary Cairns)
Manufacturing still continues on the LEM motors throughout the mid 90’s. Around 1994, John Fiorenza, an engineer with Briggs and Stratton, encounters a Lynch motor for the first time. Impressed by the efficiency and power in a relatively easy to manufacture package, he implores his bosses to license the technology for small outboard motors and electric lawn mowers. They agree, and the Briggs and Stratton E-Tek was born.
Utilizing rotors and laminatons from LEM’s largest motor, the LEM-200, John and his team completely re-deisgned the casign, shaft, and brush assembly to more closely resemble a professional automotive product. Stainless steel housings were replaced with cast aluminum, close bus bars were replaced with isolated wiring runs, and so on. We’ll see all the differences later on in this post, but on the whole, it brought powerful electric motor technology to the United States for relatively little money.
Unfortunately, the American public wasn’t dreadfully interested in electric boats or lawn mowers at the end of the 90’s, and demand for these motors was low. Once the licensing agreement for the Lynch motor expired in the early 2000’s, John Fiorenza left Briggs and Stratton to form his own company, Motenergy.
Motenergy, sometimes re-sold under the name “Mars Motors”, offered bolt-in replacements for the E-Tek that had a more conventional radial flux and brush design, but kept the strong housings of the E-Tek. The radial flux motors still suffer from the same reduced power that caused Lynch to design his axial flux motors in the first place, but several decades of technological development and improved manufacturing capabilities in China made them cost-effective and helped to close the power-per-mass gap.
More problems came to axial-flux motors in 2002, with disagreements between Cedric and the London-based investors of LEM. Cedric was pushed from the company he had founded and still holds his name. The investor’s contracts dictated that the company still owned 50% of the IP rights and could keep producing his axial flux motors without license, but that he was limited in what sort of IP he could take to his next company. LEM continues to produce what is known as “Lynch Motors” or LEMs, even though they are all based on decade-old designs of Cedric’s and he’s had very little to do with the company since.
To LEM’s credit though, they do sell a variety of sizes and wind voltages of axial-flux motors, something no other manufacturer seems to do. Their smallest is 127mm in diameter, their largest is the classic 200mm OD size and they come in a variety of KV’s at every size.
Lynch himself joined up with the Saietta Group later that year and began producing a nearly identical motor to his larger LEM-200, which they called the Agni. Saietta and Agni motors focus more on application and performance engineering than LEM, but produce more refined designs. Lately, their focus has been on custom-building performance motorcycles and production electric vehicles rather than making just the raw motors for resale
There are several other axial flux motors and companies that have popped up since the early 2000’s, but they tend to be prohibitively expensive or unreliable enough for combat robot applications. Off the top off my head I can think of YASA and EMRAX, both of which offer brushless versions of the axial flux technology. The shorter inductance paths aids these style of motors by increasing their efficiency and power ratings dramatically over radial flux, but make the motors incredibly difficult to control as feedback on low-inductance motors is a tricky thing and getting switching time right on these current hungry motors usually ends in deep-fried controllers
That’s all for now, feel free to drop me a line if I got anything horribly wrong or there are additions and experiences you’d like to add! Stay tuned for part two where I actually take apart a LEM-200 and Briggs and Stratton E-Tek side-by side!