BRUSHLESS Motor Design

Written by James Wang A conversation with Rainer Hacker As seen in the May 2020 issue of Model Aviation.
rainer hacker and the author
01. Rainer Hacker, and the author, at the Hacker factory near Munich. Hacker produces more than 100 different brushless motors.
hacker keeps a healthy inventory that includes motors
02. Hacker keeps a healthy inventory that includes motors, ESCs, servos, and accessories.

Hacker is a German company that is world famous for making high-quality, reasonably priced brushless motors. Most Hacker brushless motors are designed for RC airplanes, and the rest are for RC cars, model helicopters, and industrial applications. I use Hacker motors in my airplanes and helicopters.

The Hacker Turnado Series of motors are the company’s best-selling helicopter motors. They are designed for 500- to 700-class electric helicopters. The Turnado A50 Long Stator versions can develop 4,000 watts (5.4 hp) of power and cost roughly $200 each.

Rainer Hacker founded Hacker Motor in 1999. His passion is designing brushless motors, and he personally leads the design at Hacker Motors. I first met Rainer 20 years ago at the International Toy Fair in Nuremberg, Germany. We became friends and I have visited his factory a few times.

this stator is made of many laminated sheets and each sheet can be as thin as 0.2 mm
03. This stator is made of many laminated sheets and each sheet can be as thin as 0.2 mm. Hacker uses the best stator material from Japan to minimize eddy current.
copper wires are wound around the stator to produce magnetic flux
04. Copper wires are wound around the stator to produce magnetic flux.
hacker balances each outrunner casing in order to produce
05. Hacker balances each outrunner casing in order to produce a smoothspinning motor. These are finished industrial motors.

The Hacker research and development center is located 25 km northeast of Munich Airport in Germany. During my last visit, Rainer graciously spent a few hours explaining the intricacies of designing and manufacturing brushless motors. Besides developing motors and controllers for RC hobby use, Hacker partners with other companies to provide customized motor solutions for automotive, electric-powered vertical takeoff and landing aircraft, and other industries.

James Wang: Hello Rainer. I’m glad to see you again. Can you start by telling our readers how you got started in the motor business and what your modeling experience is?

Ranier Hacker: It’s very nice to see you, James. I started flying models [when I was] 8 years old. Roughly 20 years ago, while engine-powered models were the mainstream of modeling, I decided, "Why not fly electric?" I built a 3.5-kilogram Adriana sailplane and powered it with a brushed motor. The motor was getting so hot that I had to cool it by spraying it with Freon. There was no speed control; it was switched on by contacting copper posts together for full power.

I investigated and identified that the brushes in brushed motors are part of the limitations. I remember from my technical training that there were such things as brushless motors. At this time, there was no internet and I was working for my father in his grocery store.

During my free time, I went to the library at the University of Munich to do my own research on brushless motors. Twenty years ago, the US company Aveox was one of the pioneers in making brushless motors for RC hobby use. Aveox was making "sensored" brushless motors. Robbe in Germany also had some brushless motors; however, I wanted to build high-performance "sensorless" brushless motors.

This was a pioneering time and there was a lot of room for new motor improvements and experimentations. I knew Harald Konrath, who was the founder of Kontronik, and Harald was knowledgeable about ESCs. He and I both flew F5D Pylon Racing RC models 20 years ago. I built brushless motors for Harald to use, and he would build ESCs for me. The market is big and we are not competitors. It is good to push each other and learn from each other. Customers always need to have another choice.

JW: Is motor designing a black art?

RH: There is a science in motor designing. We use a special computer simulation program developed in Germany to do calculations first. Motors are becoming bigger and more powerful, and we have to be able to calculate and predict the performance before we build a motor.

Just having a computer tool does not mean the computer can do all of the work in designing a motor. Tremendous experience and judgment are required.

I am an avid guitar player. If you give someone a guitar, it does not mean he or she can immediately play music. The person needs to learn to control the strings, practice hard, and put in feeling.

We receive new ideas from customers every day. We are constantly learning from customers, and the learning never stops. We combine customer input with our own judgment, and then decide which knobs to turn to improve motor performance. Computer simulation is a great tool, but only a tool. Experience takes years to develop.

I have an engineer who helps me run the computer simulation, then I advise him what to tune. A computer might generate unrealistic results and tell you to make a 1-foot long motor to provide the torque you asked for.

JW: How have motor requirements changed throughout the years?

RH: Many things have changed in the last 20 years. In the past, it was just to make magnets better and improve the skill in winding the wires, but now we learn about new applications and new technical requirements from customers.

Mobile phones are good examples. Mobile phones have existed for many years. They were invented for talking, but users demanded more capabilities. People wanted [a] touch pad, texting, a camera, video, memo, and internet features. This brought the mobile phone to the age of a smartphone. We began our business years ago by focusing on motor performance, but now we must also focus on applications.

JW: An electric motor seems to have an infinite life. What can cause an electric motor to fail?

RH: In theory, only the ball bearings in the motor might not last forever, but modern industrial bearings can last thousands of hours.

We also make motors for automotive factory use. In three or four years of continuous usage, these motorized robots have assembled 1 or 2 million screws and the only maintenance to these industrial motors was to replace the bearings.

JW: Can the magnets in the motor fail?

RH: Neodymium magnets can work [in temperatures] up to 85° Celsius without any problem. They start to demagnetize at above 150°. I recommend that the maximum temperature the outside casing of a brushless motor should reach is 70°.

After a motor run, one should be able to touch the motor casing with the fingertip for 2 to 3 seconds without getting burned. Usually, there is no worry about the copper coating melting because the wires are double coated and can handle up to 210°.

JW: A few years ago, there was a magnet shortage around the world. How did that affect the hobby motor business?

RH: [The] majority of the magnets in the world are produced in China. In the last few years, the price of magnets has increased six times, but the price of our motors has not increased because motors are not made only from magnets. The price of magnets today has dropped to only double of that from few years ago.

JW: What is the difference between an outrunner and an inrunner brushless motor?

RH: Most RC helicopter motors are outrunners, because they provide excellent torque. In an outrunner motor, the motor casing spins and magnets are glued to the inside wall of the motor casing. The extra inertia from the spinning motor casing helps with torque.

For inrunner motors, the motor casing does not spin; the magnets are mounted on a rotor that sits in the center of the motor. Inrunner motors have lower inertia and can spin very fast—even up to 50,000 rpm. They are popular for on-road RC race cars.

JW: Can you please explain to our readers what a stator is?

RH: [The] stator is the iron core that the copper wires are wrapped around (for outrunner motors, the stator looks like an octopus with short arms). The purpose of the copper winding is to use electric current to generate magnetic flux. The magnetic flux then pushes and pulls on the neodymium magnets to cause the motor to spin. The copper wires are wound in loops around the stator poles to generate a focused pattern of magnetic flux.

The iron stator helps confine and concentrate that electromagnetic flux. For an electric motor to operate, the direction of the magnetic flux must constantly reverse direction thousands of times a minute. A stator is made from a special iron material where the molecular structure inside the iron is able to reverse direction and realign rapidly.

hacker manufactures its own motor components with cnc
06. Hacker manufactures its own motor components with CNC machines.
rainer stands next to the highly guarded motor research
07. Rainer stands next to the highly guarded motor research and design test room.

High-quality stator material allows the internal molecules to change direction quicker. An iron-nickel (NiFe) alloy is a typical stator material. Currently, Japan manufactures the best treated iron material for motor stator use. Good stator iron is expensive, requires a special manufacturing process, has gone through the right hot-cool cycle, is stable with temperature, and has no oxygen inside. We buy our stator material from Japan then stamp it to the shape we need in Germany.

JW: Why is the stator made as a stack with many thin sheets, roughly 0.2 mm thick, rather than made as a big, thick, solid iron block?

RH: Using a lamination of many thin sheets to make a stator stack and separating each thin sheet with an insulation coating will help reduce "eddy currents" inside the stator. Eddy currents are swirling currents induced inside a conductor to oppose any change in magnetic flux.

Eddy currents are generated in the stator every time the magnetic flux changes direction and want to oppose change. When we force a change, we force the kinetic energy of the currents in the stator to convert to undesirable heat. The shape and thickness of the stator sheets are influenced by the performance requirement of the motor. This is where experience comes in.

Depending on the application, the winding method, wire thickness, quality of stator, and grade of magnet will all impact motor performance and efficiency. There is also more to a motor than simply efficiency. The motor must fit the application. A Porsche and a tractor may both have a 300 hp engine with the same efficiency, but their operating environment and design requirements are different.

JW: A motor with a higher Kv (rpm per volt) rating will have higher, steady rpm for a given voltage. Is it true that if we want more torque and better acceleration we should use a motor with a lower Kv? How do you control the Kv when designing a brushless motor?

RH: When designing a new motor, Kv is controlled by the number of stator poles and wire turns. We have to explain what the poles and turns are. If you look down from the top of a stator, on an eight-pole motor, a wire has to wind around eight arms as each wire goes around the circumference of a stator.

We may use many wires, but if each single wire wraps around each pole twice, then that is a two-turn motor. In general, fewer poles give higher motor rpm, and more poles give more torque. Only two turns give a higher Kv and higher rpm, and more turns give a lower Kv rating and more torque.

JW: The Turnado A50-10L V3 and the Turnado A50 Edition 530 V3 motor both have 530 Kv. Why does the Limited Edition 530 V3 motor use a few thick copper wires while the standard Turnado A50 V3 motor uses many thin wires?

RH: It is not always the thicker the wire, the better. To use many thin wires or few thick wires depends on the application. For example, we could make a motor with 90 thin wires or with 10 thick wires. In some cases, thicker wires could be worse. That is why we make many different motors.

The wires in the Limited Edition 530 V3 motors are wound by hand, and the standard Turnado A50 wires are wound by machine. If we find something we can do better, we always want to offer the best value for the customer.

JW: What does "timing" mean for brushless motors, and why can changing the timing setting in the ESC change the motor power?

RH: Let’s use a combustion engine as an analogy. If a spark plug ignites every time that the engine piston reaches the top dead center, the engine timing is 0°. However, an air-fuel mixture needs time after the spark ignition to reach a maximum burn. Therefore, the engine designers need to advance the timing to ignite the air-fuel mixture before the piston reaches the top dead center.

the standard turnado a50-10l v3
08. The standard Turnado A50-10L V3 motor uses thinner copper wires and is machine wound.
the turnado edition 530 v3 must be custom ordered and uses thick copper
09. The Turnado Edition 530 V3 must be custom ordered and uses thick copper wires and is hand wound in Germany.
a row of new motors is ready to be finished
10. A row of new motors is ready to be finished.

For electric motors, timing means how early the electromagnet (stator) should be energized to pull the motor shaft forward. A magnetic field likes to remain at status quo. There is a time delay between when the current changes direction and when the magnetic flux changes direction in the stator; remember eddy current and stator material.

Advancing an electric motor’s timing will start changing the current direction earlier. There is an optimal timing for each motor design and stator material. If the optimal time for a particular motor is 10° and you set the ESC at 5°, it will not hurt the motor. You will likely achieve slightly less torque and a lower Kv.

It is safer to set the ESC to a lower timing. If the optimal timing is 5° and you set the ESC at 10° (you start the current change too early), you might hear some squeaking noise because of bad commutating.

JW: Are sensorless brushless motors more suited for RC use than sensored brushless motors?

RH: Sensorless design is easier for RC applications and is easier on the ESC design. Sensor brushless motors use Hall effect, contactless sensors to measure and know the angular position of the motor rotor at any given instant.

Sensor motors are more useful for industrial applications when you must know the precise angular position of the motor armature at any given instant. It is easier to control the torque on sensor motors; they are useful for motorized tools where precise torque control is required down to even zero rpm.

JW: Are all Hacker motors manufactured in Germany?

RH: Hacker produces different grades of brushless motors, and all are designed in Germany. On the highend Hacker motors, the copper wires are usually much thicker and hand wound by German technicians. The benefit of hand-wound motors is that the wires can be packed denser, hence producing more magnetic flux and power.

In other motors, the copper wires are machine wound around the stator. Depending on the motor type, some are manufactured in Germany, and some are manufactured in China under close supervision. Regardless of where they are manufactured, they must pass Hacker standards. Hacker uses high-quality stator material that can handle high-frequency change in magnetic flux and minimizes eddy currents.

JW: Thank you, Rainer, for the very informative explanation.


Hacker Motors

(913) 214-6995

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I thought your motors were made in china by sunray technologies.

I am reaching out to your company to see if you would be able to build a custom motor/actuator prototype? I have a design sketched and my small model as given me enough information request a more professional design. Please let me know.

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