Model motors: AXI external rotor brushless motors

PRODUCT REVIEW

A new spin on brushless motors

Every once in a while, a new approach changes the way we do things. Over the last few years, electric power systems have been moving to brushless motors for all but the smallest and lightest models. This move has accelerated with the recent Introduction of relatively Inexpensive brushless motors such as the Jeti and Mega Motors.

Now a new, even less expensive "external rotor" design is making inroads in electricpowered models. Just what is the external rotor? In all the brushless motors we use, the windings are stationary and the permanent magnets rotate. In an external rotor, the magnets are attached to the outer case (can) of the motor, and the case with the magnets rotates around the fixed windings. This results in greater torque, and the motor can swing a larger propeller on direct drive versus a conventional brushless motor.

MODEL MOTORS LINEUP

The Czech firm Model Motors now produces two sizes of external-rotor-type motors, each in two winds-its AXI series. The first motor was the AXI 2820/10. It's about the size of a Speed 500 can motor and weighs 5.7 ounces, including wiring and 3.5mm bullet connectors. It's 1 3/8 inches in diameter, has a 3/4-inch-long stator and has a 10-turn winding. This motor is also available with a 12-turn winding. Later came the 4.9-ounce AXI 2814-a smaller version available in both 10- and 12-turn winds.

All the motors have a hefty 5mm-- diameter shaft that is about 3/4 inch long. The front endbell is a finely finished casting, and the three power leads exit from it. The motor must be solidly mounted from the front, and there must be clearance all around it for the rotating case. Two pairs of 3mm mounting holes on 1-- inch centers are provided in the front case. This is the standard mounting-hole spacing for most motors of this size and larger.

SPEED-CONTROL REQUIREMENTS

Most sensorless brushless controllers with sufficient current rating work well with the AXI motors. My gold-label Jeti 40-3P controller works well with the AXI (older white-label controllers will not), and the Jeti Advance 40-3P also works very well, and it gives a bit more power when in the "hard timing" mode. Other possible controllers include the MGM ComPro TMM-- 40e-3ph and the Castle Creations Phoenix 35. I'm sure there are other suitable controllers in the Schulze Future and Kontronik Smile and Beat lines as well. Let's take a look at what these remarkable little motors can do.

APPLICATIONS

I have been flying an AXI 2820/10 for several months now in my JK Aerotech Big T. It's a cabin plane made of foam and Coroplast that has a 6-- foot wingspan and weighs 5 1/4 pounds. I fly it primarily on a 10-- cell pack that consists of Panasonic 3000mAh NiMH cells. After a little experimenting, I found that the APC 12x6 thin electric prop is best for this application. The motor turns this prop at more than 8,000rpm while drawing a little less than 40 amps at full throttle. My Big T, with 400 watts of power from the 2820/10, gets off a grass runway in three or four airplane lengths, climbs steeply and can do very large aerobatic maneuvers. In the hands of pilots more capable than I, the plane has flown extended knife-edge flight. Mind you, the Big T is intended to be a primary trainer, not an aerobat. After zooming around like this for 7 to 10 minutes, the little motor is rather warm to the touch, but not dangerously so. I operate it pretty close to its upper limits and beyond the manufacturer's recommendations, but with good cooling, I have had well over 50 flights and no problems. It does benefit from being mounted out in the breeze. Also, I don't fly whole flights at full throttle but rather a combination of aerobatics, cruising around and touch-and-go's. I can only imagine what this motor will be able to do in a 3 1/2 to 4-pound sport aerobatic airplane.

I also bench-tested but did not fly the smaller 2814/10 motor. This dynamite 6-- to 8-cell motor swings a 9- to 11-inch-- diameter propeller at a healthy rpm. I can think of a bunch of 3-pound or so planes that use direct-drive speed 600-type motors and would get a big performance boost (as well as shed about 3 ounces of weight) using this motor. Models such as the Great Planes ElectroStreak and ElectriCub and any of the 6- to 7-cell direct-drive 2-meter sailplanes come to mind. The 2814/10 is small and light enough to also provide more power in planes designed for geared Speed 400 and 480 motors, such as the SR Batteries X-250.

The 12-turn motors of the AXI family-- the 2820/12 and 2814/12-have similar capabilities but run at lower currents (35A peak rather than 40A) on more cells. They might be more suitable where longer run times are important, or where higher-cell-- count packs are already being used, or where a larger-diameter prop is desired, such as on a sailplane or a scale model.

FINAL THOUGHTS

If I sound excited by the AXI motors, it's because I am. The simplicity of direct drive along with their low price make them very attractive to sport and scale modelers like me. They are equal to or better than the best geared brushed motors but have none of the maintenance requirements associated with them. They lower the price and raise the performance of electric model airplanes, and the simplicity and silence of direct drive really appeal to me. If these properties also appeal to you, I suggest you seriously consider the Model Motors AXI family of motors.

Copyright Air Age Publishing Feb 2003
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