Darren's Blog

Motors 101 Part II

Motor specifications and safe operation

Electric motors are damaged by both over-heating and over-rotation. Over rotation is the simplest to manage, so we’ll start with it.

Every motor has a limit to how fast it will spin before flying apart. Every motor is different, but most inrunner brushless motors reach their limits around 50,000 RPM; Outrunners 25,000 RPM. Brushed motor max ratings vary widely due to differing methods of brush construction, but most top out by 40,000 RPM.

To ensure your motor cannot exceed its rotation spec, multiply your intended batterys maximum voltage (not the nominal voltage on the pack) by the Motors Kv. This number should not exceed the Rated RPM for maximum safety. The spec can be pushed into the yellow line by going 10-15% above rating (since battery V will be lower and load will slow the motor some), but over 15% should be used with extreme caution as motors in flight (particularly in a dive) can spin up to their no-load RPM.

Example:

I want to fly a Razor RZ400 in a Wild Wing with 3s li-poly packs. With a Kv of 3650 and a rated max rotation of 50,000, I can use up to (50,000/3650) 13.7V safely. The maximum voltage of a 3s li-poly pack is 12.6V, so I am within spec. If I decide to push it to 4s (16.8V max) my potential RPM shoots to (3650 * 16.8V) 61,320 -- 22% above rated max RPM!

Power and Heat

Heating comes from a number of sources inside a motor, but the primary culprits are electrical current and motor rotation – technically referred to as electrical and magnetic losses. A motor is running at maximum efficiency when its electrical and magnetic losses are equal – but in most our lower voltage (sub 20V), high current applications, current losses dominate, so start there.

Most motors come with a maximum constant current rating – which is a good baseline to use for safe operation. However, extra current itself does not ruin the motor, heat does. So depending upon your motor’s cooling, the ambient temperature, and throttle usage, this spec can be pushed farther with additions of heat sinks and cooling air, or must be scaled back in applications with no cooling air or insulation (such as foam or balsa) surrounding the motor.

No matter how closely your power system is designed to the maximum current rating, it is important to keep initial flights short (5 mins) and check temps. It is also good to check your motor temps periodically after flight (after each flight where practical) to ensure it is not too hot. Too hot is usually defined as instantly hot to the touch. Some brushless motors can be damaged at 60 degrees C (140 F). On a hot day, it does not take much to heat from 100F to 140F. The cobalt motors and some of the Neodymium alloy motors (including the Himax & clones which use Cobalt rotors) can operate as high as 100C (212F) before damage.

It is particularly important to check motor temps when using a gearbox as any foreign material, bend in the shafts, wear of the gears, etc may cause binding and drastically increase amp draw and motor temp.

Rotational heat is generated by magnetic resistance and eddy currents as the motor rotates. Just as copper and gold have electrical resistances, cobalt and neodymium have “magnetic resistances”. The motion of magnetic fields generates heat. Neodymium has a lower resistance than cobalt, which is why most BL motors chose rare earth or Neodymium. The efficiencies are higher.

Usually, if you design a system to operate at its Max RPM spec with unloaded voltage and near its max Current spec with prop and gearing, you’ll have the highest safe power output with good efficiency.

Example: I have a Himax 2025-5300 motor, some 3s TP2100 12C rated packs and I want to fly a GWS Corsair with good speed and thrust. A Himax has a max RPM of 60,000 and max current rating of 25A. My unloaded voltage is approx 12.6V, max RPM unloaded will be 66,780 – right at the 10% red line so I’ll want to be careful to throttle back during dives.

Looking at the data chart for 3s li-poly cells provided on the manufacturer website, a 5.33:1 ratio and a 9x6 prop comes it at 15.9A. For the Corsair, that will provide a good balance with 36oz of static thrust (on a <20oz plane) and good speed with 6 inches of pitch. It also provides ~150W on a <1.5lb plane so I have greater than 100W/lb. I’m also well below the max rating of the motor.

That will be my starting point. From there I can increase my prop size or pitch to add more speed or thrust as desired.

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