Heat management (1)

The heat management of the motor is essential for an effective use of the energy. About 50% of the electric energy is wasted into useless heat. The heating of the motor and components reduces their efficiency and causes more loses (resistance).

There are mainly 3 components which dissipate the heat:

• motor
• ESC
• wires

My strategy is as follows:

• reduce the thinnest wires (ESC to motor) to a minimum
• increase wire diameter from battery to ESC to a maximum 
• have the ESC cooled externally
• have the motor cooled externally

The result is that I changed the battery cable to and placed the ESC on the motor rod. This reduces the wire length from ESC to motor and gives the ESC a cooling from the air flow of the propeller.

I also installed some additional propeller blades to cover the center of the 40" propeller.
The cooling of the motor was really bad before, as the direct driven motor does not get a good air flow from the 30" or 40" propeller.
With the additional 2 small blades (about 10", from a helicopter), I cover just the center of the propeller. Looks cool (and it is) and should give 500g more of thrust :-)
You can see the ESC (red circle) and the additional blades (green circle). The propeller is still well balanced after the change.

RPM measurement

Today I did some RPM measurement to calibrate the throttle control. As I have a Arduino to produce the input signal for the ESC, I can put it easily to any desired value and watch the RPM count from the Hall effect sensor.
I made the measurement with the 40" propeller and here is the result. The input signal varies from 0-179 and the RPM output has the expected curve. The max. RPM for this motor is 5800. At 5400 RPM I measured a thrust of about 13kg which is in accordance with the previous measured valued.
The output signal is now limited to 150 to not exceed this power setting.

Hall sensor for motor RPM

To know the RPM of the motor is a good thing. As the throttle control is done by a Arduino, I can use the RPM value to prevent the motor from overheating and adapt the throttle to different propeller configurations by software.
I made some research how to install a RPM sensor in a model motor. There are basically three posibilities:

• getting a signal right from the 3 wires of the synchronous motor --> that needs some soldering and signal processing
• putting a optical sensor for the propeller RPM --> possible, but too much components for electronics
• Hall effect sensor with magnets glue to the motor --> needs a least 2 additional mangets

I than realized that the motor has yet a lot of magnets inside. There are 40 of them ... so why do not use the Hall efect sensor with them?
This magnets are glued to the inner side of the moving motor housing. I have to get as close as I can to get a good signal. I tried some configurations with a standard Hall effect sensor and it worked.

The Arduino software checks now every 200ms the counts of the magnets and calculates the RPM.
Nice and simple. The value can be used to adapt the ESC signal to control motor speed.
The photo shows the position of the sensor, glued to the motor mounting. You can also see the motor mounting rod, made of carbon fibre. There are some wires left for 2 additional temperature sensors: one for the motor and one for the ESC.

 

Maiden flight

Today was the big day, the maiden flight of the ascending aid. I choose a small hillsite near Toledo (Magan, Cerro del Aguila) for this first test as the height is only 90m and I had to go back by walking with 15kg of equipment and 4kg of motor/battery.
The wind direction was perfect for this site, north-west with about 5-8 km/h. The test was conducted with the old battery set and the 30" propeller with 10kg thrust.
The ground handling is very easy, the battery pack does not disturb too much. The start procedure is like a normal paraglider. The motor is switched on when airborne to prevent any hazzle with the lines. The motor is also switched of before landing for the same reason.
I flew 2 nearly parallel tracks to compare the sinking speed with and without motor. You can see the details in the track log. The 10kg thrust gives me a ascending rate of about 1m/s. It could not compensate the 1,2 m/s sink rate at this moment, but it would have made the flight much longer if I wished.
Next time I will try the 40" propeller with 15kg thrust. In this configuration, a positve ascending rate should be possible.
Here is the link to the video :

And here the track log:

Arrival of new batteries

Today I received also a new battery pack. The new Turnigy Lipos are 15C rated and weight a little more than the first ones (only 10C). Now I have 6 x 8000mAh 6s batteries. Next test will be conducted to compare the performace of each one. With all batteries I should have a autonomy of about 20min full power. The battery weight is 6kg and the housing motor 2kgs as before. Here a photo of the two battery types:

New BEC

Today I got a new BEC to power the Arduino. The former setup was a smaller BEC rated 6s, which burned down after 5 minutes. Than I installed a 9V battery. with the new beter BEC I do not have to check for the voltage of the 9V battery. as far as there is a 6s 24V LiPo connected, the Arduino has power. Here is the photo of the old and new BEC (old, small one is already junk):

Ground test with video

In this viedo you can the the assembly of the eHayabusa to the standard harness Swing Connect Reverse. The test was conducted with the 30" propeller. Than I tried the 40", which gives >15kg of thrust. But in this configuration the heating of the wires is to much, after 60 seconds I stopped the test. I will install a new software version for the 40" propeller to limit time of full throttle to prevent any overheating.

https://www.youtube.com/watch?v=C8QBHXXR0wA

Continous run test for 6:00 minutes

One of the most important test was conducted today, the continous run test. I started full throttle with nearly full batteries to check the power consumption of the motor and the heat dissipation.

The result is very convincing: at full throttle with the 30" propeller I operated the motor for exactly 6:00 minutes. The cells were depleted afterwads to 22.7V that means 3.78V (LiPo 6S).

That is a fairly good value and there would be still some reseserve power. As the cells had seen some short tests before, I estimate the total operational time of about 7:30 minutes.

That is also more than initially expected.

The ESC has a shut-off controll for low voltage to protect the battery, so there is no problem of damaging it with to low voltage depletion.

Another important point is the temperature disipation. I checked with an infrared thermometer and the motor housing is getting to 78ºC at the end of the test. The power cables are also getting warm, around 40ºC. The battery itself only changed to 35º, also the electronics is not getting any warmer.

Static thrust on ground

I measured the static thrust on ground with a normal digital scale. The measured result was 10.1kg with the 30" propeller and unbelievable 14.9kg with the 40" propeller.

From the photo you can see that the axis has a inclination of about 25º. That means the real thrust in propeller direction is about 10% higher: 11kg for the 30" and 16,5kg for the 40".

That is far more than I expected for the 40".

Measurement setup with a sand bag to avoid the movement of the housing on the scale: