The Story

From brand new, after 3 flights, the problem was that the motor would just shudder and not rotate.

There was that burnt electrical smell and one of the three speed controller (ESC) mosfet chips had a fine crack.  One of its two mosfets was shorted.

The small controller board includes a DSM2 receiver, flight stability controller and motor ESC.  The model has four servos, two for ailerons, elevator and rudder.

ESC Repair

The board was remove from the model and cleaned of silicon and super glue.

The parts arrived after a week from RS Components and the circuit board was repaired.  All 3 chips replaced. Each chip contains a pair or mosfets; P and N channel.

The test runs were using a current limited power supply set to 7.4V rather than the battery.  This prevents further damage if there is another fault or something goes wrong.

Once repaired and tested, the board was coated with liquid electrical tape.

There is still a problem

It runs up to 1/4 throttle then makes squealing noises and stops.  Pushed rapidly to full throttle it runs up to speed but is not smooth, and stalls as soon as the throttle is reduced.

It seems the speed controller doesn’t like the motor.

A Faulty Motor

My first guess (based on past experiences) was a faulty motor.  So I connected another small motor; a brand new HK AD100L 12g (I think). Other than the motor being horribly out of balance, it all worked.

I noted that it is possible to stall the motor and the ESC gets a bit upset for a short time.  The ESC does detect the overload and shut off.  With a battery connected, there is a chance that the ESC could self destruct.  It’s an abnormal situation, but not uncommon when a landing goes wrong. Just remember to cut the throttle whenever there is a problem.

Next step was to measure the three winding inductances of the brushless motor.

• black-blue = 20.27uH
• blue-red = 3.66uH
• black-red = 22.22uH

There appears to be a fault on the blue-red winding. Probably shorted turns.

A New Motor

The only new motor available had short leads. After testing the new motor to be sure it was a good one and measuring the winding inductances, the longer leads from the old motor were grafted to the new motor..

• black-blue = 44.6uH
• blue-red = 48.3uH
• black-red = 40.0uH

Other than the winding inductances being a lot higher than the readings from the old motor it all worked. A few current readings with the new motor and a 7.5V bench power supply current limited at 4.5A.

• 63mA = idle current draw
• 240mA = full throtle with no propeller
• 0.8A = half throttle with propeller
• 2.2A = full throttle with propeller

The controller board does get warm when running the motor at half throttle or more.