Motor heat: does it matter?
Why temperature is important at an electric motor?
Is it a new trend now? Is it really important? Why do many pilots measure the temperature of the electric motors? One question and all the answers to know.
If you have ever flown with a nitro helicopter, you know that, checking the temperature of the engine is crucial. When temperature is not good, engine is weak and it can damage badly or even worse. Tweaking the carburation affects the temperature and the performance of the engine. This is why electric motors made helicopter flying easier. Measuring temperature during and after flights is unnecessary for ordinary pilots. However it is a really important information for caring manufacturers.
Sponsored and team pilots measure the temperature of their motors, because this is also a feedback for the suppliers. But why is it so important? Let's try to understand it.
The principle of these motors is the same: we are using brushless 3 phase motors for the helicopters. One main component of these motors is the neodymium magnet on the rotor (and this is not the rotor made by blades, this is the rotating part of the motor). These magnets are the strongest known magnets, way stronger than the iron magnets. They contain iron, boron and neodymium (Nd), because the neodymium itself is a very rigid, sensitive material, it gets oxidized quickly, breaks easily. The neodymium alloy magnet's heat tolerance is about 80°C, however with appropriate manufacturing this can be increased to 230-240°C maximum.
The magnetic strength gets weaker by higher temperature. The only way to achieve the same power from the motor next to the higher temperature is increasing the current. This is why high amp drawing doesn't mean the motor is strong. It can mean anything, even bad config or stiff gears. Matter is the power output and better motor draws less amps for the same power.
Increasing the current draw means more loss, like the copper loss, electric loss and so on. Energy cannot be destroyed just transformed. And the energy loss has to turn into something, this is heat typically. At the end of the day, loss turns into heat, so this is a positive feedback like the nuclear chain reaction. And winding of the motor cannot be loaded with current endlessly, there's a point where it gets full, it saturates and doesn't produce stronger magnetic field even if the current is higher.
Balance between the current and the temperature is crucial. As balance between the stator strength and the rotor strength also highly important. In an ideal world they are as strong as the other one, otherwise the absorbed energy from the stator to the rotor is not 100%. Quality of the winding, design of the motor, materials used, airflow are all very important. And this is why the team pilots and sponsored pilots check the temperature after the flights especially after different flying styles, then they send these results back to their factories. Heat is the public enemy for the motor.
If a motor is hot once, magnets get weaker temporarily. But as it happens more and more times, the loss of the magnetic strength turns into permanent, by the time the motor gets weaker unavoidably, it gets demagnetized. Back in the time when the Align T-Rex 500 was released, the Align RCM-BL500L motor wasn't too powerful, but it was okay. Then after many flights it got weaker and weaker. There was one symptom known by everyone: the motor was extremely hot, even the frame got the heat. After 200-250 flights the motor was gone, it didn't work anymore with no obvious reason.
In an ideal world there is no friction and there is no electric resistance at all in a motor, which all result loss. And this loss occurs in the form of heat. The aim when a motor is being designed is to minimise the loss and the heat produced as much as possible.
If you have been in the hobby for long time, you remember, when the Scorpion began producing electric motors for helicopters. It was a trend, everyone replaced the factory default Align RCM-BL500L motors to Scorpion ones in the really popular T-Rex 500s, because they were much more powerful - we liked them, but they were really hot after the flights. This heat had a less liked consequence: the bearings got worn out in really short times. After 30-50 flights all were gone, and the sign was a squeaky noise.
We have tested all bearing brands available from the cheap Chinese ones through SKF to Japanese EZO, but none of them lasted longer than 50 flights. After a while we've got so experienced in the replacement, I'm still able to replace the bearings in my old Scorpion motor of the old T-Rex 500 in 5 minutes (including the getting off and putting back). The motor is still working (after almost 10 years), but it demands this caring time-by-time. I keep a set of bearings at home always.
This wearing out is caused by the heat. The outer ring is not expanding as much as the inner ring and if this wasn't enough, the balls are also expanding by the heat in the smaller space produced by the differently expanding rings. The bearing gets tighter causing more friction and finally the balls don't take the forces anymore, then the bearing makes a bad noise and makes an extra vibration, until it stops working. After all the motor shaft starts slipping in the bearing or even worse, the entire motor stops.
Also the insulation of the wires in the motor gets damaged in the motor resulting a short circuit, killing the motor - and if you were not lucky, the ESC followed it and died.
And if you didn't get to this point, still the constant high current draw shortens the lifetime of the battery as well. Yes, they can supply high current for a short time, but it matters how many times it has to deal with these peaks. If the motor is hot, something is not good. It can be a construction problem or just a simple timing issue, but hot motor means short battery life rather than high performance of that particular motor. Also high current is not equivalent of good motor. Better motor is which can produce the same power output on lower amps. Because that means the loss is lower, the motor is more efficient, the battery lasts longer, the motor will work well longer.
These all are affected by the heat. In the early times the only question with these motors was: how to make them stronger. The current question is how to make them stronger without making them hotter. All bits are about this balance. All components - like the alloy of the iron, boron, neodymium - are top secret, and each manufacturer is working on these parameters. And there are no exact rules, building a good motor is an experimental science. Each combination has to be tested again and again before the motor leaves the factory as a final product. The only difference among the factories is the point: when gets the motor declared good.
In fact the low temperature itself is not the only indicator of a good motor. The power provided is the ultimate indicator and the life time given by these expensive power sources also very important. But all indicators are affected by the temperature after landing.
Leader motor factories are more or less similar in order to quality: Egodrift, Kontronik, Scorpion or XNova are all put this level very high, and even the less popular companies in RC helicopter hobby (which can be very popular in other markets), like KDS or Hacker are working by similar standards.
When you choose a motor to buy, the temperature is not the only parameter you have to consider. First: does the factory make a motor for your model? Then it counts how loyal you are, what your friends and favourite pilots use and the experiences they have got. How is the warranty, the customer service. And finally but last: how "thick" your debit card is. However the temperature measured after a hard flight predicts how long the motor will last for your hardly earned money.