cjOregon

What happens to make motors overheat and burn up when you use too large a propeller? You’ve got the same battery, ESC and wiring as with a smaller prop, what makes the amperage increase until something breaks???

The electrons start from a battery, follow the wire to the ESC which closes two switches for an appropriate period of time, the electrons proceed to the motor and enter one “armature” of the motor windings and proceed through that armature to the “WYE” juncture and proceed out the second armature of the motor windings and back to the battery to form a complete circuit.

Question #1, are these concepts correct so far?

When the electrons flow through the windings around the armatures of the motor they create electro magnets out of the armatures which repel away from one magnet and attract toward the adjacent magnet. The windings of each armature, if they were to be “unwound” and stretched out straight and connected up to that circuit would form a short and create amperage limited only by the maximum capacity of the circuit. The reason a short circuit doesn’t occur is that there is a resistance formed within the coils while the electromagnetic field is being formed and has to “fight” the adjacent overlapping magnetic fields also being formed by each wire. Is this resistance called reactive capacitance?

Question #2, is the theory of the second paragraph correct?

What basic understanding am I missing?

Matt Kirsch

My Feedback: (21)

Wow. Well, I wouldn't exactly call those "basic" questions, with respect to R/C. You're certainly way deeper into the technical aspects than the average modeler. Let's try to address the question about why motors burn up when you put too large of a prop on them, though.

A motor is designed to spin at a certain RPM on a certain voltage, with no load. When you put a load on the motor, you prevent it from reaching the RPM it was designed to run at. The bigger the load, the farther from the designed RPM.

Because the load is slowing the motor down, it's spending more time with each electromagnet energized as it pulls/pushes the motor on to the next phase. The bigger the load, the longer each electromagnet has to remain energized. Technically, each phase is in fact a short circuit, and the longer each short circuit is energized, the more heat is generated due to the resistance in the wire. You see an increased current draw at the battery because in essence, that is the AVERAGE current being drawn. At the instant any of the electromagnetic phases is energized, the current is actually determined by the voltage of the battery and the resistance of the windings.

This is probably a bit oversimplified, but I believe it's close enough to reality to give you a basic understanding of what is going on.

Red B.

An old saying says "Explain things as simply as possible but not simpler", Matt is close to overdoing it :-)

Old Michael Faraday figured it all out centuries ago. Its all about magnetic induction.

Because of magnetic induction a rotating electrical motor produces a voltage known as back-EMF when it is rotating. It is a voltage that tends to neutralize the voltage applied by the battery/ESC. The back-EMF is proportional to the RPM of the motor, the higher the RPM the higher the back-EMF will be.

The current flowing through the motor depends on the net voltage (i.e. battery voltage subtracted by back-EMF). The lower the net voltage, the lower the current will be. Ohms law applies: Current = Net Voltage / Resistance. In this case the resistance is that of the windings in the motor.

The result of all this is that the current decreases as the motor is allowed to rotate faster (assuming constant battery voltage). The lowest current will be obtained when the motor is running at full speed without any load.
If a load is applied, the motor will slow down, the back EMF will decrease, the net voltage will increase and as a result the current will increase. The maximum current will occur when the motor is loaded to the the point that it is stalled (zero RPM).

The heat generated by the motor depends on the current through the motor and the resistance of the windings. Again Ohms law does apply and Power = Current squared * Resistance, i.e. increasing the current by a factor of two will yield a four-fold increase in heat output.

/Red B.

cjOregon

Sincere Thanks Matt. It's starting to make sense now<grin>. I didn't understand that ESC time frames went longer when you begin to lose RPM (load the motor). That explains why a motor with no load pulls so few amps and the more load the more amps - delightful to finally understand. At the end of my second question I pulled up the wrong word from my feeble memory, perhaps coil resistence to change is called ?inductive reactance? Perhaps if an ESC could be designed that would limit the maximum current then motors wouldn't burn up?? It was bugging me and it's fun to know, thanks again for your reply.

cjOregon

Thank you very much Red B! Maybe the RCU University could eventually put your words down for people like me to read! As in school lots of times many others have the same question but don't ask...