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Normally, a dual-motor plane has two ESCs but could I connect two brushless motors to one ESC? What would happen to the torque and speed of each of the two motors connected to the ESC?

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Technically, you could connect 2 motors if they're exactly the same and it will probably work. However, there is a good possibility of various weird things happening due to this.

The problem with connecting more than one motor to a single ESC stems from how these motors are driven. An electric motor works by using electromagnetic coils to pull permanent magnets and create a force on the rotor. However if you just supply current to the coil and leave it as is, after the nearest magnet gets attracted to the coil, it will just sit there in the nearest position. To create a continuous rotation, you need to stop the current as the magnet passes by and reverse it to now repel it and add more rotational momentum to the system. In brushed motor this switching is done mechanically using contacts (brushes) inside of the motor.

A brushless motor does not have this kind of mechanical switching mechanism, so the switching must be done externally, and this is the brushless ESC's job. To do the switching at the correct moment it must detect when the magnets are passing the coils. To do that it constantly measures the voltages on its three terminals (as a magnet passes by an electromagnetic coil, it creates a voltage in it), and uses that information to switch the currents in the motor in perfect sync with the motor's rotation, speeding up the current pulses as the motor speeds up and slowing them down as it slows down. Sometimes, a special situation called desynchronization, or, colloquially, desync can occur if the ESC is incorrect in its assumptions about what the motor is doing or how it's going to react and sends pulses that don't correspond to the motor's actual state. With quadcopters this is most common when the motors are spinning at very low RPM, loaded by a reverse airflow (e.g. falling out of the sky bottom-down) and need to suddenly speed up, all of which mess with the ESC's expectations.

Now, if you connect two motors, the ESC will still think it's one motor. If they are the same brand and model, with the same propellers, it's possible for them to rotate in sync, in which case they will produce more or less the same induced voltages in their wires and react the same to what's supplied by the ESC. Even then, nothing is perfectly matched. One motor will likely be slightly ahead of the other, or otherwise slightly off, which is probably not enough for triggering a desync, but means a more confused ESC and less efficiency overall, as one of the motors (or both) is driven suboptimally. Further, when you mount the motors on an actual airplane, conditions may be even more unequal. Suppose that the plane is flying in a crosswind, or even that it's just in a banked turn, so one wing moves through the air slightly faster than the other. Then the load on the motors is even more unequal, and one of them might get just enough ahead of the other to confuse the ESC into either giving up, or just thinking the wrong thing about this weird combined "motor" to send the wrong signals that might, for example, slow down one of the motors and speed up the other, likely ultimately ending in one of the motors desyncing and stopping entirely. This, in turn, can potentially lead to very unpredictable behavior of the model in flight.

If you connect two different motors, the most likely thing is that one (or both) of them won't work. Different motors will both create voltages of different magnitudes when rotated, and react with different speed to currents flowing through their coils, so one of the motors will invariably get far ahead of the other in response to a current pulse, making synchronization of the two near-impossible.

So, to conclude this long-winded explanation, the answer to your question is:

Don't. Just don't.

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From a technical standpoint it is possible, but there are a number of considerations:

  • Only need one ESC so lighter
  • ESC needs to have a higher current rating to fully power the motors
  • Motors must be identical (more of a problem if home-building with what's on the bench)
  • Loss of redundancy (if a prop jams or motor fails, you have no thrust)
  • Cannot use differential thrust

Which of these are pro's or con's will depend on what you're building. For example, if you are building something for the fun of the build, to see what you can make and you're happy with it not surviving the day, then doubling up the motors could get you in the air quicker. If you are building a much-anticipated scale model that you would like to fly for years to come, then the redundancy benefits probably make it worth picking up the extra ESC.

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You can, but it's not recommended. First of, you will need an ESC that can handle twice the current of what one motor consumes. The motors will also be seen as one single motor by the ESC, so of one of them has an error or becomes out of sync, both of them will stop.

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