Q-85: Can I wire two small brushless motors into one Castle Creations Phoenix-10 ESC?
A-85: Recently I noted several articles on multiple electric motor
scale aircraft, and articles explaining how to hook up such motors (such as
series or parallel and tying into a single ESC). I went back over these articles
I realized that nothing referenced operating two brushless motors from a single
brushless ESC. I was curious so I posed the question to Castle Creation’s
Marketing/Support Director, Shawn Palmer. The bottom line on this: it is not a
good idea!
This is what Shawn had to say on this subject: “This is a very common question,
and yet the full answer is still poorly understood. As you know, a sensorless
controller relies on the back EMF (electrical feedback) from the motor to 'know'
its rotor position, acceleration state, etc., in order to accurately commutate
the motor.
"When a controller is connected to a single motor, the back EMF signal is
'clean' and easy for the controller to read, so the motor runs accurately and
efficiently. When there are two motors connected, the controller’s
lifeline of information is not 'clean' since it expects only one signal. The
controller essentially averages both signals together to run the 'single' motor
it's seeing. The observed effect is very simple: neither motor is being
run accurately. This results in waste heat from the motors not commutating
properly which quickly builds. The controller will run hotter because of the
higher partial currents of one motor running too advanced and the other too
retarded.
"The net result under best-case scenario is inefficient running of both motors—increasing motor heat dramatically in some cases—and a hotter running controller. The more electrically and operationally identical the motors are, the less the observed effects, but no two motors are electrically and operationally exactly the same. They will always have phase differences, magnetic differences, resistive differences, and slightly differing Kvs. Even with two identical motors, the heating effects and inefficiencies will still manifest.
"Resistive differences from solder joints, wire lengths, and connectors come
into play, as well as differences between propellers and gearboxes on the two
motors. This also presumes that both motors start absolutely reliably together
every single time as is required for safe flight. Rarely is this the case in
observed systems."
Thank you, Shawn, for a good, honest answer that is easy to understand. The
bottom line is that if you plan on a multiple-motor aircraft and use brushless
motors, you should consider a separate brushless ESC for each motor. There
doesn't need to be much of a weight penalty, since these controllers are getting
smaller and lighter in weight every day, but it certainly is going to cost you!