Question: Can You Run Two Brushless motors from a Single ESC? Answer: No
I am in the process of building a
Douglas DC-3
control line model that uses a pair of
ElectriFly Rimfire .10 (35-30-1250kV) motors, and wanted to know
whether it would be possible to use a single electronic speed controller (ESC) for them.
Unlike brushed DC motors with which you can - and I have in the past - gotten away with
powering two motors from a single ESC, the brushless motors use a three-phase signal
that is both amplitude and pulse width modulated. Such a waveform (see my
video from 2007) is not likely to be able to drive more than
one motor properly, particularly given the motor's interaction with the ESC due to its
time-variable complex impedance.
I did a fairly extensive Internet search trying to find a definitive answer as to
whether it can be done, but all were just guesses. Many people seemed
very knowledgeable on brushless motors and their controllers so I didn't doubt their
expertise, but no hard evidence was presented. Now if anyone else searches for empirical
testing that answers the question of whether two (or more) brushless motors can be driven
with a common electronic speed controller, he (or she) will hopefully find this page
- and/or my video - and get the answer: No, it cannot be done!
December 2108 Update
Website visitor Richard Percifield was kind enough to provide this information:
I read your article about driving two BLDC motors from one controller. Looking at
the motors you are using they are BLDC Sensorless versions. Being sensorless they monitor
the Back EMF to determine the phase that the rotor is transitioning to allowing the controller
to turn on the correct transistors in the 1/2 H Bridge drive. Thus, since it is monitoring
the Back EMF from the rotor rotating past the stator when you have two running in parallel
they both contribute Back EMF and thus place the controller in the wrong state for the
motors to be commutated properly. There are 6 states for the motor from the three phase
drive system.
If you had an open loop control system you could have driven both at the same time.
The system would work like a stepper drive system just rotated through the 6 step commutation
sequence and then the motor would synch up at the correct point, and frequency would
control RPM. Many systems use this method for driving identical motors in synchronous
fashion. You could also have used stepper motors driven synchronously to perform this
as well. This type of system is used in locomotives with good results. The only issue
is that you now lose the feedback and thus cannot detect skipped steps and locked/torque
overloaded states in the two motors.
Given that almost every BLDC drive controller chip has a Tach Feedback output, one
would think that there would be an option to lock RPM between controllers for multi engine
systems. Maybe this is a development opportunity!
Thanks, Richard Percifield
As you will see in the video, the interaction of the two motors is completely random,
where sometimes they will start and run together at low speed, and then go out of synchronization
as the throttle input is increased. Other times the two motors will not even start rotating
at the same time. Whenever the motors are out of synch, the current goes up very quickly
and the ESC gets hot. A simple "T" (or "Y" if you prefer) type connection is used between
the two motors and the ESC.
An interesting test, one which I have no intention of going to the trouble to perform
due to the time involved, would be to place high value capacitors in series with the
motor leads to see if isolating the direct current (DC) component between the two motors
and the ESC would make any difference. For that matter, it would be interesting to see
whether even a single brushless motor would work with series capacitors in the leads.
The end result is that I will bebuying another ESC to have separate
ESCs for the two motors, and use a "T" connection from the receiver to the ESC pair.
Attempting to drive two brushless motors with a single ESC.
Complex waveform from brushless motor electronic
speed controller (ESC).
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