How ESCs Work
To make our rotor rotate, the ESC needs to switch
current between the three different sets of windings we
discussed last month. To do this, the state of the three
wires going into our motor are controlled with a set of
MOSFETS (metal–oxide–semiconductor field-effect
transistors). The windings are set to either high (connected
to the battery voltage), low (connected to ground), or left
floating.
Since there are three wires that can be in three states,
we can calculate the number of unique states. Reaching
back into our math classes, we know that the number of
permutations of a system with no repeating values is the
factorial of the number of choices. In our case:
3!= 3 x 2 x 1 = 6
If we number the inputs to our motor 1, 2, and 3, we
can chart out the state of the windings over the six states
that make up one complete revolution of the motor (Figure
1). If the sequence is played in reverse, the motor rotates in
the opposite direction. This explains why you can switch
any two wires on a BLDC motor and get it to rotate in the
opposite way!
So, how does an ESC accomplish this switching? It
needs some high current switches that can be controlled by
a microcontroller that takes care of the timing and
orientation sensing. A MOSFET voltage source inverter will
do the job.
Before we look at the entire three-phase switching
array, let’s examine a single MOSFET voltage source switch
(Figure 2). The N-channel MOSFET can be thought of as a
single-pole single-throw (SPST) switch of sorts. When the
gate voltage is at the ground potential, the MOSFET acts as
an open switch. When the gate voltage increases above a
certain threshold voltage, the MOSFET begins to conduct.
Using the configuration shown, we can connect the
The
Multi-Rotor
Hobbyist
ESC Basics
By John Leeman
50 SERVO 07.2017
Just like an engine powers your car but is controlled by a computer, the brushless DC motors
(BLDCs) we talked about last month power your quad but are also controlled by a computer. This
computer — the electronic speed controller (ESC) — can make or break the performance of your
quad. This month, we’ll go over the basics of ESCs and test one out with various firmware versions
to see the difference between them.
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magazine/issue/2017/07.
Figure 2: A single
MOSFET source
inverter circuit allows
the output to be
connected to either
the voltage source or
ground.
Figure 1: Six unique states compose a complete rotation of the BLDC
motor. Each phase can be connected to the voltage source (red),
ground (green), or left floating (orange).
