No. A brushless DC motor and a brushed DC motor are two different
A PM brushed DC motor has permanent magnets for the stator (mounted to
the motor housing) and coils on the rotor (mounted to the turning motor
shaft). Commutation (which is what produces rotating torque that makes
a motor turn) is accomplished by brushes in contact with the commutator
(which is connected to the shaft).
A brushless DC motor turns this around - the permanent magnets are
mounted on the rotor and the coils are mounted on the stator.
Commutation now occurs electronically by the brushless motor
controller. The controller "rotates" the magnetic field in the motor by
driving multiple stator coils with phase-shifted sine waves.
I've never really understood all the different types of motors that exist.
If a brushless DC motor is driven by phase shifted sine waves (which makes
plenty of sense as you describe it), doesn't that make it an AC motor? Why
is it called a DC motor if it works like that? Any clues? Maybe it's time
for me to do a bit of research on the net and learn something new...
Yes. A "brushless DC motor" is really a polyphase synchronous AC servomotor.
For historical reasons, small servomotors of this type are called
"brushless DC", and large motors (typically 1KW and up) are called "AC
servomotors". (And small motor controllers are called "motor controllers",
while big ones are called "drives").
The upper limit on motor size for AC servomotors is quite large.
The current generation of large locomotives uses AC servomotors to
drive the wheels, keeping all the wheels in sync by active control.
No more individual wheel slip.
Not true. If you feed the right signals to the feedback inputs you
should be able to use one phase of the BLDC driver to drive the motor.
Then again, that would be insane, since BLDC drivers are bigger, more
complicated, and more expensive than ordinary drivers.
That doesn't sound right. A normal DC motor will invert the connections to
the rotor as it spins so you can feed it with a constant DC voltage. A
brushless DC motor won't so the driver needs to constantly invert the
polarity of the signal sent to each phase. I don't see any way you can use
a driver that is generating an AC signal to drive a normal DC brush motor -
no matter what feedback signals you send it.
Many drivers select which phase is active based on hall effect feedback
sensors in the motor. If those feedback inputs were instead hardwired
such a driver would not produce an AC signal on its own. You are right
that this won't work for open loop drivers.
Curt Welch wrote:
Thanks for the info. I've also done some reading on the net now and have a
The "brushless DC motor" works the same as normal permanent magnet DC motor
except instead of using a mechanical switching system (the commutator) to
switch the current to the coils, the mechanical system has been replaced
with an electronic switching system. Since you no longer need the
mechanical switching, there is no advantage to making the coils spin, so
they invert it and hold the coils still and make the magnets spin. So in
both types of DC motors, the system is fed with DC. In both cases, the
switching systems converts DC to AC with the frequency matching the
rotational speed of the motor. So even a normal permanent magnet DC motor
is driving the coil with an effective variable frequency AC signal created
by the commutator. I had never thought of it like that before.
And there's no reason a normal PM DC motor needs to have the coils on the
rotor. It could just as easily put the PM on the rotor and make the coils
stationary and still use a mechanical system to switch the coils. But the
mechanical system would need extra rings and brushes if you did that to
connect the coils to the switch (at least 2 extra brushes and 2 more
rings). So if you are using a mechanical system to generate the AC current
to drive the coils, the system is simpler if you put the coil on the rotor.
And likewise, an electronic switch could have the coils on the rotor, but
you would need rings and brushes to get the current to the spinning coils.
So as long as you have the electronic switch to replace the mechanical
switch, it makes more sense to put the permanent magnets on the rotor and
keep the coils stationary so you can eliminate all the mechanical brushes.
So it's called a "brushless" motor simply because the mechanical brushes
have been eliminated by using an electronic commutator. The advantage of
course is you have replaced a mechanical system that wears and produces
nasty conducting dust with an electronic system that will last a lot longer
and not produce the dust from the brushes and rings wearing down. They
could just as easily be called, PM DC motors with electronic commutators.
The advantage of both types of DC motor systems is that they provide high
torque even at slow speeds since the switching of the drive current is kept
in sync with the rotation of the motor.
AC synchronous motors are fed with a constant frequency AC current and the
motor spins in sync with the frequency of the AC current. But they have
problems starting since the AC frequency is not slowed down to keep it in
sync with the actual rotation of the motor - so other systems are used to
start these types of motors. These seem to be used mostly in special high
power industrial applications.
Most common AC motors (like what we find around the house) are not
synchronous motors but instead, are induction motors, which act like
transformers. So the rotor has windings (or one type or another) but is
not directly powered through brushes of any type. Instead, the AC field
created by the stationary windings induce current to flow in the rotor
which acts like the transformer stationary. These actually loose power as
they get near to synchronous speed because if the rotor is turning in
perfect sync with the rotating AC field, then there is no net power
transferred to the rotor windings and the torque drops to zero. So
induction motors always run slightly slower then the sync speed.
The other major class of motor we find around the home seems to be the
universal motor. This works like a DC PM motor, except the PM is replaced
with a stationary winding energized by the same current used to energize
the rotor. Unlike the AC induction motor, this motor needs brushes which
work just like a DC motor with brushes. But since both coils are driven by
the supply voltage, you can drive the motor with either DC or AC. Which is
why they are called "universal". Spin direction is not controlled by the
supply current polarity but instead, by how the rotor and stationary
windings are connected. Invert one and the motor spins in the other
direction. These motors work like DC motors in that there upper rotational
speed is limited only by power and load. And for high power applications,
it seems they have a real problem of self destructing by trying to spin too
fast if you don't keep a load on them. But they have the advantage of much
higher speeds than the induction motors which can't go any faster than
their synchronous speeds - so higher speed applications like blenders and
drills tend to use this type of motor.
There seems to be an endless number of variations on these ideas (and as
many different names and terminologies to go with the variations), but the
above designs seem to be the major classes of electric motors that exist.
Exactly. At the windings, all motors are AC. Or they don't keep
rotating. They'd reach some position and stop, like a rotary solenoid
or a stepper.
Wikipedia has a good article on electric motors.
Well, I'm no expert, but this is what I gather from the web sites I read.
Steppers are basically a type of brushless DC motor. They have magnets
which rotate and fixed stator coils. Their design is optimized to step and
lock in fixed positions instead of being optimized for spinning. For one,
this typically means they have more stator coils and more poles on the
rotor magnets to allow it to stop in more positions for each rotation.
Because they use permanent magnets, they will generate currents when you
I'm not sure, but some of the sites I read left the impression that some
stepper designs don't use magnets. But it's unclear from what I read if
that means they have coils on the rotor connected with brushes that must be
energized to act as a magnet, or, if the rotor simply uses some metal that
is attracted to the energized stator coils. I believe it's the second.
I have often heard steppers called "very early forms of dc brushless".
Any varying magnetic field passing through a wire will cause a potential
to be induced in the wire. So as you apply torque on the shaft, you are
moving the rotor, and it's magnetic fields are varying, causing the
windings in the stepper to "feel" the varying fields, and generate
This is very much like the same principle that every speaker (motor to
move air around) can also be used as a microphone (air moves speaker,
and it generates current).
Here's an even better explanation from hyperphysics . In the case of the
stepper, the magnet is moving and the wires are still, but the
generation princople is still there, because it is relative motion that
And if you want to have some real fun, get a pair of identical steppers
and connect the like wires together. If you then spin one of them fast
enough to generate a decent voltage, the other will step along in sync.
Never tried that, but don't doubt it a bit.
The same is true of the really finely made swiss motors, with a little
bit of gearing infront of them. Hook them in parallel, turn one, the
other follows. Almost ghost like. And you don't have to turn that fast.
And that brings up a whole 'nother set of things called servos.
Servo-resolver or synchro-resolver sets. Also called selsyn when used as
a system. Anybody remember those? I hadn't thought of those in about 20
years. I had a whole tinker toy set of those things in our Mk 68 Gun
Fire Control System when I was gunner officer, U.S.S. Dewey.
The very point of those resolvers is one would exactly track the
position of another off at some distant point.
I do remember hearing or reading about those at some distant point in my
past. I think it was back in the days when I was more into electronics
before I got hooked on computers which would have been about 30 years ago.
Didn't they use external power and act as electro-mechanical amplifiers?
Or were they stand alone with no need for external power?
They used an external AC reference signal. Really they were more like
transformers than amplifiers. The phase orientation of one would
minimize the current only if the other was at the same phase. They
didn't have a lot of power. Mostly used for indicators. Here's a pretty
detailed write up about them.
You might be thinking of magnetic amplifiers, whcih was a whole 'nother
contraption (also used in that gun fire control system).
I think this is how a radar operator in a WWII destroyer tracked targets. I
was a radioman and our shack next to radar and we'd visit. I was always
amazed at how neatly the radarman could control that huge dish on top of the
mast. This was my introduction to servo technology.
Very astute. This was probably a selsyn, which is a particular type of
AC-actuated servomechanism. Very simplistic compared to modern servos
and in fact no external electronics (other than the odd resistor or
capacitor) were usually required. For a power control application these
were often referred to as an Amplidyne, as amplification of the
transmitter signal was needed to control a much heavier receiver load.
Your ship's telegraph was probably selsyn-operated as well, as were a
number of its indicators. A selsyn mounted on the shaft of the rudder
was used to indicate the actual position of the rudder, for example.
Selsysn are still used as position transmitters for feedback
on aircraft actuators. They're more rugged than absolute encoders
or feedback pots. The receiving end today is usually a resolver-to-digital
converter, not another selsyn.
An amplidyne is something else. That's an application of super-regeneration
to get extra gain from a Ward Leonard drive. It's a DC device; a selsyn
is an AC device. It's obsolete; the early stages of amplification for a high
power drive have been done with semiconductors for decades.
Ward Leonard drives, which were invented in 1893, are still around.
They're a motor coupled to a DC generator, with the field of the generator
controlled to vary the output of the generator. This was the main drive
system for elevators for most of a century, and there are thousands of them
still running. If you're in an elevator, and it's stopped, but you
hear a big motor still running somewhere above, that's a Ward Leonard drive.
Nobody installs them in new installations, but there's still a replacement