Brushless DC is the
The article on the Vectrix motorcycle has a point in asking
whether the motor should properly be called a "brushless DC motor". In fact, the
motor proper is either an AC synchronous motor or an induction motor. Either
type can be fed from an inverter, in principle.
But then nearly all DC motors have an armature which is that of
a synchronous motor. The commutator and brush gear ensure the armature is always
"in sync", including at zero speed.
The only true DC motor is rarely seen and is called the
homopolar or acyclic configuration. The rotor is the armature and is just a very
large flat copper disc with a brush at the axis and another at the rim. The
field is a horseshoe magnet mounted so that the magnetic field is normal to the
radial line between the two brushes.
The acyclic motor is thus very simple; the drawback is that the
armature current required is enormous, up to one million amps, depending on the
Therefore, "brushless DC motor" is arguably an appropriate term
as the inverter does exactly the same job as the commutator and brush gear. I
prefer to separate the two functions into "inverter" and "AC motor", the latter
being either synchronous or asynchronous (induction). My reason is the
combination is much more capable, albeit more expensive, than the conventional
John Waller, Plainfield,
Connecticut, 06374-1429, USA.
Don’t change settings
while power is
I read the article on the Low Voltage Adjustable Regulator
(SILICON CHIP, May 2008) with some
interest because the design is nearly the same as one I built for myself nearly
a decade ago. In my design, I used a 6-position wafer switch to change the value
of the resistor between the regulator’s adjust terminal and ground. However, the
switch was a break-before-make type and that led to the demise of a perfectly
good Walkman cassette player.
I realised after I’d plugged the Walk-
man into the
regulator that I had set the voltage selector switch to 3V instead of 6V.
Without thinking of the consequences, I changed the voltage output setting with
the switch, without disconnecting the Walkman. For the brief period while the
adjust terminal is disconnected from ground via a resistor, the output of the
regulator rises to almost the input voltage, in my case 24V. Ouch!
The result was one dead Walkman. I think it would be a good
idea to issue a warning in the next edition of SILICON
CHIP that removing the jumper shunt with a load connected is asking for
trouble. Otherwise it’s a great project that will be very useful for many of
Peter van Schaik,