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Letters and emails should contain complete name, address and daytime phone number. Letters to the Editor are submitted on the condition that Silicon Chip Publications Pty Ltd may edit and has the right to reproduce in electronic form and communicate these letters. This also applies to submissions to "Ask SILICON CHIP" and "Circuit Notebook".

Brushless DC is the
correct motor description

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 application.

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 DC motor.

John Waller, Plainfield,

Connecticut, 06374-1429, USA.

Don’t change settings
while power is applied

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 your readers.

Peter van Schaik,

Tenterfield, NSW.

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