- of which Taycols are the commonest example for model boating - used to be used extensively as hobby electric motors
until the Japanese started producing decent permanent magnet motors
in the 1960/70s. They are not easy to reverse, however, requiring a polarity switch of the field coils, OR armature coil, but NOT both. This means that vintage modellers cannot easily use a polarity-reversing radio control ESC to control such a motor.
A common technique to address this problem uses a diode rectifier to maintain the initial polarity on one set of coils while letting the others be driven in reverse. This is a simple to apply solution, but it has a few drawbacks - it lowers the voltage available for forward running, for instance. An ideal method of reversing would be to switch the coil connections as specified by the original designer.
Such switching could be done physically by an extra servo, but that brings its own ergonomic difficulties. A better method would be to detect the polarity change and switch the coils using a relay
. The circuit below is designed to do this, with few components (though more, of course, than the rectifier!)
The PWM signal is smoothed, then fed into an op-amp
acting as a comparator. Hysteresis around the switching point is achieved with a resistance feedback. This seems to cope with the problem of generated back-emf interfering with the polarity detection quite effectively. The output switches a 5-20A relay
(depending on the motor being used) via a mosfet. The relay
is, of course, set so that it is on for reverse, so that the only circuit drain during forward running is the op-amp
, which is a matter of uAs.
This approach appears to be novel - I can't find anyone else doing it - so I am cautious about recommending it for widespread use. I have tested it on several ESCs and motors
, and put it through simulators on a circuit forum that I follow. The circuit seems to work reliably with these component values, but I would appreciate it if someone else would trial it with their system. There are so many different ESCs out there that it would be hard to test them all!
There are some PIC-driven polarity-detector units on the market place for other purposes - reversing steam engines or water-jets, for instance. I have tried one, and found that motor interference made the logic circuits very unstable. Using a standard op-amp
avoids that problem.