Image: Antenna Rotator Control Unit.. Click on image for larger view.
I overheard a conversation recently between two radio amateurs in the Western Cape trying to scrounge a 130 mf 50-volt capacitor for a dead rotator. The rotator suddenly died after several decades of faithful service. The control unit of his Ham IV Rotator ceased to function. A postmortem examination revealed a dead alternating current motor starting capacitor.
These capacitors are hard to come by in South Africa and if you can find one, it will be very expensive. Ask me how I know that. To solve the problem, two 150 mf, 50 volt electrolytic capacitors and two 100-volt, 3 ampere silicon rectifier diodes were used in the circuit shown. The control unit was resuscitated.
For those interested in the theory of the functioning of the circuit, the explanation is simple. During any half cycle one capacitor is shorted by its associated diode. It might be presumed that having two capacitors in series, the resultant capacitance would be halved. This, however, is not the case because the diode acts as a bypass for the capacitor during every half cycle.
It is NOT a motor starting capacitor!!!
The purpose of the cap is to cause a 90-degree phase shift across the windings of the motor, which determines which direction the motor will turn. Some old capacitors dry out and the value changes. This does result in the motor not starting to turn. Replacing the cap (if you can find the correct value) will make the motor work again.
Note: A larger value cap will allow more current to flow, but will also effect the phase angle, which will actually decrease the starting torque. An infinitely big cap would be a short! So don’t damage your motor! A non-polarized cap if obtainable would also do the trick. If it is impossible to find the correct cap, well then back to our schematic.
Use two back-to-back electrolytics as indicated in the above schematic. Connect the negative sides of the electrolytics together, and connect the diodes so that the “bar” is at the positive end of each cap. You want no current to flow through the diode when the voltage at the positive end of the cap is positive.
Another idea is to move the cap/s directly to the rotator, properly weatherproofed of course. This eliminates the cable resistance in series with the cap. Now use the two wires in the cable which went to the cap, and double up on the motor wires, cutting the resistance down there to.
Image: ZS1I successful test setup. Click on image for larger view.
Hopefully this article will resuscitate those dead rotator control units.
- Compiled many years ago by ZS1I for QSX, the monthly newsletter of PEARS.
- Source 73 Amateur Radio
