Two Infinities

Wed, 15 Apr 2020

Telescope Clock Drive Motors
After near 20 years there is time for astronomy. Two telescope mounts purchased in Michigan have right ascension drives powered by electric motors.

The oldest motor, which came with a Discovery 8 inch Newtonian, has four leads and runs off 12 volts DC. My initial assumption that four leads meant it was a stepper motor proved wrong. There is also the possibility a separately excited DC motor where rotor and stator are powered separately. Such motors can maintain constant torque at different speeds, and the hand controller that came with the drive provides up to six different speeds.
Discovery clockdrive DC motor
These things came to light because the clock drive had stopped working. I blamed the controller and then the motor as I tried to replace the controller with a Phidgets stepper motor controller, all to no avail. Meanwhile I had sprayed a carburetor cleaner into the gearbox trying to loosen up the old grease. The cleaner container contains acetone which disolved the gearbox casing thus providing access to clean the gears. With freely turning gears, the clock drive works as it should.
Nevertheless, this motor really is a stepper. And I was finally able to control it with an EasyDriver stepper driver connected to a Beaglebone Black. It's a little unusual though with 48 steps per revolution where 200 steps is typical.

The second motor, which came with a pipe mount purchased from a basement machinist shop near Livonia, MI, runs off 110 volts AC. This unassuming motor, which I initially mistook for a cheap toy, is a synchronous motor, and is still available even today from Hurst electronics for over $100. These motors have the property of maintaining a constant speed under increasing loads. This came to light because I blamed the motor when the mount failed to track accurately and I searched for a replacement. It turned out that backlash was the culprit and the motor was performing as it should have.
synchronous AC motor

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