C8 Synchronous Motors Page

Background: Celestron C8 with Inoperative Motors

I purchased my Celestron C8 on-line and it arrived without the original power cord. I eventually located a replacement power cord and discovered that the fork mount motors were not operative. It took a little trial and error, but I eventually got the synchronous motors functioning, and they have continued to function without problems. This webpage documents how I restarted the inoperative C8 motors and also includes a description of how synchronous motors function, photos of the C8 synchronous motors, and information on locating replacement motors. The Celestron C8 Page contains additional C8 information and photos.

Synchronous Motors

The C8 drive base contains two AC synchronous motors. Synchronous motors are used in applications requiring very precise speed and position control, such as clocks and record player turntables. Synchronous motors use electrical frequency to regulate the motor speed. The synchronous motor contains fixed stators (electromagnets) and a rotor (also a magnet). When the stators are energized by an AC current, they produce magnetic fields that attract the opposite poles on the rotor. The magnetic poles on the rotor become interlocked with the stators. As the AC current changes polarity with time, it changes the magnetic field on the stators. The changing magnetic field moves from stator to stator, forming a rotating magnetic field inside the motor. The stator-rotor interlock causes the rotor to move or follow the rotating magnetic field; this is called synchronization. Once the rotor is synchronized with the moving magnetic field, the speed is dependent only on the electrical frequency. Astronomers used a device called a drive corrector to make small changes to the electric frequency and vary the telescope drive speed (the Celestron C8 Page contains additional information about drive correctors).

The below diagram shows how a simple two phase synchronous motor orientates the rotor as a function of a changing electric wave. At position (A): the electric wave amplitude is becoming positive, the red stator north pole attracts the rotor south pole and repels the rotor north pole, and the blue stator south pole attracts the rotor north pole and repels the rotor south pole. At position (B): the electric wave is at maximum positive amplitude and the attraction between the rotor and stator poles is at a maximum. As the electric wave amplitude decreases between positions (B) and (C), the attraction between the rotor and stator also decreases. The electric wave amplitude switches to negative after position (C), the stators switch polarity, and the magnetic field has rotated within the motor. The red stator north pole (now on the top) attracts the rotor south pole and repels the rotor north pole, and the blue stator south pole (now on the bottom) attracts the rotor north pole and repels the rotor south pole. Between positions (D) and (A), the electric wave amplitude approaches zero and the attraction between the rotor and stator again decreases (the cycle then repeats). Changing the frequency changes the time for a complete wave cycle (amplitude increase from zero to maximum, then decrease to minimum, and increase to zero) and changes the motor speed.

C8 Synchronous Motors

The C8 drive base contains two Synchron® motors; the dual Synchron® motor system was used to reduce backlash error. The C8 drive base bottom and interior are shown in the below left and right photos, respectively. The Synchron® motors attach with two machine screws and are parallel connected to the oval power receptacle. Each Synchron® motor has a 12 tooth brass spur gear that extends through a hole into the drive base and engages the right ascension axis gear. There is a small plastic inspection window in each motor cover. When the motors are turning, holes in the rotor cause a blinking effect in the inspection window.

The Synchron® motor dimensions (cm) are show in the below photos. The below right photo shows that this motor is rated for 110 V, 60 CY, 3W, and 1 RPH.

Replacement Motors

The original Synchron® motor manufacturer (Hansen Corp.) is still in business, but I am uncertain if they still sell this particular model of motor. Used Synchron® motors can often be found on the following websites: eBay, Astro Parts Outlet, Cloudy Nights.com Classifides, and Astromart.com Classifides. If you are going to purchase used Synchron® motors, you need to be sure to get motors running at the proper speed and direction (depending on your hemisphere). My C8 motors are 110 V 60 CY 3W 1 RPH motors. Non US motors will most likely require 220 V and 50 CY power. 1970's C8 fork mounts are often listed on eBay; it may be easier to just purchase a replacement fork mount or upgrade to something more modern.

Warning Prior to Any Maintenance

The following webpage section documents the maintenance and repair I have performed on my C8. The C8 operates on 110 V mains electricity (non-USA models may operate on 220 V), so there is the possibility of shock or electrocution. Never open the drive base or perform any motor maintenance while the C8 is connected to mains electricity. If you are not qualified to work on electrical devices, seek the help of a licensed electrician. There is always a risk of damage with any home repair and maintenance of optical equipment. It is always recommended to contact the manufacturer prior to performing any maintenance as this may void warranties and alter equipment performance. I assume no responsibility or liability for any injury or damages resulting from what you may do to your own equipment. Anything you do is at your own risk, so be sure you know what you are doing and accept all risks prior to beginning. If you are not qualified to work with electrical devices, get the help of a licensed electrician!

Freeing Locked C8 Motors

I believe that my C8's motor problems were due to long periods of storage and non-use; note that the motors were not physically damaged. The procedure that is described on this webpage will probably be ineffective if your C8 motors are damaged.

The first thing I tried was powering up the drive base, and nothing happened.
Next I gently tapped on the motor body and cover (tapping or hammering sometimes loosens a stuck motor).
Finally I flipped the power on-off multiple times; eventually one of the motors began to make a little noise and started running.
If the motors are running, the rotors will cause a blinking effect in the plastic inspection windows; the blinking effect is caused by circular holes in the rotor passing in front of the inspection window.

The remaining motor would not activate by turning the power on-off so I opened the motor. Because the C8 runs on mains electricity, I'm going to repeat the following warning: There is the possibility of shock or electrocution. Never open the drive base or perform any motor maintenance while the C8 is connected to mains electricity. If you are not qualified to work on electrical devices, seek the help of a licensed electrician.

I unplugged the C8 power cord and removed the power cord from the drive base.
I removed the motor cover and inspected the motor for foreign debris.
I used a non-conductive rod to spin the rotor. Despite the drive base being unplugged, I was uncertain if there was residual charge in the stator coils. To avoid the risk of shock, I used something non-conductive to spin the rotor. I replaced the motor cover before powering up the drive base.

It took several cycles of spinning the rotor, replacing the motor cover, and powering up before the motor started. Spinning the rotor seemed to loosen up the motor. Below are photos of a C8 motor with the motor cover removed: stopped (left) and in operation (right). Note: never power up the C8 drive base without the motor covers installed or with the drive base opened!