I'm after electronics help on an AC drive corrector

[mental4astro]

Mods, I'm not sure if this is the right forum for this thread, of if it should be in the Equipment forum. Please change as you see fit.

Hi all,

I've recently got my hands onto a lovely original orange tube C8. It, like my C5, run on mains power. The original C8 instruction manual, however, shows an "optional extra" a drive corrector that can vary the Hz supply from the typical 50Hz. To speed up the drive, the frequency is increased (upto 85 Hz), to slow it down, the frequency is dropped (down to 35 Hz).

The pic below shows the Orion AccuTrack, which is essentially the same thing. The Celestron drive corrector also had the Lunar, Solar, Sidereal & Automatic rate selections.

http://www.company7.com/orion/graphics/accutrak_drive_corrector_wC-8_792357.jpg

Now, would these gadgets be still available, or do they need to be made from scratch. If they need to be made, can someone guide me on how?

Cheers,

Mental.

[Barrykgerdes]

I think your best hope is to try to pick up a second hand one from Ebay or a dealer. The next best would be to try Silicon Chip and see if they have ever designed a variable frequency P/S unit. A kit would most likely then be available.

Unless you can design and build one yourself it would probably cost as much as a modern Goto telescope for someone to design and build it for you.

Essentially you will need a variable frequency oscillator (555 chip) or similar driving an inverter to generate 240 v AC from a DC source.

Barry

[snowyskiesau]

You should do a search for VFD - variable frequency drive.
These are more often used to drive 3 phase motors from a single phase supply but there are single phase versions available. Be aware that some of the single phase units are designed for particular types of motors.
As Barry said, they aren't cheap.

The best option would be to fit a low voltage, speed controlled motor either a stepper or servo. There are lots of motor/driver combinations available, quite a few of which have the details necessary to build your own.
It would also be a lot safer, not a good idea playing around with anything at mains voltages.

[mental4astro]

I hear you, Geoff, I hear you! :scared: :scared: :scared:

[snowyskiesau]

Mental,
I've done a bit of browsing on some CNC/machining forums and was given some recommendations.
As I said previously, the most common use of a VFD is to drive a 3 phase motor from a single phase supply. (having 3 phase power installed is expensive!). There are however a number of low power VFD's (1/4, or 1/2 HP) available for use where voltage and/or frequency control (motor speed) are required. Prices for these are around the $200 - $300 mark. Perhaps cheaper on eBay although I couldn't see any single phase units listed.

The VFD's are programmable for voltage/frequency either via a built in keypad or in some cases via an RS485 controller.

Here's one supplier (http://www.automationdirect.com.au/) of a number of different brands. Hitachi and Danross were both given thumbs up in response to my queries.

DIsclaimer: These are mains voltage devices and need to be installed in a suitable enclosure and wired up by a licensed electrician.

[miki63au]

http://code.google.com/p/ultima8drivecorrector/wiki/design

Regards,
Mick.

[Leeroy]

Most VFD's output a PWM generated sin wave, not a pure sin wave as in mains power. Not too sure of your telescope electronics would cope too well with that..

One could build your own fairly easily with an adjustable sin wave generator, linear transistor amplifier and a small mains transformer.

What voltage is required by the drive motor?


Cheers
Lee

[snowyskiesau]

I've run my C8 drive from a Dick Smith 12V inverter, also square wave output, with no ill effects. There's no electronics as such in the drive, just a 240VAC geared motor.

[snowyskiesau]

Just re-read the thread - I like the solution pointed to by miki63au. A VFD while a simple solution, is overkill for this application.

If you could get a small enough transformer at the required VA rating, it might all fit in the C8 base.

[miki63au]

A 5VA PCB mounted transformer sufficient.

Mick.

[AstroGuy]

Hi Alexander,

Don't know if you saw this...

Someone on cloudynights pointed to this:

http://www.parksoptical.com/product_info.php?cPath=26_142&products_id=689&pname=Dual

Is that what you are after?

[mental4astro]

Hi all,

I'm rather overwhelmed at the amount of replies, and the quality of the replies.

Eugenio, muchas gracias por el link.

Rick, your link to a drive corrector circuit diagram is brilliant. Using a built in inverter will certainly make using quasie mains power much safer.

The voltage of the C8's motor drive is 240V 50Hz. Its power rating is bugger all.

Its periodic error is very noticeable though. Much, much more than the C5 I have which uses the same power and drive mechanism, but is 10 years younger. Actually, the C5 shows next to no periodic error. If it's there, it is very small.

[AstroGuy]

For Sale: Celestron Drive Corrector-SALE PENDING $40
http://www.astromart.com/classifieds/details.asp?classified_id=88861

Alexander, maybe you could contact this person and if the sale falls through they might let you have it.

Also good link to some useful info about the original drive correctors for the orange c8:

http://skywatch.brainiac.com/used/used_sct.pdf

[AstroGuy]

I think here is anotherm Accutrack version.

http://www.astromart.com/classifieds/details.asp?classified_id=262350