G203v witih auto transformer for testing outside the case?

[Wolfesmetalfab]

Is there a problem using an autotransformer rectified and a capacitor added to test my motors with? I would probably only be using one drive and motor on my x axis (gantry) for testing at 24v to find the best supply voltage for my psu to build or buy for the ipm needed. It would not be in the case with the drives, I would run the dc output wires into the case and hooked to the drive (just for testing) and then I would build or buy the appropriate supply for what I need.

Also is toroidal the only way to go, or would a standard transformer be ok?

I was going to do this until I read geckos faq and it got me thinking, but It would not be in the case, and its only temporary, so i dont think there is a problem, is there?

The faq reads.....

Q.) Can I use a variable autotransformer with my drive?



No. Do not even think about it, for the following reasons:



1.) The Geckodrive mounting plate is connected to the 'Power Ground' terminal 1.

2.) The mounting plate hard anodizing is for the benefit of the 8 power MOSFETs, not to insulate the plate from whatever it gets mounted to. The mounting screws can and will ground the mounting plate to the mounting surface.


3.) The common anodes ('-' terminal) of the power supply full bridge rectifier connects to 'Power Ground' terminal 1 of the drive.


4.) If the 'neutral' side of the autotransformer is grounded to the chassis or the 'ground' wire connects to the chassis, it shorts-out the rectifier diode whose cathode goes to 'neutral' and whose anode goes to 'Power Ground' on the drive.


5.) With the rectifier shorted, uncontrolled current flows from chassis to mounting plate to 'Power Ground' to remaining common anode rectifier diode back to autotransformer.


6.) This current melts the aluminum plate, over-currents and destroys the remaining bridge rectifier diodes and causes amusing fireworks until the circuit breaker shuts down the fun.


7.) The resulting molten aluminum causes some people to muse about anodizing and its shortcomings.



Thanks
Brian =)

[Mariss Freimanis]

Please go with the amusing description of what will happen and try not to personally prove it is all true. Use a real transformer, an autotransformer can kill you (as in making you 'metaphysically challenged').

Mariss

[Wolfesmetalfab]

amusing it is! I guess I just dont understand what autotransormers are good to be used for, or is it the fact that they aren't good for DC? I know they are used, but evidently not for this.

How about 2 computer ATX supplies wired in series?

Or a 36v charger (it has enough amperage, about 125amps worth!!!)

And what about regular transformers as opposed to toroidal ones?
I could maybe find a standard one to make a quicky power supply for testing around 24v.

[Ozheat]

Use a regular isolated transformer meaning that the primary and secondary windings are totaly seperated.
An autotransformer has a common primary and secondary coil which means that there is no isolation from the mains if a fault occurs. Autotransformers are cheaper than isolated transformers because they only require one winding.
You can use an variable autotransformer(variac) on the primary side of a isolated transformer creating variable AC voltage on the secondary side and still be safely isolated from the mains. Rectify the AC output for an unregulated DC power supply.

[Wolfesmetalfab]

Thanks ozheat, that makes sense. So what about my options? Would the 36 volt charger be ok to use for testing?

[Ozheat]

It should be ok but put a multimeter on the output as depending on the design of the charger it may be putting out a lot more than 36Vdc unloaded.

edit. come to think of it your 36Vdc charger sounds like it maybe a forklift battery charger, if it is an intelligent charger ie. adjusts current depending on battery charge it may not work as it will not turn on without a load connected but if it a simple charger with a transformer only it will work.

[Wolfesmetalfab]

ok, i will do, the motors are ok to about 56 volts, and the drives 80 of course. whatever it is outputing, i will just adjust the math accordingly to figure best permanent supply.

Thanks

[Al_The_Man]

Originally Posted by Wolfesmetalfab amusing it is! I guess I just dont understand what autotransormers are good to be used for, or is it the fact that they aren't good for DC? I know they are used, but evidently not for this. .

Autotransformers function exactly the same as an isolated transformer, the difference is that when the primary is fed from a grounded mains system that produces a neutral, all the conductors will be referenced to ground.
If you have a piece of equipment that entails rectifying the output of the transformer and the equipment happens to have the DC common connected to ground, you will blow the bridge rectifier.
Also there is a safety issue if you do not take steps to ensure the correct primary connection, it is possible to have both secondary leads at mains voltage above ground.
So the two main reasons not to use auto-transformer are the nature of the subsequent equipment and safety issue, otherwise, in theory it is possible.
BTW, the common Variac falls into the category of autotransformer, so should not normally used for the same reason.
Al.