[Chris D]

Hi All,

I am in the planning stages of a CNC conversion using Gecko 320s and 30 Volt servos. The servos are rated at a maximum current of 24 amps, this is a lathe so there will be two servos.

I have read through the various posts on this subject and looked over many of the submitted schematics of similar designs. I have noticed that most, if not all of the transformers shown, are rated at 10 amps or so.

I do not know enough about electronics to understand this so I figure I had better ask...

If I have 2 servos rated at 24 amps ( or more importantly, 2 geckos that can handle 20 amps), shouldn't the transformer be able to handle 20 amps?

Thanks

Chris

[DieGuy]

Well probably but not exactly, Most folks feel that they will not fully load all axis at the same time.

Many here are much more versed in the electronics part that I am. Just to make this perfectly clear you have 2 servos each requiring a max of 20 amps. I suppose with heavy cuts or fully loaded rapids you might hit 20 amps X 2. Sounds like you need a pretty heavy Power Supply.

[Chris D]

Actually, I am off on my numbers a bit...


Gecko 320 are rated at 20 Amps max - so that is my limitter I guess
Motor is rated at 24 volts, 40 Amps max, 30 Amps continious.

However, the end result is, I believe I need 24V @ 20 Amps going to each Gecko. That's a lot of Amps! But, that is also what the motors want, so I believe I have to give them what they want (to achieve the performance I would like).

Chris

[Runner4404spd]

you probably shouldn't use the geckos

[ViperTX]

ChrisD...will you settle on what the servo rating are....you keep jumping all over the board? Do already have the Geckos? If you don't then I'd use the Rutex. If you have the Geckos, you can use them, but know that you won't get all the torque the servo can deliver. If using Geckos, set their current limit to something below 20 amps.

Also your power supply should be at least 40 amps....or have 2 supplies, one for each servo.

[Chris D]

ChrisD...will you settle on what the servo rating are....you keep jumping all over the board? Do already have the Geckos? If you don't then I'd use the Rutex. If you have the Geckos, you can use them, but know that you won't get all the torque the servo can deliver. If using Geckos, set their current limit to something below 20 amps.

Also your power supply should be at least 40 amps....or have 2 supplies, one for each servo.

I have two different sets of motors.

Motor #1
30.3 Volts
12.6 Oz in. / Amp Torque constant
1.3 Ohms Resistance
.21 no-load current
23.0 Peak current (Stall)

Motor #2
24 Volts
10.0 Oz-in / Amp Torque constant
.59 Ohms Resistance
.30 no-load current
40.4 Peak current (Stall)

So, I have two different sets of specs to refer to, sorry if I "Jumped all over the board".

Chris

[ViperTX]

No problem Chris....do you already have the Geckos?

If you limit the max current to 20 amps, Motor #1 will max out around 252 oz-in of torque, Motor #2 will max out at about 200 oz-in of torque.

[Chris D]

No problem Chris....do you already have the Geckos?

If you limit the max current to 20 amps, Motor #1 will max out around 252 oz-in of torque, Motor #2 will max out at about 200 oz-in of torque.

No, I didn't buy the Geckos just yet. As you can probably guess, the reason I want the Geckos is that they are the best priced units that I could find. The Ruetex drives also seem nice, but over twice the price which really hurts the budget on this project, a 9 X 20 lathe conversion. Being such a small lathe, rapid speed isn't as critical as torque. As such, I plan on about a 4:1 reduction through timing belts.

Even at 200 Oz-in on the smaller motor, multiplied by 4 gets me up to 800 Oz In which should be okay for this machine - I think :-)

Chris

[ViperTX]

Chris....yes that should be quite powerful.

[Chris D]

Based on what I have been hearing on other boards and the little that I actually know about electronics, it appears as though to make a power supply for 24 volts, I need the following then...

For each Gecko, I will need 24 volts & 20 Amps. Rather than one big power supply - it may make more sense to use two. Using a transformer, bridge rectifier and filter cap as the basic components....

I will need a 16 volt transformer that can output 20 amps of power. I know some folks have wound their own transformers etc. and would be willing to do that (if I knew how), but does anyone have any sources for transformers meeting this requirement?

Also, would it be okay to go to 18 volts - more commonly available than 16 volts, but, if my numbers are right, I would have about 26 volts as opposed to 24 volts.

Thanks again!

Chris

[ViperTX]

ChrisD...since you'll have 2 supplies. I would go with a 24 volt and a 30 volt supply. I believe mpja.com has some transformers that will fit your needs, you can also use toroidal transformers if you can't find the right transformers at mpja.com. The TT can be found on ebay.

[Chris D]

Hi Viper,

If I went with a 24 volt transformer, I would end up with about 36 volts on the far side of the rectifier, isn't that a lot of overvoltage for a servo motor?

Chris

[ViperTX]

A 24 volt transformer would work for 30.3 volt servo...since you're feeding a bridge and that would be 2 diode drops or about 1.4 volts less....and when you apply a load it will drop a little more since the supply is unregulated.

For the 24 volt servo go with 16 to 18 VAC transformer...