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richards
09-10-2007, 01:29 PM
Here's some trivia that I picked up about the Gecko G202 and the G203 stepper drivers:

- The G202 was designed to work best with stepper motors having an inductance rating in the range of 1mH to 40mH.

- The G203v was designed to work best with stepper motors having an inductance rating in the range of 0.5mH to 20mH.

- The PK296A1A-SG3.6 motor (very similar to the motors shipped on many PRT machines) has a full-coil inductance rating (series connection) of 31mH and a half-coil inductance rating (unipolar connection) of 8mH.

- If the PK296A1A-SG3.6 motor is used with the Gecko G203v drive, the motor should be connected to the drive using the half-coil wiring connections.

- Using the formula SQRT(inductance) X 1000, you can find the highest voltage at which to run the motor. For instance, the square root of 8mH is 0.0894 and 0.0894 X 1000 is 89.4 volts. Since the Gecko stepper drivers are limited to a maximum of 80V, you could use a power supply with that motor up to 80V, but a power supply of 65V would still give you excellent performance with much less heat.

Many thanks to Gerald Dorrington for passing along that information. That data corresponds exactly with the tests that I've run during the last two years with the G202, the G203 and lots of different stepper motors. Gerald also forwarded on part of an email from Mariss at Gecko who had wired up a PK296A1A-SG3.6 motor using half-coil connections and a current limiting resistor to let the motor pull 2A from the G203v. Mariss reported, "The 8mH version of that motor performs flawlessly with a stock G203V and outputs 82Watts mechanical at 65VDC compared to 39V for the 1A, 31mH motor."

Although I use the PK296B2A-SG3.6 motors at 25VDC to 35VDC, it looks like I could have gotten excellence performance from the PK296A1A-SG3.6 motor at 65VDC (as long as I wired it half-coil).

pfulghum
09-10-2007, 02:10 PM
Mike, double check your math...

You say
"the square root of 8mH is 0.0894"

Square root of 8 is 2.82???

-- pat

richards
09-10-2007, 02:42 PM
Sorry, Pat, I should have written 8mH as 0.008H to make the math clearer.

br928
12-11-2007, 10:58 PM
Mike,

How would you go about wiring the PK296A1A-SG3.6 in the half coil configuration? This is a 6 wire motor right? Use the two unused 'center tap' leads instead of the full coil?

Is the current still set @ 1.0 Amp? You mentioned 2A w/ a current limiting resistor. Is this a different configuration?

I have a spare PRT 7.2 reduction motor that I need to drive with a Gecko G203.

richards
12-11-2007, 11:47 PM
Stan,

The PK296A1A-SGxx motor is a six-wire motor. To connect the motor in half-coil mode, you can use the YELLOW wire and EITHER the BLACK wire OR the GREEN wire for one coil. You would then use the WHITE wire and EITHER the RED wire or the BLUE wire for the other coil.

Although Mariss tested the motor at 2A, the unipolar current rating of that motor is 1.5A. Personally I would use 1.5A. The current limiting resistor would be: 47 X 1.5A / (7 - 1.5A) = 12.8K (use the nearest standard 1/4w resistor that you can find at your local Radio Shack or other electronic supply store).

Maximum voltage for that motor in half-coil mode would be 1000 * SQRT(7.7mH), so 1000 * SQRT(0.0077) = 87V. Of course the maximum voltage for the G203v is 80V. I don't like to fry my fingers when I touch a motor, so I would run that motor somewhere between 50V and 65V.

br928
12-12-2007, 10:26 PM
My spare PRT 7.2 stepper motor only has 4 wires leaving the case. I don't have the model number, but it is the standard issue ShopBot stepper. Is it possible to remove the case safely to see it the other two connections are available? If not I will just run the motor full coil with the Gecko 203V.

richards
12-12-2007, 11:56 PM
Stan,
You may have a problem getting smooth operation with that motor wired as Bipolar Series with the G203v stepper driver. That motor has 30.8mH inductance, which is more inductance than the G203v was designed to handle. The G202 stepper driver can handle the higher inductance.

I just learned today from someone else that his motors also appear to only have four wires. That would limit your choice of connection method to Bipolar Series.

None of my motors have inductance beyond the range of the G203v, so I can't give you any first-hand information on what to expect with that motor/driver combination.