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A six wire motor can be used as bipolar or unipolar, so can you tell from the wiring which it is? As unipolar, the two center taps are connected to the power supply but as bipolar, they are left open.
I don’t understand your question about “required current”. The maximum steady state current is specified by the motor manufacturer and is set by the amount of heat that can be safely dissipated by the windings and motor frame. The motor can become quite hot when running at maximum current, but it should still be safe in the long run. Normally, you would use less current, as determined by the torque requirements of the application. In any case, you should be safe if you use the motor power supply as delivered.
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It sounds to me like the driver is actually unipolar, but no matter.
To make a stepper move quickly, you need a chopper style driver and a high voltage, high current power supply. It is not uncommon for voltages in the range of 50-100 V to be used. Naturally, the driver must employ active current limiting, which most modern chips allow. Pololu has a very nice range of well-designed boards, but I think all of them are limited to less than 2 amperes/winding steady state. If you need more than that you will have to buy an industrial driver, like the Gecko http://www.geckodrive.com/geckodrive-st … rives.html
To estimate the maximum steady state current that your motor will tolerate, use a large adjustable power supply and apply a voltage to both windings in parallel, leaving the center taps open, and monitor the current. Monitor the temperature rise of the motor frame over time and increase the voltage in steps (with 10 or more minutes between steps). Stop the experiment when the motor frame reaches about 60 degrees Celsius. Most industrial quality stepping motors are designed to tolerate such a high running temperature in the long term, but run the motor at less current if it makes you more comfortable.
Of course, if you are just infrequently opening/closing the valve, and some mechanism other than the motor holds that setting (so that you can turn off the driver), you can go to higher currents.
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The backplate on the motors I use states that they are rated for 60 degrees C temperature rise, so the running temperature can be as high as 80 C or so.
It sounds like the current is not high. You could try this driver from Pololu http://www.pololu.com/product/2133 with a 12 to 35 volt, 2 ampere power supply, being sure to set the motor current to 1.5 amperes/winding as per the instructions on the Pololu site. It is hard to beat the price, and you are unlikely to damage anything at those power levels. Note that the actual winding current can be higher than the current drawn from the power supply – these drivers act as step-down switching regulators.
I would recommend using only full step mode until you understand the performance of the system.
You will need a microcontroller like an Arduino to command the steps. This would be an open loop system, so you might want to monitor the voltage on the feedback element attached to the motor while the original system is opening and closing the valve, in order to figure out how the closed loop system works.
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I would use the six-wire motor as bipolar, leaving the center taps open. There are many more options available with bipolar drivers.