Shanki:

For one motor, power supply needed is :

Current => 1.4A x 2 = 2.8A

Voltage => 12volt

# (Since I first need to test with one motor)

For three motor, power supply needed is :

Current => 1.4A x 2 x 3 = 8.4A

Voltage => 12volt

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Which is the best device to supply the power, as I need variable power supply?

Mark is right. You haven't converted the voltage.

Yes, the quadrature drive means it uses 1.4 times the single-coil current but 2 is a good number to use to give some headroom (spare capacity) for the power supply, which allows for conversion losses and anything else you didn't think of.

You need to know the power that the motor is using. A typical NEMA-17 motor has a resistance of 1.65 Ohms per coil. At 1.4A, this only takes 2.3V to drive that current. So the coil is using 3.2W of power. Multiply by 2 (1.4 plus headroom) and multiply by the number of motors (3). I get 19W in my calculations. Find a power supply of at least 20W, maybe 25-30W would be good. For a 12V supply, that would be 2A to 2.5A. Much smaller than 8.4A.