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I bought a dozen types of stepper motors with rated voltages ranging from 4v to 12v and currents from 2A to 0.3A. I am unable to get anywhere near the rated torque on all motors except those with rated voltages 10v and higher. My setup:

Power supply: Agilent E3610A set to output 12V (current limited to 2A)

MCU: Teensy 3.1

Stepper Driver: Pololu A4988 (Cant provide link due to low reputation)

Now, for steppers like this one with rated voltage of 4.2v and current 2A - no matter what I do with current limiting pot on the surface of the driver - I simply cant get the motors draw more than a fraction of their rated currents. Its a single turn pot - so I fall off the adjustment space before current (measured at power source AND motor coil) reaches any appreciable amount (usually it maxes out around 0.1-0.2A). Torque too is very low, naturally. I tried voltages 8-17V (the range limited by the driver board and my power supply) - but still, cant get the motors produce more torque.

All 4 wire steppres are in bi-polar config.

Interestingly, it is not a problem with steppers rated 10-12v, for instance CanaKit stepper (cant provide link due to low reputation) rated at 0.33A and 12v - I can get them draw more current, although even in that case - there is a huge mismatch between the current they ought to draw based on VREF pin reading, and what the actually draw. For instance, measuring the VREF (~1v)and applying the formula - I should have gotten 2.5A (or slightly less due to 70% energization) - but I really measure only 0.29A on the coils.

Everywhere I read, it looks like I should not have this problem. I.e. read the threads on SE (cant provide links due to low reputation) What am I missing? All is wired up together on a breadboard. Wiring is solid, but could this be a problem? Should I consider a different driver?

Appreciate any tips!

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    \$\begingroup\$ Recall that the motors are rated at 4.2V which is less than your supply voltage, so the drivers will use PWM to limit the current to what you set on the potentiometer. This will act like a step down regulator (with the inductor being the motor winding) meaning the current drawn from the supply will be proportionally less than the winding current. Also the A4988 is rated for 2A max with sufficient cooling, so you may not get 2.5A even if you tried (without potentially melting something!). \$\endgroup\$ – Tom Carpenter Jun 28 '15 at 21:09
  • \$\begingroup\$ Makes sense, I do not intend putting 2.5A through the motor thought - I am merely dialing the trim pot up to increase the current in the motor winding (measured at motor winding , not the supply) - but it maxes out long before reaching its rated current. \$\endgroup\$ – grishik Jun 29 '15 at 21:39
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Some high current steppers should have much greater power, some as many as 30 to 80 volts. The best you can start with (without no datasheet) is to measure coil resistance and calculate.

Something here you might use.

You'r drive A4988

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  • \$\begingroup\$ Thanks for responding, but not sure what you mean. Calculate what? If the stepper is rated, say 1.5A at 4V - and I current limit it using the chopper driver (like A4988) - then with a supply voltage of 12v it should still draw enough current to produce rated torque, no? \$\endgroup\$ – grishik Jun 28 '15 at 19:20
  • \$\begingroup\$ If you measure a consumption at 0.29A by 12V (12V / .29A = 41 ohm), then each coil is around 41 ohms, which means that you have to get up at 82V to consume 2A (41 ohm * 2A = 82V) So to be sure, measure the resistance of one of the coil. \$\endgroup\$ – JustMe Jun 28 '15 at 19:32
  • \$\begingroup\$ I measure the resistance, and its quite low, for instance 2.3 ohm, still, unable to have that motor draw more than 0.6A (measured winding current). Supply is at 12v, 0.1A (limited to 1A, but never coming close to reaching it, obviously) \$\endgroup\$ – grishik Jun 30 '15 at 0:18
  • \$\begingroup\$ Have you written this "pot on the surface of the driver" is this built in the stepper? If yes, try to separate it and disconnect it, and measure the resistance directly on the wires to a coil. With 2.3ohm and 12V it should take 5.2A and requires only 4.6V \$\endgroup\$ – JustMe Jun 30 '15 at 6:00
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    \$\begingroup\$ Interesting experiment, never thought of it. Yeah, connecting power supply directly into the coil results in 0.9A drawn at 2v ... So, I guess I've proven Ohm's law works :-) Great holding torque too. Of course its a driver (or knowing how to work with it). Hence my original question :-) \$\endgroup\$ – grishik Jun 30 '15 at 21:06

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