I would like to compute maximum speed (RPM) of my bipolar stepper motor 42BYGHM809 -- rated voltage 2.7 V, rated current 1.78 A, phase resistance 1.6Ohm, phase inductance 2.5 mH.

I know how to compute speed if I use maximum rated voltage for this motor (cca 6.6 ms per step -> 0.38 rotate per second).

But how to compute speed if I am using A4988 stepper driver and voltage is 12 V?

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    \$\begingroup\$ it takes 400 steps per rev so at 6,6mS per step it will take 2.64 seconds to make 1 rev or approx 22.7 r.p.m \$\endgroup\$ – JIm Dearden Jun 1 '13 at 14:03

At 12 V the speed will soon be zero since something will overheat and break. That's if this A4988 thing can even supply the necessary 7.5 A per phase. If not, then it will probably get hot and break. Either way, this is not a good idea.

There is one exception to this, which is if the 12 V is only applied for short periods of time to overcome the inductance of the windings, with the voltage then quickly brought back down to spec before the current exceeds spec. That sort of drive can be useful for steppers because the current in the coils switches faster, which allows the motor to run faster. However, care must be taken to not exceed the rated current. Unless this A4988 thing is specifically designed to do this and you can set a current limit at the 1.78 A maximum the motor is rated at, the points in the first paragraph apply.

  • \$\begingroup\$ Wouldn't it be related to back-emf? If you spun the motor at max speed (with a drill or something), it would generate 12V? Or break. \$\endgroup\$ – Bobbi Bennett Jun 1 '13 at 14:23
  • \$\begingroup\$ I should mention this. A4988 can detect current to phase coil and adjust voltage. So I can set 1.78 A as maximum current. \$\endgroup\$ – vasco Jun 1 '13 at 15:08
  • \$\begingroup\$ @Bobbi: Stepper motors are rather inefficient and therefore make crappy generators. You would have to spin it really fast to get rated voltage, let alone over 4x rated voltage. But yes, back EMF does subtract from the diving voltage, but there won't be significant back EMF from this motor relative to 12 V, at least in any "normal" usage. \$\endgroup\$ – Olin Lathrop Jun 1 '13 at 15:49
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    \$\begingroup\$ The a4988 has built in current limiting and can drive up to 1A per phase with no heat sink or up to 2A per phase with sufficient heat sink/forced air cooling. I use these drivers pololu.com/catalog/product/1182 on a homebrew 3d printer to drive stepper motors rated for 3V from a 12v supply. The Pololu boards have a pot on them for setting the current limit. \$\endgroup\$ – jwygralak67 Jun 1 '13 at 21:57

Increasing the motor supply voltage while using current limiting like the A4988 provides does increase maximum pulses per second a stepper motor can handle because a higher voltage causes the coil current to ramp up more quickly. There isn't an easy way to know how well your stepper motor will respond to an increased voltage because it depends on the construction of your particular motor.

I recommend testing your motor under the loads you expect to see in your application with different pulse rates to see what actually works in practice. Also, keep in mind that you can often achieve higher speeds if you slowly and smoothly ramp up the stepper motor speed.

Also, as JIm Dearden points out in a comment, if you do not increase the pulse rate, the stepper motor will go the same speed whether you use 2.7 V or 12 V for the supply assuming it moves in both cases.


protected by W5VO Jun 2 '13 at 17:51

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