@@ -119,30 +119,3 @@ The most technical ways of choosing the stepper motors are mentioned in [Motor S
---
## Resistance and rated voltage
---
- These are simply the resistance per phase, and the voltage drop across each phase when the motor is stationary and the phase is passing its rated current (which is the product of the resistance and the rated current).
- These are unimportant, except that the rated voltage should be well below the power supply voltage to the stepper driver.
> The 0.4 A stepper motor has the rated phase voltage of 12 V (phase resistance_30 ohms * rated current per phase_0.4 A). Providing 12 V supply voltage to the **DRV8825** stepper motor controller IC, we must use the 1.8 A rated current stepper motor. Because it has the rated phase voltage of 2.7 V (phase resistance_1.5 ohms * rated current per phase_1.8 A), which is much lower than the supply voltage (12 V) compared to the 0.4 A stepper motor.
## Inductance
---
- The inductance of the motor affects how fast the stepper motor driver can drive the motor before the torque drops off. If we temporarily ignore the back emf due to rotation and the rated motor voltage is much less than the driver supply voltage, then the maximum revs/second before torque drops off is:
-**Example**: a 1.8 deg/step (i.e. 200 steps/rev) motor with 3.2 mH inductance run at 1.5 A using a 12 V supply, and driving a GT2 belt with 20 tooth pulley would start losing torque at about 318 mm/sec. This is the belt speed, which on a CoreXY or delta printer is not the same as the head speed.
- In practice the torque will drop off sooner than this because of the back emf caused by motion, and because the above doesn't allow for the winding resistance. Low inductance motors also have low back emf due to rotation.
> What this means is that if we want to achieve high speeds, we need low inductance motors and high supply voltage. So, we must choose the 1.8 A stepper motor having lower inductance (3.2 mH) compared to the 0.4 A stepper motor (58 mH).
