Even if the motor current is greater than the maximum card current, the motors can develop a torque that remains sufficient for your use, the drop in torque not being proportional to the current. Dito, for 2/3 of the current, you will obtain a torque which will be greater than 80% of the nominal torque.
What type of food?
If you choose a PWM-based card, which is more expensive than a card without regulation, you will have the great advantage that, due to the continuous regulation of the current, it is not necessary to use a stabilized power supply.
Thus, a simple transformer, with a rectifier bridge and a large capacitor (10,000 microFarads) is sufficient. Go for the Metalworks there.
What voltage for this power supply?
Powered by a chopper board, your stepper motor will need a supply voltage of at least four times its rated voltage. The maximum voltage is generally equal to 20 times the nominal voltage of the motor.
- Higher voltage improves speed possibilities (as long as your stepper motor is capable of it).
- Higher voltage gives better torque at good speed (but torque at low speed remains unchanged).
- However, there is a penalty. When you increase the supply voltage, it tends to increase the losses (iron losses) in the motor, so it gets hotter.
- This heating is progressive with the supply voltage / nominal voltage ratio, it increases faster than the ratio.
- Heating becomes significant beyond a voltage ratio of 10.
- As, moreover, many of our motors are mounted on wooden supports, they are not very well cooled.
- Also, a good compromise is a voltage ratio between 8 and 12.
- Be careful, it is normal for motors to heat up. A temperature of 50 to 60 ° C is not abnormal.
Be careful, the rectified and filtered voltage downstream of your transformer will be appreciably higher than the average voltage on the transformer wires, it will approach the voltage on the peak of the ripples.
Approximately:
Voltage at rectifier output = 1.4 * Transformer voltage -1.6V (1.6V is the voltage drop in the rectifier bridge)
- At full power, we will obtain a ripple which will drop the average voltage a little, but at low power this tension will be present.
- For a 24V transformer, the filtered rectified voltage is therefore 24V x 1.4 -1.6V = 32.1 V.
- Also, for a motor such as the one already mentioned (2.6V), the maximum recommended voltage for the transformer is 24V, which gives a rectified voltage of 32V, i.e. a ratio of 12.4.
- Be careful, your card has a maximum operating voltage.
- You must leave a margin between your supply voltage and the maximum voltage of your card.
- For a stabilized power supply, it is recommended not to exceed 80% of the maximum voltage of the card.
- Also, if your board has a maximum voltage of 46V, it is recommended not to exceed 46 * 80% = 37V
- A 24V transformer giving an output voltage of 32V is therefore close to the maximum.
