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Are the below statements correct? Marked in Bold and Italic are my doubts!

Unregulated power supply (Battery?)

If supply current rating > motor current rating --> No problem since motor pulls only required amount of current from supply

If supply current rating < motor current rating --> Heats up or damages supply

If supply voltage > motor voltage --> Damages motor, since current increases more than motor current rating

If supply voltage < motor voltage --> Reduces motor performance (speed and torque both?)

Regulated power supply (Regulated bench power supply)

If supply current rating > motor current rating --> No problem since motor pulls only required amount of current from supply

If supply current rating < motor current rating --> Heats up or damages supply

If supply voltage > motor voltage --> No problem, since supply limits maximum current?

If supply voltage < motor voltage --> Reduces motor performance (affects only speed, since current will be regulated by supply and hence same torque?)

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  • \$\begingroup\$ You forgot to mention which stepper motor driver. If you use a constant current chopper driver or you use just transitors to switch without current regulation, differs a lot. \$\endgroup\$ Apr 29 '16 at 16:34
  • \$\begingroup\$ Realistically you need a chopping drive and several times the rated voltage if you want the motor to deliver useful torque beyond slow speeds. There are existing questions here which cover this well. \$\endgroup\$ Apr 29 '16 at 16:39
  • \$\begingroup\$ say regular constant current driver (ex. L298 H-Bridge?) or take DC motor only to bring driver confusion in between !:) I will edit my question \$\endgroup\$
    – Hegde
    Apr 29 '16 at 16:45
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Well, first we need to clear-up the differences between the two power-supplies. What you are implying is that the "regulated power supply" is current-limited and that the "Unregulated power supply" is not current limited. "Current-limited" is different from "regulated", which typically refers to the output voltage.

Also, I will be discussing a brushed DC motor, as no other type of motor really applies to your questions.

Having said that, most of what you list is true. I will refer to each point as 1 through 8:

1) True.

2) Not necessarily true. The Motor only pulls the current necessary to provide the needed torque (torque is directly proportional to current). If there is not much load on the motor, the motor will speed-up until the back-EMF and motor resistance balances the power-supply and limits the current at equilibrium. If you increase the load, it decreases the speed, which reduces the BEMF, which allows more current to flow, which increases the torque. So it is the load on the motor that will determine if you exceed the power supply's maximum current.

3) Also not necessarily true. Again, the current that the motor draws is determined by torque, and increased voltage may or may not cause an over-current. If the current is controlled, as it is with a motor-amplifier, it is customary to have a power-supply that exceeds the motor's voltage specification. I typically drive 24 volt motors with anything from 24 volts to 160 volts.

4) Mostly true, but it depends on the performance that is required. Reducing the voltage reduces the speed, since the voltage is required to counter the BEMF as the speed increases. A lower voltage does not lower the torque, which is solely determined by the current.

The current limited supply does not make much difference:

5) Again, true.

6) Well, no. The supply is current limited, so nothing bad happens. You simply cannot get your rated torque out of the motor.

7) True. It also limits the torque.

8) Mostly true (and mostly true for 4, as well). The only thing to keep in mind it that it is the motor's torque requirements that is limiting the current, not the power supply (unless the motor tries to exceed the supply's current limit).

Hope that helps

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  • \$\begingroup\$ Thanks Mark.. That helps me in choosing the right power supply for the project! \$\endgroup\$
    – Hegde
    Apr 30 '16 at 6:20

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