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I want to buy a lab power supply. However (30 V, 5 A).

However, when a motor starts (inrush it's called afaik), it uses extra current. But I don't know really how much this typically is. Or if there s there some capacitor inside a power supply to take care of this small extra current spike.

I would like to know how much I need to take these current spike/inrush into account when buying a power supply.

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    \$\begingroup\$ If you have a motor driver sitting between the motor and the supply, then it's the driver's capacitive inrush you have to worry about. If you're going to connect the motor directly to the supply, just turn it up from zero volts instead of having the voltage turned up and flipping the power switch on. \$\endgroup\$ – DKNguyen Dec 30 '19 at 20:08
  • \$\begingroup\$ @DKNguyen probably I will use a motor driver (at least when I would use it in a project). I think e.g. a stepper motor always needs some motor driver to have it move. \$\endgroup\$ – Michel Keijzers Dec 30 '19 at 20:10
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    \$\begingroup\$ The design of the power supply matters imho, dave reviewed several PSUs in his EEVblog YT channel with details/tear-down. \$\endgroup\$ – ElectronSurf Dec 30 '19 at 20:26
  • \$\begingroup\$ @ElectronSurf Thanks I will check it out \$\endgroup\$ – Michel Keijzers Dec 30 '19 at 20:52
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    \$\begingroup\$ Just use a large capacitor across the supply, though this negates current overload protection for equipment (not the supply). \$\endgroup\$ – DrMoishe Pippik Dec 31 '19 at 4:12
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Lab power supplies generally have a constant current limit, so if a motor tries to draw more the supply will simply lower its output voltage to keep current down to the limit. This is different from switch-mode supplies used in computers etc., which usually cut off completely or 'fold back' to a very low current upon detecting a 'short circuit'.

How much 'inrush' current a motor draws depends firstly on the type of motor, and secondly on the particular construction.

A brushed DC motor's stationary current draw is limited only by its internal resistance, which is usually very low. Stall current is typically 3-5 times higher than the current draw at rated output, but can be as much as 10 times higher for very efficient motors. In a permanent magnet or shunt-wound DC motor, torque is proportional to current. If the power supply's output current is sufficient to get the motor spinning it will still work, but could be slow starting up. The associated momentary voltage dip could also affect other circuitry being powered by the supply.

Brushless DC motors are always operated through a controller, which usually limits startup current and/or acceleration to avoid damaging itself when starting the motor. Some controllers also have low voltage detection. This may cut the motor completely, or just reduce power output until the input voltage comes back to normal.

A stepper motor draws about the same current (or less) when holding as when stepping, so 'stall' current isn't a problem. Stepper motors also have very high inductance, so the 'inrush' current rises relatively slowly.

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  • \$\begingroup\$ Thanks for this elaborate answer ... good to know also stepper motors will have not so much issues. Probably those I will use mostly (since I have a few NEMA17's). \$\endgroup\$ – Michel Keijzers Dec 31 '19 at 15:02

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