4
\$\begingroup\$

I see the definitions of them are:

  • Rated current: maximum current the motor draws (or some says the maximum current the motor can endure?)
  • Stall current: the current the motor draws when hold physically
  • Inrush current: the current the motor draws when it just starts to rotate

I have a motor and it only says a rated current. Would it be safely larger than the other two? In other words, if I make sure the circuit can stand the rated current, would it be safe enough in words of current?

Also, is there some general rule like, the stall current is larger than or equal to the inrush, or vice versa?

\$\endgroup\$

2 Answers 2

Reset to default
7
\$\begingroup\$

I have a motor and it only says a rated current. Would it be safely larger than the other two?

For nearly all motors, the rated current is much less than stall/inrush current. Most motors are rated for continuous operation at rated current. A typical motor can withstand stall current for no more than a few seconds. DC motors that are rated in the kilowatt range typically can not withstand stall current at all. They must always be started using some external means of limiting the current. When such motors were widely used for industrial applications, the de facto standard for the controller short-term current rating was 150% of rated current for 1 minute.

There are motors called torque motors that are designed to operate an extended period of time at stall current.

\$\endgroup\$
3
  • \$\begingroup\$ +1 To your points, I've picked a random small DC motor. I was aiming for something with good documentation, but otherwise run-of-the-mill. Max continuous current is 2x to 4x smaller than stall current. \$\endgroup\$
    – Nick Alexeev
    Commented Nov 17, 2017 at 6:58
  • \$\begingroup\$ +1 but "DC motors that are rated in the kilowatt range typically can not withstand stall current at all" might need rewording. They can not withstand the coil resistance stall current, but can withstand a maximum start current for a short while, which, as you say, needs to be controlled, but will be more than the rated current, or you could never get it going in the first place. \$\endgroup\$
    – Trevor_G
    Commented Nov 17, 2017 at 14:28
  • 1
    \$\begingroup\$ @Trevor: answer revised \$\endgroup\$
    – user80875
    Commented Nov 17, 2017 at 15:11
3
\$\begingroup\$

[disclaimer: I'm not an expert on electric motors.]

Is there some general rule like, the stall current is larger than or equal to the inrush, or vice versa?

No, there is no general rule like that. Once in my previous life, I've had to deal with a motor1 which could overheat and burn if it remained stalled for an hour. The maximum allowed stall depended on the ambient temperature and air flow. For this reason, some motors are equipped with thermal cutoffs.

1 Unable to recall the model or datasheet at this time.

  • Stall current: the current the motor draws when hold physically
  • Inrush current: the current the motor draws when it just starts to rotate

The current which the motor draws when it just starts to rotate is equal to the stall current.

enter image description here (source)
The spike around 300 is spin-up (inrush) current.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.