DC motor surge current - Page 1
Forum section: electronics / beginners

author: engineheat
author_rank: Frequent Contributor
timestamp: September 21, 2019, 04:23:55 am
content: Hi,We have a current tester (power supply) that outputs 1.1V to DC motor and measures the current for a few seconds. It has to be within a range (around 500mA) while the motor is spinning. I get a lot of rejected (failed) motors for too high of a current (The tester always reads 1A when this happens)When I took the failed motor, powered it with a 1.1V AA battery, and measured the current, they were within range. I did notice a quick spike up to 1.6A when the motor first spinning and once up to speed the current is around 500mA.Upon further investigation, I noticed that the current tester power supply has current limit set to 1A, which is why the tester only reads 1A. I guess the motor wants to draw more current but it is "clipped" at 1A. I wonder if this causes the motor to "stall" because it can never reach the speed needed to have the current settle at 500mA, because the 1A max limit is not enough for the motor to start spinning.Please advise if my hypothesis makes sense.thanks

author: Circlotron
author_rank: Super Contributor
timestamp: September 21, 2019, 05:33:11 am
content: Does the motor never get up to speed with a 1 amp current limit no matter how long you leave it? Not after even five seconds or so? Sounds for sure like the current limit needs to be raised if you can in fact get good results when you run it off an AA battery,

author: ledtester
author_rank: Super Contributor
timestamp: September 21, 2019, 06:33:06 am
content: Quote from: engineheat on September 21, 2019, 04:23:55 amYou might try manually spinning the shaft to see if that starts the motor up.

author: Zero999
author_rank: Super Contributor
timestamp: September 21, 2019, 12:58:07 pm
content: Yes DC motors draw a large current surge, when starting, The surge should be equal to the supply voltage driviered by the resistance of the motor windings. Look at the data sheet of the motor for the resistance or measure it with a multimeter.If the motor is jammed then it will draw the high current continuously and will overheat if the power dissipation is too high, but in circuits powered by AA cells, the voltage normally drops, before the motor gets chance to overheat.

author: EEEnthusiast
author_rank: Frequent Contributor
timestamp: September 21, 2019, 01:39:58 pm
content: Add a Large cap at the output of the power supply. That should supply the surge current and will cause the motor to spin

author: mikerj
author_rank: Super Contributor
timestamp: September 21, 2019, 03:29:38 pm
content: Quote from: EEEnthusiast on September 21, 2019, 01:39:58 pmIf the output of the supply is switched, then a capacitor will increase the amount of current it has to provide, i.e. make the situation worse.

author: rstofer
author_rank: Super Contributor
timestamp: September 21, 2019, 03:34:21 pm
content: The inrush of a DC motor will be on the order of 6 times full load current. You can easily get the real number: Just measure the DC resistance of the motor when it is not running. That winding resistance is the only thing limiting inrush current. You know the applied voltage and you now have the resistanceI = E / RSome ideas about DC motors:RPM is proportional to voltageTorque is proportional to currentSee Motor Velocity Constant and Motor Torque Constant here (about half way down the page):https://en.wikipedia.org/wiki/Motor_constants

author: Benta
author_rank: Super Contributor
timestamp: September 21, 2019, 06:33:01 pm
content: Quote from: rstofer on September 21, 2019, 03:34:21 pmA couple of corrections:1: The start/locked current of a DC motor is only defined by its ohmic resistance. The start current you mention (~6x) applies to induction motors.2: Current is proportional to torque, not the other way round.