DC Motor wont start up  - Page 1
Forum section: electronics / beginners

author: TheMotorMan
author_rank: Newbie
timestamp: February 08, 2014, 08:32:11 pm
content: Hello EEVblog Forum ! Here i have this quite large DC motor that i got out of a portable 12V air compressor . On first when i hooked it up to a 1.5A 12V Adapter i scrapped from an old internet routerit worked nice , it did take it a bit longer time to spool up but did have alot of torque and end speed . Now when i try to hook it up and start it it just tries to spin and then stutters like it cant get a full revolution and start spinning. Now i checked the comutator and its nice and shiny no visible damage marks or something like that . No severed wiring and the resistance is quite low so thers no problem there .I tryed hooking up a 1000uF 25V cap in parallel with it and it gives it a bit of torque but the problem still remains .Is it something wrong with the supply im using ?Id appriciate help !

author: denelec
author_rank: Regular Contributor
timestamp: February 08, 2014, 08:58:03 pm
content: The motor will need a large current to start.And the commutator will generate a lot of voltage spikes.You probably overloaded your power supply.Or the voltage spikes damaged the electronics.Try starting it from a car battery. If it works, you will need a larger power supply and you will need to protect it from the voltage spikes. Probably with capacitors, zener diodes and resistors.

author: moepower
author_rank: Contributor
timestamp: February 08, 2014, 09:02:11 pm
content: Denelec is right. That dc motor will appear as a near short circuit across the supply terminals until it builds up a back EMF as it spins up. Usually you would need starting resistors or ramp up the supply voltage slowly. Do you have the motor nameplate specifications and perhaps a photo of it?

author: TheMotorMan
author_rank: Newbie
timestamp: February 08, 2014, 09:13:12 pm
content: Well il take a pic of it with my mobile phone il upload it as soo as i can . Though no nameplate or anything. Can i use a a LM555 as a PWM circuit isolated with a LM317 to act as a power stabiliser with some caps ?

author: TheMotorMan
author_rank: Newbie
timestamp: February 08, 2014, 10:01:47 pm
content: Here it is with the cap and the momentary switch

author: Strada916
author_rank: Frequent Contributor
timestamp: February 08, 2014, 10:36:52 pm
content: under load that motor will need almost 7A at 12v to start. no load minimum of about 2A 12v. That should get your started.

author: TheMotorMan
author_rank: Newbie
timestamp: February 08, 2014, 10:41:41 pm
content: 7A ? Damn. Well i guess i need a better power supply then :/

author: Psi
author_rank: Super Contributor
timestamp: February 08, 2014, 11:25:56 pm
content: yep, those are the sort of motors we used to run in RC cars many years ago using 7.2V NiCad battery packs able to supply many 10's of amps.There's still common in other things, lots of electric drills/screwdrivers use them.It will either be an RS540 or an RS550

author: moepower
author_rank: Contributor
timestamp: February 09, 2014, 02:25:12 am
content: Yep, a motor of that size at 12V will easily peak around 10 amps starting under load. The capacitor won't help either. You need a beefier supply.

author: TheMotorMan
author_rank: Newbie
timestamp: February 10, 2014, 07:14:22 pm
content: Ok now i got a little bit better supply . its a 16V 4A Switching Supply . For some reason , hooking up the motor , again with the 25 V 1000uF cap just causes it to make some kind of clicking noise inside the supply ( my guess its the relay ) , and still the motor wont start . Just vibrates and tries to do a few turns but nothing more. I got it running for a very short time once and it caused big sparks on the commutator so i stopped trying since im worried i might fry it . Its the biggest DC motor yet for me :/Maybe there is some kind of damage to the motor that i cant see till opening it ?

author: Nerull
author_rank: Frequent Contributor
timestamp: February 10, 2014, 11:59:47 pm
content: 4A is still less than the starting current of that motor. The constant relay switching in the power supply is likely due to its overcurrent protection kicking in. A stalled electric motor looks like a dead short across the terminals. Will the motor run from a (sufficiently beefy) battery?Some sparking is not abnormal, but excessive sparking can indicate a problem in the armature windings, or bad brushes.

author: Greorge
author_rank: Regular Contributor
timestamp: February 11, 2014, 04:05:36 am
content: If you don't have power supply that will give 10A or about, you might try to spin the motor by hand while connecting to 4A power supply.As mentioned above starting current is way too high for your power supply to start it.But it might be just enough to keep going without load.(Make sure gear doesn't get your skin once it kicks in)

author: electrolux
author_rank: Regular Contributor
timestamp: February 11, 2014, 10:28:06 am
content: The power supply might have some sort of overload protection, It happens with my 12v air compressor, try another power supply.

author: TheMotorMan
author_rank: Newbie
timestamp: February 11, 2014, 02:14:53 pm
content: OH well , it actually came up to be a bad motor . Got the 12V car battery on the bench , fully charged and the motor wouldnt budge . taken a similar motor too just so it maybe wasnt the battery or my connections .Well too bad i guess :/ It was a pretty big motor and wouldnt think it could get damaged like that . Is there any way i could repair it or something ?

author: AlfBaz
author_rank: Super Contributor
timestamp: February 11, 2014, 02:23:35 pm
content: I'd start with something simple like spraying the crap out of the brush gear. It's not uncommon for the brushes to become jammed in the brush boxes and eventually not make proper contact with the commutator.

author: G7PSK
author_rank: Super Contributor
timestamp: February 11, 2014, 04:34:41 pm
content: Most likely the brush gear if the motor wont start with a bit of help by turning it by hand. If giving it a nudge starts it you have winding blown.