Hey Guys!

Hope someone can assist me here... I have a 12V DC motor that works fine when connected to a battery source, but when connecting it to a 12V 3A wall adapter, it starts and stops repeatedly.

After a lot a searching online I did figure out its because the "draw current" when starting is too high when starting the motor causing the power supply to cut out. To confirm this I also added another 12v fan in series which causes the motor to work, but of course the 12V is now being split to the motor and fan (also just a motor really) and I both dont want to run both all the time and think it might not be good to run 2 x 12v motors in series with a 12v psu - but I might be wrong - if it is safe and Im happy with the reduced speed can I run it like that?

I also seen it mighty be possible to add one or more resistors (in series) which might also fix the issue, but I have no idea which resistor to use nor how many (Im an out-of-work software developer learning electronics). Should this be the best fix, how do select the right resistors? I have tried a few already but to no avail - I suspect they might not provide enough resistance (could be too much as well - again I have no idea).

Without purchasing an expensive new PSU that can handle the current, is there another solution that someone can possible explain to me please?

Thanks in advance for the help!

 

What current does the motor draw when connected to a 12 volt battery ? (I am assuming your 12 volt battery is something like a car battery. ) Also provide some information on the motor and the nature of the mechanical load you are driving with it. Any resistor will slow the motor down and it will make the speed drop more when it is loaded than it would when connected to a stiff power source.

Les.

 

What current does the motor draw when connected to a 12 volt battery ? (I am assuming your 12 volt battery is something like a car battery. )

I am reusing the motor from an old 12V inflator (used for inflating camping matresses). The motor has no writing to indicate any further info. From web searches the inflator motors are usually around 50w, which confirms it requires 4A.

Also provide some information on the motor and the nature of the mechanical load you are driving with it.

Im building an inline duct-fan, so the motor will have very little load on it. I mounted the blades from an old pc fan to it.

Any resistor will slow the motor down and it will make the speed drop more when it is loaded than it would when connected to a stiff power source.

Les.

Which resistor should I be trying - I realis3e it cannot be calculated without more info on the current the motor draws, but I can try various resistors if I only knew where to start.

 

May be 2Ω would be a good starting point, but note that it will need to a BIG resistor, say 25W, as it is going to dissipate a lot of power.

 

If it does take 4 amps at 12 volts 1 ohm resistor would be a reasonable starting point. That will drop the voltage by 3 volts at a current of 3 amps. As the motor does not behave like a simple resistive load it may not reduce the current enough. This resistor would need to be at least 10 watts rating and will get hot dissipating 9 watts.

Les.

 

Instead of a series resistor I would try a series inductor.
Take a spare power transformer and put the primary windings in series with the motor.

 

Instead of a series resistor I would try a series inductor.
Take a spare power transformer and put the primary windings in series with the motor.

Sounds like something I can do without burning the house down - hehe - if possible, can you PLEASE post an image of what these windings should look like - and most importantly, where I should look for one (what type of electronics)?

 

Sounds like something I can do without burning the house down - hehe - if possible, can you PLEASE post an image of what these windings should look like - and most importantly, where I should look for one (what type of electronics)?

Show us a photo of what the motor looks like against a ruler or scale so that we have a sense of dimensions.
What is the current and rpm of the motor at no load.

I should be able to find a similar motor and then I will examine the current requirements at startup.
Then I will determine how much inductance you need for slow startup.

 

Right now to get the motor out would hard, but I did search for it before and I am 95% sure it's the "775" - image below:

 

You can try an ICL (inrush current limiter).



Here is an example:
https://www.digikey.com/en/products/detail/ametherm/SL12-10006/1873481

At startup at 25°C the resistance is 10Ω.
At full load while conducting 6A the resistance drops to 0.12Ω.