Author Topic: Question on switching power supply running DC motor  (Read 8637 times)

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Offline good_noob

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Question on switching power supply running DC motor
« on: October 11, 2012, 04:52:18 pm »
Newby here guys...greetings to all!

Here's my quandry...I'm building a small project that has me puzzled and it's been 30+ years since I took electronics class in high school.

I'm trying to run a 12v 1.5Ah DC motor with a AC-to-DC 12v 25w 2.1A switching power supply (CUI VSK-S25-12U-T).  When I connect the dc motor to the psu, the motor runs for 1/2 sec then off, then on then off, then on...you get the picture.  Now, even being a noob, I understand that the switching psu is made to maintain the volts at a constant 12v and the circuit is being turned on/off to regulate the voltage produced.  But, my question is - how do I keep the motor constantly running on this switching psu?

1) Is the motor drawing too much amperage at start up and causing the overload circuit to reset?  I put an ammeter on the psu (without the motor) and the DC amps fluctates just like I'm seeing with the motor.

2) Is this just the way the switching psu works and I've bought the wrong type - do I need a something like a laptop battery charger to function as my psu?

3) Is there a way to add a capacitor or something to the circuit to maintain a constant amperage, or function like a starter capacitor?

I'm stumped and just need to get this issue resolved so I can move forward and not waste another 4-5 hours on Google and YouTube searching for an answer...

Thanks for any help... good_noob
 

Offline Kremmen

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Re: Question on switching power supply running DC motor
« Reply #1 on: October 11, 2012, 05:14:31 pm »
What is the load of the motor? Does it run at all, without any load?
Sounds very much like some kind of overload protection. Any SMPS (switchmode power supply, that is) switches with far far higher frequency so you are not seeing that.
So alternative 2 is definitely out, or is not the cause just because of the switching action.
Alternative 3 would only make things worse because the SMPS would not only need to supply the intial current of the motor, but the inrush current of the cap as well.
I suspect alt. 1 as the root cause. I could try if interweb has anything to say about the specific SMPS type you gave. Anyway, looks like you need either a more robust supply unit or a controller to ramp up the motor speed from zero. Based on the numbers the SMPS should be able to handle the motor but maybe not a heavy start.
Nothing sings like a kilovolt.
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Online IanB

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Re: Question on switching power supply running DC motor
« Reply #2 on: October 11, 2012, 05:28:49 pm »
12 V 1.5 A DC motor

An important thing to understand is that this is not the whole story. An electric motor draws less or more current according to the load it is driving. Unloaded it will draw very little current, but if subjected to a heavy load it may draw too much current and overheat. This is especially true if the motor is starting up under load and is struggling to begin turning. (This is why you may see the lights dim when the air con turns on--the compressor motor draws a huge current as it starts trying to turn the compressor.)

If you have an ammeter, you can put it in series with the motor and measure how much current the motor draws under different loads.

I have a feeling your power supply is undersized and you may need a slightly bigger one with more power.
I'm not an EE--what am I doing here?
 

Offline good_noob

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Re: Question on switching power supply running DC motor
« Reply #3 on: October 11, 2012, 05:37:09 pm »
Kremmen - thanks for the feedback.

As far as load...nothing, other than the motor connected directly to the psu just to test and make sure it works.  Free wheeling it just to test the connections were correct without adding any load to it.

The motor is a Dewalt drill motor that I've connected directly to the 12v Li-Ion battery from the matching drill and it works fine...the motor runs like a champ at about 1500rpm.  So, I know the motor is functioning.  This is my first attempt to build something with a SMPS and I'm not familiar with its characteristics.

I purchased a PWM DC Motor Contoller to control the rpms that I haven't put in the circuit yet...I'll try that and see if it solves the problem, starting from 0% and ramping up from there.  Guess I should have tried that before now...

Thanks, I'll let you know if the PWM Controller solves the problem...
 

Online IanB

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Re: Question on switching power supply running DC motor
« Reply #4 on: October 12, 2012, 05:30:56 am »
A drill motor is going to draw far more than 2 A or 25 W. I would estimate 100 W+ and tens of amps peak draw. I'd say you are going to need a much more powerful power supply. At a guess, look for something in the 100 W - 200 W, or make some measurements to get hard data.
I'm not an EE--what am I doing here?
 

Offline Kremmen

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Re: Question on switching power supply running DC motor
« Reply #5 on: October 12, 2012, 06:28:41 am »
Yes i was also going to comment that but got distracted. You (OP) write the motor is 1.5 Ah but that is not current, that is capacity (of a battery or cell). So are you sure that is the nominal current of the motor? I have measured no-load currents for cordless drill motors with planetary reducers of 3-5 amps. Your SMPS is not going to be able to supply that, never mind any actual load that could increase the current draw to 10-20+ amps easily.
Actually, you do not need a carefully regulated supply for such use. A high quality power supply is more or less wasted because just a simple rectifier + filter cap thing would do nicely. All the more so if you have a PWM controller to drive the motor. The incoming supply voltage is not critical provided you do not grossly overdrive the motor by the PWM controller.
Nothing sings like a kilovolt.
Dr W. Bishop
 

Offline Jon Chandler

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Re: Question on switching power supply running DC motor
« Reply #6 on: July 01, 2013, 01:13:54 pm »
You would be well served to start your own thread on this rather than tacking it onto an old thread that may or may not be related.
 


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