# EEVblog Electronics Community Forum

## Electronics => Beginners => Topic started by: Miguel Angelo on July 01, 2013, 03:41:47 am

Title: Supplying a DC motor with a switching power supply
Post by: Miguel Angelo on July 01, 2013, 03:41:47 am
I posted on an older thread and it was recommended that i start a new topic so here it is:

I am trying to run a DC motor ( 12V, 1A as https://www.sparkfun.com/datasheets/Robotics/Other/spec%20sheet.jpeg (https://www.sparkfun.com/datasheets/Robotics/Other/spec%20sheet.jpeg) ) with a switching power supply ( 12V 4A as EOS VLT100-4003) and the motor will move a few degrees every second or so, never really going. The power supply has a minimum load for 12V of 0.1A so that should be covered.

If I try to use the motor with a bench top power supply, it runs perfectly at 12V and roughly 1A current. Using a recording multimeter (Fluke 289), I can see that the maximum current is 1.5A then and it runs at roughly 900 mA. A 4A rated power supply should be able to handle a 1.5A spike.

Is 4A too little to handle the initial spike for a 1A DC motor? Would it be safer to have a diode across the motor to prevent damage to the power supply? Any other aspects need to be considered here to get the motor to run properly with that power supply?
Title: Re: Supplying a DC motor with a switching power supply
Post by: Rerouter on July 01, 2013, 05:12:12 am
definatly a diode and also you may be drawing too much current at startup, to test it measure the resistance while powered off, as while charging up the inductance its going to draw as much as the resistance in the coils will let it have,

so if its less than 3 ohm you have found your culprit,
Title: Re: Supplying a DC motor with a switching power supply
Post by: eKretz on July 01, 2013, 07:09:43 am
Yeah, that's my guess. Startup current is almost always going to be higher than 1.5A on a motor that runs unloaded at 1A. I'm kind of surprised that you only saw 1.5A if you logged it. I would have guessed it would have been several times higher than that. You could try adding a start capacitor.
Title: Re: Supplying a DC motor with a switching power supply
Post by: Simon on July 01, 2013, 10:09:46 am
you could try an inductor in series to "choke" the sudden current spike but i'm no expert in that particular field
Title: Re: Supplying a DC motor with a switching power supply
Post by: Miguel Angelo on July 01, 2013, 01:28:57 pm
definatly a diode and also you may be drawing too much current at startup, to test it measure the resistance while powered off, as while charging up the inductance its going to draw as much as the resistance in the coils will let it have,

so if its less than 3 ohm you have found your culprit,

The resistance is 2.2 Ohms so initial current would be 5.45A and therefore higher than the 4A rated. I will see if I can find one rated at 6A. Thank you.

Would a starting capacitor be of any value on a DC motor?
Title: Re: Supplying a DC motor with a switching power supply
Post by: Miguel Angelo on July 01, 2013, 01:31:15 pm
Yeah, that's my guess. Startup current is almost always going to be higher than 1.5A on a motor that runs unloaded at 1A. I'm kind of surprised that you only saw 1.5A if you logged it. I would have guessed it would have been several times higher than that. You could try adding a start capacitor.

The meter may not be able to catch the spike because I have not yet configured from the standard setup. The sampling may be too slow. I will try to configure it so I can see the 5.45A spike as predicted by the resistance.
Title: Re: Supplying a DC motor with a switching power supply
Post by: Simon on July 01, 2013, 01:36:30 pm
you would need to use an oscilloscope with a small series resistor set to capture on a rising edge
Title: Re: Supplying a DC motor with a switching power supply
Post by: Miguel Angelo on July 01, 2013, 03:47:37 pm
you would need to use an oscilloscope with a small series resistor set to capture on a rising edge

I will need a bit more information to try something that complex.  :o

I do have an Oscilloscope (old Tektronics two channel) and I guess that I would be measuring the voltage across the small series resistor acting as a shunt.

I have rudimentary knowledge of how to use the oscilloscope but I need to take the course on Oscilloscopes in the forum on how to capture that elusive rising edge without risking life or limb.
Title: Re: Supplying a DC motor with a switching power supply
Post by: drm on July 01, 2013, 04:55:36 pm
Hi Miguel Angelo.

Here, u can see a margherita washing machine universal motor starting, DC pulse width modulation feeding from 310 vdc.

DC motor starting inrush current (https://www.youtube.com/watch?v=C4v-LzcRBGY#)

theres more videos, of  motor running from 0 to 260 vdc at my channel.
Title: Re: Supplying a DC motor with a switching power supply
Post by: eKretz on July 01, 2013, 06:23:34 pm
definatly a diode and also you may be drawing too much current at startup, to test it measure the resistance while powered off, as while charging up the inductance its going to draw as much as the resistance in the coils will let it have,

so if its less than 3 ohm you have found your culprit,

The resistance is 2.2 Ohms so initial current would be 5.45A and therefore higher than the 4A rated. I will see if I can find one rated at 6A. Thank you.

Would a starting capacitor be of any value on a DC motor?

If you put it in parallel it should help get the motor started I'd think. Since you are using PWM I think it will work.  Try it out, can't hurt anything but a cheap cap at worst I would think.
Title: Re: Supplying a DC motor with a switching power supply
Post by: Miguel Angelo on July 01, 2013, 07:07:52 pm
Try a switch between the motor and the supply. The supply might be having difficulty starting up into essentially a short. Let the supply come into regulation then switch in the motor.

I do have a switch between the two, so the power supply is already "going" before connecting to the motor.
Title: Re: Supplying a DC motor with a switching power supply
Post by: w2aew on July 01, 2013, 07:09:22 pm
definatly a diode and also you may be drawing too much current at startup, to test it measure the resistance while powered off, as while charging up the inductance its going to draw as much as the resistance in the coils will let it have,

so if its less than 3 ohm you have found your culprit,

The resistance is 2.2 Ohms so initial current would be 5.45A and therefore higher than the 4A rated. I will see if I can find one rated at 6A. Thank you.

Would a starting capacitor be of any value on a DC motor?

If you put it in parallel it should help get the motor started I'd think. Since you are using PWM I think it will work.  Try it out, can't hurt anything but a cheap cap at worst I would think.

I fail to see how a cap in parallel with a DC motor is going to help at all.  To me, it looks like it will just exacerbate the problem by providing a heavier transient load for the DC supply to overcome.  Not only are you asking the supply to provide the stall current for the motor until it gets moving and generates a back-emf, but now your asking it to charge the capacitor too.
Title: Re: Supplying a DC motor with a switching power supply
Post by: Miguel Angelo on July 01, 2013, 07:10:19 pm
Hi Miguel Angelo.

Here, u can see a margherita washing machine universal motor staring, DC pulse width modulation feeding from 310 vdc.

DC motor starting inrush current (https://www.youtube.com/watch?v=C4v-LzcRBGY#)

theres more videos, of  motor running from 0 to 260 vdc at my channel.

That is really helpful. Thank you. I am going to take the refresher course on oscilloscopes and get testing.
Title: Re: Supplying a DC motor with a switching power supply
Post by: Miguel Angelo on July 01, 2013, 07:15:49 pm
definatly a diode and also you may be drawing too much current at startup, to test it measure the resistance while powered off, as while charging up the inductance its going to draw as much as the resistance in the coils will let it have,

so if its less than 3 ohm you have found your culprit,

The resistance is 2.2 Ohms so initial current would be 5.45A and therefore higher than the 4A rated. I will see if I can find one rated at 6A. Thank you.

Would a starting capacitor be of any value on a DC motor?

If you put it in parallel it should help get the motor started I'd think. Since you are using PWM I think it will work.  Try it out, can't hurt anything but a cheap cap at worst I would think.

I fail to see how a cap in parallel with a DC motor is going to help at all.  To me, it looks like it will just exacerbate the problem by providing a heavier transient load for the DC supply to overcome.  Not only are you asking the supply to provide the stall current for the motor until it gets moving and generates a back-emf, but now your asking it to charge the capacitor too.

I know that the capacitors in AC motors are there to create a different phase in one of the windings to get it going. My question was more towards using the capacitor to deal with the surge but I guess that is not a trivial solution.
Title: Re: Supplying a DC motor with a switching power supply
Post by: IanB on July 01, 2013, 07:17:07 pm
Try putting a 2 to 5 ohm resistor in series with the motor while it starts going (use one of those big rectangular 10 watt kinds). Switch out the resistor once the motor is running. You could possibly automate this with an NTC resistor.
Title: Re: Supplying a DC motor with a switching power supply
Post by: w2aew on July 01, 2013, 07:24:55 pm
definatly a diode and also you may be drawing too much current at startup, to test it measure the resistance while powered off, as while charging up the inductance its going to draw as much as the resistance in the coils will let it have,

so if its less than 3 ohm you have found your culprit,

The resistance is 2.2 Ohms so initial current would be 5.45A and therefore higher than the 4A rated. I will see if I can find one rated at 6A. Thank you.

Would a starting capacitor be of any value on a DC motor?

If you put it in parallel it should help get the motor started I'd think. Since you are using PWM I think it will work.  Try it out, can't hurt anything but a cheap cap at worst I would think.

I fail to see how a cap in parallel with a DC motor is going to help at all.  To me, it looks like it will just exacerbate the problem by providing a heavier transient load for the DC supply to overcome.  Not only are you asking the supply to provide the stall current for the motor until it gets moving and generates a back-emf, but now your asking it to charge the capacitor too.

Yes, in that case, the cap would be in parallel with the supply, before switching in the motor, so that the initial surge/stall current could be partially supplied by the stored charge on the cap.  This would work provided the supply can be turned on early enough in the sequence to fully charge the cap before the motor is switched on.
I know that the capacitors in AC motors are there to create a different phase in one of the windings to get it going. My question was more towards using the capacitor to deal with the surge but I guess that is not a trivial solution.
Title: Re: Supplying a DC motor with a switching power supply
Post by: Miguel Angelo on July 01, 2013, 07:47:45 pm

They say min load on the 5V rail is 3A. I would guess from that the 5V rail is the  regulated output or weighted between the 5V and 12V rail, maybe 80-20 split Try loading the 5V rail with 15-20W. Or try an inrush limiter like Ian suggest.

A 100W SMPS should have no problem supplying a brief inrush. The capacitance on the 12V rail is likely a couple mF.

Yes, that is the one - 4003 model. Tonight I will try the different suggestions given above.

This forum is superb!  The oscilloscope training sticky is out of this world - I learned so much on the first 60 minutes alone I can't wait to see the rest - I have already subscribed to w2aew youtube channel. ;D
Title: Re: Supplying a DC motor with a switching power supply
Post by: Simon on July 01, 2013, 09:17:05 pm
If your using PWM to run this is there the possibility of doing a "soft start" by making sure the supply is on and ready before attempting to start the motor, have a large output capacitor on the supply and then soft start the motor with very small and increasing duty cycles, that way the big output capacitor (maybe an extra one with a filter coil in series) provides the current for the spikes and as the motor eases into running the duty can increase.
Title: Re: Supplying a DC motor with a switching power supply
Post by: Miguel Angelo on July 02, 2013, 03:36:29 am
Try putting a 2 to 5 ohm resistor in series with the motor while it starts going (use one of those big rectangular 10 watt kinds). Switch out the resistor once the motor is running. You could possibly automate this with an NTC resistor.

I tried with a 2.2 Ohm and two 2.2 Ohm in series for 4.4 Ohm and the motor did not move. Increasing the resistance does not seem to get this power supply to work.

If your using PWM to run this is there the possibility of doing a "soft start" by making sure the supply is on and ready before attempting to start the motor, have a large output capacitor on the supply and then soft start the motor with very small and increasing duty cycles, that way the big output capacitor (maybe an extra one with a filter coil in series) provides the current for the spikes and as the motor eases into running the duty can increase.

I am not planning to use PWM. This is a simple vacuum pump, tank and meter setup to test the player piano pneumatics that I am rebuilding. It also includes a vacuum transducer  ( http://www.freescale.com/files/sensors/doc/data_sheet/MPX4250A.pdf (http://www.freescale.com/files/sensors/doc/data_sheet/MPX4250A.pdf) ) so I can log the vacuum curves for each of the 88 actuators in Excel via arduino - this part is done and working.

They say min load on the 5V rail is 3A. I would guess from that the 5V rail is the  regulated output or weighted between the 5V and 12V rail, maybe 80-20 split Try loading the 5V rail with 15-20W. Or try an inrush limiter like Ian suggest.

A 100W SMPS should have no problem supplying a brief inrush. The capacitance on the 12V rail is likely a couple mF.

Putting a 5.6 Ohm resistor on the 5 V rail got the motor to run on the 12V rail :-+ . Removing it made it stop again. Maybe there is something more that the power supply needs to work properly. By the way, the capacitor is indeed big as it will make the motor do two or three of those jumps (every second or so) after the power to the supply has been removed.
Title: Re: Supplying a DC motor with a switching power supply
Post by: G7PSK on July 02, 2013, 08:56:55 am
That motor is trying to start on full load, the motor could be trying to take up to ten times the running current on start up so if there is any type of overload protection on a 4 amp SMPS it will shut down. You need something like a super cap or a battery in the circuit to meet the initial demand, messing around with resistors and plain capacitors will just come to grief.
This motor starting problem is something I see frequently in my line of work, customer "why cant wont my welder/ compressor run on my generator, after all the generator is 5 KW and the welder/ compressor is only three and a half, the generator must be faulty" I then have to go into long explanations of motor startup current demand when all I want to do is shout at them I told you you needed a bigger unit than that and why are you coming to me to complain I told you that those cheap Chines pieces of crap are not up to the job which is why I wont sell them.
Title: Re: Supplying a DC motor with a switching power supply
Post by: IanB on July 02, 2013, 09:15:22 am
Putting a 5.6 Ohm resistor on the 5 V rail got the motor to run on the 12V rail :-+ . Removing it made it stop again. Maybe there is something more that the power supply needs to work properly. By the way, the capacitor is indeed big as it will make the motor do two or three of those jumps (every second or so) after the power to the supply has been removed.

I didn't look closely at the power supply details before, but the 4003 model is designed with the 5 V output as the main output. The 12 V output is a secondary output to be used in addition to the main 5 V. Putting the resistor on the 5 V rail therefore makes sense. The power supply will require this to start up and regulate.

If you only want to use 12 V the 1001 model would make more sense. Even if you need 5 V for a micro or other electronics you could get it from the 12 V with a step down regulator like a 7805.
Title: Re: Supplying a DC motor with a switching power supply
Post by: Miguel Angelo on July 02, 2013, 03:14:14 pm
I didn't look closely at the power supply details before, but the 4003 model is designed with the 5 V output as the main output. The 12 V output is a secondary output to be used in addition to the main 5 V. Putting the resistor on the 5 V rail therefore makes sense. The power supply will require this to start up and regulate.

If you only want to use 12 V the 1001 model would make more sense. Even if you need 5 V for a micro or other electronics you could get it from the 12 V with a step down regulator like a 7805.

That explains a lot. I had this power supply from another project and that is why I decided to use it (it also does not need a fan). I did not realized the 5V is the main output aspect from the datasheet, but now that you have brought it up it seems logic. Thank you.