27 Replies Latest reply on Feb 16, 2020 2:55 PM by jw0752

    Why didn't this Vacuum Power Supply work?

    makerkaren

      I'm building a project using this 12VDC shop vac. It's designed to be used in a vehicle and plugged into the cigarette lighter.

      Its specs say it is 12V, 5A. So I bought this 12V 5A power supply:

      12V Shop Vac                                                   12V 5A Power Supply You can also find it here.

      When I hooked this power supply up to the vacuum, all I would get was a very soft and inconsistent *whir*. At first, I thought I had a problem with the relay circuit I was using to switch it on, but then realized that it did the same thing when connected directly. So I started digging around in my bin of random power supplies, hoping to find a few 12V ones. I did! But a lot of them also didn't work, or were weak. I tried a 100mA, a 1A, a 1.5A, a 350mA. One of those sort of worked (I don't remember which one), but the plugged was wired in the other orientation, so it made the vacuum run backwards and I didn't want to mess with that, so I kept looking for a better power supply.

      This is the one that finally worked. It's a 12V 850mA leftover from who knows what.

      12V 850mA power supply

      What I don't understand is that while all the power supplies are rated for 12V, the 100mA, 1A and 5A supplies didn't work, but this 850mA power supply did. That seems like bad math to me, so I'm assuming there is a factor I'm missing. What is it?

        • Re: Why didn't this Vacuum Power Supply work?
          shabaz

          Hi Karen,

           

          I'm not sure, but maybe it's not really 12V 5A that is needed, but much higher initially, since the motor in the vac is not initially spinning. That surge might not be possible with some power supplies. I'm just speculating really. But I've seen a similar issue driving motors, where on the top-level specs it should work, but in practice it needs a higher power supply, due to the higher power requirement until the motor is spinning.

          Regarding the one it is working with, that is a transformer-based supply (as opposed to a AC-DC converter supply) and maybe it doesn't implement fast shutdown on overload. It might not last long, so you may still need to source an alternative higher current supply. The reason is, if the transformer gets too hot, sometimes they have a thermal fuse built-in to the transformer, and if it activates, then the situation is unrecoverable and the transformer needs to be thrown away.

          6 of 6 people found this helpful
            • Re: Why didn't this Vacuum Power Supply work?
              makerkaren

              Interesting. I tried to find more information about the vacuum, but even the manual that comes with it is sparse on information.

              As for the current power supply(850mA), the vacuum is only getting turned on for maybe 10 seconds at a time, maybe a little longer, but not much. Do you think that will be better or worse for the transformer inside the power supply? I don't know if sustaining power puts more strain on it, or recurring starting and stopping.

               

              Happy to hear any recommendations for power supplies that would be more likely to work. Cause if this one does die, then my project will be a bit dead in the water and I'll be hitting up my local Restore to buy all the random 12V supplies they have in order to try to make this work.

                • Re: Why didn't this Vacuum Power Supply work?
                  shabaz

                  Hi Karen,

                   

                  It could be worth ordering these parts:

                  CL-50CL-50 and CL-60ACL-60A and CL-70CL-70   they are inrush current limiters, and they may work with your 12V 5A supply. They work by looking like a high resistance initally, so that there's no sudden surge at switch-on. I don't know which one would work, so it's worth getting all three (they are around $1-$2 each), maybe even a couple of each. When it is in series with the vac, then initially the vac will start up slowly, and then as the inrush current limiter gets warm, the resistance will drop, to allow the vac motor to speed up.

                   

                  If you wished to try it without that, one way is to use a 10 ohm (maybe) power resistor, and switch it out after a few seconds with a relay or manually with a switch. It will get hot, so you'll need to watch out for that, it can burn.

                  7 of 7 people found this helpful
              • Re: Why didn't this Vacuum Power Supply work?
                Bald Engineer James

                In addtion to shabaz's excellent NTC idea, you might consider a PC ATX power supply. They have enormous current capability on the +12V rails and are (or at least were) designed to drive inductive loads like motors. If your makerspace has a parts wall, there is probably at least one in it.

                 

                They usually need a load on the +5V and +12V rails to operate. The 12V rail is fine with your vac. The 5V rail might need some LEDs or a power resistor. The really cool thing is that you could power your Arduino with the standby supply rail and then have the Arduino toggle the "PS_ON#" signal to active the pump.

                 

                If you went that route, you might test it with a NTC just to help keep the supply from having such a huge in-rush current.

                9 of 9 people found this helpful
                • Re: Why didn't this Vacuum Power Supply work?
                  kmikemoo

                  makerkaren To answer the question why the 5A didn't work but the 850mA one did... Old School Technology - or lack thereof.  Your 5A power supply is a regulated switch mode model.  Lots of cool circuitry.  No capacity for abuse - like inrush.

                  My guess is that your old 850mA wall wart is transformer based with less circuitry.  The open circuit voltage on the 850mA is probably around 18VDC.  The voltage drops to 12VDC as you load it up.  It is likely that the transformer windings share the neutral making it an autotransformer.  The higher inrush current creates its own version of reduced voltage starting.  While transformers are incredibly efficient devices, during the moment of inrush, they do have some inductive reactance and the cores can saturate - resulting in clipping of the transformed voltage much like a varistor.  The clipping is usually undesired, but in this instance it serves our purposes.  Depending upon how the transformer is wound, the secondary winding may even buck the primary winding resulting in a form of voltage regulation similar to old school magnetic amplifiers.  You may still want to up-size your power supply.  Go for the heavy, less regulated ones.

                   

                  EDIT:  Should have clicked "Show earlier"... yeah, what shabaz said.

                  8 of 8 people found this helpful
                  • Re: Why didn't this Vacuum Power Supply work?
                    jw0752

                    Hi Karen,

                     

                    You could experiment by using the 12V 850 mA transformer based adapter to get the motor rolling and then without turning it off or disconnecting it you can parallel the 12V 5 A switching supply to make the vacuum run normally. As long as you maintain proper polarity matching this should work. From my experience the speculation, by the other guys as to the cause of your observations, is correct.

                     

                    John

                    5 of 5 people found this helpful
                    • Re: Why didn't this Vacuum Power Supply work?
                      michaelkellett

                      If only they taught a bit of motor theory to everyone - the Uni course I dropped out of in 1972 had a bit too much but never mind.

                       

                      A permanent magnet DC motor (not steppers or brushless)  has an armature resistance R.

                      When it's not turning the current through the motor will be V/R.

                      When the motor turns it is spinning a coil in the magnetic field so a voltage is induced in the coil - this is the back emf  and it is proportional to the motor speed (revs per minute or whatever units you like).

                      The back emf = K * f where K is a constant for the particular motor and f is the rotational speed

                      The motor current (spinning or stopped) is (V - Kf) / R -so you can see that the current will be lower when the motor is turning.

                      The useful work done by the motor is I * Kf, and the loss in the motor is I^2 * R.

                       

                      Just to make life worse your fancy power supply may well have foldback current limiting - which means that it can put out 5A at 12V output, but if the load pulls the output down to far the current will drop, so it might only be able to put 1A into a short.

                      This is OK for a lot of things but not for starting  a motor.

                       

                      You can measure your shop vac with a meter to find the motor resistance - that will allow you to work out the start up current at any given input voltage.

                      You could make a soft starter for it with a resistor and a relay - but it might be cheaper (and certainly easier) just to use a more gutsy power supply.

                       

                      MK

                      4 of 4 people found this helpful
                      • Re: Why didn't this Vacuum Power Supply work?
                        dougw

                        Just to add to the ideas already presented... Some switching supplies have trouble starting with a heavy load.

                        Have you tried turning on the 12 V supply before attaching the motor?

                        1 of 1 people found this helpful
                        • Re: Why didn't this Vacuum Power Supply work?
                          DAB

                          Hi Karen,

                           

                          When you are dealing with motors, there can be a very large inrush current.

                          A car battery can supply this high initial current demand, but a power supply would not be able to supply the instantaneous current need.

                          The result would be a slower or limited current draw that would not be sufficient for the motor to work properly.

                           

                          If you are not going to power it with a battery, you should put it on a good power supply and look at the initial current draw profile, it would be very enlightening.

                           

                          DAB

                          5 of 5 people found this helpful
                          • Re: Why didn't this Vacuum Power Supply work?
                            kmikemoo

                            makerkaren  Another old school trick to possibly help your motor starting dilemma - that may even make your 5A power supply work - is to add a capacitor in parallel with your power leads at the vacuum.  I don't mean a tiny capacitor.  I mean a motor run capacitor, 40MFD, 370VAC cap.  We sometimes still use them in emergency generators to keep digital controllers alive during engine cranking.  Low tech but very effective.

                            1 of 1 people found this helpful
                              • Re: Why didn't this Vacuum Power Supply work?
                                makerkaren

                                I just looked this up and whoa, that's intense.

                                • Re: Why didn't this Vacuum Power Supply work?
                                  michaelkellett

                                  Sorry Mike, I don't get this - how would a 40 uF cap across the motor help (in the case of Karen's DC motor), and why would it need to be rated at 370V AC ?

                                   

                                  From Wiki:

                                  https://en.wikipedia.org/wiki/Motor_capacitor

                                  A motor capacitor,[1][2] such as a start capacitor or run capacitor (including a dual run capacitor)[2] is an electrical capacitor that alters the current to one or more windings of a single phase AC induction motor to create a rotating magnetic field.

                                   

                                  MK

                                    • Re: Why didn't this Vacuum Power Supply work?
                                      kmikemoo

                                      michaelkellett  In this instance, DC, the capacitor is merely an energy storage device.  The larger the capacitor, the more energy it stores - ergo why the recommended size of the capacitor.  During the motor starting event (inrush current), the capacitor returns some of its stored energy back into the power system supplementing the power source.  By placing it near the motor, there is just less line loss ( I2R ).  Old school momentary UPS.  Ever wonder why some power supply / regulator circuits have one or two 1000MFD or 2000MFD electrolytic capacitors on the output?  In my world, we call it "ride through".  I'm sure there are other phrases.

                                      I hope this helps.

                                        • Re: Why didn't this Vacuum Power Supply work?
                                          michaelkellett

                                          Putting a great big capacitor across the supply might help start the motor if the power supply was turned on and the capacitor charged before the motor was connected.

                                           

                                          But you suggested a 40MFD, 370VAC capacitor - which is way too small (I'm assuming you meant 40 micro farads (which is available in 370 V AC rating).

                                           

                                          MK

                                          1 of 1 people found this helpful
                                            • Re: Why didn't this Vacuum Power Supply work?
                                              jw0752

                                              Hi Michael.

                                               

                                              Another reason that the 40 uF 370 VAC looks big physically is that it has to be designed to be non-polarized due to its use in AC applications. The combination of the high voltage (370 V) and the nonpolarized configuration make it large in size but small in capacitance. The energy that can be stored in a 40 uF 370 VAC cap is quite large due to the higher voltage. E = 1/2  CV^2. When the voltage comes down to 12 volts however the capacitance has to increase, as you have pointed out, in order to compensate for the current draw on a motor. One other observation is that the Start Cap in an AC motor is used in quite a different manner than what is being discussed here as a current boost for getting a SMPS past the initial start current of a DC motor.

                                               

                                              John

                                              3 of 3 people found this helpful
                                              • Re: Why didn't this Vacuum Power Supply work?
                                                kmikemoo

                                                michaelkellett You are correct that the power supply will need to be energized so the capacitor can be charged prior to starting the vacuum. 

                                                 

                                                As to the "MFD", that is used by many US suppliers for microfarad when it comes to capacitors used on motors.  As jw0752 stated, it's way more about the voltage rating of the capacitor than the capacitance value.  The reason that manufacturers use the 40MFD instead of a 20MFD is that it is physically bigger for essentially the same cost - so it clearly must store more energy.  As to how much energy it really stores... enough to scare the dickens out of you when you're working on a control system that has one installed and you didn't know it.  The arc is enough to burn through powder coated paint and leave all the characteristic traces.  The next technician will definitely know that something happened there.

                                                 

                                                So we're clear, this isn't my great idea.  It is used in literally thousands of applications.  When I remembered it, I jumped on the forum fully expecting jw0752 to have already posted about it.  It's a tried and true solution - meaning 'old school trick'.

                                                  • Re: Why didn't this Vacuum Power Supply work?
                                                    michaelkellett

                                                    @Mike,

                                                    As JW said, motor start capacitors used in AC motors have a totally different function from the parallel capacitor across a DC supply which may be there to provide short term extra current under transient load conditions.

                                                     

                                                    This diagram shows how a start capacitor is connected to an AC motor.

                                                     

                                                     

                                                    There is also a run capacitor which has a different function.

                                                     

                                                    Neither capacitor is connected across the supply but both must be AC rated and will have values in the tens to low hundreds (in the case of a very large motor) of uF.

                                                    The caps in this case are not being used to store energy but to provide phase shift in the start winding.

                                                     

                                                    A DC motor (like Karen's vac) has no start winding and no place to connect a start capacitor.

                                                     

                                                    Here the basic spec of a 50W ish DC motor (Transmotec D8095 - a bit upmarket of the vac motor I expect but it'll do as an example.)

                                                     

                                                    Nominal voltage (V) 12 24

                                                    No load speed (rpm) 3500 3600

                                                    No load current (A) 1.50 0.70

                                                    Nominal speed (rpm) * 3100 3200

                                                    Nominal torque (mNm) * 145 145

                                                    Nominal current (A) 5.0 2.4

                                                    Stall torque (mNm) 1422 1464

                                                    Starting current (A) 35 17

                                                    Output (W) 47 49

                                                     

                                                    The no load current is 1.5A and the starting current is 35A (for the 12V version)

                                                     

                                                    Let's assume that the vac motor is similar - it expects to be powered from a car battery so 35A is possible for a startup current.

                                                     

                                                    Unfortunately this doesn't tell us how much current the motor actually needs to start - it doesn't need 35A, it just needs enough current to get it turning, and the only way to find that is to measure it.

                                                    If you put a capacitor across the output of a switching supply, connect up the motor and switch on - it probably won't help the psu to start up  - it needs now to drive the stalled motor AND the capacitor to get any volts across the motor.

                                                     

                                                    It may work (probably will) if the switch or relay that connects the vac is after the cap (so the cap is charged up before you connect the motor) - a common rule of thumb is to use 1000uF per A or load current - which suggests 4700uF might be a good value to start with.

                                                    You don't want to use a bigger cap than you need because it is giving the PSU a harder time starting up with it connected  - and some PSUs object and won't start at all into big capacitive loads.

                                                     

                                                    MK

                                                    3 of 3 people found this helpful
                                                      • Re: Why didn't this Vacuum Power Supply work?
                                                        kmikemoo

                                                        michaelkellett  wrote:  @Mike,  ... the parallel capacitor across a DC supply which may be there to provide short term extra current under transient load conditions.

                                                         

                                                        Yup.  That's exactly the purpose.

                                                         

                                                        makerkaren has a series universal motor in her vacuum.  It will run off of AC or DC.  Unlike the single-phase AC motor that needs a phase shift in the start windings to create out of phase flux for starting, the series universal motor uses a commutator and brushes to accomplish this.  See any one of the cordless drills you own.  Exact same setup.

                                                        Series Universal Motor

                                                        The cap goes on the source side of the switch, not the load side.  As you state, putting it on the load side of the switch only increases the issue.

                                                         

                                                        Because it's a series universal motor, makerkaren could even use an SCR controlled bridge rectifier with an AC wall wart and a simple ramping circuit to control the gate - and therefore the applied voltage and, thus, the inrush.  If she really wanted to get fancy, she could use a servo to drive a variable resistor similar to the trigger in the cordless drill.  Or PWM.  Over engineered?  Probably.

                                                         

                                                        We already know that the 850mA power supply works so the inrush isn't as significant as we all imagine.  It only needs to be mitigated a bit.  Also, series universal..,. the electromotive force and the counter electromotive force are always in the same proportion and at the same phase angle.  We get a much better starting characteristic than we do on AC motors with across the line start.  Since it's been decades since I've played with DC motors (other then starters), I'm also curious what the real draw is.  What I know is that you can start a 6-1/4 HP circular saw with a series universal motor on a 3200w inverter generator but you can't start a 1 HP table saw with an induction motor on the same generator.

                                                         

                                                        Mike

                                                        2 of 2 people found this helpful
                                                          • Re: Why didn't this Vacuum Power Supply work?
                                                            michaelkellett

                                                            How do you know it's a series universal motor ?

                                                             

                                                            You certainly can't tell from the spec:

                                                             

                                                            https://www.stanleytools.com/products/home-improvement/cleaning/wet-dry-vacuums/1-gallon-dc-portable-vacuum/sl18125dc

                                                             

                                                            Hose Length4 ft
                                                            Peak Motor Horsepower2

                                                             

                                                             

                                                            2 HP = 1500W, and we all know that amount of power is NOT coming through the cigar lighter socket.

                                                            I'm a bit amazed at Stanley being so sloppy with the spec - this must be an error.

                                                             

                                                            I'd expect a little permanent magnet motor - but unless Karen will pull the vac apart (come on Karen !) we aren't going to know.

                                                             

                                                            It's all getting so interesting (and I could use a 12V vac to clean up after the dog) that I would buy one - but they don't seem to be available in the UK.

                                                             

                                                            MK

                                                              • Re: Why didn't this Vacuum Power Supply work?
                                                                kmikemoo

                                                                michaelkellett  Fair question.  Great discussion.  I could be wrong on the motor. makerkaren's vacuum could have a permanent magnet motor.  She wasn't joking when she said that there isn't any real information to be found about this item - so I dug into my DeWALT vacuum that runs off of a 12V battery.  The market isn't huge for these items so I expect that they are all built the same.  A permanent magnet motor should draw even less starting current.

                                                                 

                                                                As to the peak motor rating... I wondered the same thing about the 6-1/4 HP circular saw.  How could that tiny motor output that much power?  The answer that makes the most sense... marketing mathemagic.  It doesn't.

                                                                I read it on the Internet so it has to be true.     Okay, but it does make the most sense.  Nothing else did.

                                                                HP = (torque X RPM)/5252  for ft-lbs.  The RPM value is the theoretical maximum unloaded speed for the motor - probably in excess of 30,000 RPM.  My Dremel says that it will do 35,000 RPM.

                                                                The torque is the maximum calculated gross torque (all losses removed).  I read one time that the torque is calculated from the maximum theoretical momentary force from the shaft multiplied by the maximum distance to the edge/tip of whatever is being spun.  At 20,000 RPM, Karen's motor would only have to produce 0.53 ft-lbs (0.71Nm) of torque to make 2 HP.  Crazy small.

                                                                 

                                                                Mike

                                                                2 of 2 people found this helpful
                                                  • Re: Why didn't this Vacuum Power Supply work?
                                                    luislabmo

                                                    I say the only way to be 100% sure is to first measure how big the inrush current of the motor is -probably this needs a quality instrument to do so to get a reliable measurement. If I were you and those were my choices, I wouldn't pick something exact, or too close to the specs of the motor, to account for the margin of error of all parts involved, and also to give plenty of room for peaks of power consumption (inrush current, motor being force by some reason, stall, etc). so TL:DR: I'd pick the 10A  if those 3 were my options.

                                                     

                                                    Luis

                                                    1 of 1 people found this helpful
                                                    • Re: Why didn't this Vacuum Power Supply work?
                                                      dougw

                                                      A rule of thumb for inrush current is to multiply the voltage by 1.41 and divide by the resistance of the motor, so about 17 divided by the resistance in your case.

                                                      If the resistance of the motor is more than 2 ohms, the larger power supply should be adequate.

                                                      4 of 4 people found this helpful