Wiring a pair of 15 amp vacuum motors.

Discussion in 'Burners and their construction' started by oldironfarmer, Jun 22, 2019.

  1. oldironfarmer

    oldironfarmer Silver

    Very well put Kelly. Just like compressors, multi stage blowers, well designed, achieve higher pressure at better efficiency than single stage but with blowers the design work is so well advanced efficiencies are about to theoretical limits.

    I do disagree on your 220v comment and would like confirmation either way so I don't make a mistake. My assumption has always been if you run a 220 circuit to two blowers and have one on one leg and the other on the other leg you will get 15 amps in the neutral when only one is running and close to zero on the neutral when both are running. Am I wrong there?
  2. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    This is a continuation of the discussion started in another thread, I tried to move the previous replies immediately but completely screwed it up and lost some of the posts so starting over…..about various options for wiring/powering a pair of the vacuum motors pictured below. I use them for a host of tasks but at the moment primarily vacuum assisted lost foam casting and eventually a source of forced air for burners.

    They are 120vac universal motors and draw 15amps. I want to be able to run one or both, and pneumatically be able to connect them in series or parallel. In (pneumatic) parallel they of course have twice the flow potential but in series, they become a 6-stage vacuum source that can generate a 1/2atm of vacuum.

    2, 3-stage in series.jpg

    I was originally going to speed control each motor to control the vacuum signal, but the motors are air cooled by a separate fan and decided it would be easier/wiser to just throttle the inlet to control the vacuum signal.

    So obviously I could just drag out two drop cords from two separate 120vac circuits. -Easy enough. Then I thought why not just use a single 240vac circuit, a NEMA 14 series (2 legs, neutral, with ground) plug and receptacle, and wire each motor between a leg and neutral. Like this.

    Vac Motor.jpg

    It was mentioned it may be analogous to this but only when both motors were running.

    Vac Motor Series.jpg

    But I thought that meant the neutral would carry 30 amps and defeat the convenience of being able to use a single 12gage (3 conductor w/ground) drop cord. It would also need to tap a 240vac wall circuit wired for >30 amp capacity. I have a few of those but most are 20amp circuits. It seems like opinions varied on what the current would be in the neutral leg.

    I didn’t/don’t see how neutral can be 0 amps, “balanced” or not. Looked like 30a to me. Aside from the current, there were cautions made about creating imbalances between the loads on each leg. Isn’t that (imbalance) what occurs with every individual 120vac circuit in your service? Every 240vac circuit breaker I have is two-pole, so each leg is fused anyway. What’s the problem in that respect?

    Carry on


    Attached Files:

  3. oldironfarmer

    oldironfarmer Silver

    The best way I can describe it is since it is alternating current opposite phases meeting each other eat the other phase making neutral.

    It's not your style, but you could do a farm boy test, hook the hot wires from two 15 amp single phase breakers which are on different phases and disconnect the neutral wires at the box, tie them together, and rig up a 15 amp breaker between the two lines and the neutral terminal strip. Start one blower then start the other blower and see the ground breaker does not trip.

    From your breaker box to the utility any load mismatch between phases goes back to the transformer in the neutral. If you have a 150 am service and you run 100 amps of 120v on one leg then you only have 50 amps of 220 you can use. If you balance the load and have 50 amps of 120v on each leg you can add 100 amps of 220v.


    "Circuit Balance
    To understand how balancing works, imagine that you have two 120-volt circuits with single-pole breakers. One circuit supplies a refrigerator that draws 8 amps; the other circuit supplies a chest freezer that draws 7 amps. Both appliances run all the time, year-round. To balance the load of the two circuits, the breakers should be on different hot bus bars, or "legs," of the service panel. That way, the amperage of the two circuits cancel each other out when the power returns to the utility on the neutral. In this case, the current on the neutral would be 1 amp: 8 – 7 = 1. If both appliances pulled 8 amps, the current on the neutral would be 0. The goal is to have the current on the neutral be as low as possible—for safety, energy-efficiency, and other reasons.

    On the other hand, if you placed both circuits on the same leg of the panel, the loads of the appliances would add together, resulting in 15 amps of current returning on the neutral. That would be an unbalanced load and preferably avoided."
  4. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    First, apologies to anyone (namely FishbonzWV and PatJ) for losing your posts in the move. It was unintentional.

    OIF, I think what you are saying is at anytime, as long as the two loads look identical, there is equal and opposite flow of current in the respective phases..... examples-of-phase-shifts.jpg

    So if you clamped the neutral leg with an amp meter you would measure net zero. If you turned either motor off you would measure 15amps.....is that right? But I take it the Utilities power meter still records the 15amps coming in through each leg irrespective of phasing....balanced or not.

    If they were two separate 120vac circuits that were in phase, and you tied the common together you would measure 30amps in the common/neutral in the cord returning to the receptacle.

    Still kind of hard for me to get my head around that, but it would say there would be no problem with the ampacity in the neutral line if it were the same gauge as the load legs, as it carried no net higher current than either of the phased legs that were each 120v above/below ground. In other words 12ga 3-conductor w/ground would due. Hmmmmm.

  5. oldironfarmer

    oldironfarmer Silver


    It's hard for me to think AC instead of DC. But I like A/C.

    The meter measures kwh through either phase. My understanding is the meter does not measure the neutral (not that it's relevant) but all the power is measured.
  6. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    I recall 240vac driers/appliances that had a combo of 240/120 vac circuits and loads used to be 3-prong and they just tied neutral to ground and then some time ago the 4-prong became code.

    So in the same example above, if you only had 3 conductor cable/cord, what scenario/hazard is being eliminated compared to the low side of each motor is tied to ground instead of neutral?

  7. FishbonzWV

    FishbonzWV Silver Banner Member

    This is the simplest way I can explain this.
    You have a 1hp motor that consumes 748 watts of power at full load (it's a giant resistor). That power is consumed on the positive side of the sine wave and on the negative side.
    I think everyone is thinking like it's a flip-flop circuit and it's only using power on the up stroke.
    Now add another motor on a separate leg, it's consuming 748 watts and if it's sharing the neutral, double the current.
    Now figure in what they call a locked rotor, that's when the motor is off and you apply power. you can get an amp surge of up to four times the normal run amps until the rotor starts turning.

    (and that 1 amp net on the neutral on that web site is a bunch of BS, you balance your hot sides)

    And, you wouldn't believe what all the new generation power meters report to the power company, it's in real time now, you pull your meter, they know instantly.
  8. _Jason

    _Jason Silver

    Most of the meters I've seen generally had four lugs - an in/out for each of the hot legs. The neutral entered the panel from the service wire and was bolted onto a two sided lug inside the meter can. Your house neutral bolted to the other side of that lug. The newest electronic kWh meters may touch the neutral but I wouldn't get overly concerned about it. Also, it is best to keep your neutral and ground separate once you leave the breaker panel.

    I'm with OIC on this, in that I think you can get away with the shared neutral. Try it and see though.

    If I were wiring this, since this won't be a fixed installation somewhere, I would wire a 240V receptacle at the wall, then run a length of 12/4 SEOOW to my fans and split it from there. 12/3 w/ground ROMEX would work too in a pinch but SEO is a portable cord and much more flexible as well as being rated for outdoor use.
  9. oldironfarmer

    oldironfarmer Silver

    It looks like we're not going to settle this until some neutral party does a test. Somebody who already has two blowers.

    Out of curiosity I asked the question on the tractor forum I participate in. A wide range of topics gets discussed there and some very good advice, especially the attorney/electrical engineer. Five people responded quickly. Two didn't understand the question but answered anyway. Three said yes, the neutral will not carry any current. One said his garage was wired with a 12/3 Romex service line on two phases with two circuits in the garage and it worked.
  10. OMM

    OMM Silver Banner Member

    I’ll try to explain this the best I can.

    Power stations generate three-phase. One of the lines is distributed to each house/subdivision . 98% of North America should be getting 230 V to 252 V not 220.

    Kelly as explained it pretty good. Neutral is a centre tap to ground.

    To run two 120 V motors in the series you have to be very careful. Yeah it can be done. In this situation I would add the neutral as the two motors will be definitely doing different work if the airflow is also in series. The motor that is doing more work would throw the additional amperage on to the neutral instead of making the second motor work harder for no reason.


    I would also like to mention you cannot use ground as a centre tap. It has to be a dedicated neutral line. So you would need to use a four prong plug and if you’re on a 15 a Circuit this would be a 14–15 . The breaker would need a tie handle for 240 V.
    Last edited: Jun 24, 2019
  11. PatJ

    PatJ Silver Banner Member

    I am still at the Grand Canyon, using the little phone, so I can't really do much commenting till I get back.

    The north rim view is spectacular.
  12. oldironfarmer

    oldironfarmer Silver

    That looks pretty clear. Two equal loads on different phases results in zero flow in the common.

    Having a fourth wire is safety in case the neutral fails, right? They would be tied together on each end. I think it is a Code issue. He was talking about using a 12/3w/g.
  13. oldironfarmer

    oldironfarmer Silver

    Best response from the tractor forum. This guy is a lawyer as well as an experienced EE (now retired) and is unbelievably clear and practical in his responses for his lofty credentials.

    "Andy, another great electrical question (they draw the most responses). In a nutshell and for most cases (my short answer) YOU ARE CORRECT. Any time I attended NEC Seminars and other training and in my years designing power distribution systems BALANCING LOADS AMONG DIFFERENT PHASES WAS A MAJOR DESIGN CRITERIA. Since the two legs in a 120/240 Volt Single Phase Three Wire service are 180 out of phase from the other, if BOTH legs are drawing the same current NEUTRAL CURRENT IS ZERO. Anytime current is carried in a conductor there are I Squared R Heat Energy losses, so if there's no current those are avoided.

    THAT BEING SAID there's more to the story!! While the above holds true for resistive loads, if the loads were inductive and if the inductance isn't the same on both legs the circuits may not actually be quite "perfectly" balanced. When I considered utilizing what are referred to in the trade if I recall correctly "Multi Wire Branch Circuits" (single common Neutral for two legs) if there were complex multiple (never the same) highly inductive loads I was trained to avoid their use.

    All in all and even though what you spoke of sure will "work" and your basic premise (if loads are balanced Neutral current is zero) is true, WHEN DESIGNING MOTOR CONTROL BRANCH CIRCUITS in our shop and in the training I received multi wire branch circuits where a common Neutral was used for two legs wasn't favored.

    But don't let this discourage you, a couple small motor loads isn't all that much and any such resulting minor mis matches isn't gonna break the grid lol...........What I was talking about was bigger and more complex power distribution NOT a couple small motors in a shop."
  14. _Jason

    _Jason Silver

    In a four conductor cable, such as SEOW 12/4 or 12/3 w/ground ROMEX, you'll have two hots, a neutral, and a ground. With cable like SEOW, there'll be black, red, white, and green wires. With Romex, you'll have black, red, white, and bare copper wires. The black and red are the two hots, white is the neutral, and green/bare is ground.
  15. joe yard

    joe yard Silver

    I a not good at explaining things clearly but here it goes.
    Think of the phase thing as a 360 degree circle or wheel with spokes. In a single phase such as a 130V leg you are always 180 degrees out of phase with the hub center “neutral” . In a 2 phase system such as 220 the phases are 180 opposing each other and balance the hub or neutral when the load is the same. In a 3 phase 277 system the phases are all opposing 120 degrees but the analogy works the same with neutral in the center. If you add in any spoke without adding to the other side more than the other. You will pull off center and increase the load on the center or neutral.
    Ground is NOT neutral In a perfect world it would be because the neutral at the power plant and at the load in the U.S. system is tied to an earth ground. Unfortunately the earth is anything but a perfect ground and answers to resistive loses just as any other material in nature. The earth the ground conductivity is not something that can be easily controlled like in a known consistent conductor
    Now comes the part that will keep many scratching there heads for a long time and cant be covered in a single form, AC Ohms law states that voltage leads current in a capacitive circuit and current leads voltage in an inductive one.
  16. OMM

    OMM Silver Banner Member

    Joe you are right in many ways. But there are very few Two Phase. I believe the last of the two phase is in and around Boston. Two phase is 90° separation, Where three phase is 120°. Two phase requires four conductor wires and three-phase only requires three conductor wires. It is commonly confused that single Phase when they pass it through a poly phase transformer with a centre tap, some think this is now two Phase.
  17. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    I’m going to have to wire the two motors to the common neutral on a 240vac circuit just to see for my own eyes 0 amps in the neutral leg.

    I’m reconsidering just using two separate 120vac circuits though. The reason being I’ll probably only use one of the motors the majority of the time. The times I don’t will just require a another drop cord from a separate circuit and I do have several receptacles within drop cord length of where I cast.

    The idea of one cord seems clean but to do so, I’d need to set up a dedicated circuit and make a custom cord with unique (at least for my shop) plug/receptacle that would be good for nothing except running this dual motor contraption. Right now the only circuit I have configured for such is the high power one to run my resistive electric furnace. It’s NEMA 14-50, and I’ll always be using that when I want to use the vacuum motors.

    I have lot’s of 20a 240vac circuits in my shop, and I do occasionally need a 240vac drop cord, but all of these circuits are two legs and earth ground, as is the case in USA 240vac single phase. I could use a 3-conductor drop cord and run both motors to ground, and even though it would work, I don’t want to run the motors to Earth ground.

  18. PatJ

    PatJ Silver Banner Member

    It sounds like Matt has it right.
    It can get pretty confusing because as I stated the other day, you make all sorts of electrical connects that are unsafe and not recommended, but they will actually work assuming you don't touch the wrong thing, or a short circuit does not happen.

    A penny in the fuse box will work perfectly as long as there is not a short circuit somewhere in the line.

    Most standard US residential services are single-phase that are fed from a center-tapped single phase transformer.
    It is not 2-phase (as Matt says), but rather two "lines", and a neutral.

    I have seen several people convert a 120 volt circuit into a 220 volt circuit, and as long as you do not need the neutral, you can do this, however, the wire color code will be wrong, and that could hurt someone.
    I have seen people use the ground conductor as a neutral, and this is very dangerous.

    A neutral conductor can carry current under normal conditions, is a fully insulated conductor, and is attached to fully insulated neutral busses.
    The ground conductor does not normally conduct current, is not normally insulated, and is connected to a ground bus that is electrically bonded to the metallic panelboard enclosure, and often bonded to other metallic parts of the electrical system.

    The neutral and ground busses are only bonded together at the service entrance panel.

    So what all this means is that if the neutral is carrying current, it will be insulated all the way back to the service entrance panelboard.
    If you use a bare ground wire as a neutral, it will carry current, and the panelboard enclosure and any other metallic item such as conduit, junction boxes, etc. can also carry current.
    So when using a ground as a neutral, all you have to do is tough something metallic and get into the current path, and you are toast (you will have 120 volts and full current flowing through you).

    The purpose of a ground conductor is to provide a low impedance path back to the circuit breaker for fault current, so that the circuit breaker will trip a faulted circuit off line.

    For instance, if a dryer were operating without a ground wire, and one of the heating elements or the motor winding faulted to the metallic enclosure of the dryer, the dryer could still run perfectly, and the circuit breaker feeding it would not necessarily see any fault current, and so would not necessarily trip.
    But if you happen to touch an ungrounded dryer enclosure, current can flow through you and back to the ground rod outside the house, and ultimately back to the neutral tap point on the transformer.
    Thus the reason for all metallic enclosures needing to be grounded.

    Generally speaking, if you touch a circuit, you will get 120 volts to ground through your body.
    If you touch a 240 volt circuit, generally you will still get 120 volts to ground through your body, unless you happen to grab the two line conductors at the same time, in which case 240 volts will flow through you.

    And low voltages like 120 volts kill far more people than higher voltages.
    It is the disruption of the heart electrical signals when exposed to low voltages (like 120 volts) that kills most people.

    Indeed, defibrillators use a high voltage to slam the heart muscles via contraction, and then start normal beating afterwards, so high voltage does not cause the hear signal problems.

    I have seen a lot of non-standard connections over the years, and the most common is the corner-grounded delta.
    For low voltage, I use 480Y/277 and 208Y/120 three phase systems, and 120/240 volt single phase systems.
    For medium voltage systems, I typically work with 23KV, often with 23:4.16 KV for 5KV motor loads.

    Low voltage in the US is generally defined in power distribution systems as 600 volts and less.
    The break point between medium voltage and high voltage is at 69,000 volts, and I think medium voltage is up to and including 69,000 volts.

    Unfortunately the term "high voltage" is plastered all over almost all electrical equipment, and so most people refer to anything above about 24 VDC as "high voltage".

    Any electrical system that I run across that is not one of the standards I listed above, I give the owner two options:
    1. Change/upgrade the system to one of the standards listed above.
    2. Find another engineer besides me.

    Since an ee is liable for whatever he designs, then designing or changing a non-standard electrical system is a dangerous game of Russian roulettte, and sooner or later it will not end well, so I don't play that game.

    I do know some people who have/do practice voodoo electrical, but generally they don't have to buy liability insurance and be responsible for the outcome as I do.
    Last edited: Jun 28, 2019
    joe yard likes this.
  19. OMM

    OMM Silver Banner Member

    LOL voodoo. I do voodoo all the time. But I have no leaks. I have 600 V three-phase in my shop. From 240 single phase.
  20. PatJ

    PatJ Silver Banner Member

    If I had a nickel for every voodoo electrical connection I have seen, I would be rich.
    But from a liability insurance standpoint, you don't want to go voodoo, since that does not play well in court.

    Almost anything can be done with electricity.
    I recall purchasing a small book when I was in grammar school, and it had a coil in it.
    When you opened the book, it would shock the beejesus out of you.
    I had great fun shocking fellow students and some teachers, but I was never well liked, and am still not well liked, but I am respected, especially around books.

    I remember some other shocking device that we played with, and I remember we would all stand in a circle and hold hands, and get shocked.

    And I remember the kid next door showing me (when I was about 5 years old) that you could stick a paper clip in the big slot on an electrical outlet and not get shocked, but if you stuck it in the short slot, it would shock you). He would do demonstrations to the other kids.

    Electric drills had the neutral connected to the metal frame of the drill, and all you had to do was reverse the plug in the socket, and you got shocked, especially if your feet were damp/wet.

    How we survived those days, I have no idea.


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