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Accurate voltage control for motor

  1. Jun 20, 2012 #1
    I have an air turbine, and it's critical that the motor speed be precisely regulated.
    It's very voltage-sensitive, so the fluctuations in the line voltage between 120-126 A make the motor speed too variable.
    How can it be regulated?
    A UPS might work. But it would have to output voltage accurate within +/- 1%. THere's a rheostat on the motor, so the UPS could just regulate up, say, so it wouldn't be necessary to have both a step-up and step-down transformer. The motor uses 1-2 A at the normal speed. It uses max 9 A, but I never use it at that speed. I don't know, if you get a voltage regulator that's rated for a certain current, and more current goes through it, if it damages the voltage regulator, or it just can't regulate the voltage right.
    It also needs battery backup for when the power fails.
    Can anyone point me to such a UPS?
    It would be great if the input voltage could be regulated by the actual airflow. The air turbine is powering an airline respirator, the airflow is 4-5 CFM through a hose maybe 3/4" inner diameter, with garden hose fittings. If anyone can help with that I'd appreciate it.
    Laura
     
  2. jcsd
  3. Jun 21, 2012 #2

    NascentOxygen

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    Hi Lark. You are going about it the wrong way, wanting to tightly fix the voltage. This means you are placing blind faith in the motor maintaining a constant angular velocity for a fixed supply voltage. What happens when the motor ages, or atmospheric pressure changes, or thermal effects show up?

    Much better to use a feedback arrangement and try to maintain a constant airflow. But measuring airflow accurately may be difficult, easier is to measure speed of rotation and alter the voltage to the motor in an effort to maintain a fixed angular velocity. You can accurately sense the speed by sensing the passage of a pattern of dots around the shaft. Or maybe there is somewhere to measure pressure, and try to maintain that at a constant level?

    Sorry, I don't have circuits in mind. Just pointing out what I reckon you should be looking out for.

    If you're lucky, others may have some recommendations. Good luck!
     
  4. Jun 21, 2012 #3
    Is it easier to tightly control DC voltage? It's a universal motor, it likely can run on DC just as well ...

    The motor has a rheostat, so slow changes in the airflow/voltage ratio are OK. The variation I notice is from voltage changes.

    It would be accurate enough to measure the airflow within about +/- 5%. A small change in voltage causes a big change in airflow, that's the problem.

    I agree that it would be nicer to have a feedback arrangement, and I'm interested in how to do that. But it's likely more practical to control the voltage.

    Laura
     
  5. Jun 21, 2012 #4

    sophiecentaur

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    Control the voltage by measuring the airflow - using an anemometer as the sensor - and use that as your feedback signal. An open loop system is almost bound to be less satisfactory and there is no need to avoid feedback just on the grounds of complexity. Feedback rules in most circumstances where you need control.
     
  6. Jun 21, 2012 #5
    It would work well enough if I could tightly control the input voltage. However I don't know if there are voltage regulators that output voltage within +/- 1% that are reasonably priced. I asked about one, it costs ~$2700, which is too much. It likely has a lot of features I don't need, though.

    It would be enough to step up the voltage to a precise value, since there's a rheostat to decrease it again.

    About the air speed sensor, the air speed in the hose is about 10 m/sec. I would need a sensor that can fit in a 5/8" inner diameter hose and output a signal giving air speed +/- 5%. I don't know how to find such a thing, and once I had it, I wouldn't know how to use it to control the voltage input to the motor. I wonder if it could be done by software, send a signal giving airspeed to a computer, which then would send a signal to the motor.

    Laura
     
  7. Jun 22, 2012 #6

    sophiecentaur

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    I can see you have a problem. It seems you want a 'black box' approach to this as you are short of experience or facilities for building your own equipment. This usually means that you need to spend money to get what you want!! (A pain)
    The question with using open loop control is can you be sure that a particular voltage (however accurately controlled) will correspond to a particular air speed unless temperature and humidity are also controlled?
    If you are worried about air speed measurement then surely you will be measuring it anyway?? Else, why are you bothered?
    Would it be daft to suggest using the turbine speed as your feedback signal? If all you need is for it to be constant, irrespective of air flow, then a control loop from speed sensor, via a voltage regulator to the motor supply would do what you want.
    You may be between a rock and a hard place with this - either you have to put up with a system you aren't happy with or you need to learn about (or have local help with) control systems. You are right that a computer could be used. You could look into the Arduino system. It's a microprocessor board system which is aimed at home and student construction projects. It has many different interface modules for controlling virtually anything you could want. It is very easy to get started with loads of software and support available at reasonable cost but, of course, it represents another 'overhead' on your main project. (An excellent addition to the CV of a prospective job applicant, though).
     
  8. Jun 22, 2012 #7
    It's an airline respirator, it pumps clean air to a facemask that I wear. If the air speed is too slow, the mask leaks. If the air speed is too fast, the turbine gets very loud. It's an industrial safety device that I'm using for a different purpose. Usually people when they use it, would just turn it up high, and let it be noisy.
    Not at all daft, regulating the turbine speed might well work fine, and it might be easier to find a motor speed sensor than an air speed sensor. I'm not sure if air temperature or pressure variations would make enough of a difference in the air flow/turbine speed ratio, to matter.
    Is there a motor speed sensor that I could attach to the metal housing of the turbine? I don't know if something could be attached to the shaft.
     
  9. Jun 22, 2012 #8

    sophiecentaur

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    From what you say, an air pressure sensing, rather than air flow sensing would suffice - because it'll be pressure that governs whether the air escapes from the mask. Did you even consider having a pressure relief valve on a T-off from the main air hose? I'd bet that would be the cheapest and most straightforward solution. As long as the volts supplied to the pump motor are always sufficient to give you 'enough' then, if they are too high, the valve would open and keep the pressure where you want it.
    Just another, alternative idea.
     
  10. Jun 22, 2012 #9

    jim hardy

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    is there a hands-on technician in your plant who might help you?

    Treadmills abound in junkshops now. Most have a 6000 RPM 90 volt permanent magnet DC motor capable of 2hp, and a power supply for it that gives steady speed control via a knob.

    I hesitate to advise a novice to mess with that kind of electrical power
    but it's a good source for a speed controller for like $10 instead of $2700.

    You'll need some practical help .




    old jim
     
  11. Jun 25, 2012 #10
    Again, I'd probably do fine with a sensor for the line voltage, then something like a rheostat that would be adjusted based on the line voltage.
    That's what I'm doing right now by hand. I have a voltmeter, and for a given line voltage, I know how the rheostat should be adjusted. It seems fairly consistent.

    The turbine has a universal motor, it has brushes. Are speed sensors available for universal motors?

    Most people who use airline respirators use them at a high enough speed so that even with variations in voltage, they're always getting enough air. But I'm using it almost full time, I even sleep in it. So the noise it makes, which increases drastically with the air speed, matters.

    Laura
     
  12. Jun 25, 2012 #11

    sophiecentaur

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    That's worth knowing - a critical factor. You really do need to be running the motor as slow as possible and at a reasonably constant speed. It doesn't appear that the pressure / flow needs to be as well regulated as your original scenario suggested - you aren't using it for precise measurements, for instance. Is this a 'safety of life' issue or a way of keeping yourself isolated from ' room air'? The cost would relate to the answer to that question.

    Basically, I would say that there isn't an easy DIY solution to this.
    I see the solution in two parts: a regulated power supply (DC our would be cheaper) and a UPS. A whole range of UPS units exist from around £200 upwards and you would have to decide what down time it would need to cover. The regulated power supply is harder as it's not an everyday 'consumer' requirement.

    There is one scary solution and that would be to have ten 12V lead acid batteries in series, each one connected to its own charger. The chargers would need to be, say, 5A continuous rated and the batteries would need to have whatever capacity (in AHr) that you need to cover supply outages. The batteries wouldn't be cheap but the chargers could be ordinary auto battery chargers. You would need fuses all over the place and a well ventilated room to keep it all in but it would achieve a good, constant supply voltage for your motor and the UPS facility all in one go.
     
  13. Jun 25, 2012 #12

    jim hardy

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    Have you tried an incandescent light dimmer ? Like goes in a wall for ceiling lamp?

    I get a double outlet box, plastic from hardware store, and mount a dimmer in one side and an outlet in other side, nail box to a plank.. Wire dimmer to control one side of outlet. That'll control speed of a cheap old single speed electric drill which is a universal motor.. It'll give more constant voltage than does a rheostat. It'll run cooler too.

    old jim
     
  14. Jun 26, 2012 #13

    sophiecentaur

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    Great solution, Jim. Time was when you could get speed controllers with mains in and out connectors already fitted but now they're all inside the tools.
    She still has the problem of a UPS, though. No cheap solution there, I fear.
     
  15. Jun 28, 2012 #14
    I get sick for several days when the turbine fails (allergies). But it's not life-threatening.

    Is there a good speed sensor for a universal motor?

    The utility voltage is pretty good - the engineer I talked to said it's usually between 120 and 124 V. Perhaps since the input voltage is already good, a less expensive voltage regulator would be needed to make it exact within 1%.

    There are slight symptoms of resistance in the neutral to ground connection in my house, and if it's reasonable to eliminate that, the line voltage would be more constant. When I turn on my 1 KW microwave, there's a 1.1 V neutral to ground voltage at the outlet that the microwave is plugged into. Also, the voltage to the turbine increases by about 1 V. And, the voltage at the side the turbine is on, is usually about 4 V higher than the voltage at the microwave side.

    The turbine draws max 9 A, and dimmer switches are 5 A max. I decided to use the rheostat that comes with the turbine, as a remote control, because of problems like this.

    Laura
     
  16. Jun 28, 2012 #15

    sophiecentaur

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    Imho the battery solution could be good for you because you could even buy a cheap lab bench supply unit for the lowest step in the 'Voltage ladder' and have, say, a continuously variable, stable 0-20V (say) range of control.

    The neutral volts would not be relevant in a system like that and many bench supplies have an external sense, which could easily be arranged to look at the 120V line.
     
  17. Jun 28, 2012 #16

    jim hardy

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  18. Jun 29, 2012 #17

    sophiecentaur

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    They are usually open loop control, aren't they so how would you use it in a regulator circuit, Jim?
     
  19. Jun 29, 2012 #18

    jim hardy

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    indeed they're primitive. My thought was since it controls firing angle it would make a stable voltage source .

    Poking around i found bigger 'lamp dimmers' selling to home shop enthusiasts for router speed controls.
    I think this 10 amp one is $39. A bit pricey yet just for an experiment.
    http://www.wolfautomation.com/assets/15/ac03.pdf [Broken]


    To close the loop is a significant jump in effort.

    Of course nowadays there's IC's made for that, here's an analog one and a microcontroller based one.
    http://www.onsemi.com/pub_link/Collateral/TDA1085C-D.PDF
    http://www.mouser.com/catalog/specsheets/stevalill004v1.pdf

    Gee- today's hobbyists have a cornucopia of gadgets !
    Probably there's something on the hobbyist market based on these IC's just i haven't found it yet.

    Closing loop on a lamp dimmer would be interesting challenge - but for this application one should stay with ready made parts , enclosed and safe.



    old jim
     
    Last edited by a moderator: May 6, 2017
  20. Jul 1, 2012 #19

    jim hardy

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    aha lamp dimmer with IR remote
    so control is not connected to mains
     
  21. Jul 2, 2012 #20

    sophiecentaur

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    Good thinking Batman. Pass the soldering iron. ;-)
     
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