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slowing down single phase motor

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  • J Tiers
    replied
    If you run a given motor at double speed, with double voltage, you get double power, IF the motor is good for the volts. In US, a 230/460 motor would be OK for 460 if set for 230 and run 460V/120Hz. Same volts/Hz.

    If you had a 4 pole motor and did that, it would run at the speed of a 60 Hz 2 pole and have double power for same size..... but that isn't the OP's question.


    But this does not vary speed, and if you could, you'd have a VFD, so what's the point of the comment about poles and so forth?????

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  • wmgeorge
    replied
    Huh and huh. All the guy wants is a practical way to vary the motor speed on his drill press/milling machine whatever. Take the single phase motor off, replace with a 3 phase motor, used ones are cheap. Put on a VFD that can take a single phase input and output the 3 phase he needs. Job done. Been there and done that.

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  • dian
    replied
    huh:

    "But the benefit of a higher pole count is that the drive frequency can be increased, and more power obtained at the same speed."

    i probably should go to bed.

    if the frequency increases, the speed increases, i thought. but how can that deliver more power (torque) at the same speed?

    or are you saying you can overdrive the motor more as the pole count increases?

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  • PStechPaul
    replied
    A two pole motor develops torque according to the tangential force exerted by the misalignment (or slip) of the rotor with the rotating field of the stator. This is a vector sum of all the magnetic fields along the gap, and since there is essentially one electromagnet with a N and S pole, the force will be greatest at the center and then diminish to nearly zero at perhaps +/- 45 physical degrees. So you have one effective torque-producing sector that covers perhaps 25% of the circumference (2 * 45 / 360).

    A four pole motor has two pole pairs, each of which have perhaps a +/- 30 physical degrees of magnetic field. But there are two pairs, so with the same current in each pair, the torque may be greater, perhaps 4 * 30 / 360 or 33%. An 8 pole motor might get 8 * 20 / 360 or 44%. The power depends on the speed, so with 50% more torque but half the speed, the power is about 75%, which explains why my 4 pole motor is 1.5 HP compared to 2 HP for the 2 pole.

    But the benefit of a higher pole count is that the drive frequency can be increased, and more power obtained at the same speed. You can double the power of a two pole motor by doubling the current for more torque, but resistive losses are quadrupled, so a 90% efficient (10% losses) motor will have 40% copper losses and 60% efficiency, requiring a 25% duty cycle. But magnetic losses are more like a square root function of frequency, so 10% losses at 60 Hz and 1800 RPM might be 14% at 120 Hz and 3600 RPM.

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  • J Tiers
    replied
    Originally posted by PStechPaul View Post
    For smaller motors there is also a trade-off of power versus torque versus size. I have three motors of approximately the same physical size and weight, but the two pole is 2 HP, the four pole is 1.5 HP, and the eight pole is 1 HP. I have not found exactly why this is so, but I think it has to do with winding efficiency and overlap of adjacent poles.
    ..............
    But another possibility might be remove the PSC and run the third phase to that winding. I think the capacitor does not provide a full 90 degree phase shift because of the resistive series element of the winding to which it is connected. It is probably about 80 degrees at maximum torque (at which point the winding voltage will be low), and then closer to 60 degrees, or less, when the load is very light.
    The diameter of rotor, combined with the possible current in the pole windings will limit torque.... you might get more torque, but would overheat the windings. Larger diameter makes same pull produce more torque....

    The motor will have it's "physical phase" (winding position) aligned with the electrical phase. Giving it a different electrical phase may not work well

    Leave a comment:


  • jlevie
    replied
    Originally posted by dian View Post
    of course if its a futile attempt, i would like to be told. as well about the torque issue. if its only going to slow down the motor with constant torque, its useless. also the drill press is in a location, where i cant easyly connect it to 3 phase power.
    If, as it sounds, you are looking for torque to increase along with a slower speed, you won't get that by just changing motor speed. At best you could hope for the same torque at the lower speed.

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  • PStechPaul
    replied
    For smaller motors there is also a trade-off of power versus torque versus size. I have three motors of approximately the same physical size and weight, but the two pole is 2 HP, the four pole is 1.5 HP, and the eight pole is 1 HP. I have not found exactly why this is so, but I think it has to do with winding efficiency and overlap of adjacent poles. But another possibility is that higher pole count motors have higher torque and thus the frame and shaft need to be stronger. I have rewound several motors for the highest possible pole count, and for a 24 slot stator it is four poles and for a 36 slot stator it is six poles. I also found that when I reversed one of the phase windings the motor ran at half speed, but with very little torque. I think this was due to the "salient pole" phenomenon, or due to the fact that each pole of the stator has a phase angle and amplitude determined by the vector sum of the phase currents flowing in each adjacent slot.

    BTW, a three phase VFD might be able to run a small single phase motor on just two of its phases, although it might kick out due to sensing phase imbalance. So a very small motor on a large VFD might be OK, especially (perhaps) if you added some load to the remaining phases. Or possibly put a 240/120 autotransformer across A and C, and connect the motor from the center tap to phase B.

    But another possibility might be remove the PSC and run the third phase to that winding. I think the capacitor does not provide a full 90 degree phase shift because of the resistive series element of the winding to which it is connected. It is probably about 80 degrees at maximum torque (at which point the winding voltage will be low), and then closer to 60 degrees, or less, when the load is very light.

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  • gvasale
    replied
    Thanks for the explanation.

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  • vincemulhollon
    replied
    Originally posted by gvasale View Post
    Question then: What's different about a single phase motor that runs at 1125 rpm? Most are 1725 or 3450...
    rpm = 120 * hz / poles so you've got a six pole motor there.

    You could wind a 10 pole or 12 pole motor if you wanted. Of course at some point it becomes simpler or at least more economical to use a gearbox if you want lower RPMs.

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  • rythmnbls
    replied
    An 1125 rpm motor is wound with 6 poles, the other speeds are 4 and 2 pole respectively.

    Steve.

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  • gvasale
    replied
    Question then: What's different about a single phase motor that runs at 1125 rpm? Most are 1725 or 3450...

    Leave a comment:


  • J Tiers
    replied
    The VFDs specifically mentioned are single phase output. But ONLY for "PSC" and shaded pole motors. NOT for split phase types. While they might work on a split phase motor that is already spinning, only the PSC and shaded pole can be started by the VFD.

    As for torque, it may depend on the motor. But torque can be the same, or even more.

    Many 2 speed motors have lower current draw in slow speed mode, which will certainly make the power less, and possibly the torque less as well. Power will depend on the speed and torque, so at constant torque, the power will reduce in proportion to speed. If the motor has lower power by more than the speed proportion, then the torque must be lower as well.

    Leave a comment:


  • vincemulhollon
    replied
    Originally posted by dian View Post
    the drill press is in a location, where i cant easyly connect it to 3 phase power.
    "most" little VFDs are normal 1 phase in, 3 phase out for just the kind of thing you're trying to do. Be careful as there are some 3ph in / 3 ph out models out there, sometimes the different is only 1 digit in the model number.

    Totally respect the educational purpose, but go into it with the idea that you'll probably be accepting scrap price for the old motors and doing the VFD thing in the end.

    I was recently surprised to discover Hitachi is now selling a 1/2 HP 220 1ph in, 220 3ph out vfd for a bit over a hundred bucks brand new. I do believe that we're probably going to see plain old cordless drills and plain old woodworking tools with little full featured VFDs inside them, relatively soon.

    Forest's answer is correct that used VFD of unknown problems and unknown model and no manual and no support is cheap, but it might be a overwhelming place to start, especially since a known working, new one with manuals and a support phone # and web sites is not much more money.

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  • SGW
    replied
    You will likely run into problems with the starting circuit. I think most single phase motors rely on centrifugal force to disengage the starting circuit at around 75% of full rpm. If you reduce the motor speed by 50% the starting circuit will never disengage. So you could reduce the strength of the springs by 50%, I suppose....

    If you want to experiment, have at it. Tinkering is a most worthy way to occupy one's time. If you want to solve your drill press speed problem though, the two ideas suggested -- a center jackshaft with reduction pulleys, or a 3-phase motor with a VFD to convert single-phase to 3-phase -- are eminently more practical.

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  • dian
    replied
    well as i have two identical motors (4 pole) i dont need, i thought i would sacrifice one of them for my education.

    unfortunately there is nowhere to go "scrounging" in this part of the world, no surplus stores or techticalyy oriented fleemarkets. the local ebay is full of crap at prices higher than if you go and buy it new.

    of course if its a futile attempt, i would like to be told. as well about the torque issue. if its only going to slow down the motor with constant torque, its useless. also the drill press is in a location, where i cant easyly connect it to 3 phase power.

    Leave a comment:

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