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DC treadmill motor question about power supply

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  • darryl
    replied
    The treadmill motors I've been using will run over 7000 rpm with full rectified line voltage. If the controller can handle actual DC, which would be the bridge rectifier with a filter cap, giving around 150 vdc, the no-load rpm can go a bit higher. However, a realistic rpm under load is about 4500, so a 3 to 1 reduction would give 1500 rpm as a 'working' speed. On my lathe, that's about what I get. I seldom use the full rpm, so I could have used an even higher step-down ratio. I almost never change the belt position on the pulleys- speed control is done by applying lower voltage derived from something akin to a variac. So basically I have a three-fold increase in torque automatically, and I've never had an issue with falling power at low speeds.

    What I'm suggesting is to figure out what rpm you can live with as your high speed, then use the maximum reduction ratio you can so you can still achieve that rpm. You could pick say 1000 rpm as your high speed, then use a step-down ratio of 5 to 1- that means a 5 times increase in torque at any lower rpm. That might be enough for your needs. If you need more torque, then sacrifice your high speed and use a higher step-down ratio.

    As others have suggested, you can easily smoke the motor if you're trying to use the motor directly and have it dialed in for a very slow speed. You can always use a separate fan to keep airflow up, but you're really testing the brushes at the high current levels that would be required to get a high torque output without a reduction drive. Mostly what I see is that the brush holder area isn't designed to dissipate heat very well, and the brushes can get hotter than stink if you continually push it- and the same goes for the armature. Far better to work out the maximum reduction ratio you can live with- let the motor run faster and with less current, not pushing it too close to it's rated current.

    An interesting side note- my Unimat came with what I think they called a 100 watt motor. Series motor, wound field. When I upgraded the bed I also decided to do something with the motor, so I rebuilt it to use permanent magnets (neodymium) instead of the field windings. I changed the brushes to use more of the width of the commutator, but didn't do anything to the armature except to recondition the commutator. At this point the motor must now use direct current, so I used a bridge rectifier straight from line voltage. Originally I figured the armature and the field winding would be roughly sharing the voltage applied to the motor- now the armature is getting full rectified voltage, or roughly twice what it worked with before. My worry was arcing at the commutator, but that turned out to be ok. With the stronger magnets, the back emf is higher, and that means the motor doesn't over-speed, even with the higher input voltage. But does it ever have torque now-

    I know that if I use that torque on a semi-continuous basis, the armature will overheat. I just have to be smart enough to not cook it. The main benefit I get is that the motor rpm doesn't drop nearly so far under load as it did previous to the mod. I use the same kind of power supply I use with the larger lathe, a transformer with many secondary taps, so the spindle rpm depends on the voltage I select. And as far as using that extra torque- so far I'm still using the original pulleys with the O-ring belt, and it will just slip and squeal- a change to a toothed belt and pulleys to match will be my next upgrade to this lathe.
    Last edited by darryl; 02-15-2020, 06:29 PM.

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  • nickel-city-fab
    replied
    Wow you want to do a mag drill with it? Gonna need about 8-1 gear reduction. Every commercial drill that I have ever seen tops out at a very heavy-duty 600 RPM. And tapping would be even slower. That gear reduction would be in addition to any variable speed control. Not saying it can't be done, but it is going to require some thought and some doing.

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  • PStechPaul
    replied
    The circuit apparently expects a variable resistance to adjust phase angle (and thus voltage and speed). You should connect the wiper arm (probably center terminal) to one of the other terminals so that it functions as a rheostat (variable resistor). It would normally be connected so that a clockwise rotation increases speed. The reason for connecting the wiper to one of the ends is so that there will not be a momentary open circuit as the wiper traverses the resistive element. This was probably more important for low value wire-wound pots.

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  • john b
    replied
    Hello. At the start of my 2x72 grinder build I purchased a scr 60, bridge rectifier and a potentiometer shown in the video in post 3. Now that I am ready to hook up the potentiometer, I have a question. The potentiometer on the scr has two leads, the new one has three terminals. Which terminals do I use. Thanks John b.

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  • J Tiers
    replied
    The theoretical general curve is as you say. The background for it may be different. And that is not quite a "practical limit", but a "maximum capability". Somewhat like the specs on a vacuum cleaner, they are not ones you can actually use. How different they are depends on the motor design.

    The torque is produced by the current through the windings. Provide that current, and the torque is there. it is the "providing the current" that changes.

    At zero speed, ALL the supply voltage is available to produce current through the motor. There is NO guarantee from the theoretical curves, that the motor can stand that current for long without "letting out the smoke". The current required may be 2 or 3 times more than the rated current, giving 4x to 9x the normal heating effect, at the same time that the cooling capability is reduced due to speed reduction.

    At maximum speed, the back EMF is high enough that only enough current can flow to supply the small torque needed to overcome friction and windage.

    In between those, the torque is whatever the difference between the voltage and the back EMF can provide.

    A KEY POINT THAT MUST BE REMEMBERED: The maximum torque curve is ONLY for the condition where the rated voltage is provided, and the motor is loaded down so heavily that it slows to the speed noted. THAT CURVE DOES NOT APPLY FOR THE CASE WHERE THE VOLTAGE IS REDUCED TO SLOW THE MOTOR DOWN TO THE SPEED YOU WANT.

    The ACTUAL motor curve for continuous operation would be one where the torque increases as you move down from the maximum no-load speed, and then reaches a maximum around the rated power point. From there down to zero, the "PRACTICAL" torque is going to be limited by the capability of the motor to get rid of heat at each speed, and will probably decrease to a lower value at zero.

    Servo motors are a special case. The heat lost is resistive heating, and a servo is often made with larger wire than necessary, so that the heat loss at any current is less than typical, and may have heatsink fins etc.. That allows a higher continuous current (and so higher torque) at zero speed than most motors could take.

    Check this spec

    https://www.hansen-motor.com/pdf.php...1inDCMotor.pdf

    Notice that there is a "stall torque" and a "stall current", that are different from the specs under running conditions. If you do the "motor math" you will find out that the "stall current" is just the rated voltage / motor resistance. They do not claim that is a usable motor running condition, they just say that the motor can do that for some unspecified time as a maximum.
    Last edited by J Tiers; 02-13-2020, 11:14 AM.

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  • MaxHeadRoom
    replied
    Just about all the reputable manufacturers of DC motors I have used, publish a spec sheet that shows the continuous torque curve and the momentary operating curve.
    The continuous curve typically showing maximum at zero rpm.
    This is necessary when the motor is typically operating in a servo application and requires a constant current at zero rpm to hold a position.
    I did come across a T.M. that operated with a Universal motor and used the TDA1085 Motorola IC for control, originally intended for washing M/C motor controller, it had a 6 pulse/rev opto rev indicator back to the IC. in order to maintain the speed.
    Typically T.M. controllers have a built in acel/decel rate and cannot be fed full voltage from zero.
    Max.

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  • J Tiers
    replied
    Originally posted by MaxHeadRoom View Post
    If you use a PWM controller such as one of the T.M. MC2100 versions, they will control down to zero rpm, they normally have to control a varying load such as low belt speed and heavy user.
    A DC motor typically has maximum torque at zero RPM.
    Often the simple Triac with bridge versions as shown in the video have very little built in features.
    Max.

    The statement that "maximum torque is at zero RPM" is kinda true, but maybe not. It depends whether you are talking about the maximum it can ever produce, or the maximum it can produce steady-state, etc. And it depends on the type of motor.

    There is torque and there is power and there is rpm. Motor designs vary, and treadmills vary. But there is one constant fact.... the motor will normally put out max power at the design "base speed", the speed it goes with the design load and full voltage applied.

    A series motor does produce fantastic torque at low speed, and that is why those were always used as "traction motors". Treadmlls do not use them, typically, although some might.

    Most of the treadmill motors are permanent magnet "shunt type" motors. Those have a base speed that is set by the design. That is where they produce the most power. Torque is generally constant at any speed, because the magnet is a constant field strength, and the coil wires can only take so much current before they overheat. (That gets much worse at low speeds.....)

    Power is force x distance per unit time. So power is related to torque and rpm. If RPM is cut, so is power, unless torque can go up. There is a dramatic demo of this in the "This Old Tony" demo of the minilathe. That has all speed control via the electronics. At a low, but reasonably credible speed, he basically could not take a cut without stalling the lathe.

    And, there is the problem.... If you cut RPM in half, power is cut in half too, because with a shunt motor, torque is pretty much proportional to current. Current COULD be held constant, BUT, typically the motor has a cooling fan, and it does not cool as well at half speed. So actually, the torque you can USE goes down, unless you put an external blower on it. At zero speed, you cannot run much continuous current or it will overheat, unless you have a blower on it.

    Bottom line is that without a blower, you probably should not cut speed much below 1/3 of design base speed, if that much, if you expect to maintain torque. Yes, you can exceed that for short times, but you cannot be sure how short a time you have at any current.

    To get the sort of torque needed for tapping with larger taps, you really need to run the motor as fast as possible and if you need a slow speed, use belt or gear speed reduction as your basic method to get there. Not only is that needed to make it work, but also, that will probably give you the best speed control.

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  • MaxHeadRoom
    replied
    If you use a PWM controller such as one of the T.M. MC2100 versions, they will control down to zero rpm, they normally have to control a varying load such as low belt speed and heavy user.
    A DC motor typically has maximum torque at zero RPM.
    Often the simple Triac with bridge versions as shown in the video have very little built in features.
    Max.

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  • Dan_the_Chemist
    replied
    The thing about the treadmill controller that I frazzled is that it provided good power at all speeds, and many of them were pretty darn slow. Treadmills can go just a fraction of an mph, but it still has enough umph to move a 250 pound buffalo standing on the tread. It does have a roughly 3:1 belt ratio in it's favor, but that's all it has. That is what made me favor the idea of using a treadmill motor for low speed applications. If I can't get the low speed for power tapping then the real advantage of this project is gone along with the magic blue smoke....


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  • Tundra Twin Track
    replied
    I bought one of those Black Box Motor Controller,had no luck with it giving strong or smooth power.

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  • J Tiers
    replied
    Heh.... That ad has USA all over it, but the price says CHINA in big letters...... Might not be getting any for a while either, "china" is pretty well shut down due to the virus, so say folks who are there now.

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  • PStechPaul
    replied
    An SCR/TRIAC controller with a full wave bridge rectifier may work to some extent, but it will tend to run the motor at a speed determined by the phase angle, especially under light load. It will work better when under a constant load. Another way to get better control would be by adding an inductor in series with the motor, but then you may have problems with inductive transients. Best way is true PWM, like this:

    https://www.ebay.com/itm/6V-90V-15A-...P/282704941288

    Click image for larger version

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  • mattthemuppet
    replied
    yeah, you'll need a crazy azz amount of belt reduction to get down to tapping speeds. I tap on my drill press alot and have (I think) an 8:1 reduction via a countershaft with the speed dialed down to just above stalling. That's mostly for small stuff 1/4-20 and below, as I don't have a dedicated tap holder and the taps will spin in the chuck (I know, I know) for bigger stuff.

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  • J Tiers
    replied
    Those are generally high rpm, you will need a belt reduction. I'd do as much belt reduction as possible, because torque and power suffer as you reduce motor rpm. You want to keep the motor at as high a speed as possible, and just have the spindle slow.

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  • Dan_the_Chemist
    replied
    This will be the motor for my magnetic drill project. I want to use it for both drilling the holes and then machine tapping, so I need to have high and low speeds. The high speeds would range over those speeds appropriate for the tap drills for 10-32 to 1/2-13, although I will be using mostly 1/4, 5/16 and 3/8. The low speed just should be low enough for machine tapping. I figure I will use a DPDT switch to reverse the drill to remove the tap.

    So I don't think the RPM has to be very accurate - just somewhat in the right ball park for the large and small drill sizes, and much slower for tapping.

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