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  1. #1
    la vache fantôme phantomcow2's Avatar
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    Will an electrically savvy person help me out?

    I am having a problem with a DC motor controller I have. There is no schematic available that I can see.
    It requires a 5k potentiometer (which I bought from Radioshack). 120VAC input, and 110vDC output. There are 4 LED's on the circuit board:
    AC voltage
    SCR Trig
    Cur Lim
    Sped Cntrl

    AC voltage LED is always on, so I assume this means the presence of AC voltage. SCR trig is on as long as the motor is. Sped Cntrl lit up when the Pot was wired up properly, so I know that is good. Cur Lim is current limiting, only activated when the motor is trying to draw over the rated 11amps of the controller.

    Now here is the thing, I turn the shaft of the potentiometer and the motor variates its speed fine. But then about half way through the rotational travel of the shaft, the motor comes to a fairly sudden halt. Also, the SCR trig LED is no longer active. I thought maybe it was something to do with the 2HP motor I am driving, so I tried it with another 4/5HP DCmotor I have sitting around. Same thing! My thought was that maybe the motor has reached its max RPM, but that does not make sense. The max for the 4/5HP motor is 5000RPM, and max for the 2HP motor is 6750RPM.
    Not quite sure what to do here, any ideas?

  2. #2
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    I'm a little confused by your desription. You're saying the motor shaft does not
    make even one full rotaion before it stops? Or that it revs up to a certain speed
    and then stops?
    I’m not familiar precisely with exactly what I said, but I stand by what I said whatever it was.

  3. #3
    Senior Member pmseattle's Avatar
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    I, too, am a bit confused. Do you mean that as you turn the pot, the motor speeds up, then stops suddenly when you reach 1/2 of the pot's maximum resistance ?
    Do the motors have separate connections for field and armature windings ? Or are there just two wires connecting the motor to the controller ? When you run the motors, are they under load ?

  4. #4
    Footballus vita est iamlucky13's Avatar
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    First thing I'd check is the voltage coming out of the controller as you ramp up the potentiometer, although it sounds like that's going to be good then cut out, which only confirms that it's the controller. Perhaps though it has some of overvoltage protection and the potentiometer span is off. I don't know much about motor controllers so I'm pretty much just tossing stuff out there.
    "The internet is a place where absolutely nothing happens. You need to take advantage of that." ~ Strong Bad

  5. #5
    Senior Member DannoXYZ's Avatar
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    What's the slope of the pot? I suspect a Radio Shack part might have a S-shaped audio curve rather than a linear one....

  6. #6
    Senior Member nodnerb's Avatar
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    Can you figure out what the section of the circuit is that the pot is connected to, specifically relative to the scr? Is the drive circuit pulse width modulated?

  7. #7
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    Your motor controller is DC, but the motor is AC?? ....sounds like you're providing the wrong type of electrical signal to the motor.

  8. #8
    2-Cyl, 1/2 HP @ 90 RPM slvoid's Avatar
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    I may be an idiot... but if you're trying to power an AC motor, which takes in a plus minus sine wave, and power it with a DC supply, which slams it repeatedly with a square wave, won't that not work?

  9. #9
    Senior Member pmseattle's Avatar
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    Quote Originally Posted by slvoid
    I may be an idiot... but if you're trying to power an AC motor, which takes in a plus minus sine wave, and power it with a DC supply, which slams it repeatedly with a square wave, won't that not work?

    I think phantomcow means the controller uses AC as it's power source, and outputs ( rectified ) DC to a DC motor ? But he's not responding . . . I hope he did not electrocute himself.

  10. #10
    la vache fantôme phantomcow2's Avatar
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    Okay let me clarify:
    The shaft of the motor is spinning fine. It is half way through the rotation of the pot that the motor shuts down.
    I turn the knob to the pot, half way through, SCR trig led shutsdown, motor comes to a halt (it was spinning at several thousand RPM before). Motor is not under any load.
    Motor has two wires, black and red.
    No I am not trying to power an AC motor with DC. This is a real live DC motor, which has worked in the past with other DC motor controllers. The controller itself is 120vAC input, and 110vDC output.

    Controller calls for 5k Ohm pot. So I bought htis one from Radioshack:
    http://www.radioshack.com/product/in...entPage=search

    Here is the controller:
    http://www.surpluscenter.com/item.as...tname=electric

  11. #11
    la vache fantôme phantomcow2's Avatar
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    Quote Originally Posted by pmseattle
    I think phantomcow means the controller uses AC as it's power source, and outputs ( rectified ) DC to a DC motor ? But he's not responding . . . I hope he did not electrocute himself.
    You got it . I've been slow these past 2 days to reply as we have company. I feel like I dont fit into the family conversations however hard I try.

  12. #12
    Senior Member nodnerb's Avatar
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    Have you directly measured the current it is drawing at the point it cuts out? Most meters are only capable of 10A readings though so this may be difficult since the controller is capable of 11A.
    Or, measure the resistance of the motor coil while it is not attatched to the circuit, and the voltage across the motor just before cutout while connected and running, then caculate the current. Voltage at cutout divided by the coil resistance will give you the current it is seeing at that instant.

    YOu may just be hitting the 11A threshold of the controller.

  13. #13
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    Quote Originally Posted by nodnerb
    YOu may just be hitting the 11A threshold of the controller.
    It sounds that way to me too. My electronics skills are rusty, but I think 1 horsepower = 746 watts. So, a 2HP motor would draw 1492W / 110V = 13.56A of current at full power. The link you gave also said that the controller's output was only 8A.

  14. #14
    2-Cyl, 1/2 HP @ 90 RPM slvoid's Avatar
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    I may also be a bit slow... but in this case, if it's feeding dc to the motor in pulses, then the faster the pulses are, the faster the motor goes but at some point, it'll be going faster than the motor can keep up with and it might not generate enough EMF to keep pushing and it'll just stop when you turn it high enough.

  15. #15
    la vache fantôme phantomcow2's Avatar
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    Oh, my bad. Its 8A actually.
    THe thing is, I know that hitting the current limit is not an issue. FIrstly, motor says it draws under 2amps at 110vDC and no load. This happens under no load.
    Secondly, I tried it with another motor which is rated @ 3amperes.

    I am almost wondering if when it stops, it has reached the motors max RPM. I will see if I can borrow my neighbors device that tests the RPM, the name of this device escapes me now.

    I think slvoids response seems quite probable, going to ask a guy at school tomorrow.

  16. #16
    Senior Member pmseattle's Avatar
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    Quote Originally Posted by phantomcow2
    Oh, my bad. Its 8A actually. THe thing is, I know that hitting the current limit is not an issue. FIrstly, motor says it draws under 2amps at 110vDC and no load. This happens under no load. Secondly, I tried it with another motor which is rated @ 3amperes. I am almost wondering if when it stops, it has reached the motors max RPM. I will see if I can borrow my neighbors device that tests the RPM, the name of this device escapes me now. I think slvoids response seems quite probable, going to ask a guy at school tomorrow.
    Slvoid's response is an unlikely scenario; the RPM in a PWM-controlled DC motor is not controlled by the pulse rate. Slvoid is probably thinking of an AC induction motor for which the RPMs are very much affected by the frequency of the applied AC. Also, your DC motors should not draw much current running unloaded, since they are not doing any work. The DC motors probably are not running anywhere near their maximum rated RPM due to the counter EMF. When the motors stop, are they coming to a near-instantaneous halt ? As in, stopped dead in their tracks instead of spooling down ?

  17. #17
    la vache fantôme phantomcow2's Avatar
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    Well its not stopped dead in their tracks like connecting the + and - leads, but its much faster of a stop than turning off power immediately. I'de say this motor (keep in mind it has a 17mm shaft, so there is some inertia) takes about .5-.75 seconds to stop.

  18. #18
    Senior Member pmseattle's Avatar
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    Quote Originally Posted by phantomcow2
    Well its not stopped dead in their tracks like connecting the + and - leads, but its much faster of a stop than turning off power immediately. I'de say this motor (keep in mind it has a 17mm shaft, so there is some inertia) takes about .5-.75 seconds to stop.
    It sounds like the DC current source to the motor is being switched off and the armature leads are being shorted, which will exert a high braking torque. Do you have any more data on the controller than was at the sales web site ?

  19. #19
    2-Cyl, 1/2 HP @ 90 RPM slvoid's Avatar
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    Quote Originally Posted by pmseattle
    It sounds like the DC current source to the motor is being switched off and the armature leads are being shorted, which will exert a high braking torque. Do you have any more data on the controller than was at the sales web site ?
    Why would his controller start electrically braking the motor beyond a certain speed? And that's a lot of heat he's going to be dissipating.

  20. #20
    Senior Member pmseattle's Avatar
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    Quote Originally Posted by slvoid
    Why would his controller start electrically braking the motor beyond a certain speed? And that's a lot of heat he's going to be dissipating.
    I don't know why it's happening, or really even if it's happening, but that's what it sounds like from his description. The heat will come from the rotational kinetic energy of the armature and shaft which is not that much for a small unloaded motor. I work on variable speed drives that run motors up to 400 hp and move loads up to 30 tons. They generate some real heat during dynamic braking, enough to melt the resistors and their steel frame if the cooling fans fail.

  21. #21
    la vache fantôme phantomcow2's Avatar
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    No other information on the controller. I will see if I can find a schematic somewhere, but I doubt it.
    I think my motor is at a high enough RPM for my needs before it shuts down though, so it might all be okay.

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