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Thread: Don't know where to begin

  1. Don't know where to begin

    I've been tasked with a project to create a hand held remote control device (radio based) to control the following DC motor's speed and rotational direction. I accepted the task because it looks like a great learning experience.

    7.5 HP 1765 RPM 3PH MOTOR

    New, Baldor Electric model M3710T 3 phase motor. Ideal for compressors, pumps. Mount in any position.

    • Power 7.5 HP
    • Voltage 208-230/460 AC
    • Amperage 22-20.2/10.1 Amps
    • Speed 1765 RPM
    • Rotation Reversible
    • Service Factor 1.15
    • Bearings Ball

    • Enclosure TEFC
    • Duty Continuous
    • Frame 213T
    • Mount Rigid welded base
    • Shaft 1-3/8" dia. x 3-3/8" w/5/16" keyway
    • Size 14" x 9-1/2" x 10-1/2"
    • Shpg. 145 lbs.
    Since I am a novice, this is a request for some guidance to accomplish my task. Maybe I first need to buy some instructional kit that shows a direct example of steps necessary to do my task, and if so please direct me to where I need to go.
    It would seem like I'd need some microcontroller(which one?), but really I'm somewhat lost after that.
    I looked at the "Controlling DC Motors" tutorial video, but it didn't seem to fully apply to my task; no computer is used (other than a microcontroller) and it would seem that the size of this motor would need more heavy-duty controller circuitry(?!)
    Any guidance is appreciated.


  2. #2

    Re: Don't know where to begin

    Looks like you have an AC motor? The company "Baldor Electric" sells motor controllers. You might consider taking a look at their support site. I'm not sure if this motor has a controller attached.

    Since your requirement is to use a radio controller I imagine your control signal is PWM (Pulse width modulation). That's pretty common. If you have a controller you can use a standard RC system. The kind for RC airplanes. Or you could go with a servo controller.
    Last edited by MikeG; 11-09-2008 at 11:44 AM.

  3. #3
    Join Date
    Jul 2008
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    Re: Don't know where to begin

    Controlling a 3-phase AC motor at 22 amps is, to put it gently, not novice material.

    These motors can be controlled basically in the same way as brushless DC motors.

    Meaning, you need to create a three-phase sinusoidal signal, with a variable frequency.

    Personally I feel I have quite a bit of electronics experience, and I shudder at the idea of taking this on from scratch.

    I'm not trying to dissapoint you or put you down in any way, I'm merely stating the high level of difficulty involved.

    Your best bet would be, as stated before, getting a ready-made motor controller (back here there literally called frequency convertors, don't know how they're called in english though)
    Be prepared though, they're not cheap.

    One other question, since it's supposed to be remote controlled, are you going to be using batteries or will it be a hybrid (eg gas engine supplying electricity through an alternator)?
    Artificial Intelligence is no match for Natural Stupidity

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  4. #4
    Join Date
    Apr 2008
    Sacramento, CA, USA Area
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    Re: Don't know where to begin

    I think ScuD has the right of it here. I sat down for a few minutes to think through the requirements, then moved to the whiteboard for a few more minutes, then erased it and thought "Glad this isn't my cross to bear..." It's really the scale of the components involved when you're moving 20+A at high voltage - they're hard to source in onsie-twosie quantity. You also need to think about isolation - the spacing of the components, the trace sizes, where your wires route... That kind of power will arc around like mad if you get it wrong. I hate to be discouraging, but better to be realistic on the front side than the expensive back side, right?

    I'd go buy a motor controller. This is one of those projects that screams to me "if you have to ask where to start the answer is 'way before this project'" I think you're in for an expensive couple years of building-up your electronics and electrical theory training before this project can be reasonably grasped.

    On a safety note:
    I've stuck my hands in three phase 480v one times too many over the years. That stuff bites like a bad dream... A 480v panel was what led me to inactivate my electrician's license - I only needed to see one experienced electrician get blown across the room, three inch holes melted in the bottom of his rubber-soled workboots, unrevivable, to realize that playing with small DC motors was more fun. That said, I've still arc-welded steel frames with shorted wires even from 24v DC circuits fed by large batteries. Be careful when you're playing with stuff like your project. It won't hesitate to kill you dead if you make even a small mistake.
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  5. #5
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    Sep 2008
    The devil steals my soul when I go to San Bernardino
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    Re: Don't know where to begin

    Page 857 of the McMaster-Carr catalog (sorry, can't figure out how to link to their pages (stupid AJAX)). Bottom of the page is a speed controller, takes 115 or 230 VAC single phase and outputs speed-controlled 230VAC 3-phase (someone who knows more than me; is that likely variable volts, or PWM?).

    Listed with it is the Input Kit which "allows controllers to accept a 0-5 VDC, 0-10 VDC, or 4-20 mA input. The kit mounts easily over the bottom of the controllers."

    That input kit sounds like it was designed to be attached to a microcontroller.
    Earth's got chinese food and microphones.
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  6. Re: Don't know where to begin


    I am taking all your kind and sage words very seriously.

    Unfortunately, my benefactor wasn't forthcoming about the motor's usage until now (duh).
    She told me (after great deal of prying) the motor will be used to continuously drive a piston at 200 strokes per minute, and that the motor needs a minimum of 1/12 horsepower with at least 20 in-lbs of torque and should be powered by a regular 110/120V AC outlet available in the USA. I'm guessing the original motor specs (handed to me by someone probably as novice as I am!) is therefore completely wrong for that task, so I convinced her to let me select a different motor(blind leading the blind...I know).

    Now that we know the motor's intended usage, what type of motor and controller would you (SCUD and ADRENALYNN) suggest I use instead? I'm aware that maybe all I need to do is ask a motor manufacturer for an answer, but I'm looking for a "robotics perspective" to this problem (if there's any credence to having such a perpective about this task).

    Adrenalynn, thanks for the warning!! I certainly don't want to become a fried idiot!

    I told her I could take a crack at creating the RC device to control an appropriate motor. Maybe I could even use something available through Trossen(?!) ...or......maybe just realize I can't do it yet ....but I really like challenges so it's hard for me to give up trying.

    Thank you all so much for helping me thus far. If you could point me to a more practical solution, it would be great and then I'll shut up. I certainly don't want to belabor this issue, since I know, to you experts, it all may appear somewhat ridiculous.

    Thanks again.

  7. #7
    Join Date
    Apr 2008
    Sacramento, CA, USA Area
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    Re: Don't know where to begin

    Thanks for understanding the perspective!

    Knowing the desired output makes this a *much* easier prospect. The power outputs you need are fairly trivial. Stepping down a 120v AC line to something like a 24v DC motor gives us everything we need and becomes fairly easy.

    I've got a couple of 4+HP 36v scooter motors sitting here on the desk that would put out more than 20 FOOT POUNDS. They're small, weigh less than ten pounds, push about 1200 RPM, and only cost a couple hundred dollars. A $300 motor controller will drive two of them and whistle a happy tune doing it. So all you're looking at is a marginally high-current step-down transformer mated to a moderately high current DC bridge to feed the motor controller...

    The motor controllers I'm using are RS80D, 80A/channel bursting to over 120A/channel.

    I can't get the Banebots motors page at Trossen to load, but 20in-lbs is about 320oz-in... Lots of motors that will handle that. So you just have to consider how you're interfacing to the piston and then do the calculations for how much RPM you'll need.

    The 18v Hitachi drill motor/gearbox in my good commercial/construction drill does over 600oz-in at 400RPM and stalls around 35A.

    The motor in your first post was so much overkill even *I* wouldn't spec it.
    Last edited by Adrenalynn; 11-10-2008 at 02:26 AM.
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  8. Re: Don't know where to begin

    Thank you so much Adrenalynn!
    You're truly great at this.


  9. #9
    Join Date
    Sep 2006
    Carol Stream, Illinois
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    Re: Don't know where to begin

    page is now fixed

    sorry bout that.

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    - Charles Darwin

  10. #10
    Join Date
    Apr 2008
    Sacramento, CA, USA Area
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    Re: Don't know where to begin

    Thanks, Alex! Much better!

    Sam - thanks for the vote of confidence!

    Have a look at this bad-boy:

    We're talking almost 3x the torque required at 600+ RPM loaded. (assuming you don't need step-up or step-down with whatever attachment system you use to get it to the piston) - and it weighs only a couple hundred TIMES less than your first spec'd motor and only costs ~$40.

    Ok - so we have a worst-case scenario of 42A for a few seconds (stalled at 42A, never a good place to be unless you enjoy frying motors. ) We only have one channel to control, and we don't need continuous power much over 30A...

    So we find:

    This is ready to talk to an R/C receiver out of the box. And we only spent $150 for 40A continuous power, with a nice 30A breaker that should be about perfect to keep from toasting our widdle motor.

    The whole package will weigh in a bit over a pound and could be slapped in a project box - and (excluding power supply) we're spending sub-$200 which I promise is a fraction of what that first motor you linked costs...

    Then we go find ourselves a good industrial-grade power supply, might as well not skimp there right?

    The first good 12v @ 40A supply we find has overvoltage/overcurrent/over-temp protection, ISO 9001 certified, can take input anywhere from 90-264VAC auto-switch. Woot for that! How bad is it going to hurt? $120. Sign me up, twice on Tuesdays. No way I can build it for that...

    We're done. The whole project goes out the door with good quality components for around $300... (Assuming you already have the radio and receiver)

    As a rule of thumb, I don't play with motors that require a crane hook unless I have a really good reason (that's what that eye-hook is for on the motor you posted... )
    Last edited by Adrenalynn; 11-10-2008 at 07:18 PM.
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