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Thread: Club's "mustard" doesn't cut it anymore....

  1. #1
    Join Date
    May 2007
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    Orem, Utah
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    Club's "mustard" doesn't cut it anymore....

    At our last turning club meeting, our esteemed president was slated to demo some basic bowl turning techniques. He put a good-sized chunk of green wood on the club's Powermatic 3520A, at a low speed, and started roughing it down to round.

    Whenever he "leaned into it" the lathe would spin WAY down and all the torque would disappear. If he "let up" it would eventually speed back up again ... but after 3 or 4 repeats in even fewer minutes, he threw in the towel.

    I was filming from one end of the lathe, so I couldn't see the digital readout on the controller box, but someone in the audience mentioned to me later that each time it would show what appeared to be an "overload" indicator.

    Anyway, the 220 plugs were bared, inspected and reassembled; the controller was cracked open and peeked into; the circuit breaker was thrown and reset ... all to no avail.

    One of the club members said that his PM had acted that way before. At some point in the power cord / connection he found that one of the wires was only attached by a few strands. He figured that when a load was introduced the supply couldn't satisfy the demand and the controller shut things down.

    Does that sound like a reasonable explanation? The "quickie" inspection didn't find anything, but perhaps a more thorough one will. Anything special to look out for?

    Our poor president; he had to do the rest of the demo using a white board, a couple of pre-prepared bowls at different stages of turning (whew!) and some hand-waving. I dare say it won't be one of the more-requested club videos....

  2. #2
    Join Date
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    I'm not a VFD expert but the possibility of having the wires worn so that only a couple of strands are attached does not seem likely to me. To greatly reduce the speed a lot of voltage would have to be dropped in those few strands of wire. That voltage drop and the amount of current flowing into the motor would generate a lot of heat and probably melt the wires. Then the motor wouldn't run at all.

    I think most VFD's anticipate the speed/torque curve and make modifications to the voltage/current when it detects that more of a load is placed on the motor. When a load is placed on the motor, the motor has to generate more power to maintain the speed (or close to the speed). To generate more power, the motor has to take more power from the line. The VFD should detect the increased current flow or the voltage drop (depending on how the VFD does its control) and make adjustments to supply more power to the motor. The motor will slow down because there will be a greater slip but the slow-down should be fairly small.

    So - to give you a short answer, my guess is that you have a problem in the VFD. But I'm not a VFD expert by a long shot.

    Mike

    [and as Ken suggest below, certainly check the wires just in case. It's quick, easy, and cheap to do.]
    Last edited by Mike Henderson; 01-11-2008 at 05:21 AM.
    Ancora imparo
    Go into the world and do well. But more importantly, go into the world and do good.

  3. #3
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    Thanks for the feedback, Mike. I think "bad VFD" is the most likely answer, too.

    I'm sure the president is looking into it ... just thought I'd "go fish" in case this had happened to someone here, and they could speak from (sad) personal experience.

  4. #4
    If a power cable is reduced to just a few strands from what is normally hundreds.....It will in fact, limit the current and cause the motor to have limiited power and it would probably slow down under high torque conditions. Believe me, in nearly 40 years of working in electronics and electro-mechanics I've trouble shot those problems.
    Ken
    ------



  5. #5
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    Most VFD have an "Error Code" type of thing on them, that you can look up in the manual, was there an error code?

    If you do go to replace the VFD, get a Mitsubishi, they are far and away the best out there, also, I'd suggest, if possible, mounting the VFD on a wall, not too close to the lathe, the vibration etc from a lathe can, possibly, cause real trouble with a VFD, then can be somewhat fragile. You seldom see VFD in industry mounted on the machine they are controlling, they are usually a short distance from the machine.

    Just some thoughts.
    The pessimist complains about the wind; the optimist expects it to change; the realist adjusts the sails.
    William Arthur Ward

  6. #6
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    Kerry - I went and reviewed my motors book - the section on inverter fed induction motors. I was generally wrong in my earlier post.

    However, you could have a problem with your VFD only on low speed operation. According to the book, most VFDs use a voltage boost at low frequencies (low speed) and if that is not working the motor will not have much torque at low speed.

    You can test for this by doing some work on the lathe at "normal" speed. If the motor doesn't slow down (much) when loaded at normal speed, the voltage boost at low speed is probably the issue. If it does slow down when loaded at normal speed it's something else - maybe that wiring problem.

    Mike
    Ancora imparo
    Go into the world and do well. But more importantly, go into the world and do good.

  7. #7
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    Quote Originally Posted by Ken Fitzgerald View Post
    If a power cable is reduced to just a few strands from what is normally hundreds.....It will in fact, limit the current and cause the motor to have limiited power and it would probably slow down under high torque conditions. Believe me, in nearly 40 years of working in electronics and electro-mechanics I've trouble shot those problems.
    Cool - the "voice of experience". Thanks, Ken.



    Quote Originally Posted by Stuart Ablett View Post
    Most VFD have an "Error Code" type of thing on them, that you can look up in the manual, was there an error code?
    I won't swear to any given code ('cause I didn't see it myself) but I thought the person said it was "OL". Time to look for an online 3250A manual....



    Quote Originally Posted by Stuart Ablett View Post
    I'd suggest, if possible, mounting the VFD on a wall, not too close to the lathe, the vibration etc from a lathe can, possibly, cause real trouble with a VFD, then can be somewhat fragile.
    Thanks Stu, that's good info. Unfortunately...

    ...this lathe "lives" in a workroom / showroom at a hardwood supplier. Our club usually meets there every other month; the locations for the other months are spread all over the Wasatch Front. I had no idea until Tuesday night that the lathe even belonged to the club, but the former president was talking about how the club had paid to have a special single-phase 220 outlet put in, etc. (It's marked "LATHE ONLY".)

    I thought he even said that the VFD has been replaced before, and I wouldn't doubt that vibration was the cause ... the lathe gets picked up by fork lift (or something) and moved to the "demo spot" on club meeting nights. It's not necessarily the best spot for the lathe, but it's the only place that works for the 50-60 person audience.

    Still, all the more reason to put the VFD on its own stand or something, eh? I'll try to bring that up with the president, and I'll pass on your suggestion about getting a Hitachi unit in case it turns out that the VFD is indeed "blown".

  8. #8
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    PS - I found [an online manual] and learned some interesting things.

    First, the "box" with the readout display (item 21 on the diagram on page 20) is described as Inverter (Programmed 3520A) on page 18.


    Second, the TROUBLE-SHOOTING section on page 17 includes the following info:

    Code:
    PROBLEM                    POSSIBLE CAUSE                 SOLUTION
    ==============================================================================
    [...]
    
    Motor or spindle stalls.   1. Excessive cut.              1. Reduce cut depth.
                               2. Defective motor.            2. Replace motor.
                               3. Excessive belt wear.        3. Replace belt.
                               4. Improper belt adjustment.   4. Readjust belt.
    
    [...]
    
    Motor fails to develop     1. Power line overloaded.      1. Correct overload
    full power.                                                  condition.
                               2. Undersize wires in supply   2. Increase supply
                                                                 wire size.
                               3. Low voltage.                3. Request voltage
                                                                 check from power
                                                                 company and
                                                                 correct low voltage
                                                                 condition.
                               4. Defective motor.            4. Replace motor.
                                  system.
    
    [...]
    
    Lathe runs at one speed.   1. Electronic AC inverter      1. Replace AC 
                                  defective, not programmed      inverter,
                                  properly or loose wiring.      reprogram, or
                                                                 check wiring.
    
    [...]
    The only mention of the inverter is in relation to the lathe being stuck in one speed.


    Third, no mention in the whole manual about any codes or what they might mean.


    Fourth, the electrical schematics for all three model variants (pages 28, 30 and 31) indicate that the lathe can be configured for 3-phase power. Hmmm, I'll have to mention that.....
    Last edited by Kerry Burton; 01-11-2008 at 05:20 PM.

  9. #9
    That is a nice piece of equipment your club has, the first thing that came to mind was.... Why not contact Powermatic with the problem?

    As with my car, I prefer to have someone look it over who is knowledgeable of the beast. Yes I have done all the motor works from rebuilds to repairs but with the new gismos on newer autos I haven't a clue. Same goes with your Lathe, It is obviously beyond your membership's knowledge base, so rather than screw around, go to the source and get the correct answers.

    Not saying less of your club members (I do not intend to belittle anyone) but they or this forum is NOT the place to seek help. Although, I guess if one of the forum had a similar situation he/she might know where to start.

    IMHO Take what info you already have and present it to Powermatic for an answer

  10. #10
    Join Date
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    Quote Originally Posted by Bill Simpson View Post
    That is a nice piece of equipment your club has, the first thing that came to mind was.... Why not contact Powermatic with the problem?
    You're absolutely right. I'm totally "out of the loop", so I don't know what avenue(s) the club officers have gone down at this point. My thought was to shoot an email to the pres. with info from someone who'd been through it before and had come up with a slam-dunk solution. Kinda like you said:


    Quote Originally Posted by Bill Simpson View Post
    I guess if one of the forum had a similar situation he/she might know where to start.

    So, thanks to everyone for the feedback. I'll send what I've learned to the president and hopefully hear back about what the problem turned out ot be.

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