Band saw with DC motor project

Discussion in 'The Projects Forum' started by Quasar71, Jan 30, 2013.

  1. Quasar71

    Thread Starter New Member

    Jan 30, 2013
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    Hello all,

    This is my first post, sorry for the length of it but I wanted to provide what I hope is a detailed and clear explanation of my situation. I have a project I’m working on which is beginning to exceed my paltry IC/electrical knowledge. While I’m fairly tech savvy and have built numerous PCs over the years, I’ve never had to deal with the circuitry itself – have never soldered circuitry onto boards, for example. I’d really appreciate any feedback or suggestions…thanks in advance…

    Anyway, here goes:

    QUESTIONS/CONCERNS:

    I’ll put the questions first for those less inclined to read lengthy posts – but please continue on to understand the context of the questions.

    1. The specs on the LM2577s I’ve been looking at say 3-34 volt input, but I’ve also read on other forums that it’s closer to 3.5 volt minimum for these boards. Is that true? If so, it won’t work for my 3v input voltage.

    2. Amperage – the LM2577s says “Output Current: 2.5 amp max” (or 3, depending on model) – which makes me think the amperage is adjustable, but from what I can tell, only the voltage has an adjustment potentiometer. How could I adjust the amps? – I’d need to keep it to 0.9 max for the fan per the specs.

    3. Do I need anything else? Perhaps something to regulate the output voltage and amperage and keep it within spec? One ad suggests putting a fuse at the input point- do I need to do that? “Short circuit protection: None (Please Install fuses and protection circuits at Input port.)”

    4. Also, some ads say “Short circuit protection: current limiting, since the recovery” – What does “since the recovery” mean?

    BACKGROUND:
    I have a benchtop bandsaw that had an AC motor, which I quickly discovered was never meant to cut metal, which is what I needed it for. I didn’t want the larger footprint of a horizontal metal cutting bandsaw since I only have a small shed for a workshop. So a quick Google search turned up numerous tutorials in other online forums of others in my situation who swapped out the AC with DC motors from old treadmills, giving the ability to adjust the speed and slow it down enough to even cut steel plate. So, I’ve done the same. All has gone well and I’ve tested the saw which now has the DC motor along with the transformer, potentiometer and a small IC board which appears to be a step down converter – all appears to be working well - at least the saw and light operate.

    The only thing left to do is wire in a 12v DC 80mm 80cfm PC fan which I’ve had to custom mount on the opposite end of where the original pulley wheel/fan had been on the motor shaft (had to remove original fan because it was in the exact spot where the motor needed to mount to the saw). Here are the specs of the motor and fan:

    Motor: Permanent Magnet DC, 1.5HP @ 100v, 14 amps.

    Fan: 80mm 80cfm Delta (model FFB0812EHE) 12v DC rated (output voltage range: 7.0 to 13.8), input current: 0.9 amp, input power 10.8 watt

    ISSUE:
    As seen in the specs, the motor puts out 100 volts and 14 amps so the voltage and amperage need to be stepped way down. Fortunately, the saw already has a step down (Buck?) converter for the on-board LED work light. According to my multimeter, approximately 3 volts is coming out the end of the board going to the light (I’d been hoping for 12), and 0.2 amps.

    So, what I’m thinking is to install an LM2577 Boost converter (one of those 3-34V to 4-35V adjustable ones from China, perhaps? – the price is certainly right at a couple bucks on eBay) and placing it after the existing step down board, boosting the voltage from 3 up to 12 volts, amperage from 0.2 to 0.9 and wiring it out to the fan. Please see above for questions.
     
  2. Kermit2

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    Feb 5, 2010
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  3. Quasar71

    Thread Starter New Member

    Jan 30, 2013
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    Thank you for your response. I've purchased a vertical benchtop bandsaw (one of those $100 Skil models found at Lowes) and, as described in my post, have replaced the AC motor with a DC. It is working well - just need to wire in a fan to cool the motor. My post's questions center around the use of an LM2577 Boost (step up) converter and it's appropriateness of usage given the specs listed in the post.
     
  4. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    My apologies for not reading your post closely enough...

    The problem you will have is trying to draw almost 1 whole amp from the step down which is only providing .2 amps

    Are you planning on using this voltage for the saw motor, or the fan motor?


    If its for the fan, then how about something like this instead... http://www.skycraftsurplus.com/3-18metalmuffinfan-120vac.aspx
     
  5. Quasar71

    Thread Starter New Member

    Jan 30, 2013
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    For the fan.

    That's an interesting link you posted - I didn't realize they made small fans that ran on 100 volts. I'm keeping that in mind as a possible option. The only downside I see is that the fan I have is 80cfm vs this one's 19cfm. The author of the tutorial that inspired my idea of doing this DC motor conversion said he first used a lower air flow fan but found it necessary to change up to the same 80cfm fan I have. He, though, simply used a wallwart. That's another route I'm considering doing - perhaps I could modify one of those universal adapters for my use.

    An option I initially considered was using a step down to drop from 100v to 12v (and 14 amps to .9), but it was nearly impossible to find anything that had that degree of voltage range. The only thing I found was for a golf cart.

    Thank you again for your response.
     
  6. tubeguy

    Well-Known Member

    Nov 3, 2012
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    You have the correct idea as far as having to drop the voltage, but current is a different matter.
    Current is drawn by devices, not supplied as voltage is. In your house the different wattage light bulbs all receive 120 VAC (if in the USA), but each bulb draws only the current it requires. A 12 VDC regulator with a 2.5 amp maximum can supply 12 VDC at UP TO 2.5 amps.

    But it can also supply 12VDC at 1 amp or even much less current. It depends on the current drawn by the device(s)/components(s) connected to it.
     
    Last edited: Jan 31, 2013
  7. Quasar71

    Thread Starter New Member

    Jan 30, 2013
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    Thanks Tubeguy! Didn't know that...guess I'm finally starting to learn some electrical basics...:D
     
  8. Quasar71

    Thread Starter New Member

    Jan 30, 2013
    17
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    Hello all,

    Here's an update on my project. After taking your comments into consideration, reviewing my options and discussing the matter with some "techies", I decided to go with a 12v AC/DC adapter (it basically looks like the kind laptops use) - it's now set up with the fan, tested and is working fine. After doing that, however, I thought to myself that it would be nice to only have 1 ac cord to plug into the wall instead of 2, so I'm planning on splicing the ac cord of the adapter into the ac cord of the motor.

    In addition, I got another idea: With the existing set-up, the fan will run as long as the ac cord is plugged into the outlet. It would be nice for it to only come on when it's needed - when the motor is running. Instead of adding another manual switch to have to remember to turn on/off, I started researching the use of a relay to trigger the fan circuit only when the motor was on. I tracked down the point where power was controlled via the potentiometer knob - it's the point where the 2 wires from the motor attach to the PWM motor controller board. Using my multimeter, I was able to chart out the voltage and current at that interface point under 4 motor operating conditions: stopped, startup, low speed and high. I've attached the chart of my findings along with my crude, non-schematic drawing of my plan (I'm such a beginner that I had to Google what it meant to test DC current "in series"! LOL). As can be seen from the table, while stopped, the interface point voltage is at its peak of nearly 10.5 volts, while the amps are 0. The 2 reverse as the motor is sped up.

    As a side note, I measured the + (red) wire first - began making the same current measurements in series of the - (black) side when the test lead slipped and I managed to burn out my PWM board - sometimes I learn the hard way! :mad: Bought another one on eBay for $40.

    So now, mishaps aside, I'm trying to find the appropriate device that will trigger (close) the circuit when the voltage drops below 10 volts or so.....or....one that will trigger (close) the circuit when the current goes above say 1.9 or 2 amps. Undervoltage relay? Current sensing relay? I'm not sure what would best meet my requirements.

    One in particular I looked at is a KBAP-240D "Current Sensing Relay and Overload Protector" (http://www.kbelectronics.com/data_sheets/kbap240d.pdf). My question with it is this: When it trips, is it opening or closing the circuit? It has a 2 amp trip (on up to 20 amps) but I'd need it to close (complete) the circuit - I have a hunch that it's opening the circuit when it trips instead.

    Does anyone know of other devices that would perform this function? Any comments or opinions of my plan? Thank you again for your help and continuing feedback.
     
    Last edited: Feb 6, 2013
  9. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Why not just wire the adapter to the AC switch that turns on the PWM board? They would turn on at the same time then. I'm talking about the AC side of the adapter not the DC out put.
     
  10. Quasar71

    Thread Starter New Member

    Jan 30, 2013
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    Thank you for taking the time to respond to my post. There isn't an AC switch that turns on the PWM board - simply plugging the motor's ac cord into the outlet supplies 10.45 volts to the board (at the interface point where I took the voltage measurements) - when the motor is not running. In other words, as long as the cord is plugged into the wall outlet, the PWM board has power. The potentiometer controls the motor speed and turns it on/off, but there is always power at the PWM board. I only want the fan to come on when the motor is running.

    Regarding the 2 ac cords (1 = motor, 1 = ac/dc adapter) I do plan on splicing them together so there is only 1 to plug into an outlet.
     
    Last edited: Feb 6, 2013
  11. Kermit2

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    Feb 5, 2010
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    you could add a simple small switch. The switch might be put where it is activated by the movement of the saw. Say, when you lift up the moveable portion of the saw to operate it the switch operates, and then opens again when the saw is lowered to its rest position.

    This would require a normally OPEN switch, so the power would be OFF when the switch is pressed down and ON when the switch is opened.
     
  12. Quasar71

    Thread Starter New Member

    Jan 30, 2013
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    That's an interesting idea. The moving parts would be the 2 guide wheels that drive the blade and the motor shaft & pulley wheel - (see photo here for interior view). Could you please expand on your idea? I'd have to figure the best placement for the switch in relation to the aforementioned moving parts (and it's a tight fit inside the narrow body of the saw). What is the term for that kind of switch?
     
  13. Kermit2

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    Feb 5, 2010
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    I am sorry sir.

    Nothing I have suggested has been of any help to you.

    Again, in this case, I have had a different 'picture' in my mind of your saw. Without a moving axis on which to pivot the rotating blade, my switch idea will not work. The saw in this case remains stationary and my suggestion to have the fan switched when it is raised is obviously out of line after viewing the photo of your saw.
     
  14. Quasar71

    Thread Starter New Member

    Jan 30, 2013
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    Nonetheless, thank you for taking the time to read and comment on my post anyway Kermit. I should have posted a picture from the beginning - it would have made things clearer.
     
  15. Metalmann

    Active Member

    Dec 8, 2012
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    I think Kermit thought you were talking about a horizontal bandsaw, as was I;

    Why can't you use a toggle switch?
     
  16. tubeguy

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    Nov 3, 2012
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  17. shortbus

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    You really should have an on/off switch before the PWM board. If not to keep the PWM from being powered all the time, but for safety reasons. How many machine tools do you see that don't have a switch? Counting on just the plug is not very safe.
     
  18. Quasar71

    Thread Starter New Member

    Jan 30, 2013
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    Hi all,

    Sorry for delayed response but have been tied up with other pressing issues. Thanks again to all those who took the time to post their responses.
     
  19. Quasar71

    Thread Starter New Member

    Jan 30, 2013
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    Hi Metalmann, I could, but would rather have the fan control be automatic so there is one less switch to have to remember to turn on or off.

    "Instead of adding another manual switch to have to remember to turn on/off, I started researching the use of a relay to trigger the fan circuit only when the motor was on." (quoting my post)
     
    Metalmann likes this.
  20. Quasar71

    Thread Starter New Member

    Jan 30, 2013
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    That's a possibility....I'll investigate further & look for switches having the appropriate temp range. Thanks
     
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