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Modifying Line Voltage

  1. Jul 15, 2008 #1
    Let me just start by saying this is all a little above my head. I recently purchased a Bosch 1276D Belt Sander. The label rates it at 12.5 amp with an input power of 120vAC at 60hz. I've been digging for days, trying to find ways to juice this thing up a little more. I'm trying to get it rdy for my first belt sander race and will be running against lighter, and faster Makita's and DeWalts. In my research I have heard about Full Wave Bridge Rectifiers which should be able to take the standard 120Vac 60hz and take it to around 169 Vdc 120hz. Needless to say I don't know much about altering circuitry so I'm trying to find some help from people on what rating of bridge rectifier to buy and how's I go about hooking this thing up. I've also read that it's a good idea to apply capacitors to the bridge to smooth the DC output. Also, is this DC voltage going to smoke my belt sander's motor and what effect is doubling the hz going to have on the motor. The average race last between 2.5 and 3.5 secs...so I wouldn't be running it very long. But, for lining up the sanding belt and other various pre-race preparations, it would be nice to be able to switch the rectifier off when I don't need it. Any suggestions would be much appreciated.

    Don't worry, any help you offer would not be considered enabling me to cheat. Multiple racers that come to this event every year, juice there's up but aren't willing to divulge their secrets. I'm just a simple carpenter lookin for a little help. Thanks alot.
  2. jcsd
  3. Jul 15, 2008 #2


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    1) You can't simply take and AC motor and power it with a DC voltage.
    2) Trying to modify an electrical piece of equipment to function beyond its design is very dangerous, especially by inexperienced people. I would recommend that you don't attempt this for your own safety.
    3) DC voltage doesn't have a frequency associated with it other than a very small ripple - assuming it has been rectified from an AC source.

  4. Jul 15, 2008 #3


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    If you boost the voltage too much then the windings will just rip out of the armature and the thing will self destruct instantly.

    Safety concerns ignored.
    All hand power tools (like the belt sander) I've ever seen are AC/DC motors and will work on DC power. Bench tools are different and use AC motors.

    Adding a bridge rectifier alone probably won't make much difference.
    You need the capacitor (big ones at 13 amps) to boost the effective RMS voltage, since the peak to peak voltage in a 120v AC line is about 340 volts anyway.
    Something suitable in the cap line will be difficult or impossible to find.

    You are better off staying with AC and using a boost transformer or a Variac.
    A Variac is something you can buy off the shelf and just plug in.

    Rewinding the motor is another and probably a more effective option.

    How do you keep the thing from just flipping over?
  5. Jul 15, 2008 #4
    Hrm, thanks for your input and concern for my safety stewart. Perhaps the rectifier isn't the proper solution for a novice like me. Though, I'm intriuged by Notime's option for the Variac. What is it? Where do I get it? How do I hook it up? Ohh, and as for the flipping of the belt sander...that is easily remedied with the application of a wheelie bar made of angle metal and wheels from a pocket door.
  6. Jul 15, 2008 #5


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    For starters, you really should be familiar with rewiring electronics, or get somebody who knows about it to help you.

    lf the motor will run on DC, a bridge rectifier and smoothing cap might work and boost the speed. That would be a lot easier (and lighter) than adding a step-up transformer which handles well over 12.5 amps inside the sander.

    HOWEVER, you do risk burning out the motor and other electronics if you do this. Ask yourself if you're willing to ruin a perfectly good belt sander.

    p.s. The cap should be large enough to supply 12.5 amps in 1/2 cycle or 1/120 seconds:

    Q < C V, or
    C > Q/V
    C > (12.5A) x (1/120 sec) / 170 V
    C > 600 micro Farad

    These cost about $11 from DigiKey:
  7. Jul 15, 2008 #6
    Even if I'm running the sander for 2.5 to 3.5 secs I could still burn it up with a bridge and this cap?
  8. Jul 15, 2008 #7


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    I didn't realize hand tools had universal motors! Learn something new everyday!

    If that's the case then you can obviously use DC power like NoTime indicated.

    BTW here is a quote from Wiki on Universal motors just for your reading pleasure that highlights some of the pros and cons of them:

  9. Jul 15, 2008 #8
    Assuming I wanted to take the risk of blowing up my Bosch. What would you recommend for a rectifier. Belly already pointed out a cap. If I just have these things and have a general idea of what to do with them, I can take them to my wifes grandfather who is a retired boeing engineer and perhaps we can fashion them together and win some races?
  10. Jul 15, 2008 #9


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    The speed of a universal motor can be controlled with an autotransformer or a solid-state control circuit. The effect is this: Increasing the applied voltage increases the armature current, which increases the developed torque, and hence increases the speed.

    Essentially universal motors are series motors that are design to work with either DC or single-phase AC (typically 60-Hz or less). If using AC, the RMS voltage should be approximately equal to the design DC voltage. If you use higher voltages, you run the risk of burning the motor up. You may be wondering how fast. That will depend on the applied voltage and the original design of the motor. So I would just be guessing since I don't have those details. Of course not all motors are designed the same way so take this with a grain of salt as it is just an elementary explanation to give you an idea of how to proceed assuming you are will to take the risks as previously point out.

    The best alternative would be to just buy the correct size motor! I'm not sure if your rules allow it, but that is the most logical thing to do.

    BTW, how are you planning on powering this motor since it is mobile?

  11. Jul 15, 2008 #10

    The belt sanders are fed power from heavy gauge extension cords extended down the side of the track to the starting area. Where, when the belt sanders take off, the just peel the cord up from the side of the track.
  12. Jul 15, 2008 #11
    If I applied a 4 diode rectifier capable of handling up to 14 amps and 400volts (seeing as the suggestion for voltage rating should be 3 times the rms value of the power being fed to it 3x120 360....400). Then if I apply a smoothing electrolytic cap across the DC supply, my DC output should be equal to 1.4x(RMS voltage being input). So, 1.4x120VAC would give me 168 VDC roughly. Now, that might be more voltage than I'm willing to throw at my Bosch...so, between the smoothing cap and the motor, could I then install a small voltage regulator to turn it down to around 140-150? I think I'm starting to understand this a little more after days and days of research....or maybe not, I don't know. Does that sound kosh?
  13. Jul 16, 2008 #12


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    The 400 v would be an absolute minimum, with line noise and whatnot 600v is a better choice.
    Peak current is not going to be 14 amps.
    Basically, its 170v / (ESR cap + Line resistance + diode resistance).
    The three resistance values might add up to an ohm or 2.
    Diodes have two ratings, peak current and RMS current.
    So as a guess you need a diode that can handle a peak of 90 - 170 amps.
    Diodes are very sensitive to peak currents even though they last for a very short period.
    Adding a snubber coil like found in a light dimmer switch can help reduce the peak.
    The cap will increase RMS current so if you develop 170 v DC then you are looking at
    1.4x14A for that, but for 3-4 seconds run time it probably doesn't matter much.

    Redbelly computed the cap on a charge basis, but the voltage drops exponentially.
    About 60% in 1/5 of 1/120 sec, so you end up with a sawtooth output.
    So if IIRC then to get about 165v DC effective, then you need a cap around 5000uf.
    With 600uf I suspect you end up around 130v to 140v effective somewhere.
    A 600uf @ 200v is a biggish cap.
  14. Jul 16, 2008 #13
    So, in your opinion a


    P14076-ND CAP 680UF 400V ELECT TS-HC

    Wouldn't be sufficient? Or would fry? Or wouldn't produce a big voltage increase from 120VAC?

    And doing this from a heavy-gauge extension cord...am I just going to pop their GFCI? BTW, thanks to all for the comments and suggestions...it's been very informative and educational.
  15. Jul 16, 2008 #14


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    You might even get a boost with no cap, just the bridge ... this will require some experimentation on your part. I'm not familiar enough with these universal motors to know what happens with just a bridge rectifier.

    WAcarpenter, it's good to have your wife's grandfather available, two heads are better than one.

    Note to everybody: WA just needs a power boost sufficient to beat the other race entrants. A large capacitor to get the AC ripple wa-a-ay down shouldn't be necessary. Also, any modifications to be added INSIDE the sander housing. No external variable transformers allowed! And forget replacing the motor, the current motor already mounts and fits nicely inside the present sander housing. A modest power boost should not damage the motor if run just a few seconds at a time.

    So ...
    ... the main issue right now is what to use for a bridge rectifier? If we take 168 volts, or 1.41 times the design voltage, we roughly expect 1.41 x 12.5 A or 17.5 A current. I would try to find a rectifier rated for at least 20 A or even better 30 Amps.

    Here's one rated for 35A, 400V, for $3.75:

    According to the datasheet, it's about 1-1/8" square. Thickness is 7/16" thick, or 1" including the leads. WA, will that fit somewhere inside the sander housing?
    http://www.diodes.com/products/inactive/_data/ds21303.pdf [Broken]

    Edit added:
    For a 1200 micro-F capacitor, the voltage will drop to about 1/2 it's peak value after a half line-cycle. I'm basing that on a 9.6 ohm effective load (120 V, 12.5 A) and the usual exponential decaying voltage. That may be enough to give this motor a boost. Anybody more familiar with universal motors welcome to jump in here!
    Last edited by a moderator: May 3, 2017
  16. Jul 16, 2008 #15


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    Frankly, I don't know if just the rectifier alone will make a difference (better or worse).
    I would tend to doubt it.

    One thing does occur to me though.
    That's about a 2HP motor on the sander.
    I doubt that the outcome or the race will be torque limited.
    So I think the key is upping the belt speed or in other words the motor rpm.

    DC motor rpm is largely determined by field coil strength.
    So IIRC bridging out the field coils should increase the rpm at the expense of torque.
    Putting resistors across each of the field coils might do the trick.
    As a guess each of the field coils might have about 30 volts on them.
    Perhaps a 15 ohm wirewound resistor.
    That works out to about a 60w resistor, however for this type of resistor even a 20w one will last for several seconds before it melts.
    Pick the largest wattage that will fit. The resistors may not be kind to plastic cases.
    I might start with a somewhat larger resistance and experiment a bit.

    Changing the motor/belt gearing is a possibility.
  17. Jul 17, 2008 #16
    Yes redbelly, I am pretty sure I could fit something that size either in the handle or in the side of the motor housing (seeing as how I removed the blower fan that drives the sawdust into the collection bag.....added resistence, added weight...had to go). As for changing the gearing, that has already run through my head. The sprocket on the side of the motor is very small, and the one on the side of the driving roller is very large. as far as i can see< they are not interchangable>>>plus changing them around< i would have to alter the case design as it already accomadates the sprocket set up now> i think i may try redbelly"s idea and do some testing with a thirty five amp rect at my wifes_grandfathers shop> thanks again for the help>
  18. Jul 17, 2008 #17


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    After reading up a little on universal motors, I will agree with that statement. In fact the diode drop in the bridge could make things a little worse. I am now thinking at least a 1200 uF cap (which gives rippled voltage, varying between 50% and 100% of the 168V-or-so peak, would be desired.

    I would think it's the product of both field coil and armature current. For a wound field (as in universal motors), this would essentially be the square of the current, no? So wouldn't increasing the mean-square voltage address this?

    I didn't follow what you're describing here. But I think of myself as only "casually knowledgeable" about motors in general; I'll have to read up on this stuff more, if only I can find the time to do that.

    You're welcome! Just to repeat what I said earlier in this post, I now think a capacitor (perhaps 1200 micro Farad) across the bridge output is needed.
    This capacitor is 1.38" dia. x 1.78" long:


  19. Jul 17, 2008 #18


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    For a permanent magnet motor, rpm is pretty much a constant for a particular voltage.
    Higher voltage (greater armature current) higher rpm. The field is constant.
    For series wound motors (like the one being discussed) speed also increases with voltage, but at a fairly slow rate, since the field current also goes up.
    I tried a few of my power tools at 140v AC and they do speed up some, not a lot.
    It may however, be possible to design one that slows down or keeps a constant rpm with voltage changes.

    I'm not a designer, but I have worked with a few.
    I know from working with some parallel wound DC motors that reducing field strength, reduces back EMF in the armature with the effect of increasing armature current and rpm.
    They are started with high field currents to get better torque.
    Reducing field strength in the series wound motor by bypassing the field coil current directly to the armature should have the same effect.
  20. Aug 14, 2008 #19
    At 120VAC In and 170VCD out should it include a bleeder resistor? If so what size is appropriate?
    Using "Bridge Rectifier 400V 35A MB-35 PN: MB354-ND" and Smoothing Cap "CAP 1200uF 250V ELECT TS-HB PN: P10133-ND"

    Can the DC output be measured in Hertz? If so would it be ~120Hz?

    Is there a solution for keeping the input and output 120vac but doubling the hertz to 120 on the output without using a drive? Wouldnt increasing hertz increase RMP but loose torque.
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