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Thread: 120 to 220 bandsaw conversion?

  1. #46
    After blowing a little dust off my head and looking at what your trying to say I think those numbesr you have are true horse power. I don't believe it would be correct to say maximum rated load. True horsepower I believe and I'm certainly no expert on this would be mathmatical horsepower not under any load. I'm willing to bet that is where Rick was going so I was most likely mistaken that he had a typo.
    Ever wonder what happens if you get scared 1/2 to death twice ?

  2. #47
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    ......and another good 3/4 horse *beat to death*!

    The guy really just needed to get his motor wired correctly......
    [/SIGPIC]Necessisity is the Mother of Invention, But If it Ain't Broke don't Fix It !!

  3. For some reason I had not seen that there were several new postings in this thread.

    I have confirmed the math I mentioned in a previous posting. If you were referring to the table in the NEC for amperage versus horsepower, it is very inaccurate and does not correlate to any actual motor's values.

    The full (non-estimated) formula for determining a motor's horsepower is:
    hp = I x V x pf x eff / 746

    The powerfactor (pf) is how much phase shift exists between the current and voltage at rated load. Typically this is 0.8, but may be as high as 0.9 if the manufacturer rates the motor higher into its loading curve, which is possible if it uses better insulation in the windings. The less the motor is loaded, the lower this value. For example, an idling motor will have a powerfactor down near 0.2. This is why an idling motor will still draw about 50% of its FLA value even when it is doing no work. (By the way Roger, it is FLA not FLC, even though FLC is grammatically more correct.)

    The efficiency (eff) is the various losses in the motor and is not dependant on loading. This is due to factors such as windage, air gap, and hysterieses in the iron core. This too is approximately 0.8 for a typical motor.

    So working this formula backward, the current for a typical 3/4 hp motor is:
    I = .75 x 746 / 120 / 0.8 / 0.8 = 7.3 amps

    If it were a premium motor, I use 0.85 for both values:
    I = .75 * 746 / 120 / 0.85 / 0.85 = 6.5 amps

    Oh, the formula for horsepower is not an estimated formula; it is the true and accurate formula for determining horsepower. However, unless stated on the motor nameplate, the powerfactor and efficiency are estimates, but they are fairly accurate estimates, nevertheless.
    Last edited by Rick Christopherson; 02-12-2009 at 3:11 AM.

  4. #49
    My apologies Rick my mind set just wasn't thinking that way at the time. I sorta thought after I replied that I should look at what you said closer and run the hp formula, especially after Mike replied. But you have to live with what you say.. I just didn't feel even doing so would result in a nearly 7 amp difference from table value flc. So I just learned something.

    I wonder why the training for sizing motor overload (nameplate fla) and ocpd and conductors would be from such inacurrate values? Would you happen to know why the NEC uses tables using such high values? If the tables are using the same terminology max FLA or FLC being the same why would the two methods differ so much??
    You can PM me (if you find the time) as I know this is off subject for the thread.
    (By the way Roger, it is FLA not FLC, even though FLC is grammatically more correct.)
    Thats what I thought and shot from the hip that you had pulled your figures from the NEC table when I saw that 230 volt FLC was 6.9 amps....I've seen that mistake simple as it is many times but in this case I was wrong...but dog gone it Rick I forgot you were an engineer you guys have a different way of looking at things.... albiet probably more acurrately.

    Regards
    Last edited by Roger Frazee; 02-12-2009 at 11:27 AM.
    Ever wonder what happens if you get scared 1/2 to death twice ?

  5. Roger, I have no idea how the NEC comes up with their numbers, but the smaller the motor, the worse they become. In order to come up with their amperage for a 3/4 hp motor, they would need to have used a powerfactor of 0.4, which is ridiculously low, and that is why the amperage is double what it should be.

    To compound this even further, if you had two identical products that used the exact same motor with the exact same nameplate data, but one of those products had a UL sticker (which is extremely expensive), then you are permitted to use the nameplate amperage from the motor, but for the other product, you must use the artificially inflated values in the table.

    This would be fine if the table were more realistic. Keep in mind that the NEC mandates that all motors carry this nameplate data in the first place, but then prevents you from utilizing the very data that they make mandatory to appear.

  6. #51
    Quote Originally Posted by Chip Lindley View Post
    ......and another good 3/4 horse *beat to death*!

    The guy really just needed to get his motor wired correctly......
    That's very true and I agree we need to reach a consensus to tell a guy/gal who asks a question like this a simple yes you can or no you can't without all this motor terminology and formulas.. I think if I had it to do over again (I really didn't add much to this thread anyway) I would simply say.. sure.. but I don't think on a 3/4 hp your going to create any magical performance improvements other than those things Tom mentioned. That would let the poster decide if those things Tom mentioned would be to his liking to go ahead and make the conversion. Just make sure you understand the conversion at the motor terminals... don't have the mind set that I think it's right.. most are very simple... but ya gotta be sure..

    BTW I have been forced to look back at some of my replies and I am very guilty of complicating things more than they should be....I'm a retired electrician by trade and new to the forum so trying to get my adjustment meter tuned to the needs of this forum...so bear with me....I just turned it down at least two notches....
    Last edited by Roger Frazee; 02-13-2009 at 10:49 AM.
    Ever wonder what happens if you get scared 1/2 to death twice ?

  7. #52
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    Don't get me wrong Roger! I appreciate very much the expertise of professionals on technical subjects in these forums. But, many here get carried away with expounding on a subject far beyond the *purpose* of the thread, and the *need* of the OP. An *in-depth* explaination is wasted energy if it falls on deaf ears!

    *Personal Opinion* is always plentiful in these forums! But, also, at times, *Too Much Information*! It's not all bad! The *tricks of the trade* are learned two ways--personal experience, OR recalling off-hand what we heard (or read) once upon a time. Best Regards, Chip!
    [/SIGPIC]Necessisity is the Mother of Invention, But If it Ain't Broke don't Fix It !!

  8. #53
    Chip, I absolutely agree. This isn't an electrical forum. A couple things I know.. we need to avoid the cord and plug arguments, those are just silly. And 120 to 240 on power tool motor conversions need to stay within reason... there really is no harm either way.

    Regards
    Ever wonder what happens if you get scared 1/2 to death twice ?

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