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Thread: plug in amp meter question

  1. #1

    Default plug in amp meter question

    quick question. i seen some plug in amp/volt meters and wondered if this would be a good way to see how much of a load the motor is taking when machining. I have been doing alot of turning on my lathe lately and dont wanna burn the motor up. its 1hp 110v 750watts/15.7amps i believe. i used the kennemetal calculation chart and have a ballpark of how much HP i am using at times......but say my motor is rated at 15.7 amps wired at 110v. If i used a meter and stayed under 15 would i never have to worry about overloading it? Would a meter help me in anyway? Any suggestion would be appreciated. thanks!

  2. #2
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    I am sure your motor can handle much more than the rated current for short periods of time, especially when starting and I also suspect that many motors would overhead if run too long at their maximum rated current.

    BTW, 15 amps at 110 volts is a way lot more than 750watts so something is awry with your figures.

    I think what would be much more useful would be a temperature reading from inside the motor, if thats possible.
    Last edited by The Artful Bodger; 06-02-2011 at 10:47 PM.

  3. #3
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    If you don't "bog down" the motor you should not have a problem.

    Bogging down indicates excessive "slip", which shows overload, and will draw excess current.

    BTW, 1 HP often IS about 15A in a 120V motor, because the motor has a fairly crummy power factor even at full load. It may even be more. My "handy book" shows 16A.

    A meter is probably not going to help you much... if you bog down a 1 HP motor you will know it pretty quick.
    Last edited by J Tiers; 06-02-2011 at 11:19 PM.

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    If your motor circuit has a thermal overload relay, you may sleep well.
    Overload relays are the best protectors.

    Also, if it's a 15 amp motor and you run it on a 20 amp breaker, the breaker may trip if you seriously overload your motor. This, however, is not a good or reliable way to protect motors from overloads, therefore it's not considered to be acceptable for this purpose.

  5. #5
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    Not disagreeing with any of the above advice, but being able to relate an ammeter reading to a machining operation is handy. As others have said in different words, you'd be able to relate the differences in the sound of the motor with how much power is being used or applied to the machining operation. You end up getting a feel for how much feed to apply, or how much depth of cut you can use, by listening to the motor laboring. For some it would be more appropriate to look at an ammeter for the same cues.

    Our wide belt sander at work has an ammeter, with which you gauge how much to crank the table up. Trying to hear the motor loading down doesn't give any clues on this machine.

    On my lathe, the amount of loading is judged by the loudness of the hum coming from the motor. It runs from a dc power supply, but the dc level fluctuates at line frequency the more it's loaded down. Hearing this gives me an indication, as does the motor slowing down, and I learn at what point I have to either back off on the feed or the depth of cut. But there are other things making noise in the shop, so something other than an audio cue would sometimes be good. If you're going to be looking at a meter for these cues, then an ammeter is the one that makes the most sense.

  6. #6
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    Load meters are convenient. An AC ammeter isn't veryt eapensive but the lower end of the scale is sadly compressed.

    http://cgi.ebay.com/Vintage-Simpson-...item256382e549

    Look before you leap. Some ammeters require a shunt to work properly and direct connecting the meter in series with the load without the shunt will fry it.

    A cheap analog clamp meter set to the proper range, clamped on ONE line conductor and semi-permanently affixed in a readily visible place (a verboten thing I'm suggesting. The Framers of the NEC will burn me at the stake if they heard of this proposal) will let you monitor motor current as things procede. The meter is also handy to have around the shop for other things.

    http://cgi.ebay.com/Clamp-Ammeter-An...item2c5ce8942f

    Something to think about: Induction motors draw 30 to 40% of its full load current at idle and slowly increasing until about 50% load is reached, Then the motor current goes up in rough propotion to mechanical load. It's a peculiarity of induction motors and the power factor Mr Tiers mentioned. IOW it's normal.
    Last edited by Forrest Addy; 06-03-2011 at 01:49 AM.

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    Well if your worried about burning up your motor then perhaps its the quality of the motor you are more worried about.

    I know from experience a Baldor or Dayton 1 Hp motor can take exponentially more abuse then 1 of those "liberally rated" Chinese motors. Judging from the 15.7 amp 1 Hp rating I tend to think you have one of the better motors. Try reaching down and feeling how hot the motor is running periodically. First after running 30 -45 minutes of light duty cutting and then after the same period of heavy duty cutting.

    Rule of Thumb - most motors (American motors) are capable of maintaining operation under a 40c rise in temperature.

    Additionally the alignment plays an equal role in the life expectancy of your motor. If the mounting assembly is flimsy and allows the motor to become sideways during start up or operation this will wear the shaft bearings prematurely.

    BTW: I have a Dayton 1/4 Hp motor that has been running the hydraulic pump on my surface grinder at 150 psi for years and years with 100s of hours on it - and it shows no signs of giving up the magic smoke any time in the near future.

  8. #8

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    since its summer and much hotter in my garage would that play any difference in a motor bogging down easier? I run the same parts over and over when i machine. same speed/feed/tooling....etc. lately my motor has been bogging down and tripping my breaker. i only have my lathe plugged into it so its not sharing power with anything else when its running. its on a 15amp breaker. so i had to back off on my feed a little to get this last run of 25 parts finished. could the rise of temperature in my garage play a factor? its probably 85 degrees compared to 55 degrees a couple months ago.

  9. #9
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    A couple of things to consider, is the motor 110/220v?
    And do you possibly have 240v or 2 120v circuits in the garage that you could swing over to a double breaker and look at running off of 240?
    This may solve your bogging down problems.
    Max.

  10. #10
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    My opinion only, but it's more likely that the average voltage that your shop is being fed is lower now. It's unlikely that the motor is responding to that temperature change. If it was, then you would have noticed the bogging getting worse during a run of parts as the motor heated up, and it would have been fairly consistent.

    I could be wrong- it has happened before

    Because this is something you are concerned about, it might be a good idea to set up an ac voltmeter for awhile, just to see if you can correlate the bogging to a drop in supply voltage. Either you can, or you find that it isn't related, and you can drop that line of reasoning.

    What about the lathe itself- is it possible that it doesn't like being warmer? Maybe something is on the verge of binding- not enough end play on a shaft possibly, something like that.
    Last edited by darryl; 06-13-2011 at 07:23 PM.

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