Newsgroups: rec.crafts.metalworking Subject: Re: Motor Question From: John De Armond Date: Fri, 04 Feb 94 06:44:11 GMT bq340@cleveland.Freenet.Edu (Michael Burlage) writes: >Thanks, but I need a way to check that is not guess work. >I think amperage is more likely to tell me what is going on. Nope, sorry. The amps approach FLA quite a bit before the motor is fully loaded. What changes near FL is power factor. The only way to tell without guessing is to put a wattmeter inline and measure the power consumption. Ain't no other way. Lacking a wattmeter, measuring the winding temperature is the next best bet. Oh, and try not to quote whole articles in order to add 2 lines. Thanks. John From: John De Armond Subject: Re: Motor Question Date: Sun, 06 Feb 94 09:09:41 GMT gary_preckshot@lccmail.ocf.llnl.gov (gary preckshot) says: >In article <11!4!vq@dixie.com>, jgd@dixie.com (John De Armond) wrote: >> >> The only way to >> tell without guessing is to put a wattmeter inline and measure the >> power consumption. Ain't no other way. > >That's only one way of determining power input to the motor. You can use a >voltmeter, and ammeter, and some way of determining phase, such as either a >phase meter or an oscilloscope, to accomplish the same purpose. However, >power input is irrelevant to motor heating. > >> Lacking a wattmeter, >> measuring the winding temperature is the next best bet. > >Motor heating is caused by I**2 * R heating in the windings, I**2 * R >heating due to eddy currents, hysteresis losses in the iron, and friction [A bunch of boring and irrelevant crap deleted.] >Consequently, motor current *is* the appropriate variable to measure. What say we look at some real data to evaluate that claim. I picked a nondescript 1/10 hp split-phase motor out of my junkbox and instrumented it. It is small enough that I could load it by dragging a shop towel against the attached flywheel but large enough to produce an upscale indication on my instruments. Instruments consist of a Westinghouse in-line electrodynamic wattmeter with a spec of +- 1/4% full scale. Ammeter is a Fluke 88 DVM. The numbers: Line voltage: 130 Condition Watts VA PF Amps %FLW %FLA -------------------------------------------------------------------- idle (no load) 20 65 .31 0.5 22% 71% ~50% of full load 45 78 .57 0.6 50% 85% Full load * 90 91 .99 0.7 100% 100% * Determined by measuring the RPM with a strobotach and loading it until the RPM drops 10%. (motor lacked a nameplate.) FLA = Full Load Amps FLW = Full Load Watts VA = volt-amps PF = power factor It is pretty obvious that the FLA is NOT a sensitive indication of loading. It is also obvious how the power factor changes as the motor is loaded. Though I didn't take the time to measure it, I know from years of experience that the winding temperature rises approximately proportional to load. When a motor is at its maximum load (which may be more than its rated load), the temperature will rise approximately that allowed on the nameplate. If the motor has a service factor of 1.0 (typical cheap compressor motor) and the rated rise is 80 deg C, the windings will approximate that rise above ambient at full load. If the service factor is 1.2 (typical of quality motors), the max winding rise will occur at about 120% of full load. Note that this is the actual winding temperature and not the outer shell temperature. All these specifications are on the nameplate. I repeat, the best way of determining full load is with a wattmeter. The second best way is to measure winding temperature. A distant third and worst way is to look at the amps. John Newsgroups: rec.crafts.metalworking Subject: Re: Motor Question From: John De Armond Date: Tue, 08 Feb 94 08:11:53 GMT gary_preckshot@lccmail.ocf.llnl.gov (gary preckshot) writes: >[Bunch of electrically ignorant crap deleted] >Except for the following really ignorant crap: too bad, preckshot, that you didn't let it lay while you were ahead. >> I know from >> years of experience that the winding temperature rises approximately >> proportional to load. [and much more of the same] >From 22 years of education, including a Ph.D. EE, and years of experience >at Grand Coulee Dam as a field engineer instrumenting motors and generators >of various sizes until I could do it in my sleep, John, you're full of it. John's 1st law of debating - the first fellow to quote credentials as an argument has just lost. Congrats. BTW, I have you beaten in experience, education in the field, power plant worked and projects managed but that's irrelevant to this discussion. >The main source of motor heat is winding current, followed by either eddy >current losses or hysteresis losses. And of course, rotor losses, particularly the el-cheapo chinese wundermotors. Measure it sometime. I have, as part of using an electric motor excited with DC as an eddy current dynamometer brake. >> I repeat, the best way of determining full load is with a wattmeter. >> The second best way is to measure winding temperature. A distant third >> and worst way is to look at the amps. >> >Well, repeating it doesn't make it so. It just shows determined ignorance, >although the above sentence is technically correct but you don't know it. Wow. I'm correct but I'm ignorant and don't realize it. What an argument. What logic. >Full load is usually taken as motor shaft horsepower output, which is >related to heating only by loss mechanisms, not shaft output. A wattmeter >shows you mechanical output plus all losses, but in almost any motor of >normal efficiency, shaft horsepower dominates. A small calculation should >convince all but the most determined fool: motor efficiency is normally >above 90% for most induction motors. I suppose the fools at GE must be pretty determined too. A quick waltz through their Five Star Motor Catalog (the first one my hands landed on) shows that they rate their premium, super-high efficiency motors ($uper$aver trademark) at 85 to 88 percent efficient. Their high efficiency (Energy$aver trademark) motors are rated at 82 to 85% efficient. Their regular line (no trademarked name) is rated as "less than 82%". These motors are Type KCR, Type KCP and KC respectively, single phase motors in the range of 1/4 to 10 horsepower. The table in the front of the catalog doesn't indicate a $uper$aver motor efficiency of 90% until 20 hp in their 3 phase line. >Consequently, at full load you're >seeing at most a 10% variation in wattmeter reading due to heating, >friction, and windage. Given a nominal 1% accuracy for an analog >wattmeter, you couldn't determine losses to better than 10%. And about >half the losses are not functions of current or shaft load, they are >essential constant. Get smart, dude. >Now to blow away a bunch of other misinformation: >1) it's claimed that you ought to measure winding temperature rather than >believing the nameplate because somehow manufacturers just fake up the >motor ratings - >Don't be ridiculous! How do you think manufacturers determined motor >heating characteristics in the first place? They instrument the windings >on test examples, including either RTDs or thermocouples. The ratings on >the nameplate reflect their findings and are to NEMA standards. You think >you're good enough to do better? I don't. I haven't seen a post in this >entire thread (besides mine) that indicates that any of you understand >electrical machinery. How you expect to do better than a pro can only be a >triumph of wishful thinking over reason. It's real easy, Preckshot. First off, what you read off the nameplates on the motors at Grand Coulee during your visitor's tour have little to do with the small single phase motors that are the subject of this thread. The person trying to maximize the load on a particular motor is trying to maximize the output of THAT motor and not the typical motor of that design, which is what the name plate represents. Even the nameplate can be suspect and/or confusing. Consider the following from the same catalog for 1 hp, 240 volt motors: Type FLA Service Factor --------------------------------------- KCR 4.2 1.2 KCP 8.0 1.0 KC 7.1 1.0 KC compressor 7.0 1.0 (air compressor service KC NEMA SF 6.2 1.25 I see FLA varying almost 2:1. I see a difference in varieties of the same basic design, the KC, varying by almost 15%. I know from working in a friend's motor rewinding shop that the GE KC motors of a given HP are the same on the inside and use the same number of turns on each pole. They vary only in the nameplate rating. This is kinda obvious when you compare FLA and service factor. The main factor in shortening the life of a motor is the winding temperature, which should be obvious. Indeed, GE defines the term "temperature rise" as "the amount by which a motor, operating at its rated output, is hotter than its surrounding temperature." In other words, the nameplate temperature rise is the limiting factor if long life is desired. If I want to make sure the motor is running at its rated output, I will instrument it with a wattmeter and compute the FLW (if not specified) from the motor's rated PF and VA. If I want to push the motor beyond its nameplate rating, as the original questioner wanted to do, I'd measure watts if I had the instrumentation but more importantly, I'd measure the winding temperature. The measured amps is a very insensitive indication of load as I demonstrated in my previous post. >2) It was claimed that foreign motors dissipated more heat because of wider >air gaps, and a variety of other arcana - >An inaccurate air gap causes problems, but because of decreased flux >linkage between the armature and the rotor. This causes increased >parasitic inductance which increases the motor's vars, but doesn't directly >increase losses. Gee you just contradicted yourself. On one hand you claim that heating is directly proportional to amps (in your previous post) and now you claim that increased VARs (which at a constant voltage means more amps) don't directly relate to increased losses and heat. One way or the other, dude. Can't have it both ways. BTW, the previous poster said nothing about non-uniform gap in foreign motors, he noted the much larger gap and he was absolutely right. My el-cheapo drill press with the motor that stinks of hot insulation at rated load and which will smoke on stall before I can hit the switch has almost a quarter inch of gap visible from through the end bell. These things have less of eveything - copper, iron, rotor, bearings, cooling. While I'm on the subject of motor temperature, I'll relate a nifty little truism taught to me by the resident Westinghouse motor expert at the Sequoyah NP, an old fellow who knew all there was to know about motors without the impediment of a PhD. If you want to approximate the shell temperature of a heavily loaded motor, slap your hand firmly against the motor shell and leave it until you can't stand the heat anymore. As your hand makes contact, say the following at about 2 words a second (normal conversation): "Damn, this son of a bitch is really, really hot." For every word you get said, subtract a degree from 80 degrees C. This will get you amazingly close, assuming your pain threshold isn't too high :-) This also assumes the motor doesn't have 40 years' worth of paint or grunge on it. John Newsgroups: rec.crafts.metalworking Subject: Re: Motor Question From: John De Armond Date: Wed, 09 Feb 94 07:42:36 GMT bq340@cleveland.Freenet.Edu (Michael Burlage) writes: >I didn't mean to start a war or anything, I just want to know for sure how >to check the load ACCURATELY, so what is the answer??? >My modem crashed for a few day, but, now I am back on-line. >all this B.S. is getting boring. One guy says yes, one says no, >But, it is very interesting.... >What about a prony brake? Load to 1 hp, and then check what? As I recall your original question, that you wanted to gear your compressor up a bit to gain a bit more capacity, you can't answer it even with a brake. The real question you're asking is "How much power can I get out of this motor without burning it out?" Unless it's a real cheap ah-so piece of garbage (in which case you might melt the rotor - I've seen it happen), your limiting factor is the winding temperature. You can safely load the motor until the winding temperature rises by the amount specified on the nameplate. The permitted temperature rise is related to the insulation class of the windings. An air compressor motor will typically be specified at either 60 or 80 deg C rise. This is a somewhat archaic method of specifying the limit. If the ambient is 35 degrees, you can't just let the motor rise 80 deg above that. The rise is spec'd against 20 deg. So if your motor is rated at 80 deg C rise, you can let the temperature rise to 100 deg C and not exceed the mfr's spec. I can describe a number of ways of estimating the reserve left in your motor but the easiest way is to just buy a larger motor pulley, install it and measure the winding temperature. A compressor motor is almost never fully loaded because the compressor almost never runs against the highest pressure. If you're drawing air off fast enough to run the compressor (almost) continuously, the only operational mode to be concerned about, the compressor will spend its time somewhere around the cut-in setupoint or below. You can amp-clamp the motor if it makes you feel better but it won't tell you much. Note that when you measure the winding temperature, you have to actually get on the winding. This is easy to do with a small thermocouple. Back it with a piece of Duct Putty /AKA gorilla snot. That insulates and secures the thermocouple. If you have to use some other indicator, you should de-rate your maximum by a few degrees to account for the heat gradient across the stator and case. John Newsgroups: rec.crafts.metalworking Subject: Re: Motor Question From: John De Armond Date: Wed, 09 Feb 94 08:11:54 GMT garyp@tcsegp1.PEN.TEK.COM (Gary R Pimm) writes: >John, What is the phase of the current vs voltage at power factor 1. In phase, by definition. >Or better yet what is the phase angle range of a induction motor as it >goes from no load to full load. Power factor is simply cos(theta) or the cosine of the phase angle between the E and I. power = E * I * cos(theta) >If I understand correctly (slim cance!), a motor that is lightly loaded has >a fairly large phase angle between the voltage and current. 90 degrees? >This angle decreases as the load icreases. It won't go to 90 degrees because there is always some real current being dissipated in the winding resistance and the laminations via eddy currents. Neither will it go completely to unity at full load because there is always some magnetic leakage. >I have a Amprobe with a broken meter movement in it. I don't think it would >take much to modify it so that I could use it as an oscilloscope current >probe. Doing this I could watch the phase angle change under load. Knowing >what the angle at power factor 1 I then would know when the motor reached >"full load". Is this correct or am I missing something. The motor would be at full load at the mfr's specified PF. This can vary widely according to the type of motor. Easier than trying to scope this is to simply measure the amp and volts to get volt-amps and then measure true power with a watt meter. The PF is then simply true power/VA. BTW, contrary to Prickshot's claims, surplus lab-grade wattmeters are commonly available in the $10-20 range. I have several that I bought in that price range. All are at least rated at 1/4% of full scale. They typically come in gorgeous furniture- grade wooden cases. I have a very expensive GE wattmeter calibration standard whose calibration is NBS traceable. All the meters I've bought surplus, including the one last calibrated in 1942 (!) were within specs. These meters contain no magnets to weaken over time so they should retain their calibration indefinitely. Such a meter is VERY handy to have around, particularly when trying to chase down the source of high power bills. There are a couple of lurkers over in sci.electronics who collect old instruments. Indeed, there is a mailing list, though I don't know the address. Ask over there. Someone will pop up with a meter. I know from talking to one of these guys that there was recently a LARGE lot of meters dumped on the market in, I believe, Ma. BTW, if you need a fairly accurate load with which to check your setup, the 500 watt quartz-halogen bulbs such as are used in outdoor lighting and those torchiere lamps are REAL close to 500 watts when exactly 120 volts are applied. I have a number of these things set up in a switchable bank for loading gasoline generators, inverters and the like. I've yet to measure one that varies more than 2 or 3 watts. Plus or minus 5 watts would be one percent so that's not bad at all. Same holds for 300 watt bulbs, though I've not tested as many. John Newsgroups: rec.crafts.metalworking Subject: Re: Motor Question From: John De Armond Date: Wed, 09 Feb 94 09:03:00 GMT iorio@oasys.dt.navy.mil (Vincent Iorio) writes: >1. Motor experts never agree 100% about motors and ratings. I'm still >waiting for two people to give me the same answer to the question - > "What does servce factor mean?" I can't imagine why anyone would argue over this term. (OK, OK, I take it back.) Quoting from the GE catalog again: "A measure of the overload capacity designed into a motor. A 1.15 SF means the motor can deliver 15% more than the rated HP without injurious overheating. A 1.0 SF motor should not be overloaded beyond its rated horsepower. Service factors will vary for differetn HP motors, and for different speeds. Standard NEMA Service factors for various HP motors and motor speeds are shown in the table below for easy reference." And the table: HP 3600 1800 1200 900 ---------------------------------------------------- 1/20-1/8 1.4 1.4 1.4 1.4 1/6-1/3 1.35 1.35 1.35 1.35 1/2 1.25 1.25 1.25 1.15 3/4 1.25 1.25 1.15 1.15 1 1.25 1.15 1.15 1.15 1.5-up 1.15 1.15 1.15 1.15 John |
|