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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.


From: John De Armond
Subject: Re: Motor Question
Date: Sun, 06 Feb 94 09:09:41 GMT (gary preckshot) says:

>In article <11!4!>, (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.


Newsgroups: rec.crafts.metalworking
Subject: Re: Motor Question
From: John De Armond
Date: Tue, 08 Feb 94 08:11:53 GMT (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,

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.


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.


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.


Newsgroups: rec.crafts.metalworking
Subject: Re: Motor Question
From: John De Armond
Date: Wed, 09 Feb 94 09:03:00 GMT (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


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