Subject: Motor Question

From: [email-masked] (Michael Burlage)
Date: 3 Feb 1994 04:39:26 GMT

How do I know if my compressor motor,which is 1 hp, is
operating near it max?
I need to squeeze more cfm by increasing speed by changing
pulley size, but don't want the motor to overheat!
Should I measure the running amperage, or will the starting current 
be the thing to look at?
it is not cap start or run.
it bogs alot when it starts, so I may not be able to change
much. Has anyone else gone thru this??

-- 
"Onward through the fog"
                         - Power-Hitter
[email-masked]
[email-masked]

---

From: [email-masked] (David Jenkins)
Date: 3 Feb 1994 12:00:12 GMT

>From article <[email-masked]>, by [email-masked] (Michael Burlage):
> 
> 

A typical compressor with a 1 hp motor is *very likely* to be running at
maximum capacity.  A good check is to run it for 10 minutes or so, perhaps
running a tool or something, then try putting your hand on the motor casing
*carefully* - beware, it may be too hot! - if you can comfortably rest your
hand there but it is warm, then it's within its rating, but close to the
limit.  If it's cool, then it's not struggling at all and might be happy
with extra load.  However, if it's reluctant to start, then you've had it.

CAUTION!  Approach the motor casing with care, as a compressor motor is
likely to be very hot, right on the limits.  You have been warned... don't
come crying to me... etc.  Also, if the machine isn't well made or
maintained, there may be stray mains voltage about...

David Jenkins

* I'd rather be steaming... *

---

From: [email-masked] (Michael Burlage)
Date: 3 Feb 1994 16:36:38 GMT

>* I'd rather be steaming... *

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.
-- 
"Onward through the fog"
                         - Power-Hitter
[email-masked]
[email-masked]

---

From: [email-masked] (James W. Swonger)
Date: Fri, 4 Feb 1994 13:23:59 GMT

The previous suggestion about checking the motor case for heat is
about the best way to go. See, cheap compresors use cheap motors and
run them right at the limit; often they specify a duty cycle maximum
at rated output, and if you look at the poor little motor on a Sears
compressor, it'll say "Compressor Duty" which seems to be a euphemism
for "thermal cutout in 10, 9, 8, ....". Checking the motor current
draw may tell you whether it's pulling near the rated (nameplate)
current, but says nothing about the load vs. actual capability.
Case temperature rise and the smell of roasting varnish are pretty
reliable indicators of overstress.
-- 
##########################################################################
#Irresponsible rantings of the author alone. Any resemblance to persons  #
#living or dead then yer bummin. May cause drowsiness. Alcohol may inten-#
#sify this effect. Pay no attention to the man behind the curtain. Billy!#

---

From: [email-masked] (Michael Burlage)
Date: 4 Feb 1994 17:04:36 GMT

>reliable indicators of overstress.

With ammeters,voltmeters, and watt meters available, why all the guesswork?
Current draw varies DIRECTLY with the load, and is a excellent indicator of
what is happening. This motor cost me $150.00, and I do NOT want to
destroy it to find the maximum load. The ratings printed on the motor are
there just for the reason of telling you what should be going on.
My compresser is not  a 'cheap' one, I just need a couple of more cfm's
from it....

---

From: [email-masked] (John De Armond)
Date: Fri, 04 Feb 94 06:44:11 GMT

>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

-- 
John De Armond, WD4OQC, Marietta, GA    [email-masked] 
Performance Engineering Magazine.

"Dr. Kevorkian, please report to the Oval Office."

---

From: [email-masked] (gary preckshot)
Date: 4 Feb 1994 22:02:53 GMT

In article <[email-masked]>, [email-masked] (John De Armond) wrote:
> The only way to 
> 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, 

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
and windage.  Of these, power input is only indirectly related to the major
component, which is I**2 * R losses.  Only the first loss is variable to
the first approximation.  Eddy current losses and hysteresis losses are
functions of voltage and frequency, and friction and windage losses are
functions of rpm, which is indirectly a function of frequency.  Current
heating in the windings is directly a function of current and indirectly a
function of power insofar as power increases as input current.

Consequently, motor current *is* the appropriate variable to measure.

However, if the compressor is well-designed in the first place, installing
a larger motor pulley to run the pump faster is probably a bad idea.  There
is no way you can get more net average power out of the motor without
increasing the heating, and if the motor is running at rating when the pump
shuts down at highest pressure, the only advantage a larger pulley will
give you is more CFM for a short time at the expense of lower efficiency,
smaller percent-on duty cycle, and more rapid and higher temperature
excursions.

Therefore, it's appropriate to measure motor current just before shutoff
and compare that with motor ratings.  The motor should be rated (if it's a
good motor as claimed) typically for 40 degrees C rise in a 40 degree C
ambient at 1.15 load factor (to allow for start stop operation).  You can
run the motor more heavily loaded in a low ambient, and you can use the
1.15 load factor if the motor will run continuously.  However, if the
compressor designers did their job correctly, they will have already
factored this into the design.  Very probably, all you will achieve is
lower average CFM, with slightly higher peak CFM, and shorter motor and
compressor life.  Beats me why you want to Searsify your compressor.

G.

---

From: [email-masked] (Michael Burlage)
Date: 4 Feb 1994 23:58:52 GMT

>compressor life.  Beats me why you want to Searsify your compressor.

Thanks for an answer that does'nt require smoke on my part!
I just want to be sure I'm getting the most out of my motor.
I've learned to ask someone who's done it, before I do it!!!!!!!!!!
Thank to all who have written and posted.

---

From: [email-masked] (John De Armond)
Date: Sun, 06 Feb 94 09:09:41 GMT

>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
-- 
John De Armond, WD4OQC, Marietta, GA    [email-masked] 
Performance Engineering Magazine.  Email to me published at my sole discretion

---

From: [email-masked] (Andy Dingley)
Date: Sun, 6 Feb 1994 19:53:34 +0000

Isn't measuring winding temperature the *best* way to work ?

Think about it. The little brass plate on the back of the motor isn't gospel,
it's just a recommendation of maximum power from the makers (measured in W,
VA, HP, furlongs-per-fortnight or whatever). How the makers determine this
maximum rating is by looking at what sort of power will damage the motor, and
reducing that for a safety margin. For "average" motors, the damage factor
isn't going to be snapping the output shaft, over-speeding the bearings
or over-currenting (sic) the brushgear, it's going to be overheating the
windings. Measuring the winding temperature is thus not only more convenient
than the problematic measurement of AC power, but it's actually a *better*
guide to the health of the motor.

NB - This assumes you're measuring the winding temperature, not the outside
case temperature. I'm sure that the moderately devious can work something out
with small thermocouple probes.

-- 
Andy Dingley   Codesmiths of Newcastle   [email-masked]

          Save the whales !   Collect the whole set

---

From: [email-masked] (David Jenkins)
Date: 7 Feb 1994 09:00:40 GMT

In article <[email-masked]>, [email-masked]
(Michael Burlage) writes:
|> 
|> In a previous article, [email-masked] (James W.
|> Swonger) says:
|> 
|> > The previous suggestion about checking the motor case for heat is
|> >about the best way to go. See, cheap compresors use cheap motors
|> and
|> 
|> With ammeters,voltmeters, and watt meters available, why all the
|> guesswork?
|> Current draw varies DIRECTLY with the load, and is a excellent
|> indicator of
|> what is happening. This motor cost me $150.00, and I do NOT want to
|> destroy it to find the maximum load. The ratings printed on the motor
|> are
|> there just for the reason of telling you what should be going on.
|> My compresser is not  a 'cheap' one, I just need a couple of more
|> cfm's
|> from it....
|> 
-- 

Thanks James, for the support  ;-)

The reason why ammeters, etc. will not help you is that the greatest
factor in motor efficiency, all other things being equal, is the build
quality.  Good quality European and USA motors will have a very small
gap between the static field coils and the rotating parts.  This
minimises power loss.  Far-eastern motors are not built to such fine
tolerances, and the resultant large gap leads to wasted power and
hence heat.

The end result is that a good-quality motor will give a far greater
torque before stalling than the cheaper one.

The only "scientific" way to go is to apply a calibrated braking
mechanism to the shaft with an ammeter on the power and a thermometer
in the windings, and find out the power output you can get without 
exceeding the manufacturor's rated max. temperature.  Now take a 
calibrated power source and measure how much power input is needed 
to drive your compressor before and after conversion.  
Now compare the two.

I think putting your hand on the case is easier. 

David Jenkins

---

From: [email-masked] (gary preckshot)
Date: 7 Feb 1994 17:37:32 GMT

> [email-masked] (gary preckshot) says:

[Bunch of electrically ignorant crap deleted]

Except for the following really ignorant crap:

> I know from
> 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. 
The main source of motor heat is winding current, followed by either eddy
current losses or hysteresis losses.

> 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. 
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.  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.

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.  The major increase in heating that can be caused by air
gap irregularities is because the rotor drags on the armature iron, a
situation immediately noticeable to all but the drugged.  The author of
this particular bit of information seemed to think it was some sort of
progressive disease, which, with the exception of deteriorating bearings,
it isn't.  If you've got a Taiwanese or other foreign motor, and it works
ok now, it'll probably keep cranking just like it is for a long time.

---

From: [email-masked] (Andy Dingley)
Date: Mon, 7 Feb 1994 23:04:37 +0000

In article <[email-masked]>
           [email-masked] "gary preckshot" writes:
> heating characteristics in the first place?

Isn't that just what I said in my original posting ?

The *problem* we have here is that someone has a motor which they may or may
not be overloaded, and needs some way of measuring this. There is a magic
number of HP stamped on the makers plate, but there isn't a simple means of
measuring power available. You can't describe the motor's present power as a
percentage of rated power, if you can't measure what it is !

Hook up an ammeter (your recommendation) and accurately measure the I2R
losses in the windings ? Great, we now accurately know a figure that is
completely meaningless to us. What are these losses supposed to be at full
rated power ? What proportion of those do we have at the moment ?

In the absence of a way of measuring the present value of the rated figure,
or a comparison figure for the power losses we *can* measure, why *NOT*
use the winding temperature as a guide to health ?

> I haven't seen a post in this entire thread (besides mine) that indicates
> that any of you understand electrical machinery.

Such modesty !  When I first saw that I was convinced it was a posting from
old [email-masked]  8-)

What did Jeffrey Dahmer say to Lorena Bobbit?
"If you're not going to eat that, do you mind if I have it?"

---

From: [email-masked] (Michael Burlage)
Date: 8 Feb 1994 04:58:02 GMT

>           [email-masked] "gary preckshot" 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?

---

From: [email-masked] (John De Armond)
Date: Tue, 08 Feb 94 08:11:53 GMT

>Except for the following really ignorant crap:

too bad, preckshot, that you didn't let it lay while you were ahead.

>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.

>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.  

>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.

>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

>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.

>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

---

From: [email-masked] (David Jenkins)
Date: 8 Feb 1994 14:30:02 GMT

<[email-masked] (gary preckshot) ranting deleted>

Gary,

Was it really necessary to be rude and abusive to all and sundry on
this group?  On the whole this is a friendly and constructive place to
have a conversation, without scoring points off one another.

My original comments were made from practical experience, e.g. when
you overload an induction motor you may well get:-

motor slowing or stalling, possibly leading to 
overheating
fuses blowing

Whether these result from eddy currents, losses, etc. are irrelevant
to the practical motor user.

Rather than spouting your undoubtedly vast experience and writing snotty
comments about others, why not *politely* tell others they're wrong, then
offer practical advice on how an amateur with limited measuring equipment
can safely judge whether a moter is being overloaded.

If you can't do this, then sit on your typing fingers and let others
get on with their lives.

-- 

David Jenkins

-- 

David Jenkins

---

From: [email-masked] (David Jenkins)
Date: 8 Feb 1994 14:47:16 GMT

In article <[email-masked]>, [email-masked]
(David Jenkins) writes:
|> 
|> Rather than spouting your undoubtedly vast experience and writing
|> snotty
|> comments about others, why not *politely* tell others they're wrong,
|> then
|> offer practical advice on how an amateur with limited measuring
|> equipment
|> can safely judge whether a moter is being overloaded.
|> 

...and to save others the trouble, I *do* know how to spell "motor".

;-)

-- 

Divad Jerkins, the well-known typing error

---

From: [email-masked] (gary preckshot)
Date: 8 Feb 1994 18:16:46 GMT

In article <[email-masked]>, [email-masked] (John De Armond) wrote:

[much misunderstanding of motor theory, or rather lack of any knowledge of
electrical machines theory]

In answer to the lot of you: your advice to the original poster was both
dangerous to his motor and singularly irrelevant.  John, especially, is
almost completely ignorant of the induction motor equivalent circuit,
otherwise he'd understand why FLA and load varied so differently, and he'd
also understand that motor load and motor losses are two different things -
one is only indirectly related to the other.

To take the thermocouple suggestion first - assuming that Mike took your
advice and put a thermocouple in the windings, the first thing he'd
(possibly) unforturnately discover is that due to 60 Hz magnetic fields in
the motor, the thermocouple vibrates, enough to wear insulatation on
winding turns.  There's a very good possibility he'd end up damaging the
motor, even if he didn't remove the endbells and just insert the TC through
the vents (assuming it wasn't a TEFC motor).  Secondly, since he doesn't
know the motor cooling characteristics and isn't experienced at this
endeavor, whatever he measured would be about as useful as running the
motor until he smelt burning insulation.  None of the suggestions were
anywhere near practical for somebody who isn't experienced at motor
instrumentation and has some knowlege of the cooling characteristics of the
particular frame style he has.

The wattmeter suggestion, besides being completely ignorant of motor
characteristics, also flies in the face of 100 years of experience in the
electrical industry - why do you think motor controllers have
time-overcurrent heaters to match motor characteristics?  So John was
suggesting that a wattmeter be used, a piece of equipment almost nobody but
those intimately involved with power systems have, and it's completely
unnecessary.  Motor losses have two parts - one that's directly related to
motor current (excluding magnetizing current, which is virtually constant
at constant voltage) and one that's completely independent of current. 
Neither has any dependency on shaft load except as shaft load causes
increased current.  So any of you non-participants out there can test this
easily by noting that even if you run your favorite induction motor
no-load, it gets warm.  To interpret the numbers John is so fond of, at
no-load, his motor drew 20 watts, which was all copper losses, iron losses,
and friction and windage.  No matter what shaft load was applied, he still
has at least 20 watts of losses, and about half of these heat the motor,
while the other half is expended in moving air.  So he loads the motor with
his rag, and the wattage increased to 90 watts - how much is additional
loss?  Who knows, John doesn't.  He can't separate the shaft load from the
additional copper losses due to increased current.  His wattmeter reading
is completely irrelevant unless he has a dyno.  And then what he's doing is
subtracting two big numbers to get a small one, which is the usual recipe
for inaccuracy.

So, what to do?  The most direct thing is to use the nameplate current
rating, which is directly related to motor heating.  That's been selected
by the manufacturer so that the motor, in the highest ambient, will not
exceed the insulation damage temperature *at any point in the winding*.  A
small point ignored by many posters.  As I said, you think you're better
than the designers?  I don't.  Go ahead, tell me you know where to put the
temperature probe.

So what can our compressor guy do?  Here's a way:

1) buy a candy thermometer that measures in the range 90 - 212 F.
2) make an aluminum block that fits the motor radius and has a hole for the
thermometer.
3) stick the thermometer in the block to the motor with a large blob of
putty so the aluminum temperature will rise with the motor case.  Stick
this to the motor case as close as you can get to one of the screws that go
through to the stator.
4) start the compressor and run a steady stream of air out a valve until
you either get the duty cycle rated or the CFM rated.  You can measure CFM
by inverting a waste basket in a child's wading pool so it's full of water
and squirting air under and counting seconds until the water is displaced.
5) once you're running at rated CFM or duty cycle, let it go until things
stabilize and you get a good temperature base line.  Measure the motor
current at turn-on and turn-off.  You now have a baseline to make changes
from.
6) measure the ambient temperature.

Motors are usually rated for 40 C ambient.  You can probably safely
increase case temperature by the difference between your ambient and 40 C. 
I'd do it in 10% load increments (choose a pully that pumps 10% more air). 
I'd also keep an eye on motor current.  Once you start exceeding rated
current, you're on your own, as far as the manufacturer is concerned.  You
also may need to change your overload heaters, if your compressor has a mag
starter.

You don't need a wattmeter or elaborate thermocouples, nor do you need
John's many years of misunderstood experience.  Testing compressors is
pretty simple and maybe it does take a Ph.D. to understand that.  One
that's actually done this shit before.

---

From: [email-masked] (Robert Mitchell)
Date: Tue, 8 Feb 1994 17:11:08 GMT

>of various sizes until I could do it in my sleep, John, you're full of it. 

While it is fun to watch opinionated, self-proclaimed experts slug it out,	
please remember that most of us reading this conference are
amateurs who enjoy working with metal.

Too much vitriol certainly discourages postings by lesser mortals.

 - Robert Mitchell
-- 
UUCP:  [email-masked]
DDD:   408-746-8491  Fax: 408-725-8638
USPS:  Amdahl Corp.  M/S 205,  1250 E. Arques Av,  Sunnyvale, CA 94086

---

From: [email-masked] (Vincent Iorio)
Date: 8 Feb 1994 17:11:18 -0500

I would like to  add two points to this civil discussion of motors.

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?"

2.  I measureing winding temperature, a TC (thermalcouple) or RTD (RTD),
will give only the temperature at the point it is placed.  Another
method of measure winding temperature is measureing the change in
resistance between a cold motor and hot motor.  This method will give
the average temperature.  It is not simple measurement to make.  The
change in resistance is small, and it has to be made as soon as the
motor is stopped.

Vince

Disclaimer - I did not write this post, it's not my fault.

---

From: [email-masked] (Gary R Pimm)
Date: 8 Feb 94 23:14:02 GMT

>measuring the winding temperature is the next best bet.

John, What is the phase of the current vs voltage at power factor 1.
Or better yet what is the phase angle range of a induction motor as it 
goes from no load to full load.

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. 

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.

Gary Pimm

---

From: [email-masked] (Andy Dingley)
Date: Tue, 8 Feb 1994 21:11:19 +0000

> Too much vitriol certainly discourages postings by lesser mortals.

But it's good for pickling copperwork after brazing  8-)

---

From: [email-masked] (gary preckshot)
Date: 9 Feb 1994 01:04:22 GMT

In article <[email-masked]>, [email-masked]
>  "What does servce factor mean?"

service factor: per unit extra heat dissipation for starting and reversing
service.  When starting, in a multiwinding motor, the starting windings add
considerable heat to stator and also to the rotor due to extra bar current.
 In hard starting service, this has to be accounted for.  Also, if the
motor is plugged, there may be service factor allowance for this.

duty:  a rating for the duty cycle expected of the motor, of which there
are four usual ratings.
1. continous duty - the motor will support full load continously without
damaging winding insulation
2. intermittant duty - the motor will support full load for some percentage
of the time, number of starts per hour and duty cycle must be given for a
complete rating.
3. compressor duty - a motor rated for intermittant duty but with a load
profile rather than full load.  The load profile usually matches tank
pumpup, and the number of pumpups per hour are sufficient to support rated
CFM.
4. special - almost any load profile and duty cycle, dependent upon the
application.
> will give only the temperature at the point it is placed.
True.  A significant point if you propose to measure your favorite motor.

>  Another
> the average temperature.
For the location you choose to measure.

>  It is not simple measurement to make.  The
or when the motor has achieved steady-state and is running.

Basically, the difficulty with measuring winding temperature with TCs is
that 1) you could damage the winding 2) you don't know if you've got the
hottest spot 3) if you measure air in the vent, the measurement may be
variable depending upon where you place the TC.  It doesn't tell you much
either, because you've got no calibration about what your measurements
mean.  Measuring case temperature close to where the stator is bolted to
the case is more practical for a number of reasons 1) you don't tangle with
energized windings 2) if you run the motor over it's rated duty cycle, you
get a good baseline of how the designer intended the motor-compressor
system to work 3) the case acts as a low-pass filter and incidentally
removes your temperature device from high EMI areas.  See the duty cycle
description above - if you've got a compressor duty motor, there's no way
short of contacting the designer, to get the profile to which the motor was
rated.  The closest you can come is running the machine as rated.  For this
you want slow, consistent measurements.  The machine should settle down to
an on-off duty cycle that results in the motor attaining a rated duty cycle
working temperature within its temperature rating.  If it doesn't, you
bought the wrong compressor.

---

From: [email-masked]
Date: 9 Feb 1994 00:46:23 GMT

I wish all you "sparks" or electrical engineering types would stop
your fucking flame wars on rec. crafts metalworking.  You are living
up to your name as dinky engineers, that what electrical engineers are 
call in Japan.

Peace brothers.

Dave Smucker

---

From: [email-masked] (John De Armond)
Date: Wed, 09 Feb 94 07:42:36 GMT

>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

---

From: [email-masked] (John De Armond)
Date: Wed, 09 Feb 94 08:11:54 GMT

>John, What is the phase of the current vs voltage at power factor 1.

In phase, by definition.  

>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)

>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.  

>"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

---

From: [email-masked] (John De Armond)
Date: Wed, 09 Feb 94 09:03:00 GMT

> "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