<p>Following are several old-fashioned, simple but effective non-electrical methods to estimate a motor's load-factor:

<p>1) Load estimate by slip.
This was discussed in the Fri, Dec 27, 3:30pm response to the thread "Efficiency Calculations of Old Motors!" A fluorescent lamp was used as the strobe light.

<p>2) Load estimate by resistance.
Determine stator winding resistance for "hot" and "cold" conditions. Load-factor, LF, is the ratio of measured temp-rise, Tm, to rated temp-rise, Tr. Then, for copper and aluminum windings:
<pre>
Th = (Rh / Rc) x (Km + Ta) - Km. Since,

Tm = Th - Ta, then solving for LF,

LF = Tm / Tr = (Th - Ta) / Tr, yielding,

= (Kr - 1) x (Km + Ta) / Tr, where,

Ta = ambient temp when motor is 'cold'.
Th = ambient temp when motor is 'hot'.
Tr = motor's rated temperature-rise.
Kr = Hot to cold resistance ratio = Rh / Ra.
Km = mat'l constant, Cu=234.5 & Al=228.
Rh = stator 'hot' resistance, Ohms.
Ra = stator 'cold"resistance, Ohms.
</pre>
<p>3) Load estimate by thermometer.
Where available, remove lifting-eye from motor carcass, fill hole with oil, insert old-fashioned thermometer. If lift-eye hole is unavailable build temporary reservoir with putty, or other suitable material, add oil, then insert thermometer.

<p>Regards,<br>
Phil Corso, PE<br>
(Boca Raton, FL)<br>
[[email protected]]<br>

 

My initial posting omitted the most unusual method in my 47+ years experience. I used it in my previous life when a smart-stick was all one needed! Therefore, I dedicate this method to List members that remember the... No! Not the Alamo, the... Slide-Rule:

4) Load estimate by Slide-Rule. Kr, the hot-cold resistance ratio is all that is needed. Ambient temperature and material can be ignored. So here it is:

Align the slide-rule's 'C' and 'D' scales so that 1.0 of the 'D' scale is opposite 2.6 on the 'C' scale. Starting with 2.6 representing 0 C deg rise, then each 10th division of the 'C' scale represents a 10 C deg-rise. For example, 2.6=>0 C deg-rise, 2.7=>10 C deg, 2.8=>20 C, 2.9=>30, etc. The 'D' scale represents Kr.

Example, find Kr = 1.2 which lies between 3.1 and 3.2 on the 'C' scale. This represents the range 50-60 C deg-rise. Interpolation yields a 52 C deg-rise.

Credit this method to George C. Kent, of American Photocopy Co of IL. Analogously, this method ranks up there with 10,000+1 uses for duct-tape.

May the New Year bring to you and your loved ones, Happiness, Wealth, and most of all Health!

Phil Corso

 

Further to my earlier post covering the slide-rule method:

For those of you who can't find theirs, or never felt the power of one strapped, holster-like, to their belt, or worse yet don't recognize the term, here is a simpler method than the slide-rule. It is:

5) Load estimate by direct equation:

Y = A x X - B, taken from slide-rule,

Y = deg-rise, Celsius.
X = hot-cold resistance ratio, Kr.
A = slope = delta deg-rise / delta Kr.
= 90 / 0.345 = 261, from scales 'C' & 'D'.
B = Y-axis intercept = - 261.

Thus, Deg-rise = 261 x Kr - 261, simplfying,

Deg-rise = 261 x (Kr - 1)

Of couse, the slide-rule often led to embarrasing situations on the NYC subway, but I won't go into details.

Regards,
Phil Corso, PE
(Boca Raton, FL)
[[email protected]]

 

Further to my Thu, Jan 2, 3:00pm post covering the direct equation method:

The equation covering method 5) has an error. The 'B' term coefficient should have been '+', as follows:

5) Load estimate by direct equation:

Y = A x X + B

Regards,
Phil Corso, PE
(Boca Raton, FL)
[[email protected]]

 

Dear Phil,

These are fine and remarkable methods. But I could not understand in which cases we will need them. Yet I am a new engineer compared to you (experienced for 15 years only), but never seen a single case where I cannot read at least the load current.

 

Dear Hakan,

Then, you probably know that load current is the least accurate... if voltage varies.

In my experience I have used one or the other methods to determine load-factor at least 5 times, or about once every eight years. But, that then begs the question "what is experience?"

Please note that methods 2 thru 5 are all variations on the same theme... temperature-rise.

On the other hand I used the slip method many more times to determine if broken rotor bars were responsible for severe vibration.

Regards,
Phil Corso, PE
(Boca Raton, FL)
[[email protected]]

 

Dear Mr.Phil,

I am working as maintenance engineer at a 15 MW power plant in India. I wish some education on how to analyse (from vibrations) if problem is due to defective bearings, broken rotor bars or unevenness in air gaps - in induction motors.

My office-mail address: [email protected]

With best regards,
Anand Rao

 

Responding to Anand Rao's query:

While I appreciate your kind thoughts about my experience, I want to make it clear that my mechanical vibration expertise is limited. I usually get involved when all mechanical vibration causes are exhausted.

Vibration due to mechanical failures are quite varied and you should contact a good vibration analysis company such as Bentley-Nevada! I will confine my comments to those vibrations that are the most prevalent due to electrical failures.

Vibration or noise caused by electrical failures are generally related to slip-speed. This is the synchronous speed minus the rotor speed. In most cases the failures occur in large or higher voltage motors, caused when the rotor-bar separates from the end-ring. The root-cause is usually a poor bar-to-ring joint, excessive starting-time, starting on-load, frequent starts without adequate cooling periods between starts, low acceleration torque (motor-torque minus load-torque), ignorance of rotating inertia (WK^2 or GD^2) effects, or a combination, thereof. Believe it or not, some engineers forget how to convert WK^2 to GD^2 or vice-versa, or can't express load inertia in terms of motor speed!

Sometimes the vibration is highest at start, then reduces as the motor accelerates. This occurs because the slip-rpm and subsequent rotor-current is highest at start, then gradually reduces as the motor approaches rated speed.

I want to emphasize that, in my experience, most motor failures are the result of inadequate attention paid to starting requirements.

Regards, Phil Corso, PE
Boca Raton, FL [[email protected]] ([email protected]) {[email protected]}