Motor Amps vs KW

[mrdavis1525]

Can anyone refer me to any technical papers or discussions concerning the use of motor Amps vs KW for the measurement of whether a motor is being stressed to much while it is operating. The code references Amps when sizing conductors and breakers as well as the use of overloads, but is the industry moving towards replacing that with KW as a better indicator?

Thanks,

 

[boater bill]

I am not really sure how anyone can measure the loaded KW of a motor without a sophisticated piece of equipment.

My Fluke 36 stands at the ready to check any motor at any time.

I can then convert it to KW if management needs it that way, but IMHO amps is the only way to measure the loading of a motor.

If the winding have been damaged or suspected to be, then a megger will verify the insulation integrity.

Hope this helps.

 

[tony_psuee]

We use kW as the defining means if a fan motor is overloaded, measured with a handheld kW meter. Input kW*motor efficiency=shaft hp. It is easier than measuring pF, voltage, current, and then calculating.

tony

 

[masterelect1]

conversion

conversion

for 3 phase motors - HP x .72 = KW

and KW x 1.28 = HP

comes out very close.

Most foreign made motors come with a KW rating only, Siemens comes to mind

 

[cschmid]

most all motors are made in foreign countries..I believe it is an European standard for rating..I suppose with the ISO standards that it might become a global standard but I have not read or herd of any thing like that..

 

[GeorgeB]

Original question discussed effect on motors; induction motor heating comes from 2 primary sources, "I squared R" winding losses and similar losses in the rotor based on slip. Bearings and windage contribute, but usually minimally.

for 3 phase motors - HP x .72 = KW

and KW x 1.28 = HP

comes out very close.

It doesn't matter if it is 3 phase, single phase, or even if it is a motor ... power is power. A heater, a car engine, or hydraulic motor can be rated in kW or HP, or both.

The conversion is 1 HP = approximately 745.7 watts, usually used as 746 (0.746 kW). 1 kW = approximately 1.341 HP.

As motor ratings are their output, efficiency does not come into play here; where did the 0.72 and 1.28 values come from?

 

[bcorbin]

As motor ratings are their output, efficiency does not come into play here; where did the 0.72 and 1.28 values come from?

Tony_ps was using measured input kW, not shaft output nameplate data. If so, he was correct to incorporate the motor efficiency into his calculation.

 

[Jraef]

Many new Solid State Overload Relays now offer kW monitoring as a standard built-in feature (the high-end ones, not the cheapies). In addition there are stand-alone kW monitoring devices available, such as Siemens Simrel and Load Controls Inc.

The reason you want to look at kW for motor load monitoring vs current only is that with current only, the current can rise and fall based upon incoming voltage, independently of connected mechanical loading. kW is more representative of actual mechanical load on the motor.

But you cannot use kW monitoring for protection of the motor from thermal overload, that is still done by I^2t, a measurement of how hot your windings are getting, for whatever the reason is.

 

[masterelect1]

where did the 0.72 and 1.28 values come from?

Years of experience. As I said, these numbers are a very close proximity to what the actual value would be, but it is very close.

If you ever did any motor winding, you would know that a resistance rating can be altered by extra turns in the coils, and as no-one is verifying coil specs.