Tuesday, December 10, 2013

Determining load efficiency and electric motors

The engines are responsible for a large portion of our electric bill so energy efficiency deserve preferential attention. Too often the engines are misaligned or over-sized for the load plan to serve, or have been rewound multiple times.

Many times we considered the need to replace engines as a way to save energy but do not have clear operating costs and get real savings by applying this measure. To find out we need to determine the hours of operation , the values ??of improved efficiency , and load. The partial charge is a term used to describe the actual load served by the engine compared to the rated motor capacity . in this article we briefly describe the techniques we estimate loads .

Reasons for determining the load of a motor

Most electric motors are designed to operate at 50 - 100 % of rated load. The maximum efficiency is usually about 75 % of rated load . Thus, an engine 10 H.P. works acceptably in a range of cargo ranging from 5 to 10 hp , and peak efficiency occurs at 7.5 HP Engine efficiency tends to decrease dramatically below 50 % load . An engine is considered working with an insufficient charge when working in the range where the efficiency drops significantly with decreasing load .

Overloaded motors can overheat and lose efficiency. Many engines have been designed with a service factor that allows occasional overload. The service factor is a multiplier that indicates how much can be overloaded under ideal environment conditions . For example, a 10 -hp motor with a 1.15 service factor can handle a load of 11.5 hp for short periods of time without significant damage. Although many motors have service factors of 1.15 , run the motor continuously above rated capacity reduces engine life and efficiency. Never operated an overloaded motor when the voltage is below the nominal or when cooling is malfunctioning for altitude, high ambient temperature , engine or dirty surfaces .

If we use in our operation teams with engines that operate extended periods at loads below 50% , we should consider making changes . Sometimes motors are over sized because they must accommodate peaks , such as those occurring conditions when a pumping system must occasionally meet high demands. The options available to suit varying loads include two-speed motors , adjustable speed drives and load management strategies to keep them within acceptable ranges.


Measuring the power input

Determining if our engines are loaded properly can make decisions about when to replace them and what is the best way to replace them. Measuring motor loads is relatively easy and allows us to analyze the efficiency of the motor operation .

It is advisable to inspect all engines operating above 1000 hours a year, dividing engines in the following categories :

    Motors that are significantly oversized or low-load need to be replaced .
    Engines that are moderately oversized or working at low loads but moderately efficient engines will be replaced with adequate size for models when they fail .
    Appropriately sized but standard efficiency motors will be replaced with energy-efficient models when they fail .

The easiest way to test is to use engines direct power meter with the engine loaded for which there are many fairly inexpensive multifunctional teams .

Measurement of the line current

The estimation method of load current is recommended only when the measurement is disponiblemente ampere only . The motor that drives
ampere varies approximately linearly with respect to the load , down to approximately 50% at full load. Below about 50% load due to the reactive requirements of magnetizing current , the power factor is degraded and the curve is nonlinear ampere progressively. In the region of low load current measurements are not a useful indicator of load.

The current is indicated on the nameplate of the motor is the value of current at full load and only occurs when the engine runs at its rated voltage. The effective value of the current is always correct for the voltage. If the supply voltage is below or on the rating plate of the motor, the measured ampedaje correpondientemente is higher than expected under nominal condition and must be adjusted downward . The reverse also applies.

Method sliding ( slip )

Deslizamineto method for estimating an engine load is recommended only when the speed of operation is available . The synchronous speed of an induction motor is dependent on the power supply frequency and the number of poles in the motor coil. The higher the frequency, the motor rotates faster . The more poles have a motor rotates more slowly .

The actual motor speed is less than its synchronous speed and the difference between synchronous and actual speed is called slip or deslizamineo . The amount of this slip is proportional to the load on the engine by the team.

Using a tachometer to measure the speed of the current engine , you can calculate the motor loads .


The energy efficiency of a motor is the ratio between the useful output power and total power input and is usually expressed in percentage.

By definition, a bike with a number of H. P. expected given the amount of power transmitted mechanically to the motor shaft. The motor losses are the difference between the input power and output . Once you have determined the engine efficiency and input power is known , we can calculate the output power.

Most analyzes of saving energy conservation Motor assume that the existing engine is operating at the efficiency of your plate. This assumption is reasonable above 50 % load point and that the peak efficiency occurs about 3/4 charging. Larger engines powers exhibit relatively planapor efficiency curve below 25% of full load.

It is more difficult to determine the efficiency of an engine that has been in service for a long time . It is difficult for plaque has been lost and it is impossible to locate information efficiently. The engine may have been rewound , and it is likely that the efficiency is reduced .

When information is not available on the efficiency , we can determianrla operand at the point of engine load. These data we can determine the plate or engine looking for technical details .

We can also estimate the efficiency tables in Attachment C of that article in the literature.

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