Determine the load of an electrical engine is essential for the energetic performance

The analysis is necessary because it does not have a lot of sense change an engine of CA conventional by another of greater energetic performance if it does not adapt or if sobredimensiona the engine in regard to the loads that have to bear. With too much frequency, the motors sobredimensionan or rebobinan several times, what carries to important losses of performance in the operation. It is thus imperative, before considering the change of some engine, determine the real loads of the engine together with his annual hours of operation.

The majority of the motors design to work with loads between 50 and 100% of the nominal power, with the maximum performance roughly to 75% of the nominal power. Like this then , an engine of 10 kW has a rank of acceptable load between 5 and 10 kW, with the maximum performance to 7,5 kW. Like general norm, the motors of high performance produce the greater savings when accionan a load to 75% of the nominal power and work more than 4.000 hours to the year.

The problem of the performance of the engine is that it is used to to diminish significantly with inferior loads to 50%, what reveals a negative reality if it takes into account that, because of the sobredimensionamiento, so only 20% of the electrical motors into use in the United Kingdom are working to full load. This supposes also a damage in regard to the costs of the consumption of power, since it has calculated that an increase of a point porcentual in the performance saves in cost of power during the useful life the equivalent of the price of purchase of the engine.

Although the sobredimensionamiento of motors is the most common problem, the infradimensionamiento can be equally hurtful since it is likely that an engine infradimensionado was overloaded, which produces sobrecalentamiento, loss of performance and probably the premature failure, with his costly consequences for the production. This problem can occur when the costs of a system are very adjusted and the user interpret the factors of work of the engine of a way too laxa.

The factor of service is a multiplicador that indicates the quantity by which can overload an engine in environingingmental conditions ideals. For example, an engine of 10 kW with a factor of service of 1,15 can bear a load of 11,5 kW during short periods of time without suffering any significant damage. Although a lot of electrical motors have a factor of 1,15, do work an engine of continuous way with upper loads to the nominal reduces the performance and the life of the engine. Besides, it does not have to do work the overburden engine when the tension was inferior to the nominal tension, or when the refrigeration was prejudiced by the altitude, a high temperature acclimatise or dirt in the surfaces of the engine.

Also loses performance with the engine working with upper or inferior tensions to the nominal tension. The result of a sobretensión is a factor of lower power, what reduces the general performance of the engine. The same is true for the operation with an inferior tension to 95% of the nominal tension. In this case the engine usually loses between two and four points porcentuales of performance and suffers an increase of the temperature of work of until 7 °C, what reduces significantly the useful life of the isolation of the engine and affects his reliability.

In base to all these data, recommends to the users of electrical motors that inspect and test all his motors that operate more than 1.000 hours to the year. Next, and in function of the results of the analysis, will have to group the motors in the following categories:

• Motors significantly sobredimensionados and infrautilizados: change these motors by others properly dimensionados in the first available occasion, for example the next unemployed of plant programmed.
• Motors moderately sobredimensionados and infracargados: when they fail, change these motors by others of greater performance properly dimensionados.
• Motors properly dimensionados but of normal performance: when they fail, change the majority of these motors by others of high performance.

A problem of this strategy is that often it is difficult can determine the characteristics of an engine that have been in service during some time. It is not rare that the plate of characteristics of the engine have fallen or was covered of painting. Besides, when it have rewind the engine, exists the possibility to have reduced his performance.

When it have lost the plate of characteristics or this was unreadable, the performance has to determine in the point of load of the engine. For this employ the measurements taken of power, current and slide to determine the load of the engine, and of here obtain the value of the performance to partial load. Finally, when it have of direct measurements of the power, can derive an estimate of load corrected with the values measured of the power in the terminal of the engine and of the performance to partial load as it indicates in the equation 1.

Equation 1:

In which:

• Loads = Power of exit like percentage of the nominal power
• Pi = Improves trifásica in kW
• η = Performance of work in percentage
• kW = nominal Power according to the plate of characteristics

For the motors rewinds have to adjust the values of the performance to take into account that the performance of an engine rewind is inferior to the of the original engine. To reflect the typical losses of the rewind deduce two points porcentuales of the normal performance for small motors of less than 30 kW, and a point porcentual for motors of greater power. Nevertheless, it has to indicate that some skilled companies in the rewind of electrical motors with optimum controls of quality can often realise the rewind without significant losses of performance.