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In order to achieve maximum energy efficiency in speed-controlled drives, the process must be examined in detail and the load cycles analyzed as precisely as possible. In principle, it applies to all drive components that they are most energy-efficient when their throughput is high. If the peak performance for which an electric drive system is designed is only rarely called up, the significance of the efficiency rating of the individual components is lost.
For example, an IE3 efficiency class asynchronous motor often has a higher moment of inertia than an IE2 asynchronous motor. However, if the process mainly involves acceleration and deceleration, the greater inertia means that more energy can be consumed than with a low-inertia but less energy-efficient motor.
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In addition, the individual components of the drive system can influence each other. In this way, for example, the part-load efficiency of a motor can be improved with intelligent drive controller regulation. If the motor and drive controller were only considered individually, this potential would not be revealed.
Furthermore, the combination of a motor with a variable speed drive does not automatically represent the most energy-efficient solution for a drive task. Motor starters are the most efficient solution for controlling motors in applications where the speed is fixed, and the loads are variable. Their heat dissipation is negligible, and they offer advantages when designing the control panel because they do not require additional cooling, which also saves energy. In special applications, different drive controls are also combined in order to achieve maximum energy efficiency: for example in pressure boosting systems. Multi-pump solutions, i.e. systems made up of several pumps (drives), are often used there. If only one pump is operated at controlled speed, the required pressure can be generated very precisely, especially in the partial load range. If the demand increases, additional pumps are switched on via motor starters—they then operate at a fixed speed.
To evaluate the energy efficiency of the drive system, the IEC 61800-9 is the central series of standards. Part 1 of the standard introduces the so-called "extended product approach" (EPA): In addition to the motor and the motor control device (e.g. motor starter or variable frequency drive), it also includes the driven device.
The extended product range according to IEC 61800-9-1.
Part 2 of IEC 61800-9 puts the principles described in the first part into practice. Variable speed drive designers are provided with a methodology to determine the losses of the entire drive system (PDS, Power Drive System) and the frequency inverter (or CDM for Complete Drive Module). In addition to the energy efficiency classes for variable speed drives, those for PDS in the power range from 0.12 to 1,000 kW are also defined here.
The IEC 61800‑9‑2 introduces the three efficiency classes IES0, IES1 and IES2 to evaluate the energy efficiency of a drive system. To assign an electric drive system to a class, it is compared with a reference drive (efficiency class IES1).
If a drive system has 20 percent lower losses, it can be assigned efficiency class IES2; if it has 20 percent higher losses, it is assigned to efficiency class IES0.
To calculate the efficiency of the entire drive system, it is not enough to simply combine the IE class of the motor and the IE class of the frequency inverter.
This is because when assessing energy efficiency, the losses within a system must be taken into account depending on how long the application is operated and at which operating point. The IEC 61800-9-2 therefore defines eight operating points for which the heat dissipation of the respective system components must be specified and enables the assessment of your own drive by comparing it with a reference system. The absolute losses of a drive system are determined by adding the absolute losses of the variable speed drive and the motor at each operating point - this allows the energy consumption of the drive system to be calculated in a specific application.
Efficiency classes for drive systems (PDS, Power Drive System) according to IEC 61800-9-2
Assessing the energy efficiency of the entire electrical drive system is the central goal in order to reduce the energy requirement in machines and systems. The IE classes of the motor and variable speed drives are important building blocks and orientation aids, but an optimal combination for a specific application can only be found by considering the overall system. More detailed information about (EU) 2019/1781 is provided in the white paper "Successfully increasing energy efficiency in drive technology" by the energy management company Eaton, which is available for download on the company's website.
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