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Motor starters are the most efficient solution for controlling motors in applications where the speed is fixed, and the loads are variable. The term “motor starters” generally includes combinations of contactors or soft starters with motor protection switches or circuit breakers, as well as complete devices, e.g., in the form of electronic motor starters.
Above a load level of around 60%, the use of a motor starter means that considerably less energy is consumed by the drive system than if the motor is switched or controlled by a variable speed drive. Direct motor starting is the simplest and most cost-effective way to start three-phase asynchronous motors. However, when using motor starters, it should be taken into account that a high current load in the supplying network is characteristic for the three-phase motor when directly switched on.
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Motor starters offer a total of five major advantages:
Speed can be directly controlled in drive technology via variable speed starters and variable speed drives. This means, for example, that the flow rate of a fluid or gas can be adjusted to the current requirements, instead of being controlled by valves and throttle valves, as is the case with a constant speed. Despite their own heat loss, variable speed starters and variable speed drives thus usually improve the average efficiency rating of drive systems over the entire operating range.
Variable speed drives are especially suitable for achieving energy savings in machines and systems in which the relationship between speed (n) and torque (M) is quadratic, so that the power P is related in a way that is significantly more than linear. This includes applications such as centrifugal pumps or fans. The decisive factor for energy savings is the presence of a cubic relationship between speed and power (P proportional to n), so that a pump running at 50% of its maximum speed would only consume 1/8 of the power required at full speed, for example. As a result, even a small decrease in speed results in high energy savings. This means that decreasing the speed by 20% results in energy savings of 50%, as the power input of the motor is precisely adjusted to suit the actual amount required for the process.
In a pump system, a variable delivery rate is required. Changing the delivery rate can be achieved by:
Using a throttle valve to control the pump's delivery rate results in large losses and, accordingly, in a poor efficiency rating for the overall system. These losses can be significantly reduced by the use of load-dependent speed control, leading to large energy savings and therefore a considerable reduction in operational costs.
Especially in the field of pumps and fans, multi-motor drives are also used as a way to optimise energy savings. Their basic idea is to have lower losses in the system during partial load operation. At first, a first motor is started. If it is not able to apply the required pressure or deliver the required quantity, a further motor is switched on—in this way the pressure can be increased in stages, with a further motor being switched on for each pressure stage (or switched off as the pressure requirement decreases).
The control of the various motors can look quite different: In pressure-boosting systems, multi-pump solutions ensure a better efficiency rating because the individual pumps operate at their optimum duty point. 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. In this way, a system with high energy and cost efficiency can be realized, which simultaneously offers optimal functionality.
Figure 2: Instead of covering the entire required pressure range with only one speed-controlled drive, multi-motor drives can also be used, especially for pumps and fans. They enable lower losses, especially in partial load operation.
Especially for fixed-speed applications, motor starters are not only the cheaper but also the more efficient solution compared to variable speed drives, regardless of the IE class of the motor (IE2/IE3/IE4). It is therefore always necessary to consider all system-relevant factors in order to select the best automation solution. For more detailed information, see the Eaton white paper “Successfully increasing energy efficiency in drive technology”, which is available for download on our website.
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