The intelligent alternative

Anyone involved with the energetic efficiency of electric motors is forced to deal with the IEC 60034-30 standard. This standard regulates the design of three-phase squirrel-cage asynchronous motors, which are assigned to various efficiency classes (IE1 to IE4). This applies to all electric motors in the performance range from 0.75 to 375 kW. The majority of classical, motor-driven diaphragm dosing pumps with a high delivery capacity of between 4 and 1010 l/h are equipped with a three-phase asynchronous electric motor, although their electrical output is generally less than the minimum performance threshold of 0.75 kW stated in the standard. These motors are therefore exempted from the obligation to comply with the standard. The Memdos LP diaphragm dosing pump from Lutz-Jesco, which is equipped with an EC motor, represents an energy-efficient solution.

An EC (electronically commutated) motor is a brushless DC motor fitted with dedicated power and control electronics. Its commutation, the alternating current feed to the individual stator branches, is performed via semiconductor elements and not mechani-cally by means of carbon brushes as it is with a standard DC motor. This method of electronic commutation and the absence of carbon brushes considerably extend the service life. Apart from the motor bearings, there is little mechanical wear. Moreover, brush arcing – which at high speeds can lead to increased brush wear and a shorter lifetime – is avoided. Brush arcing also causes electromagnetic disturbances due to feedback.

The permanent magnet attached to the rotor results in less inertia than a standard DC motor as well as higher dynamics and a smaller footprint. With a separate sensor for each stator branch, the motor is able to switch the current feed depending on the rotor position. The nature of the current feed in an EC motor allows high torque values even at low speeds. These design features add up to better than 90 % efficiency. According to the directive, a three-phase motor with a comparable output in IE3 design achieves greater than 82 % efficiency. If a variable-speed drive is required, the calculation must also include a suitable external frequency converter. By contrast, the design of the EC motor is such that it is inseparably linked to its power electronics in a single unit – and it is on this basis that its efficiency is determined.

The EC motor installed in the Memdos LP has one of three different power ratings depending on the model: 250, 550 or 650 W. All models except those with a 650 W motor are cooled by free convection (i.e. there is no additional fan), which contributes to their increased efficiency. Moreover, the pump can be operated for long periods at a low delivery capacity without requiring an external fan. The power electronics and motor package are located in a single housing, in which they form a single unit. In theory, the working torque is available even at the lowest of speeds. However, the Memdos LP switches to start-stop mode from a low threshold speed to make sure the dosing head is permanently filled. This threshold is less than that for a motor with an external frequency converter, as the torque is reduced above about 30 % of the maximum speed.

The fact that the drive is continuously monitored by sensors makes the diaphragm pump well-suited to highly precise dosing applications. The HP (High Performance) models, which are fitted with a specially developed TPU (thermoplastic polyurethane) diaphragm, enable the delivery capacity to be increased by more than 30 % with permanent backpressure up to 16 bar for an identical motor-gear combination.

On the control side, the Memdos LP is fitted with a letter-quality display and four multi-functional pushbuttons. The menu is clearly structured similar to a mobile telephone. On the connection side, the feed units are provided with an analogue input (0/4…20 mA), an impulse input, a fault signalling relay, an external start-stop button, a stroke feedback output and a two-stage level input. The Memdos LP also has an analogue output (0/4…20 mA) as standard. Timer and batch modes can be configured in the software. A diaphragm monitoring function and double diaphragm system are optionally available. The fully integrated process pump can be equipped with an industrial Ethernet interface which communicates using the Modbus TCP protocol.

If purely economic considerations are impor-tant, three-phase asynchronous motors are still the solution of choice in applications where a variable-speed pump is not required. The elimination of the control electronics in the form of a converter generates cost advantages. In this case, the Memdos LB convinces with its tried-and-tested motor concept and manual stroke length adjustment.

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