## 1 Introduction
In [15], the optimization of the energy consumption of three parallel pumps without rotational speed regulation is considered. In [16], the optimization of the energy consumption of a system of seven parallel pumps of various types, some of which are equipped with VSD, is considered. In [17], the energy consumption of three pumps is compared when varying the number of VSDs. In [18], the energy consumption of pumping systems employing 2-4 parallel pumps without rotational speed regulation is compared. The analysis shows that the use of variable speed drives for each pump in a multi-pump system is generally the most energy-efficient solution. A large number of proposed various control strategies achieved by using different optimization methods demonstrate the complexity and necessity to take into account a large number of parameters when optimizing the energy consumption of a multi-pump system. A series of research works are devoted to comparing the energy consumption of various multi-pump systems [6; 14; 15; 16; 17; 18; 19]. One of the issues that is not sufficiently described in the literature is the strategy for an optimal number of running pump selections in a multi-pump system for specific applications. In the case of low-power pumping units, the preferred option between single-pump and multi-pump configurations may not be obvious. In particular, despite a large amount of literature on the analysis of the energy efficiency of various schemes of multi-pump systems, the energy efficiency of a pump unit with a single pump equipped with a VSD and a system with two pumps only one of which is equipped with a VSD has not been compared earlier. This case is of great practical importance since both considered configurations can be used in small pumping stations. The pumping system with two motors and one frequency converter is a special case of multi-motor pumping stations with a single frequency converter (single-drive multi-pump systems) for fluid machinery applications. Such systems are widely used in the electric drive of pumping and compressor stations employing pump/compressor units of low rated power. This makes it possible to significantly reduce the capital costs of such pumping stations while providing the smooth regulation of the pressure/flow rate [20; 21; 22]. In order to compare the operation of pumps with the mentioned topology, this paper presents a quantitative assessment of the energy consumption of a system consisting of two parallel pumps (0.75 kW), in comparison with a system containing one single pump (1.5 kW) that is driven by a VSD. A single-pump system contains only one variable speed drive for the induction motor (IM). The multi-pump system with two pumps in parallel contains one induction motor, fed directly from the grid, and a second induction motor, fed by the frequency converter. An induction motor connected directly to the grid drives a pump with throttle control. Both pumping systems have the same fluid flowrate that is specific for open loop pumping systems. With the help of the mathematical model, data from the catalogues provided by the manufacturer, and experimental data, the energy consumption of the two types of pump system topologies is compared. Since the cost of a variable frequency drive is a significant part of the total price of a low-power pump system, the case of parallel operation of two pumps using one VSD is considered. In this study, we examine the feasibility of using two parallel pumps in the multi-pump system instead of a single-pump system, mainly taking into account energy efficiency.