E+C SPOT ON • August 2014 T he rapid growth of the cement and concrete industry has created the need for concrete products such as concrete pipes, culverts, manholes, slabs, piles, etc. Since the role of electric machines in the concrete industry is vital, this article focuses on presenting the design of an energy effcient concrete mixer drive. The novelty of the proposed synchronous permanent magnet (PM) motor is in its abil- ity to develop helical movement of the rotor. This allows the mixer blades to mix concrete compound effectively with minimal energy consumption. The machine was analysed in 3D FEM and Matlab/ Simulink software. Effciency and rated power were also determined. The energy consumption of the proposed synchronous motor drive is compared to that of a conventional induction motor. Conclusions about the effectiveness of using the proposed machine in concrete mixers were made. The advantages of the proposed motor over con- ventional induction drives used in industrial mixers are also discussed. Concrete production technology is a complicated process and electric power is used almost in every stage of the concrete prepara- tion. One of the most energy consuming stages is when the concrete is mixed in concrete mixers. There are many techniques proposed in the literature on how to make concrete mixers more effcient through control applications [1, 2]. Williamson [3] and Zhao et al [4] discuss various energy-saving technologies for concrete mixing process. Most articles are not concerned about electric motors which drive the mixers. Mechanical power of the mixer blades is developed by the electrical drive connected through a gear or directly to the shaft where the blades are attached. Hence, the electric drive effciency is important for the energy consumption of the concrete plant. The issue of PM machines versus induction motors for concrete mixers exists. Some concrete mixer producers [5] suggest apply- ing PM machines which usually have higher power density, higher effciency, but also higher cost. Induction machines, however, still dominate in the concrete production process. This article introduces a novel design of a PM drive for concrete mixers. The high effciency and simple structure of the proposed mo- tor promises energy savings and lower cost advantages. The motor discussed is not a conventional synchronous PM motor. The machine is designed in such a way that ‘To Degrees of Mechanical Freedom’ (TDMF) of the rotor is achieved by the magnet’s skew on its surface. In other words, the rotor of the machine moves helically (rotary and linear movements are combined). There are various designs of electric machines with TDMF that have been proposed in the literature. Most of the designs are induction- or reluctance-type motors [6 - 9]. Chen L et al [10] designed a PM motor with TDMF. However, there are two windings (rotary and linear) placed in one stator, which can cause mutual inductance between them. Shuang Ye et al [11] introduce a synchronous PM drive for chaotic mixing. But in this case chaotic movement of the compound is achieved by geometrical asymmetric design of the mixing tank. Design Figure 1 shows the stator core and the rotor of the proposed machine. The rotor, placed inside of the stator, is a movable part able to move axially and rotary. The rotor consists of the rotor core made of solid iron, and permanent magnets mounted on its surface. The PMs on the rotor are skewed by six tooth pitches (12 slots).The design of the proposed motor is similar to the design of a conventional PM synchronous machine. The difference in the design and principle of operation is in magnet skew on the rotor. Figure 1: The motor with TDMF. The dimensions of the motor were calculated to meet the required power at rated speed. After the dimensions were determined, the motor was analysed in 3D FEM software. During the analyses the dimensions were optimised to satisfy admissible values of magnetic fux density distribution in the stator and rotor cores. The basic dimen- sions of the motor are shown in Figure 2. Energy savings in concrete production By O Dobzhanskyi and R Gouws, North-West University The cement and concrete industry in South Africa has been growing rapidly for the last couple of years. The novelty of the proposed synchronous Permanent Magnet (PM) motor is in its ability to develop helical movement of the rotor.