International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, Volume 2, Issue 8, August 2012) 167 Performance Comparison of Permanent Magnet Synchronous Motor and Induction Motor for Cooling Tower Application Prof. H.K.Patel 1 , Raj Nagarsheth 2 , Sharang Parnerkar 3 1 Assoc. Professor, Instrumentation & Control Engg. Dept., Institute of Technology, Nirma University, INDIA. 2,3 Student, Electrical Engg. Dept., Institute of Technology, Nirma University, INDIA. Abstract - The paper discusses the basic construction and types available in Permanent Magnet Synchronous Motor (PMSM) on the basis of the arrangement of the permanent magnets. The various pros and cons of each arrangement are discussed in brief. The paper elucidates the application of PMSM in industry for cooling towers, with statistical data and various practical results concerning various important parameters such as efficiency, power factor and load current. It clearly justifies and supports the efficient use of PMSM over conventional induction motors (IM) with high efficiency, with an in-depth analysis. Keywords - Permanent Magnet Synchronous motor, cooling tower, high efficiency, low power density. I. INTRODUCTION The use of variable speed drives in industry is on increase. Highly efficient drives are costly to manufacture as well as provide difficulty in maintenance. The conventionally used 3-phase induction motor is a constant speed motor, and with the help of drives the motor can be used for variable speed applications, but at the cost of reduced efficiency.[1][2][3] The recent development in Permanent Magnet machines has provided a solution for the variable speed applications, which offer easy design for controller as well as operate at higher efficiency [1] [4]. In this paper, the basic introduction of Permanent Magnet Synchronous Motor (PMSM), its types and basic constructional features are discussed [4]. The use of PMSM in cooling tower for any industry is discussed using the various graphical data, which provides an insight of its operation in comparison to the usually used induction motor. The advantages associated with PMSM and a concept toward efficient energy systems by implementing PM machines can be concluded from the discussion in the paper.[3] II. TYPES AND CONSTRUCTION OF PMSM In principle, the rotor of PMSM is constructed based on the stator frame of a three-phase induction motor. It has rotor structure similar to motor which contain permanent magnets in rotor. The design is performed in order to achieve a sinusoidal back EMF without changing the stator geometry and winding as sinusoidal excitation used with PMSM, eliminates the torque ripple caused by the commutation. PMSM are typically fed by voltage source inverter, which cause time-dependent harmonics on the air gap flux. [1][5] Permanent magnet synchronous machines can be realized with either embedded or surface magnets on the rotor, and the location of the magnets can have a significant effect on the motor’s mechanical and electrical characteristics, especially on the inductances of the machine. As the relative permeability of the modern rare-earth magnets is only slightly above unity, the effective air gap becomes long with a surface magnet construction.[6][7] This makes the direct-axis inductance very low, which has a substantial effect on the machine’s overloading capability, and also on the field weakening characteristics. As the pull-out torque is inversely proportional to the d- axis inductance, the pull-out torque becomes very high. Typically, the per-unit values of the d-axis synchronous inductances of the PMSM vary between 0.2−0.35 p.u., and consequently the pull-out torque is in the range of 4−6 p.u., which makes them well suitable in motion control applications. The drawback of a low L d –value is the very short field weakening range, as the armature reaction with a surface magnet construction is very weak. This means that a high demagnetizing stator current component would be required to decrease the air gap flux, and consequently, there would be very little current left on the q-axis to produce the torque. Direct-axis inductance of a machine having embedded magnets becomes high, as the rotor magnets per pole form a parallel connection for the flux, while with a surface magnet construction they are connected in series. With equivalent magnets, the rotor reluctance of the surface- magnet construction is therefore double compared to an embedded-magnet construction, and the inductance is inversely proportional to the reluctance [8][9]. With embedded-magnets, the direct-axis inductance is further increased because of the higher rotor leakage flux [10] [11].