Force Analysis in Design of High Power Linear Permanent Magnet Actuator with Gas Springs in Drilling Applications Ravindra Babu Ummaneni*, Robert Nilssen* and Brennvall.J.E** *Department of Electrical Power Engineering Norwegian University of Science and Technology, Trondheim, Norway **Resonator As, Trondheim, Norway Abstract-This paper mainly describes the force analysis in the design optimization of a reciprocating linear permanent magnet actuator with interior magnet and slotless stator. This paper predicts the force characteristics according to design parameters such as pole pitch, magnet width and stator inner diameter on the basis of numerical calculations. The design is unique because of the combination of a heavy piston and gas springs. The gas springs allow the piston to oscillate at high frequency and with a long stroke length, in spite of pistons large weight. A large electromagnetic force, due to the use of permanent magnets makes a high power linear electric actuator available. The heavy piston causes a considerable vibration in the housing which is utilized in the drilling application. The load can then be attached directly to the housing and the machine can be made hermitically sealed, which eliminate leakage problems. These features make the actuator suitable as a hammer in oil drilling applications. I. INTRODUCTION A linear electric actuator is an electromagnetic device which develops directly short travel oscillatory or progressive linear motion. These devices would be advantageous in many applications like compressors, pumps, electromagnetic valve actuators, active shock absorbers, vibrators etc. In extension to these applications the oscillatory motion of linear electrical actuator can be effectively utilized in more demanding offshore oil industry drilling applications. Direct linear electro mechanical energy conversion devices offer numerous advantages over their rotary to linear counter parts, notably the absence of mechanical gears and transmission systems, which result in a higher dynamic performance and improved reliability. Among various linear machine configurations, tubular machine with permanent magnet excitation have a number of distinctive features [1], such as high force density. Recently, many papers have been published on the tubular linear machines include interior PM, surface mounted PM and halbach pattern PM. An accurate analytical field analysis of the tubular linear machines is reported in [2], based on the Maxwell’s equations in cylindrical coordinates. An overall comparison of interior and surface mounted Tubular linear permanent magnet machine is well discussed in [3]. Reviews on available drilling methods, new concept in drilling applications using gas springs and well known tubular linear permanent magnet machine technology and magnetic analysis with different magnetizations are well discussed in [4]. One of the proposed structures of the Linear Permanent Magnet Actuator (LPMA) with interior magnet and slotless stator, for example with three poles is shown in Fig.1 II. DRILLING TECHNOLOGY Even though the rotary steerable systems are evolving rapidly and delivering unprecedented rates of drilling penetration, drillers nevertheless seek technology that speeds the drilling process even more. Nowadays most of the down- hole motors with rotating drill bits are driven by positive displacement motors and down- hole turbine motors classified depend upon the type of power sections used to convert the hydraulic energy of high pressure fluid to mechanical energy in the form of torque output. In these designs the power is transmitted from power section to drill bit through different mechanical arrangements like drive shaft, coupling assembly, bearings etc. In the process of transmission part of the torque developed is wasted, resulting in a reduced efficiency. In addition, bearing stresses limits the transfer of possible forces to the drill bit. To overcome the difficulties in transfer of the power to rotating drill bit there is lot of research going on to improve the bearing designs. Furthermore, research focused on the replacement of the rotary drilling bit with linear hammer. The proposed linear permanent magnet actuator with gas springs Fig.1 Linear permanent magnet actuator 285 1-4244-0743-5/07/$20.00 ©2007 IEEE