Dynamics and Limits of Electrical Braking Can Gökçe 1 , Özgür Üstün 2 , and Ahmet Yasin Yeksan 2 1 TOFAŞ Türk Otomobil Fabrikası A.Ş. Y. Yalova Yolu N.574 Osmangazi, Bursa, Turkey can.gokce@tofas.com.tr 2 İstanbul Technical University, Electrical Engineering Dept., Maslak, İstanbul, Turkey oustun@itu.edu.tr, yeksan@itu.edu.tr Abstract Conversion between electrical energy and mechanical energy is done by electrical machines. It is possible to use most of the electrical machines as motor or generator and also it is easy to switch between these states. This phenomenon makes them preferable in any dynamic application. Due to fast torque response, ease of control and efficiency; brushless DC machines (BLDC) are widely used in applications those need both acceleration and deceleration in operation, i.e. electric propulsion. This paper investigates features and scope of using a BLDC as a motor/generator. 1. Introduction Any moving or rotating system eventually stops if no accelerating power is applied and a friction force is affecting. Actually, this is conversion of kinetic energy on the system, seen in Eq.1.1(a) for translational moving body, Eq.1.1(b) rotational moving body, to heat energy(m: mass of translational body, ϑ: linear velocity, I: moment of inertia around rotational axis, ω: angular speed).       (1.1.a)       (1.1.b) If a moving or rotating body is requested to be stopped quickly, braking systems are utilized. In this case, system frictions are trivial and braking power can be used to define amount of deceleration. New translational or rotational energy can be found subtracting E brake calculated in Eq.1.2 from relevant energy and new velocity or angular speed can be easily found using Eq.1.1.    ∫         (1.2) Braking force affecting on the system also defines stopping time. If E brake is equal to E translational or E rotational , system stops and t stopping =t 1 -t 0 . To be able to design a braking system, one must basically know maximum speed and mass of the system and requested stopping time as input. Since the above mentioned systems are decelerated by friction, difference kinetic energy is turned into heat energy and (needed to be) dissipated. In systems driven by electric machines, it is possible to decelerate the system by means of electrical braking. In this case, generated electrical energy can be stored in batteries, given back to power lines or turned into heat in resistors, etc. Possibility to regain this energy is an important input to those who design efficient dynamic systems. This phenomenon is widely used in electric and hybrid electric vehicles [1-4]. For example, battery powered electric vehicles regenerate electric from braking energy to store on their batteries or trains use braking energy to support line and supply another accelerating train connected to line. However, electrical braking has its own limitations and vehicles with electric drives cannot be braked safely and efficiently only by electrical means. This requires introduction of mechanical (generally hydraulic) braking systems. Blending of mechanical and electrical braking is a tough issue that the engineers and researchers are putting great effort to define the best way to control braking and optimize solutions. The solutions include various methods from optimal control to soft control [5, 6]. In this paper, braking characteristics of brushless DC (BLDC) machines, which are widely used on light electric vehicle applications, are studied and limitations of electric braking is investigated. An experimental rotational system is built to see theoretical features on a physical system. Several deceleration tests are realized and effects of various components are seen. 2. Characteristics of a Brushless DC Machine (BLDC) Fig. 2.1 Equivalent circuit of BLDC motor [7] BLDC is very similar to conventional DC machine. But unlike conventional DC machine, BLDC does not have slip-ring for commutation; instead, electronic commutation is realized. The position of rotor and stator is followed by Hall Effect sensors or encoders and relevant windings are triggered. Similar to conventional DC machine, there is a correlation between armature voltage and speed, current and torque as seen in Eq.2.1(a) and (b), where k e and k i are speed and torque coefficients and related with design of the machine, ε is back EMF and i is armature current.