13 th International Conference on DEVELOPMENT AND APPLICATION SYSTEMS, Suceava, Romania, May 19-1, 016 Study Solution of Induction Motor Dynamic raking Mihai Rata 1,, Gabriela Rata 1, 1 Faculty of Electrical Engineering and Computer Science, Stefan cel Mare University of Suceava, Romania Integrated Center for Research, Development and Innovation in Advanced Materials, Nanotechnologies and Distributed Systems for Fabrication and Control (MANSiD), Stefan cel Mare University, Suceava, Romania mihair@eed.usv.ro Abstract The three-phase squirrel-cage induction motors are mostly used in industrial drivers since they are rugged, reliable and economical. The DC bus voltage of the AC drives increases when the motor regenerates. The regenerating energy is usually released in the form of heat. A short mathematical description and the experimental results for different cases of dynamic brake regime are presented in our paper. This solution is recommended for student use and laboratory work to improve understanding of dynamic brake working. Keywords Variable speed drives, Induction motors, Snubbers, Electrical engineering education C O, +, + Fig. 1. Different rectifier topologies used in variable frequency drive. C O, +, + I. INTRODUCTION The most widely used motor in industry for variable speed applications has been the DC brushed motor because of its simplicity of controlling speed [1]. ut the drawbacks of this motor are high maintenance and low life-span for high intensity uses. The three-phase induction motors with short-circuited rotor (squirrel cage) are mostly used in industrial applications that require fixed speed or variable speed [, 3]. The induction motors work in two different regimes depending on the direction of energy flow as follows: motoring regime, when the motor rotor turns slower than prescribed speed. In this case, the electrical energy is transformed into mechanical energy at the motor shaft; generating regime, when the motor rotor turns faster than synchronous speed set by a drive output. In this case, the mechanical energy from the motor shaft is transformed in electrical energy. In the majority of cases the induction motors are fed by PWM frequency converters that convert the power first into DC by a diode rectifier bridge and then in AC by an IGT three-phase inverter [1]. These types of converters allow the energy to handle only in motoring direction (as shown in Fig.1.a.), and have a very low cost. During the motor is in regenerative condition, the energy from the motor flows backward through the inverter bridge diodes and the DC bus voltage increases. According to the Laws of Physics, energy is never lost or gained; this energy needs a place to transformed. In most cases (pumps, fans, etc.) where the kinetic energy in the load is small or the braking time is not an important parameter and can be increased, the regenerating energy is smaller than the power losses in the driver and in the motor [4, 5]. In applications (i.e. centrifuges, cranes, some conveyors, drives that require a very fast speed reversing, etc.) where it is necessary to brake the motor fast, power losses in the driver and in the motor are not enough for regenerating energy. There are three ways to handle it as follows: Recovering of regenerative energy in the supply system. In this case, the drive must have the ability to change the DC bus energy into fixed frequency utility power through a regenerative bridge converter (illustrated in Fig.1. or a regenerative brake. It can use a supplementary braking module with braking resistor if it is necessary to control the deceleration of the induction motor in power failure case. Using of regenerative energy from one motor/drive that works in regenerating regime by another motor/drive connected to the same DC bus line that works in motoring regime. Converting of regenerative energy in the form of heat by placing a braking module and a braking resistor, often called Chopper or Dynamic rake across the drive DC bus. The major difference between the chopper and the dynamic brake lies in the construction [6]. A dynamic brake has contains the controller (regulator circuit), the transistor (switching device) and the brake resistor, in same unit. It is used for small power and up to 0% duty cycle of dynamic brake rating (the ratio between the brake time and motor cycle time). A chopper 978-1-5090-1993-9/16/$31.00 016 IEEE 33
has only the controller and the switching device, in same unit. The braking resistors are treated as separate components that allow choosing an accurate size for the specific application. Furthermore, it permits placing the chopper in a protected case and the resistors at a distance of up to 30 m. The choppers represent a more heavy duty solution than the dynamic brake and, therefore, are more suited for a dynamic brake duty cycle greater than 0% [6]. In this paper the authors propose an efficient solution to study the dynamic brake regime at induction motor powered by a variable frequency drive. This solution can be used in laboratory work by the students and it permits them to study how the equipment works when different parameters are changed (brake time, kinetic energy of load, resistor brake, and motor speed reduction) II. LIST OF THE USED SYMOLS T required braking torque in (Nm) T m rated motor torque in (Nm) T % required percent of braking torque in (%) J tot total moment of inertia in (kgm ) a deceleration during braking in (rad/s ) motor angular speed reduction in (rad/s) n motor speed reduction in (RPM) t motor deceleration time, or brake time in (s) t C motor cycle time in (s) D duty cycle of the dynamic brake in (%) R brake resistor in () P of brake power in (W) P Rb power of brake resistor in (W) DC bus Voltage in (V) V DC III. DYNAMIC RAKE When transforming the regenerative energy into heat forms while the induction motor reduces its speed, it is very important to choose an optimum brake resistor value [6-8]. For this reason, first, it is necessary to determine the required braking torque (T ) using (1). T J a n J J t 60 t The maximum braking power can be calculated using (). P (1) n T T () 60 The necessity of using dynamic braking is determined by (3). If T % is smaller than 0%, dynamic brake is not necessary due to natural dissipation of regenerative energy in the losses in both variable frequency drive and motor. If T % ranges between 0% and 150%, a dynamic brake is required. T T % 100 (3) T m M A H J * J + J Fig.. Differences between deceleration braking and overhauling load. The duty cycle of the dynamic brake obtained by using (4), is necessary to calculate the power resistor brake. If D=1, the resistor dissipates energy continuously. t D 100 (4) t C The power of brake resistor can be calculated as in (5), which represents the continuous dissipated power (average power). D PRb P, for deceleration braking regime (5) P P D, for overhauling load regime Rb The power of resistor brake is influenced also by brake time and by how the motor is used: in deceleration braking (ventilation systems), as shown in Fig..a, or in overloading load (conveyors, cranes, elevators), as illustrated in Fig..b. We can notice that the braking energy (hatched areas in Fig.) for overhauling load is twice bigger than energy of deceleration braking. The majority of resistor manufacturers recommend the calculation of resistor power like for overhauling load regime, in the case where the motor is used in deceleration braking regime with the braking time higher than 60s. R DC V (6) P The value of brake resistor can be calculated using (6). The used brake resistors must be induction-free. IV. M A H PROPOSED SOLUTION J * J + J In this paper, the authors propose a solution for studying dynamic braking of induction motor that can be used by students in laboratory work. The motor is powered through a variable frequency drive (VFD). The proposed dynamic brake is connected to DC terminals. The block diagram is illustrated in Fig3.b. The solution adopted for monitoring the DC link voltage (V DC ) uses a sensor with galvanic isolation (LEM LV5P), an IGT transistor (model SKM5050GAL 13D from Semikron), and a driver SKHI10, also from Semikron [9]. The output voltage from the LEM sensor (V DC_1 ) is compared in two hysteresis comparators with two different threshold voltages V ref_1 and V ref_. If V DC_1 > V ref_1, the first comparator makes the brake IGT transistor to turn ON. 34
, +!, + 8 = H E = > A. H A G K A? O, H E L A 7 8 9 Variable Frequency Drive rake Resistors, + Chopper, + 0 6 4 *, + 8, + - 8 # F 0 6 8, + ) / = J A, H E L A H 5 0 1 8 H A, +, O = E? * H = A 8, + 8 H A - N J 6 H E F Fig. 3. lock diagram for dynamic brake. If V DC_1 rises above the threshold value V ref_ the VFD will trip because the output of the second comparator is connected to External Trip Digital Input of VFD. It is recommended that our solution for dynamic brake be studied in lab work than in industrial environment since students can easily understand, using a scope, how works this equipment. More than that, the electrical circuit is designed to allow students to adjust the threshold voltage values (V ref_1, V ref_ ) and to observe the changes of the dynamic brake working. An industrial dynamic brake cannot offer these features. The manufacturers do not offer the schematic diagram, the values for threshold voltages are fixed, and it is difficult for the students to measure the signals with the scope because the PC is very compact. It is very important for the students to understand the practical lab works and to get them closer to real world applications. The experimental arrangement is illustrated in Fig. 4. This solution enables the laboratory to be more practical in order to provide experience closer to the real world applications to students. For this reason, we use an industrial VFD (AEG-MICROVERTER D 10.5/380), and the connected dynamic brake that we propose in this paper. The equipment allows students to study the dynamic brake regime of induction motor in the following different conditions: different brake time, different motor speed reduction, different total moment of inertia, different value for brake resistor, different threshold voltage values. The different brake time and motor speed reduction can be obtained through VFD parameterization. The proposed solution for changing the load moment inertia is to use a load realized by different weight disks, as illustrated in Fig.4.b. c) d) e) Fig. 4. Experimental setting. These disks can be removed independently from the shaft and fixed with screws to the supports, as presented in Fig. 4.c, d and e. The students can measure the maximum diameter (D), minimum diameter (d), the weight (m), and they can calculate the inertia for each disk using (7). D d J m (7) 8 The different values of brake resistor can be very easily obtained using two resistors of the same value (in laboratory, we choose two resistors of 47 /600 W). The students should use them as follows: resistors in parallel connection (3.5 ), resistors in series connection (94), or only one resistor (47). 35
V. EXPERIMENTAL RESULTS Fig. 5 illustrates the experimental results for a braking regime without connecting the dynamic brake. The signals illustrated represent the following: Ch1 the DC link voltage of VFD, Ch the induction motor speed (measured with a tachogenerator mechanically coupled at the induction motor shaft and that has the measure constant 6V for 1000RPM), Ch3 the gate voltage of brake IGT transistor, Ch4 the current through brake resistor. In this case the DC voltage is increasing during the brake regime (when the induction motor speed is decreased). If the braking time is an important parameter that must be reduced, the DC bus voltage will continue to rise until the drive eventually tripped on a bus overvoltage fault. A solution of removing that energy from the DC link is to use dynamic brake. Fig. 6 illustrates some different cases for dynamic braking regime. In all cases the dynamic brake limits the overvoltage in DC link. Analyzing the figures in a specific order, we can see the influence of some parameters on the working equipment which we studied, as follows: Results for different inertia moment of load are illustrated in Fig.6.a (low inertia moment) and Fig.6.b (high inertia moment). When the inertia moment increases, the braking time is increasing also. Results for different brake resistor values are illustrated in Fig.6.b (47 ), Fig.6.c (94 ) and Fig.6.d (3.5 ). If the braking resistor increases, the braking current is decreasing. Results for different brake times are illustrated in Fig.6.b (t = 500 ms) and Fig.6.e (t = 000 ms). If the brake time decreases, the duty cycle of braking current is increasing. c) d) Fig. 5. Experimental results without dynamic brake connected. VI. CONLUSIONS The solution proposed by authors can be used by the students in laboratory work for understanding the working of a dynamic brake connected at a variable frequency drive that control an induction motor. Fig. 6. Experimental results with dynamic brake connected. e) 36
This solution allows studying and observing the influences that the following aspects have on the dynamic brake regime: the value of resistor brake established by different connection of resistors; the break time, that can be established using VFD parameterization; motor speed reduction by a STOP command from speed selected in VFD; the inertia moment of load established by the number of disks used. The solution proposed in this paper is more versatile than an industrial one because it offers the possibility to change some parameters, and allows students to understand how it works and the effect of each change. ACKNOWLEDGMENT The authors acknowledge financial support from the project Integrated Center for Research, Development and Innovation in Advanced Materials, Nanotechnologies, and Distributed Systems for Fabrication and Control, Contract No. 671/09.04.015, Sectoral Operational Program for Increase of the Economic Competitiveness co-funded from the European Regional Development Fund. REFERENCES [1] I.F. Soran, Sisteme de actionare electrica, Ed. MatrixRom, ucuresti, 010. [] N. R. uzatu, A. Lazar, D. Alexa, G. A. Lazar, M. Moisa, "Static Frequency Converter with RNSIC Converter and Double ranch Inverter for Supplying Three-Phase Asynchronous Motors," Advances in Electrical and Computer Engineering, vol.10, no.3, pp.66-70, 010, doi:10.4316/aece.010.03011. [3] R. Munteanu, A. Cimpeanu, A. Graur, C. Filote, G. Liuba, "Quasistationary and Transient Regime of Induction Machine Supplied by Two Stator Frequencies," Advances in Electrical and Computer Engineering, vol.14, no.3, pp.131-136, 014, doi:10.4316/aece.014.03017 [4] R. Singh; S. Umashankar; D. Vijaykumar; D. P. Kothari, Dynamic braking of induction motor - Analysis of conventional methods and an efficient multistage braking model, Energy Efficient Technologies for Sustainability (ICEETS), 013 International Conference on, pp 197-06, DOI: 10.1109/ICEETS.013.653338 [5] C. K. Zhu, S. H. Dan, "Study on Optimal Dynamic raking Resistor of Induction Motor", Advanced Materials Research, Vols. 1070-107, pp. 1-17, 015 [6] ***, What Is Dynamic raking, https://www.ab.com/ support/abdrives/documentation/techpapers/regenoverview01.pdf [7] ***, General Purpose Drive, rake Resistor Design Guide, http://apps. geindustrial.com/publibrary/checkout/det-700?tnr=installation%0 and%0instruction%7cdet-700%7cgeneric. [8] ***, Regenerative AC Drives - Understanding Regeneration, http://irtfweb.ifa.hawaii.edu/~tcs3/tcs3/misc/cfht/dome_drive_upgrad -e/drive%0education/understanding%0regeneration.pdf. [9] ***, High Power IGT Driver, SKHI10 datasheet https://www. semikron.com/products/data/cur/assets/skhi_10_1_r_l50055.pdf. 37