DYNAMIC BRAKES FOR DC MOTOR FED ELECTRIC VEHICLES Nair Rajiv Somrajan 1 and Sreekanth P.K 2 1 PG Scholar Department of Electrical Engineering, Sree Buddha College of Engineering, Pattoor, Alappuzh 2 Assistance professor Department of Electrical Engineering, Sree Buddha College of Engineering, Pattoor, Alappuzh Abstract- In todays scenario conventional vehicles that are powered by fossil fuels are got lot of environmental issues associated with it along with depleting fuel resource as limted the usage of this vehices. Electric vehicle (EV) which is simple, clean and quiet in operation is an eco friendly alternative to the conventional vehicle.ev provides significant improvement in vehicle performance, fuel efficiency and energy utilization. The energy management in the EV can be well improved through the braking system used in it. In this paper a electrical braking system i.e. dynamic braking of DC motor is discussed. Model of DC-DC converter and controller was developed. After combining these models with DC motor simulation was conducted. A speed controller was designed and implemented to the system for control purpose. Optimization of controller following with verification would complete the overall model. Dynamic braking was presented by simulation and waveform results were discussed.speed-up time and braking time was discussed. Conclusions were drawn at the end of the paper. Keywords-Electric vehicle (EV), DC motor, DC-DC converter, PI controller, Dynamic braking I. INTRODUCTION Facing the quandary of decreasing of gasoline sources in the world, scientists and researchers have been trying very hard to find a new alternative energy source for vehicles. Electric vehicles (EVs) have received much attention as an alternative to traditional vehicles powered by internal combustion engines running on non-renewable fossil fuels. With modern advancement in battery machinery and motor effectiveness, EVs have become a hopeful solution for commuting over greater distances.however the expensivness of EVs has put them off the road to a broad extent. Complexity of bidirectional power-flow has lead to the pricey EVs.Hence, research and development is focused to abridge the EV configuration so as to make it more users friendly. Thus to improve energy efficiency various developments has been taken place in field of electrical automobile industries. Studies are done to improve the braking system used in the vehicle. In conventional vehicle it uses hydraulic brakes [2]. In hydraulic brakes it uses brake fuild to actuate the brake shoes which creates braking torque when its make contact with the tires. Due to the friction between brake shoe and tire the vehicle speed is reduced. The speed of the machine is reduced exclusively by mechanical process in mechanical brakes but electrical braking is far more fascinating than that because the whole process is depended on the flux and torque directions. The attraction of electric braking is that it decreases the wear and tear of mechanical brakes and reduces the stoppage timing comprehensively due to high braking retardation. In this work Electrical brake which is almost equivalent to mechanical brakes in performance of stopping vehicle is disscussed. Dynamic brake or rheostatic brake is the electrical braking system that offers the same performance. II. DYNAMIC BRAKING Occasionally it is necessary to stop DC motor rapidly which can be achieved by implementing Dynamic braking [3].It is also known as Rheostat braking because an external braking resistance R is attached across the armature terminals for electric braking. When braking is needed the armature of the motor is cut off from the source and a series resistance is set up across the @IJMTER-2016, All rights Reserved 172
armature. Then the motor run as a generator and electric current flows in the opposite direction which shows that the field connection is reversed. When the motor work as a generator the kinetic energy stored in the rotating parts of the machine and the connected load is converted into electric energy, this energy is dissipated as heat in the braking resistance R and armature circuit resistance R a. The connection diagram of DCseparately excited motor is shown in fig.1. Fig.1. Dynamic braking connection diagram for separately excited DC motor Concept of dyamic braking is explained with the connection diagram shown in fig.2. Fig.2. Connection diagram during dynamic braking Relationship between braking torque and the motor speed for dynamic braking can be given as Armature Current, I a = E b / (R+R a ) (1) = (K 1.NΦ) / (R+R a ) (2) ( E b N) Braking Torque T B = K 2 IaΦ (3) = K 2 Φ (K 1.NΦ / R+R a ) (4) = K 3 NΦ 2 (5) Where, K 2 and K 3 are constant Therefore Braking torque T B N i.e. Braking torque decreases the motor speed is also decreased III. DC MOTOR MODELING The modeling of the DC motor [5] has been carried out with torque and rotor angle consideration. (6) Here the steady state motor torque T is related to armature current I and a torque constant K. The back emf, is related to angular velocity which is given by (7) + = (8) @IJMTER-2016, All rights Reserved 173
+ = (9) Rewrite the above equations in Transfer function using Laplace transformation. The transfer function from the input voltage V(s) to the angular velocity ω(s) is, ( ) ( ) ( ) [( )( ) ] IV. CONVERTER MODELING A bidirectional DC to DC converter[4] is used to control the motoring and braking operation in association with PI controller of DC motor drive used in the EV.The converter is sometimes called as Chopper, which is commonly divided into four types namely step down chopper, step up chopper, two-quadrant chopper and four-quadrant chopper. Two-quadrant chopper is used in this paper. It is basically combination of step down and step up chopper. It is operated in two mode of operation. When current is flowing to the load (positive), it acts as a step down chopper. When current is flowing back to the supply (negative), it acts as a step up chopper. The configuration of basic dc to dc converter in the DC drive system is shown in fig.3. (10) Figure 3.Basic model of DC drive system. There are two switches Q1 and Q2 connected across a battery. The switches open and close alternately in such a way that when Q1 is closed, Q2 is open and vice versa. Q1 and Q2 turn on time contribute to one period of switching. Diode connected parallel with the switch would block the current to flow downward but provided flowing upward.mosfet and diode connected in parallel can be simplified and represented as a single bidirectional ideal switch. MOSFET provides the current to flow downward while diode provides the current to flow upward. Turn on voltage for diode is small enough to be neglected. Hence, a bidirectional ideal switch was chosen to model MOSFET and diode in parallel. From block diagram of Simulink library, ideal switch block diagram was found and therefore used in the model. The two switch used in two-quadrant chopper were controlled by a controller as shown in fig.3. Gating signals are supplied to the switches. This gating signal is either on or off in a periodical manner. To serve this purpose, pulse generator block was used. Since the two switches were turned on alternatively. It is more efficient to use only one gating circuit, means one pulse generator. An inverter was designed to invert the gating signal supplied for second switch. V. PI CONTROLLER MODEL A bidirectional converter operation is controlled using a controller as mentioned in converter modelling. Controlling the output voltage of the bidirectional converter was the one possible control option available. A PI controller [1] is used to control the output voltage of the bidirectional converter for driving the vehicle at desired speed and to afford fast response without oscillations for hasty speed changes and it shows satisfactory result. In this control practice the motor speed ωm is sensed and compared with a reference speed ωref. The error signal is processed through the PI @IJMTER-2016, All rights Reserved 174
controller. The signal thus obtained is compared with a high frequency saw tooth signal equal to switching frequency to generate pulse width modulated (PWM) control signals Figure 4.PI controller for DC drive system Here the control objective is to make the motor speed follow the reference input speed change by designing an appropriate controller. The proportional-integral(pi) controller is used to reduce or eliminate the steady state error between the measured motor speed (ωmotor) and the reference speed (ωref) to be tracked. Simulink model of the overall system and dynamic braking is shown in figure 5 and 6 respectively. Figure 5.Simulink Model of the Drive system. Figure 6.Simulink Model of the Dynamic braking system. VI. SIMULATION RESULTS The simulations are carried out using MATLAB/SIMULINK. DC motor drive along with PI controller and bidirectional converter is simulated under different speed command. The inductor @IJMTER-2016, All rights Reserved 175
parasitic resistance and MOSFET turn-on resistance are not considered in this case. For test a separately excited DC motor model is used as load to the bidirectional dc-dc converter. The motor rated at 50 hp, 240 V, and 1750 rpm. Nickel metal hydride battery rated 48V 16Ah. A total of three cases of the drive system are studied: 1) Steady state operation: In this mode the reference motor speed is set as 120rad/sec with a constant torque demand of 10Nm. Here the reference speed is 120rad/sec which is indicted by green line. Actual speed is shown by red lines which catch up reference speed with fewer oscillations. Figure 7.Steady state speed graph 2) Transient state operation: In transient mode the drive is subjected to sudden change in speed. The motor is running at 60 rad/sec and t= 5 sec the speed suddenly rises to 120 rad/sec. The speed characteristic of drive is shown in fig.8 which is indicated by red line and green line indicates the reference speed. Figure 8.Transient state speed graph 3)Dynamic braking operation: In the dynamic braking speed characteristic, the speed rises to 150 rad/sec and settles to a steady speed of 120 rad/sec due to implementation of PI controller in the drive at time t= 5 sec.when the braking is applied the speed reduces to zero at time 5.3 seconds. The speed fall depends on the value of resistance used and characteristic curve varies according to variation in resistance.the speed characteristic of vehicle is shown in the fig.9 Figure 9.Dynamic braking speed graph @IJMTER-2016, All rights Reserved 176
VII. CONCLUSION EV has become more and more popular alternative for fossil fuel powered vehicles. One of advancement carried out in electrical vehicle is in braking system used in it. In this paper an electric braking for EV has been developed. Braking system proposed is a dynamic brake. Here EV powered by DC motor is stopped by using a rehostat brake. In this work DC motor drive fed by bidirectional DC-DC converter and PI controller is developed. DC drive system has been successfully modeled and output waveforms were obtained. From the results we can observe that a EV running at a speed of 120 rad/sec is brought to halt i.e. zero rad/sec when dynamic brakes were applied. Hence simulation results help to justify the ability of dynamic brakes to stop the vehicle within fraction of seconds, which seems to be equivalent or say more accurate than mechanical brakes. Thus dynmic braking provides a alternative for mechanical brakes in EV. REFERENCES [1] Premananda Pany et.al, Bidirectional DC-DC converter fed drive for electric vehicle system, International journal of engineering, science and technology, vol.3, No.3. pp. 101-110, April 2011 [2] R.K.Singal, Automobile Engineering, in Braking systems, Vol 3, Katson Books, New Delhi, India, 2006 ch 5,pp 121-138 [3] Dynamic or rheostatic Braking Available: circuitglobe.com/rheostaticbraking.html [4] Bausiere, R., Labrique, L., Seguier, G. 1993. Power electronic converters : DC-DC conversion. [5] Berlin: Springer-Verlag [6] DC motor starting and braking Available: www.iitd.vlab.co.in [Retrieved 2 June 2016] @IJMTER-2016, All rights Reserved 177