PLUGGING BRAKING FOR ELECTRIC VEHICLES POWERED BY DC MOTOR Nair Rajiv Somrajan 1 and Sreekanth P.K. 2 1 PG Scholar Department of Electrical Engineering, Sree Buddha College of Engineering, Pattoor, Alappuzha 2 Asst professor Department of Electrical Engineering, Sree Buddha College of Engineering, Pattoor, Alappuzha Abstract- Electric traction is a promising technology that can bring out significant improvement in vehicle performance, fuel efficiency and energy utilization. The energy management in the traction system can be well improved through the braking system used in it. In this paper a electrical braking system i.e. plugging 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. Control scheme designed and implemented to the system was a speed controller. Optimization of controller following with verification would complete the overall model. Plugging braking was presented by simulation and waveform results were discussed. Reference speed were varied and simulated. Speed-up time and braking time were discussed. Conclusions were drawn at the end of the paper. Keywords-Electric vehicle, DC motor, DC-DC converter, plugging braking. I. INTRODUCTION In recent times, electric vehicles (EVs) have received much attention as an alternative to traditional vehicles powered by internal combustion engines running on non-renewable fossil fuels These unprecedented focus is mainly attributable to environmental and economic concerns linked to the consumption of fossil-based oil as fuel in internal combustion engine (ICE) powered vehicles. With recent advances in battery technology and motor efficiency, EVs have become a promising solution for commuting over greater distances. EVs present a realistic alternative for internal combustion engine (ICE) powered vehicles. However the high cost of EVs has put them off the road in a wide scale. The expensiveness of EVs is due to the complexity of bidirectional power-flow. Hence, research and development is focused to simplify the EV configuration so as to make it more users friendly. As a part of improving energy efficiency various developments has been taken place in field of electrical automobile industries. Research works are done to improve the braking system used in the vehicle. In conventional vehicle it uses hydraulic brakes [1] or say mechanical brakes. The hydraulic braking is the process in which brake pad is pressed to brake plate which develops a braking force on the tyre ground contact area. In mechanical braking the speed of the machine is reduced solely by mechanical process but electrical braking is far more interesting than that because the whole process is depended on the flux and torque directions. The main advantage of electric braking is that decreases the wear and tear of mechanical brakes and reduces the stoppage timing extensively due to high braking retardation. In this work we are discussing an Electrical brake known as plugging brake[2] which is almost equivalent to mechanical brakes in performance of stopping vehicle. II.DC MOTOR MODELING The modeling of the DC motor [3] has been carried out with torque and rotor angle consideration. (1) Here the steady state motor torque T is related to armature current I and a torque constant K. @IJMTER-2016, All rights Reserved 352
The back emf, is related to angular velocity which is given by (2) + = (3) + = (4) 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, ( ) ( ) ( ) [( )( ) ] (5) III.CONVERTER MODELING DC to DC switching converter, sometimes called chopper, makes use of parameter called duty cycle to vary dc supply level and therefore output speed. Duty cycle is defined as the percentage of time in one period where the current is allowed to flow into the load or the voltage is supply to the load. For example, 50% of duty cycle will supply an average of half the dc supply level. Chopper is commonly divided into four types: step down chopper, step up chopper, two-quadrant chopper and four-quadrant chopper. Two-quadrant chopper would be used in the research. 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. In the research, two-quadrant dc to dc converters [4] are used. The configuration of basic dc to dc converter is shown below @IJMTER-2016, All rights Reserved 353
Figure 1.Circuit configuration of two-quadrant chopper There are two switches S1 and S2 connected across a dc voltage source ES. The switches open and close alternately in such a way that when S1 is closed, S2 is open and vice versa. S1 and S2 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. The following figure shows the Simulink model of the chopper: Figure 2.Two-quadrant chopper Simulink model. 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 gating signal supply to the switch. 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. IV. PI CONTROLLER MODEL A bidirectional converter is used to control the speed of the dc drive which is shown in figure 3. One possible control option is to control the output voltage of the bidirectional converter. A PI controller [3] is used to control the output voltage of the bidirectional converter for driving the vehicle at desired speed and to provide fast response without oscillations for rapid speed changes and it shows satisfactory result. In this control technique the motor speed ωm is sensed and compared with a reference speed ωref. The error signal is processed through the PI 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 3.Control of the bidirectional dc-dc converter 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 @IJMTER-2016, All rights Reserved 354
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 is shown in figure 4 Figure 4.Simulink Model of the Drive system. V. 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 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 two 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 yellow line. Actual speed is shown by red lines which catch up reference speed with fewer oscillations. Figure 5.Steady state speed graph 2) Plugging braking operation: In this mode brakes are applied to stop the vehicle. At a step time of 5.1 secs the plug brakes were applied and then speed changes from 120 rad/sec to 0 rad/sec. Torque changes from +10 Nm to -10 Nm. Figure 6.Plugging braking speed graph @IJMTER-2016, All rights Reserved 355
IV.CONCLUSION EV has become more and more popular alternative for non-gasoline powered vehicles. One of improvements 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 developed is a plugging brake. Here EV powered by DC motor is stopped by using a plugging. 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 plugging brakes were applied. Hence simulation results help to justify the ability of plugging brakes to stop the vehicle at the instant, which seems to be equivalent or say more accurate than mechanical brakes. Thus plugging braking provides a alternative for mechanical brakes in EV. REFERENCES [1] Nice,Karim, HydraulicBrakes",IntegratedPublishing,Available:thsmm.blogspot.com/2011/08/hydraulicbrake.html [Retrieved 18 June 2010]. [2] DC motor starting and braking Available: www.iitd.vlab.co.in [Retrieved 2 June 2016] [3] Premananda Pany, R.K.Singh,R.K.Tripathi, 2011 Bidirectional DC-DC converter fed drive for electric vehicle system, International journal of engineering, science and technology,vol.3,no.3.pp.101-110. [4] Bausiere, R., Labrique, L., Seguier, G. 1993. Power electronic converters : DC-DC conversion. Berlin: Springer- Verlag @IJMTER-2016, All rights Reserved 356