Keywords

1 Introduction

The emission of hydrocarbons not only pollutes the environment but also contributes to global warming, which melts the iceberg and increases the sea level. Using efficient Electric Vehicles (EV) and Hybrid Electric Vehicles (HEV) for transportation is one of the solutions to reducing global hydrocarbon emission. Unfortunately, the price of EV and HEV is expensive, making it unattainable for many people, especially those living in poor countries. Thus, there is a need for an efficient, compact drive system for EV and HEV that can reduce their cost and thus making them economical and affordable left [1,2,3,4,5,6,7,8] as an alternative.

It is well known that separately excited DC motors are easier to control and more stable in any mode of operation than series and shunt DC motors. To date, separately excited DC motors have been used in many prototypes or products for EV and HEV such as in the Peugeot 106, Citroen Saxo, GM EV and Lada [9, 10]. Research also has shown that separately-excited dc motor offers EV with longer distance traversed for a lower price [9] as depicted in Table 1. However, the last generation dc motors had disadvantages in their size, weight, performance and reliability. The separately excited DC motors can provide sufficient high starting torque and constant torque during operation. In the working region above base speed/nominal speed, DC motors still have higher torque compared to that of AC motors [11,12,13,14]. The motor can supply almost constant torque during their operation which is good for climbing a steep hill so that speed will not drop drastically. The motors also feature high electrical braking power due to less power loss [15, 16] and a very good speed and controllability for regenerative power. The regenerative power can be used to charge batteries or ultra-capacitors [17]. With the latest technology, DC motor manufacturers have developed better DC motors more appropriate for EV/HEV applications. Such motors are equipped with higher power output, higher efficiency, smaller size, lighter weight, longer lasting carbon brush and commutator, lower operating voltage (less than 15 V) and easier to maintain structure (using modular concept construction) [10, 11]. The advanced brush technology for DC motors allows the motor to be used at low voltages [<50 V (35 V, 4500 A, 55 kW)] [11, 12] which results in lower power loss that guarantees longer travel distance.

Table 1 Production of electric cars [2]
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In this paper, a novel four quadrants drive DC Chopper (FQDC) design shown in Fig. 1 is proposed to work with a separately excited DC motor. The system has resistive braking mode to achieve higher efficiency during braking. While the common H-bridge [15,16,17] shown in Fig. 2 utilizes at least two semiconductors at a time, the proposed FQDC uses only one semiconductor with the same operation. The proposed chopper design helps to improve power as well as to reduce ripple.

Fig. 1
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Proposed FQDC topology

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Fig. 2
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Common H-bridge topology

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2 Methodology

2.1 Four Quadrants Drive DC Chopper Operation Modes

The proposed FQDC is designed to work in five modes of operation; driving, field weakening, generator, regenerative braking and resistive braking. These modes of operation can be controlled by an AI controller such as expert system, fuzzy logic, self tuning fuzzy [2,3,4, 8], neural network as well as ANFIS. However, this paper focused only on the circuit operation of each modes of the proposed topology. Equations (1)–(4) are applicable for all chopper operation modes which represent the voltage, torque and current for the chopper. In those equations, Bemf is the back emf of the motor, Kb is the back emf constant and Kt is the torque motor constant. Ra and Rf are the motor coil resistances, ω is the angular speed and Td is the motor torque. La is the armature inductance of the motor and Lm is an additional inductance connected in series with the armature windings to smoothen the motor armature current. Vt is the terminal voltage, Lf is the inductance of the field winding and Rb is the brake series resistor.

$$ I_{a} = \frac{{V_{batt} - I_{a} \left( {R_{a} } \right) - B_{emf} }}{{L_{a} }} $$
(1)
$$ I_{f} = \frac{{V_{batt} - I_{a} \left( { + R_{f} } \right)}}{{L_{f} }} $$
(2)
$$ B_{emf} = K_{v} I_{f} \omega $$
(3)
$$ T_{d} = K_{t} I_{f} I_{a} = {\text{J}}\frac{d\omega }{dt} + B_{w} + T_{L} $$
(4)

2.2 Driving Mode

The successes of driving mode rely on the capacitor and the charge that is stored. Field and armature coil of separately excited motor has different resistance values where the field has almost a ten times or higher resistance value than the armature winding. During starting up, the back emf is zero without back emf to limit the armature current; the current would be high at the armature. Unlike power supply at home, an EC Battery has only certain limited current that it could provide. Due to big difference of armature and field winding the current from the battery is most likely preferred to flow to the armature coil living very small amount entering the field winding. So the lack of current will be topped by the capacitor current. This action is only required at the start of the motor rotation because once it has already started the motor from the armature current will decrease as a result of BEMF. In this case the current from battery is sufficient to be segregated to the armature and field winding. After releasing the pulses, the armature/load current that is governed by the preset control factor α (for the field igbt) and β (for the armature igbt) flows through the chopper. The switching of main IGBT V1 and IGBT V2 are determined by the control factor of α and β related to the duty ratio. The current path for driving operation is shown in Fig. 3.

Fig. 3
figure 3

FQDC in driving mode

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In driving mode, the control factor α is set to unity and the control factor β is varied. This is an armature control mode. IGBT V2 current will rise when the IGBT V2 gate is fired on. The current rises gradually as its rate is limited by the circuit inductance. The values of armature and field currents when IGBT V1 and V2 are fired as in driving and field weakening mode can be described by Eqs. (5) and (6).

$$ \left[ { - \left( {\frac{{R_{f} }}{{L_{f} }}} \right)} \right]\left[ {I_{f} } \right]\left[ {\frac{1}{{\left( {L_{f} } \right)}}} \right]\left[ {V_{dc} K_{v1} } \right] $$
(5)
$$ \left[ {\frac{{ - R_{a} }}{{L_{a} }}} \right]\left[ {\begin{array}{*{20}c} {I_{a} } \\ \end{array} } \right]\left[ {\frac{1}{{L_{a} }} - \frac{1}{{L_{a} }}} \right]\left[ {\begin{array}{*{20}c} {V_{dc} K_{v2} } \\ {B_{emf} } \\ \end{array} } \right] $$
(6)

2.3 Field Weakening Mode

When the field weakening is needed, the control factor β of IGBT V2 is set to almost unity. The control factor α of IGBT V1 is gradually decreased. This is the field control mode and the current path is shown in Fig. 4.

Fig. 4
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FQDC in field weakening mode

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Reducing the current distribution at field winding decreases the back emf voltage as described by Eq. (1). The value of the current in the field coil is indirectly proportional to the value of α. Meanwhile, the armature current Ia rises as the IGBT V1 duty ratio is decreased in proportion with the reduction in field current. The motor speed will increase due to the inflow of the armature current. This leads to an increase in the motor torque and speed. The mathematical equations used in the driving mode are also applicable to the field weakening mode. Only the control strategy differs between the two modes of operation.

2.4 Generator Mode

The kinetic energy that is available when the vehicle is going downhill can be used to generate electrical power to charge the batteries. The generated power is collected through the armature winding. The current has to flow through the field winding to establish the magnetic field. The IGBT V1 is fired on to generate the voltage and current at the armature winding. The flow of current in the field and armature winding is shown in Fig. 5.

Fig. 5
figure 5

FQDC in generator mode

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When contactor 2K2 is energized the armature current becomes negative due to the change in current direction. The generator mode reduces the motor speed since a counter torque motion is generated. In the generator mode, the field current and armature current are given by Eqs. (7) and (8).

$$ \left[ { - \left( {\frac{{R_{f} }}{{L_{f} }}} \right)} \right]\left[ {I_{f} } \right]\left[ {\frac{1}{{\left( {L_{f} } \right)}}} \right]\left[ {V_{dc}} K_{v1} \right] $$
(7)
$$ \left[ {\frac{{ - \left( {R_{bh} + R_{batt} + R_{a} } \right)}}{{L_{a} }}} \right]\left[ {I_{a} } \right]\left[ {\frac{1}{{L_{a} }}} \right]\left[ {B_{emf} } \right] $$
(8)

2.5 Regenerative Braking Mode

The braking operation starts with the firing of IGBT V1 until a minimum motor current is reached. As soon as IGBT V1 is fired, current flows from the battery via IGBT V1. This guarantees a quick build-up of the motor voltage and currents via armature terminal. The path of both excitation and load currents in this mode of operation is shown in Fig. 6.

Fig. 6
figure 6

FQDC in regenerative braking mode

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Every time IGBT V3 is triggered, it creates a short circuit path for the armature current to flow. Instead of going to the batteries to generate voltage, the current goes back to the armature coil which is short circuited. Due to the short circuit, a high current is induced. This high current creates a counter torque action against the motion of the motor which leads to braking or slowing down of the motor speed. The braking action reduces the kinetic energy of the vehicle and consequently the rotational speed of the motor. The armature and field current during regenerative braking can be represented by Eqs. (9) and (10).

$$ \left[ {\frac{{ - \left( {R_{f} } \right)}}{{L_{f} }}} \right]\left[ {I_{f} } \right]\left[ {\frac{1}{{L_{f} }}} \right]\left[ {V_{dc} K_{v1} /\frac{{B_{emf} }}{{\left( {1 - K_{v3} } \right)}}} \right] $$
(9)
$$ \left[ {\frac{{ - (R_{a} + R_{batt} )}}{{\left( {L_{a} } \right)}}/\frac{{ - (R_{a} + R_{f} )}}{{\left( {L_{a} + L_{f} } \right)}}} \right]\left[ {I_{a} } \right]\left[ {\frac{1}{{\left( {L_{a} } \right)}}} \right]\left[ {\frac{{B_{emf} }}{{\left( {1 - K_{v3} } \right)}}} \right] $$
(10)

2.6 Resistive Braking

The IGBT V1 is fired to build up the needed generated voltage. These actions are illustrated as in Fig. 7.

Fig. 7
figure 7

FQDC in resistive braking mode

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The resistive braking mode starts when IGBT V2 is fired. The duty ratio of IGBT V2, namely β, is varied according to the speed of the motor. IGBT V1 is fired to keep the motor current constant. The generated voltage reduces proportionally with the decreasing vehicle speed. The firing of IGBT V2 guarantees the motor current to commutate into the braking resistor Rb. The flow of the current can be regulated by controlling the duty ratio of IGBT V2. The armature and field current during resistive braking can be represented by Eqs. (11) and (12) respectively.

$$ \left[ {\frac{{ - \left( {R_{f} } \right)}}{{L_{f} }}} \right]\left[ {I_{f} } \right]\left[ {\frac{1}{{L_{f} }}} \right]\left[ {V_{dc} K_{drv} /\frac{{B_{emf} }}{{\left( {1 - K_{v3} } \right)}}} \right] $$
(11)
$$ \left[ {\frac{{ - (R_{a} + R_{bh} + R_{batt} )}}{{\left( {L_{a} } \right)}}/\frac{{ - (R_{a} + R_{bh} + R_{f} )}}{{\left( {L_{a} + L_{f} } \right)}}/\frac{{ - (R_{a} + R_{bh} )}}{{\left( {L_{a} } \right)}}} \right]\left[ {I_{a} } \right]\left[ {\frac{1}{{\left( {L_{a} } \right)}}} \right]\left[ {\frac{{B_{emf} }}{{\left( {1 - K_{rgb} } \right)}}/B_{emf} K_{v2} } \right] $$
(12)

This type of braking mode can work independently or in combination with regenerative braking as shown in Fig. 8.

Fig. 8
figure 8

Regenerative and resistive braking mode

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If it works in combination the produce braking torque would be higher than the regenerative braking suitable for emergency brake. To work in combination the braking resistor value is set close to the regenerative braking short circuit resistance. The braking power of standalone resistive braking mode is much related to the value of braking resistor. The higher the braking resistor the lower the current flow through the braking resistor and the lower the braking torque produced.

2.7 Experimental Setup

In order to test the FQDC in all modes, an experimental set-up as shown in Figs. 9 and 10 were prepared. A 650 W separately excited dc motor shaft is coupled with another motor with inertia load (flywheel). Experimental results were captured using a data acquisition software. The experimental values of motor speed, armature current and field current were transferred to FQDC model in MATLAB/SIMULINK to validate the simulation model.

Fig. 9
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FQDC for separately excited motor

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Fig. 10
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Experimental set-up for the FQDC

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2.8 Results and Discussion

The Experimental and simulation results of the motor speed, armature current and field current for each modes of operation were shown in Figs. 11, 12, 13, 14 and 15. At the beginning of the driving mode, motor speed increases until it reaches the saturation level as shown in Fig. 11. Then the field weakening mode shown in Fig. 12 was initiated causing the motor speed to increase due to the rise in torque. In this mode, the armature current increases while the field current decreases.

Fig. 11
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Driving mode

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Fig. 12
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Field weakening mode

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Fig. 13
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Generator mode

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Fig. 14
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Regenerative braking mode

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Fig. 15
figure 15

Resistive braking mode

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In the generator mode shown in Fig. 13, the field winding was charged with voltage to establish the magnetic field so that the armature coil could produce the generated voltage. In the graph the generated voltage shown has been normalized. During regenerative braking shown in Fig. 14, IGBT V3 is turned on causing a short circuit path at the armature. The armature current rose in the opposite direction resulting in the developed torque to oppose the motor motion.

This caused the braking action. In the resistive braking mode shown in Fig. 15 the generated voltage was dissipated through the braking resistor. It is thus observed that the proposed chopper design behaved like a buck converter in all operation modes except for regenerative braking. In regenerative braking mode, the chopper was observed to behave like a buck-boost converter.

The proposed FQDC offers several favorable performances such as lower power switching losses, smoother torque and current ripple, less voltage drop and higher output power, as well as better braking performance as compared to the conventional H-bridge chopper. Every switching power electronics contributes to power switching losses. Losses during switch off are far greater than during switch on. The proposed chopper has single IGBT and single diode switching on and off during its operation. Meanwhile, the H-bridge has two power semiconductors and two diodes switching on and off during its operation. Thus, the switching power losses in H-bridge would be doubled. Figures 16, 17, 18 and 19 show the turn on and turns off power losses in IGBT and diodes respectively.

Fig. 16
figure 16

IGBT turn on power losses

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Fig. 17
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IGBT turn off power losses

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Fig. 18
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Diode turn on power losses

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Fig. 19
figure 19

Diode turn off power losses

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The proposed chopper also offers smoother armature current and torque shown in Figs. 20 and 21, as compared to H-bridge due to a single IGBT firing. It is difficult to get the IGBT in H-Bridge fires at the same time. The proposed chopper used contactor so that the voltage stored in inductor winding will be easily discharged.

Fig. 20
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Ripple current

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Fig. 21
figure 21

Ripple torque

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Accordingly, voltage at the inductor winding during IGBT turn off can easily be removed in this chopper circuit as compared to voltage in the H-bridge. Voltage stored in H-bridge has to be returned to the battery. When the stored voltage is lower than the battery voltage, the clamping diode will switch off thus blocked the stored voltage avoiding it to return to its origin. When the IGBT is switch on again, the residual voltage stored in the circuit causes ripple in armature current. The influence of low ripple has current results in low ripple torque.

Every semiconductor has voltage drop during turn on. This is called the voltage collector emitter saturation. While the proposed chopper running on single IGBT, H-bridge is running on two IGBT resulted in a double voltage drop. Since the chopper has less voltage drop as compared to the one in H-bridge, therefore its armature current is observed to be higher than in H-bridge. As higher current produces more torque and higher speed, thus the chopper output power is observed to be higher. The current and output power is depicted in Figs. 22 and 23 respectively.

Fig. 22
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Armature current

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Fig. 23
figure 23

Braking effect at high SOC

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As the battery state of charge is almost 100%, the voltage difference between the charging voltage and battery terminal voltage is getting lower and resulting in a lower charging current. However, braking is required at this moment. Since the battery current is low, thus armature current is also low. Accordingly the braking torque produced is also low. In order to achieve the required braking torque, IGBT V3 is fired. The IGBT V3 caused a rise in armature current due to the effect of armature voltage short circuited. Consequently, the braking torque is increased as shown in Fig. 24. In the case of higher braking torque is needed, i.e. an emergency brake, IGBT V3 can be further controlled to produce higher braking torque as shown in Fig. 25 thus the EV speed will decelerate faster. Since the braking torque is influenced by both field and armature currents, therefore any changes in vehicle speed will affect the field current. Subsequent change in vehicle speed leads to change in braking torque. Since the braking torque produced is always nonlinear, thus a proper control of IGBT V3 provides better control in the braking torque, resulting in linear braking effect as shown in Fig. 25.

Fig. 24
figure 24

Higher braking effect in regenerative braking

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Fig. 25
figure 25

Linear braking efect in regenerative braking

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Resistive braking mode offers another type of electrical braking. In the case of a higher braking action is required, a lower braking value of braking resistor is chosen. As lower braking resistor causes a rise in armature current, hence higher braking torque can be generated resulting a drop in motor speed as shown in Fig. 26. Resistive braking mode is able to extend the electrical braking before mechanical brake takes over.

Fig. 26
figure 26

Resistive braking effect

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3 Conclusions

The proposed four quadrants drive DC chopper design has been simulated and experimented to drive a separately excited DC motor for five modes of operation. The experimental results revealed that chopper is capable to operate in all the five modes. However, the FQDC performances such as lower power switching losses, smoother torque and current ripple, less voltage drop and higher output power, as well as better braking performance as compared to the conventional H-bridge chopper were simulated using Matlab/Simulink mathematical model. In summary, the FQDC has great potential to be utilized in EV application due to simple and excellence controllability thus made it suitable for applications in this cost effective EV solution.