Week-6 Challenge: EV Drivetrain
1.Which types of power converter circuits are employed in electric and hybrid electric vehicle?
In order to supply voltages and currents in a form that is best suited for the instruments, this technology deals with processing and controlling the flow of electrical energy. Both the control and power functions are carried out by the power electronic circuit. Power electronic switches, transformers, electric choke, capacitors, fuses, and occasionally resistors make up the power part, which is responsible for transferring energy from the source to the load. The components of the converter's power section are controlled by the control circuit. This block is constructed using either digital or analog circuit assembly in a complex, low-power electronic.
The four distinct categories are as follows:
a) The AC to DC converter, also known as a "Rectifier," which transforms AC current into unipolar DC current.
b) The "Inverter" DC to AC converter, which translates DC into AC of the desired voltage and frequency.
c) A "Chopper" DC to DC converter converts high voltage DC to low voltage DC (step down) or low voltage DC to high voltage DC (step up).
d) A "Cycloconverter" AC to AC converter converts the AC from the AC supply at the desired frequency and voltage magnitude.
Except for the AC to AC converter the other 3 are predominantly used in both the HEV and EV.
AC to DC converter (Rectifier) this used for charging the traction battery. Because the source of current is AC and the voltage stored in the traction Battery is DC.
The DC current from the Battery is then transmitted to 2 different converters, DC-AC converter (Inverter) and DC-DC converter (Chopper).
The inverter is usually found between Battery and the DC induction motor, this motor is directly connected to the wheels and is responsible for the traction. Incase of Regenerative braking, this motor functions as a generator by converting the mechanical energy (from the wheels & axles) to electrical energy which is then stored in the traction battery.
DC-DC converter takes the DC source from the traction battery. The output from this DC-DC converter is used to power all the other components of the EV/HEV like HMI, Headlamps, AC, etc.
2. An Electric Vehicle's powertrain with 72V battery pack in shown in the diagram below. The duty ratio for acceleration operation is 'd1' and for the braking operation the duty ratio is 'd2'.
The other parameters of the electric vehicle is given below,
Motor and Controller Parameters:
Rated Armature voltage= 72 V
Rated armature current= 400 A
Ra= 0.5Ω, KΦ= 0.7 Volt second
Chopper Switching frequency= 400 Hz
The vehicle speed-torque characteristics are given by the below equation
What is EV steady state speed if duty cycle is 70%?
3. Develop a mathematical model of a DC Motor for the below equation using Simulink.
Let us draw a model for the above equation.
Let V = 25 Volts
Kϕ��= 0.7
Ra��= 0.5Ω
and also let consider torque as a linearly increasing signal.
4.Athors Perspective
DC Brushless Motor:
With brushless machines, the rotor includes two or more permanent magnets that generate a DC magnetic field (as seen from the vantage point of the rotor). In turn, this magnetic field enters the stator core (a core made up of thin, stacked laminations) and interacts with currents flowing within the windings to produce a torque interaction between the rotor and stator. As the rotor rotates, it is necessary that the magnitude and polarity of the stator currents be continuously varied – and in just the right way - such that the torque remains constant and the conversion of electrical to mechanical energy is optimally efficient. The device that provides this current control is called an inverter. Without it, brushless motors are useless motors.
The author also said that a brushless DC motor produces no starting torque when directly connected to fixed frequency utility power. They really need the aid of an inverter whose “phase” is maintained in step with the angular position of the rotor.
Advantage:-
The advantage of DC Brushless Motor is that there is low heat generation in the rotor and its cooling capacity is better as compared to Induction Motor. Also, peak point efficiency is generally higher for this drive. The DC brushless drive can also operate at unity power factor, whereas the best power factor for the induction drive is about 85%. This means that the peak point energy efficiency for a DC brushless drive will typically be a few percentage points higher than for an induction drive.
Disadvantage:-
As Machine size grows, the magnetic losses increase proportionately and part load efficiency drops. Permanent magnets are expensive and difficult to handle due to very large forces that come into play when anything ferromagnetic gets close to them
A brushless DC electric motor (BLDC motor or BL motor), also known as an electronically commutated motor (ECM or EC motor) and synchronous DC motors, are synchronous motors powered by direct current (DC) electricity via an inverter or switching power supply which produces electricity in the form of alternating current (AC) to drive each phase of the motor via a closed-loop controller. The controller provides pulses of current to the motor windings that control the speed and torque of the motor. This control system replaces the commutator (brushes) used in many conventional electric motors.
The construction of a brushless motor system is typically similar to a permanent magnet synchronous motor (PMSM), but can also be a switched reluctance motor, or an induction (asynchronous) motor. They may also use neodymium magnets and be outrunners (the stator is surrounded by the rotor), in runners (the rotor is surrounded by the stator), or axial (the rotor and stator are flat and parallel).
The advantages of a brushless motor over brushed motors are high power-to-weight ratio, high speed, electronic control, and low maintenance. Brushless motors find applications in such places as computer peripherals (disk drives, printers), hand-held power tools, and vehicles ranging from model aircraft to automobiles. In modern washing machines, brushless DC motors have allowed replacement of rubber belts and gearboxes with a much simpler and more reliable direct-drive design.
An induction motor or asynchronous motor is an AC electric motor in which the electric current in the rotor needed to produce torque is obtained by electromagnetic induction from the magnetic field of the stator winding. An induction motor can therefore be made without electrical connections to the rotor. An induction motor's rotor can be either wound type or squirrel-cage type.
Three-phase squirrel-cage induction motors are widely used as industrial drives because they are self-starting, reliable, and economical. Single-phase induction motors are used extensively for smaller loads, such as household appliances like fans. Although traditionally used in fixed-speed service, induction motors are increasingly being used with variable-frequency drives (VFD) in variable-speed service. VFDs offer especially important energy savings opportunities for existing and prospective induction motors in variable-torque centrifugal fan, pump, and compressor load applications. Squirrel cage induction motors are very widely used in both fixed-speed and variable-frequency drive applications.
Conclusion:
It has been concluded that DC brushless drives will likely continue to dominate in the hybrid and coming plug-in hybrid markets, and that induction drives will likely maintain dominance for the high-performance pure electrics. The question is what will happen as hybrids become more electrically intensive and as their performance levels increase? The fact that so much of the hardware is common for both drives could mean that we will see induction and DC brushless live and work side by side during the coming golden era of hybrid and electric vehicles.