Week-6 Challenge: EV Drivetrain
Aim:
To get to know the types of converters used in HEV and EV vehicles
To find the speed using torque-speed relation
To get to known about induction and DC brushless motors
- Which types of power converter circuits are employed in an electric &hybrid electric vehicle?
A motor drive consists of a power converter and the controller. The power electronic converter handles the flow of bulk power form the source to the motor input terminals. The controller process the command input and the feedback information to generate the switching signals for power convertor semiconductor switches
The power converter can be a DC drive supplying a DC motor or an AC drive supplying an AC motor
A DC/DC converter in its basics form coverts unregulated DC input voltage at a certain level to regulated DC output voltage at a different level with very high conversation efficiency. These are operated at very high frequency. The switches are turned ON and OFF by pulse width modulation (PWM)
The power converter is made of high power fast acting semiconductors devices such as a bipolar junction transistor. There are some other transistors such as MOSFET, IGBT, SCR, GTO, MCT. These devices are used as on-off electronics switch to convert the fixed supply voltage into a variable voltage and variable frequency supply
All these devices have a control input gate or base through which the devices are turned on and off as per the command generated by the controller.
There are two types of step converters. When a low voltage input is converted into a high voltage output it is a step-up converter. Where as when a high voltage input is converted into a low voltage output it is a stepdown converter
Q 2) An electric vehicles power train with 72v battery pack is show 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%?
Given:
The following input parameters are given:
Battery pack voltage (v) =72V
Rated armature current =400A
Armature Resistance (Ra) =0.5
Motor constant (K) = 0.7
Chopper switching frequency = 400HZ
Duty cycle = 70%
Solution:
Vehicle output Voltage (V)= battery voltage(V)*Duty Cycle =72*0.70
=50.4V
So the vehicle voltage is 50.4 v
Now equation for vehicle torque is:
T v=KΦ/Ra-(KΦ)^2/Ra*W………(1)
Where
T v = vehicle Torque (N-m)
V v= vehicle voltage (volts)
W = steady state speed of vehicle (rps)
K = Motor constant
Ra = armature resistance
Substituting all the values in equation 1, we get
T v = 50.4*0.7/0.5-(0.7)^2/0.5*w
T v = 70.56-0.98w^2 ………(2)
Now the vehicle speed torque characteristics is given as
T v =24.7 – 0.0051 w^2 ………….(3)
At steady state
EQ (2) = EQ (3) ………….(4)
24.7 – 0.00051w^2 = 70.56 – 0.98w^2.
0.0051w^2+0.98w – 45.85 = 0 ……….(5)
Q.4
Explain in brief about author perspective.
Wally Ripple is a long – time proponent of electric vehicles prior to joining tesla motors he was an engineer at aero environment where he helped develop the EV1 for General Motors and as featured in the documentary movie who killed the electric car? Wally has also worked for the jet propulsion laboratory on electric vehicle battery research among other projects in 1958 as a Caltech undergraduate student he build an electric car (a converted 1958 Volkswagen microbus) and won the great transcontinental electric car race against MIT
In the beginning the author describes the dominance of internal combustion engine (ICE) in the world of automobile industry. He explains that all engines were similar and it was up to the user to select according to their requirement next the author highlights topics related to DC brushless Motor and induction motor
According to him DC brush motor is replaced by either DC brushless Motor or induction motor. Also the lead acid batteries for EVS. He also quoted that there will be the future of both the technology
(DC brushless motor and induction motor) depends upon their demands and uses.
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 with in 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 a 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 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.
Disadvantages:
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
Induction motor:
The technology behind the induction motor is that it basically uses a 3phase induction motor which was invented by Nikola Tesla sometime before 1889. Unlike the DC brushless rotor the induction rotor has no magnets just stacked steel laminations with buried peripheral conductors that from a “shorted structure” current flowing in the stator windings produce a rotating magnetic field generated by the stator to produce a force which is required for rotor torque.
When a 3 phase induction motor is connected to utility type 3 phase power torque is produced at the outset. The fact that induction motors are directly compatible with conventional utility power is the main reason for their success.
Advantage:
The induction motor does not have any magnets and in this the B field is adjustable the motor has the ability to start under load and there is no need of an inverter. In an induction motor B field is proportionate to V/F (voltage to frequency) this technologies is used to reduce the magnetic losses and condition losses due to which efficiency will remain at maximum peak. This all leads to increasing the performance induction motor is very useful and efficient for high performance
Disadvantages:
An induction motor cannot operate from DC voltage supply it always requires an AC voltage supply in an induction motor the shaft speed is proportionate to line frequency. Hence when used with utility power they have limited starting torque and some what limited running peak torque capabilities when compared to DC type machines