Week-6 Challenge: EV Drivetrain

Q1). Which types of power converter circuits are employed in electric and hybrid electric vehicle?

Explaination:

Power Electronics

Power Electronics is to conversion and control of Electric power, it is a combination of Electronics and Electrical Power. The Power Electronics conversion of power from one form to another and the change from one voltage level to another by using different types of power electronic converters, that is Optimally suited for user load.

There are 3 types of converters which is widely used in Electric and Hybrid Electric Vehicle:

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%?

Solution:

The Electric Vehicle Motor and its Parameter are given below:

Rated Armature voltage= 72 V

Rated armature current, Ri= 400 A

Armature Resistance, Ra= 0.5Ω,

Motor Constant, KΦ= 0.7 Volt second

duty cycle= 70%

Chopper Switching frequency= 400 Hz

battery pack Voltage= 72V

steady state speed= ?

Solution:

The Vehicle output voltage is given by:

Vo= Vbat*duty cycle

= 72*70/100 = 72*0.7

= 50.4v

Now, Vehicle Torque Equation is given by:

Tv=(V*Kϕ/Ra) - ((Kϕ)^2/Ra)*ω

Tv= Vehicle Torque (T-n)

Vv= Vehicle Voltage (volts)

Ra= Armature Resistance (ohms)

Kϕ= Motor constant ()

ω= steady state speed of vehicle (rps)

put all values in the equation (1)

Tv = (50.4*0.7/0.5)-((0.7)^2/0.5)*ω

Tv = 70.56-(0.98)*ω -------------------(2)

Tv = 24.7+(0.0051)*ω^2 ----------------(3)

solve these above equation we get;

70.56-(0.98)*ω = 24.7+(0.0051)*ω^2

0.0051ω^2 + 0.98ω - 45.86 = 0

Now, we got Quadratic Eauation by using Quadratic Formula,

x = −b ± √(b² − 4ac) / 2a

x = -(0.98)± √((0.98)² − 4*0.0051*(-45.86)) / 2*0.0051

= - (0.98)± √(0.9604 − 4*0.0051*(-45.86)) / 2*0.0051

= - (0.98)± √(0.9604 + 0.9355) / 0.0102

= - (0.98)± √(1.8959) / 0.0102

= - 0.98 ± 1.376 / 0.0102

x=(-0.98 + 1.376)/0.0102 and x=(-0.98 - 1.376)/ 0.0102

Hete, after solving we get two x values

x= 38.82 and x = -230.98

now write 'x' is witten as 'ω'

ω= 38.82 rad/s and ω= -230.98 rad/s

But the Angular speed is a scalar quantity ---- ω=38.82 rad/s (convert it into RPM = rad/s/2π × 60)

ω= 370.89 RPM

EV steady state speed if duty cycle is 70% is 370.89 RPM.

Q3). Develop a mathematical model of a DC Motor for the below equation using Simulink.

Solution:

Vo= Vbat*duty cycle

= 72*70/100 = 72*0.7

= 50.4v

Ra= 0.5Ω

KΦ= 0.7 Volt second.

The vehicle speed-torque characteristics are given by the below:

Here, we observe that the Vehicle speed is decreasing with the increasing in motor Torque.

speed is taken on Y-axis and torque is taken on X-axis.

4. Refer to the blog on below topic:

Induction Versus DC brushless motors by Wally Rippel, Tesla

Explain in brief about author’s perspective.

Explaination:

Author Wally Rippel, he was a Principal Power Electronics Engineer where he helped develop the EV1 for General Motors and was featured in the documentary movie, Who Killed the Electric Car? In 1968, as a Caltech undergraduate student, he built an electric car (a converted 1958 Volkswagen microbus) and won the Great Transcontinental Electric Car Race against MIT.

In that odious world of gas-powered vehicles, engines are not all alike. There is no one best engine type, rather there are different types of engines to suit personal requirements, such as price and performance. But this is also true for electric vehicle drives.

Earlier, there were lead acid batteries, DC brush motors, and contactor controllers. Today, none of these remain Lead acid batteries has been replaced by lithium batteries and DC brush motor by either DC brushless or induction motor. Contactor controllers, meanwhile, have given way to modulating inverters.”.

The Author Discuss about the topic on Induction Motor Versus Brushless Motor

­Induction Motor and Brushless DC Motor, which is widely used in Electric and Hybrid-Electric Vehicle.

BLDC Motor

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.

In contrast, 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.

Back in the 1990s all of the electric vehicles except one were powered by DC brushless drives. Today, all the hybrids are powered by DC brushless drives, with no exceptions.

One of the main differences is that much less rotor heat is generated with the DC brushless drive. Rotor cooling is easier and peak point efficiency is generally higher for this drive. The DC brushless drive can also operate at unity power factor.

In an ideal brushless drive, the strength of the magnetic field produced by the permanent magnets would be adjustable. When maximum torque is required, especially at low speeds, the magnetic field strength (B) should be maximum – so that inverter and motor currents are maintained at their lowest possible values. This minimizes the I² R (current² resistance) losses and thereby optimizes efficiency. Likewise, when torque levels are low, the B field should be reduced such that eddy and hysteresis losses due to B are also reduced. Ideally, B should be adjusted such that the sum of the eddy, hysteresis, and I² losses is minimized. Unfortunately, there is no easy way of changing B with permanent magnets.

With DC brushless, as machine size grows, the magnetic losses increase proportionately and part load efficiency drops.

Induction Motor

The Induction motor of the rotor has no magnets – just stacked steel laminations with buried peripheral conductors that form a “shorted structure.” Currents flowing in the stator windings produce a rotating magnetic field that enters the rotor. In turn, the frequency of this magnetic field as “seen” by the rotor is equal to the difference between the applied electrical frequency and the rotational “frequency” of the rotor itself. Accordingly, an induced voltage exists across the shorted structure that is proportionate to this speed difference between the rotor and electrical frequency. In response to this voltage, currents are produced within the rotor conductors that are approximately proportionate to the voltage, hence the speed difference. Finally, these currents interact with the original magnetic field to produce forces – a component of which is the desired rotor torque.

When a 3-phase induction motor is connected to utility type 3-phase power, torque is produced at the motor has the ability to start under load.

The only notable uses of induction drives have been the General Motors EV-1; the AC Propulsion vehicles, including the tzero; and the Tesla Roadster.

whereas the best power factor for the induction drive is about 85 percent.

In Induction machines have no magnets and B fields are “adjustable,” since B is proportionate to V/f (voltage to frequency). This means that at light loads the inverter can reduce voltage such that magnetic losses are reduced and efficiency is maximized. Thus, the induction machine when operated with a smart inverter has an advantage over a DC brushless machine – magnetic and conduction losses can be traded such that efficiency is optimized.

With induction, as machine size grows, losses do not necessarily grow. Thus, induction drives may be the favored approach where high-performance is desired; peak efficiency will be a little less than with DC brushless, but average efficiency may actually be better.

Induction machines are more difficult to control. The control laws are more complex and difficult to understand. Achieving stability over the entire torque-speed range and over temperature is more difficult with induction than with DC brushless.

1. What do they have in common?

Curiously, the stators for the 3-phase induction motor and the DC brushless motor are virtually identical. Both have three sets of “distributed windings” that are inserted within the stator core.

Both DC brushless and induction drives use motors having similar stators. Both drives use 3-phase modulating inverters.

2. What differentiates them?

The only difference between the two machines are the rotors and the inverter controls. And with digital controllers, the only control differences are with control code. (DC brushless drives require an absolute position sensor, while induction drives require only a speed sensor).

Permanent magnets are expensive – something like $50 per kilogram. Permanent magnet (PM) rotors are also difficult to handle due to very large forces that come into play when anything ferromagnetic gets close to them. This means that induction motors will likely retain a cost advantage over PM machines. Also, due to the field weakening capabilities of induction machines, inverter ratings and costs appear to be lower, especially for high performance drives. Since spinning induction machines produce little or no voltage when de-excited, they are easier to protect.

Conlusion:

• The conclusion is that DC brushless drives will likely continue to dominate in the hybrid and coming plug-in hybrid Vehicle markets.
• where as Induction drives will likely maintain dominance for the high-performance pure electric Vehicle.

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.