Week-6 Challenge: EV Drivetrain

Aim:

1. To study the type of power converter circuits are employed in EV and HEV

2. To Calculate the EV steady state speed.

3. To Explain about Inductin motor vs DC brushless motor.

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

Answer:

• The type of power converter circuits are employed in EV and HEV are:

1. Rectifier: AC to DC Converter
2. Inverter: DC to AC Converter
3. Chopper: DC to DC Converter

i. Rectifier:

Rectifier is used to convert the AC signals to DC signals.

Example:

EV/HEV Charger:

• In the EV & HEV, the DC signals are required to charge the Battery source. But, we available the AC signals at the home or power stations. By using the Rectifier in the charger, we convert the AC to DC and charge the Battery.

ii. Inverter:

Inverter is used to convert DC to AC signals.

The power available at the Battery in the EV/HEV is in DC. But the Motor drive is required AC, to conver the DC to AC the inverter is used.

iii. Chopper:

Chopper is used to convert DC to DC.

i.e; Converting the high voltage DC to low voltage DC is called Step-Down Voltage.

Converting the low voltage DC to High voltage DC is called Step-Up Voltage.

2. What is EV steady state speed if duty cycle is 70%?

Given parameters and conditions are:

• 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 `(V)`= 72 V

Rated armature current `(Ia)`= 400 A

`Ra`= 0.5Ω,

`Kphi`= 0.7 Volt second

Chopper Switching frequency= 400 Hz

• The vehicle speed-torque characteristics are given by the below equation

`Tv=24.7+(0.0051)omega^2` ------(1)`

Answer:

Rated Armature voltage `(V)``= 72 V`

Voltage after 70% Duty cycle `= 0.7*72`

`= 50.4 V`

From Speed - Torque relation we know that,

`omega= V/(Ra) - (Ra)/(Kphi)*T`

`implies``Tv=(Kphi)/(Ra)*V - (Kphi^2)/(Ra)*omega`------(2)`

Substitute Motor & Controller parameters in Equation (2)

`Tv=(0.7)/0.5 *(50.4) - 0.7^2/(0.5)*omega`

`Tv=70.56 - 0.98*omega` --------(3)`

Equate `(1) & (3)`

`implies` `0.0051*omega^2 + 0.98*omega-45.89=0`

By using the Quadratic equation formula, we get the value of speed,

`omega=-b+-(sqrt(b^2-(4*a*c))/(2*a))`

`omega=-0.98+-(sqrt(0.98^2-(4*0.0051*45.8))/((2*0.0051)))``

`omega= -231.07 (radians)/(sec) and 38.92 (radians)/(sec)`

Here, we got two values for EV Study state speed,

The -Ve value of speed shows that the Torque is transfered to the motor externally and the +Ve value states that the motor generated torque reaches to the wheels

3. Refer to the blog on below topic and Explain in brief about author’s perspective.

• Induction Versus DC brushless motors by Wally Rippel, Tesla

Answer:

Working of DC Brushless Motors and Induction Motors:

• In DC Brushless motor, the rotor includes two or more permanent magnets that generate a DC magnetic field. 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.

• Induction 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.

Limitations Induction Motors:

• While 3-phase induction motors have great utility,
• Torque performance is low compared with DC machines.
• they have limited starting torque and somewhat limited running peak torque capabilities, when compared to DC type machines.
• They cannot operate from DC; AC is a must.
• They are constant speed machines.
• Induction machines are more difficult to control. The control laws are more complex and difficult to understand.

Advantages of Induction Motors:

• When a 3-phase induction motor is connected to utility type 3-phase power, torque is produced at the outset; the motor has the ability to start under load. No inverter is needed.
• Induction motors will likely retain a cost advantage over PM machines/DC Motors.
• 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.

By Adding an inverter to Induction Motors,

• It becomes possible to power an induction machine from a battery or other DC source.
• variable speed also becomes possible simply by adjusting the inverter frequency. Still, torque performance is low compared with DC machines.
• Induction motor is now capable of competing with DC and DC brushless for vehicle applications.

Common things between DC Brushless Motors and Induction Motors:

• Both DC brushless and induction drives use motors having similar stators.
• Both drives use 3-phase modulating inverters.
• Both have three sets of “distributed windings” that are inserted within the stator core.

Difference between DC Brushless Motors and Induction Motors:

• The only differences are the rotors and the inverter controls.
• And with digital controllers, the only control differences are with control code.
• One of the main differences is that much less rotor heat is generated with the DC brushless drive.

Conclusion:

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.