Week-6 Challenge: EV Drivetrain

1. Power converters are electrical and electronic circuits that are used to either convert DC to AC and vice versa or to convert one level of DC voltage into another one (step up or step down).

Power converters can be mainly classified as follows:

1. DC to AC converters:

This category is mainly comprised of inverters. They are used mainly between DC link and Electric motor to convert DC to AC which is essential to run the motor in case of AC induction motor.

2. AC to DC converters:

Rectifiers fall under this category. Again, this is used in between Electric motor and DC link to convert AC to DC in the events such as regenerative braking or batytery recharging mode.

The two types of full wave rectifiers are:

• Centre tap full wave rectifier
• Bridge full wave rectifier

3. DC to DC converters:

DC/DC converters are mainly used between the Energy supply source such as battery/Supercaps/Fuel cell to either step up/down the voltage level to desired level.

The two main types are:

• Non isolated converter:

Non isolated converters have a DC path between Input and output via an IC.

For Non isolated converters, step-up/step-down ratio is less than 4.

Non isolated converters can be sub categorized into:

• Buck converters (Step down)
• Boost converters (Step up)
• Buck boost converters (Step up or Step down)
• Cuk converters
• Charge pump converters (Used in applications that have low power)

• Isolated converters:

Isolated converters will have the input isolated from the output with the help of a transformer. All isolated converetrs are bi-directional.

Isolated converters can be sub categorized into:

• Fly back converters
• Forward converters
• Half bridge converters
• Full bridge converters
• Push pull converters.

2. Given EV characteristic equation:

T_tr = 24.7 + 0.0051ω²

Rated Armature voltage, Es = 72 V

Rated armature current Ia = 400 A

Ra= 0.5Ω, KΦ= 0.7 V-s

Chopper Switching frequency= 400 Hz

Also, it has been stated that the duty cycle is 70%

D = 0.7.

W.K.T the Torque speed relationship for a motor is given by the following equation:

Tm = (K∅* Es*D)/Ra – (K∅^2/Ra)* ω_m

Also, the no load speed of a motor is given by,

Tm = K ∅*Ia

The gear ratio will be taken as 1 since it is given 1:1 on the differential

Thus, G =1.

Therefore, ω_{m =} ω and Tm = T_tr

On substituting the values of the parameters in torque motor relation,

Tm = (0.7 * 72 * 0.7)/0.5 – (0.7^2/0.5)* ω

Tm = 70.56 – 0.98 * ω

Equating both the torque equation, we get a quadratic equation,

24.7 + 0.0051*ω² = 70.56 – 0.98* ω

ω² = 8992.156 – 192.151 * ω

ω² + 192.151* ω – 8992.156 = 0

On solving, we get

ω = 38.91 rad/s or -231.06 rad/s

since speed is positive,

ω = 38.91 rad/s

or

N = 371.56 RPM.

3. A careful scrutiny of Wally Rippel and Tesla’s paper on Induction Versus DC brushless motors lead to the following disparities between the two:

BLDC	AC induction motor
1. Here the real power is almost the same as apparent power. Meaning there is an efficient utilisation of all the power developed by the motor	Due to the reactance offered within the motor, there is nearly 15% power loss.
2. A need for a Converter is not mandatory unless the supply voltage has to be stepped up or down to a required level as the BLDC works on DC supplied by the motor itself.	There is a need for DC-AC bidirectional converter as an AC motor cannot directly utilise the DC offered by the power source.
3. BLDC generates lesser heat	AC motor generates more rotor heat
4. Control laws are much simpler	Control laws are bit complex adding to the development cost.
5. The use of permanent magnet rotors make the controlling of flux and magnetic field and arduous task.	The absence of PM rotors makes the magnetic field so much adjustable.

AC induction motor

Brushless DC motor