EV Drive-train

Aim: To get familiar with power converters and DC induction motors.

Objectives:

1. To know the types of power converter circuits are employed in an electric and hybrid electric vehicle.
2. To solve for the EV steady-state speed with the given parameters and conditions.
3. To Explain in brief about the author’s perspective of the article Induction Versus DC brushless motors by Wally Rippel, Tesla.

Solutions:

1.

• The various arrangements of EV power supply show that in any event one DC/DC converter is important to interface the Fuel Cell, the Battery or the Supercapacitors module to the DC-connect.
• A DC to DC converter is a classification of power converters and it is an electric circuit which changes over a wellspring of direct flow (DC) starting with one voltage level then onto the next, by storing the energy of the input briefly and afterwards discharging that vitality to the yield at an alternate voltage.
• The capacity might be in either magnetic field stockpiling parts (inductors, transformers) or electric field stockpiling segments (capacitors).
• DC/DC converters can be intended to move power just a single way, from the input to the yield. Be that as it may, practically all DC/DC converter topologies can be made bi-directional. A bi-directional converter can move power in either heading, which is valuable in applications requiring regenerative braking.
• Generally, the DC-DC converters are classified as follows:

1. Non isolated converters: The type of non-isolated converters is commonly utilized where the voltage should be ventured up or somewhere near a generally little proportion (under 4:1) and when there is no dielectric isolation with the output and input. The converters under this type are categorised as buck, boost, buck-boost, Cuk and charge-pump converters. As the name suggests the buck and boost converter are used for voltage step-down and voltage step-up respectively. The buck-boost and the Cuk converter can be utilised for either the step-up or step-down of voltage. The charge pump converter is used in low power usages for step-up or inversion of the voltage.
2. Isolated converters: Wherever there is a need for output to be completely isolated from the input, an isolated converter is used and high-frequency transformer is used generally for this purpose. In this type, the classification of converters are as follows: half-bridge converter, full-bridge converter, Fly-back converter, forward converter and Push-Pull converters which are all DC-DC converters.

2. The motor voltage can be calculated as `V_m= V_b*d` where `V_b` is battery voltage and d is the duty cycle. So, as per the given values, `V_m = 72V* 0.7 = 50.4V`

The armature resistance from the given values can be calculated as `R_a = V_a/I_a`.

So, `R_a = (72V)/(400A) = 0.18 ohms`

As we know the torque-speed relation of a DC machine `T = (V*Kphi)/ R_a - ((Kphi)^2*omega)/ R_a`

where V is Voltage supply, `Kphi`is motor constant, `R_a` is armature resistance, `omega` is motor angular speed, and T is motor Torque.

From the given parameters, we can write the relation as `T = (50.4 * 0.7)/ 0.5 - (0.7^2*omega)/0.5 = 70.56 - 0.98omega`

We have another equation of the torque which is given, `Ttr = 24.7 + 0.0051omega^2`

So, by equating both the equations we will be getting, `70.56 - 0.98omega = 24.7 + 0.051omega^2`

A quadratic equation is a result which is `0.0051omega^2 +0.98omega - 45.86 = 0`

Solution for the quadratic equation of the form `ax^2 + bx +c=0` will be `omega = (-b+- sqrt(b^2 - 4*a*c))/ (2*a)`

The solutions for the `omega` will be 38.82 rad/sec and -230.98 rad/sec. So, the correct solution for the `omega` will be 38.82 rad/sec.

`therefore` The EV steady-state speed is 38.82 rad/sec.

3. Here in this article, the author starts with the comparison of the usage of engine type from past and present. Rather the author tells that there couldn't be a conclusion for one best engine type and also states that best engine type depends on personal requirements. The author also discusses the replacement of the DC brushless motors with DC brushed and several other.

The author then draws a detailed explanation of the two technologies which are DC brushless motors and DC induction motors. So, the author draws a comparison between the DC brushless and DC induction motors in terms of inverter usage, limitations, torque performance and various others. An inverter is not needed in the case of the induction machine as the torque can be generated by using 3-phase power while the brushless machine requires the aid of an inverter for the torque production. The induction machine has some limitations such as compulsion of AC, limited starting torque and limited peak torque capabilities. But, when connected with the inverter with some feedback loops all these limitations will be rectified and the induction machine will work the same as the brushless machine.

The comparison between the DC brushless and induction motors is started with stator and rotor. The main difference is that less rotor heat is generated in the case of brushless motors when compared to the induction motors. When compared the reduction of the eddy losses is easier in the applications of induction motors as it has no magnets and the magnetic field 'B' is adjustable. So, it can be said that an induction machine with a smart inverter would be advantageous than a brushless motor. Also, the cost range will also be less with the case of induction motors as it doesn't use permanent magnets. But, the control laws of the DC induction is complex and the stability of the torque-speed range is a difficult task in induction machine when compared to brushless motors.

As the brushless machines are used heavily in the manufacturing of HEVs and Plug-in HEVs while the induction machines are used in purely EVs the author is stating that both the brushless and induction motors can be used side by side for the application of EVs and HEVs.