Week-6 Challenge: EV Drivetrain

Q1) Generaly two types of convertors are used in electric vehicle

a) DC-to-DC converters

b) DC-to-AC convertors

a) DC-to-DC converters :- The DC-to-DC converters convert one level of DC voltage to another level. The operating voltage of different electronic devices such as ICs, MOSFET can vary over a wide range, making it necessary to provide a voltage for each device. A Buck Converter outputs a lower voltage than the original voltage, while a Boost Converter supplies a higher voltage.

With the application of DC-to-DC Converters, the circuit’s efficiency, ripple, and load-transient response can be changed. Optimal external parts and components are generally dependent on operating conditions such as input and output specifications. So, while designing the products, the standard circuits must be varied or changed according to and as per the need to their individual specification requirements. Designing the circuit that satisfies the specification and all the requirements needs a great deal of expertise and experience in that field.

The step-up or step-down DC-to-DC Converters are useful in applications where the battery voltage can be above or below the regulator output voltage. The DC to DC converter must be able to operate as a step up or down voltage supplier to provide constant load voltage over the entire battery voltage range through the operation.

Advantages & Disadvantages of DC-to-DC Converters

Advantages

• It simplifies the power supply systems in the circuit.
• It provides isolation in the primary and secondary circuits from each other.
• It provides a technique to extend potential (voltage) as required.
• It is available as a hybrid circuit with all elements in a single chip.
• It is also used in the regulation and control of DC voltage.
• The output is well organized as positive or negative.
• Battery space can be reduced by using a converter.

Disadvantages

• Switching converters lead to more noise.
• They are expensive as an external circuit is required.
• Choppers are inadequate due to unsteady voltage and current supply.
• More ripple current, More input and output capacitance, higher losses, etc.

b) DC-to-AC convertors :- DC-to-ac converters are known as inverters. The function of inverter is to change a dc input voltage to symmetric ac output voltage of desired magnitude and frequency. The output voltage could be fixed or variable at a fixed or variable frequency.

ADVANTAGES

• A.C. machines are simple, robust and do not require much attention for their repairs and maintenance during their use. For example, the transformer necessary to provide alternating current power is quite affordable and involves less maintenance cost.
• A wide range of voltages is obtained by the use of a transformer.
• The magnitude of the current can be reduced by using an inductance or a conductor without any appreciable loss of energy
• A.C. can easily be converted into D.C. with the help of rectifiers.
• When A.C. is supplied at higher voltages in long-distance transmission, the line losses are small compared to a D.C. transmission.
• Efficient electric power transformation over long-distance lines can be achieved through the use of a transformer.

DISADVANTAGES

• The peak value of A.C. is high and it is dangerous to use so better insulation is required.
• It attracts a person who touches it, unlike D.C. which gives a repelling shock.
• An A.C. is transmitted from the surface of the conductor and hence need several strands of thin wires insulated from each other.

Q2)

Solution: -

Vehicle Output Voltage (Vv) is given by: -

Vv = Battery Voltage (V) * Duty Cycle

Vv = 72 * 0.70

Thus, Vehicle voltage = 50.4V

Now, Equation for Vehicle torque is given by: -

Tv=(Vv−ω.K∅)⋅K∅/Ra

Where;

Tv= Vehicle Torque (N-m)

Vv= Vehicle voltage (Volts)

ω= steady state speed of vehicle (rps)

K∅= Motor constant

Ra= Armature Resistance

Substituting all these values in equation, we get:

Tv=(50.04−0.7⋅ω)⋅0.7/0.5

Tv=70.56−0.98⋅ω

Tv=24.7+0.0051⋅ω^2

Solving these two equations,

70.56−0.98⋅ω=24.7+0.0051⋅ω^2

We get a quadratic equation:

0.0051⋅ω2+0.98⋅ω−46.94=0

Since quadratic equation, we get two values of ;

ω= 38.85 rad/s and ω= -230.95 rad/s

We take the positive value;

Steady state speed of vehicle = = 38.85 rad/s

Q4)

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. 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.

Both DC brushless and induction drives use motors having similar stators. Both drives use 3-phase modulating inverters. The only differences 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; these differences are relatively small.)

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

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.

In contrast, 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. This advantage becomes increasingly important as performance is increased. With DC brushless, as machine size grows, the magnetic losses increase proportionately and part load efficiency drops. 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.

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.
I almost forgot: 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. This means added development costs, but likely little or no recurring costs.