Week-6 Challenge: EV Drivetrain

1) Types of power converter circuits employed in electric and hybrid electric vehicle

Power Electronics is the study of switching electronic circuits in order to control the flow of electrical energy. Power electronics is the technology behind switching power supplies, power inverters, motot drives, power converters & motot soft starts.

It is basically to take average i.e. the required amount of output supply is averaged by using semiconductors & other such devices.

Control is achieved here is within the time axis, rather than limiting the supply or voltage. These switching based on time is called duty cycles and expressed in the form of duty ratio.

Power converters can be like converting AC - DC, Voltages and frequency changes, or both combined.

Types of Power Converters

1) AC to DC (Rectifier)

2) DC to DC ( Chopper / Converter )

3) DC - AC ( Inverter )

The most common power converter used in EV and HEV is DC-DC converter:

DC/DC converters can be designed to transfer power in only one direction, from the input to the output. However, almost all DC/DC converter topologies can be made bi-directional. A bi-directional converter can move power in either direction, which is useful in applications requiring regenerative braking.

The amount of power flow between the input and the output can be controlled by adjusting the duty cycle (ratio of on/off time of the switch). Usually, this is done to control the output voltage, the input current, the output current, or to maintain a constant power. Transformer-based converters may provide isolation between the input and the output. The main drawbacks of switching converters include complexity, electronic noise and high cost for some topologies. Many different types of DC/DC power converters are available :

1) Non Isolated Converters

The non-isolated converters type is generally used where the voltage needs to be stepped up or down by a relatively small ratio (less than 4:1). And when there is no problem with the output and input having no dielectric isolation. There are five main types of converter in this non-isolated group, usually called the buck, boost, buck-boost, Cuk and charge-pump converters. The buck converter is used for voltage step-down, while the boost converter is used for voltage step-up. The buck-boost and Cuk converters can be used for either step-down or step-up. The charge-pump converter is used for either voltage step-up or voltage inversion, but only in relatively low power applications.

2) Isolated Converters

Usually, in this type of converters a high frequency transformer is used. In the applications where the output needs to be completely isolated from the input, an isolated converter is necessary. There are many types of converters in this group such as Half-Bridge, Full-Bridge, Fly-back, Forward and Push-Pull DC/DC converters. All of these converters can be used as bi-directional converters and the ratio of stepping down or stepping up the voltage is high.

2) To find out the EV steady state speed if duty cycle is 70%, for the given inputs :

Let,

T - Motor Torque

V - Voltage Supply

Ra - Armature Resistance

Kphi/Km- Motor Constant

omega (w) - Angular speed of motor

Hence steady state speed is 38.91 rad/s as per the calculations above.

3) Induction Versus DC brushless motors by Wally Rippel, Tesla : Briefing

According to Wally Rippel there is no one best technology, he says there is no one best engine type, rather there are different types of engines to suit personal requirements, such as price and performance. This is also true for electric vehicle drives.

He says that without a good crystal ball, it is hard to predict the future.

He compares the Induction motors with the DC brushless motors tehcnology. Below the points listed out based on his explanations :

1) 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. The essential difference between the two machines is with the rotor.

2) In brushless motor, 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, he calls this brushless motors as useless motors.

3) He says, induction motors are directly compatible with conventional utility power is the main reason for their success. 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.

4) He also suggests to add an inverter (without any feedback control) and 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. Add some feedback loops such that the inverter produces the exact frequency that the motor “desires,” and the induction motor is now capable of competing with DC and DC brushless for vehicle applications.

5) One of the main differences between Induction and Brushless motors 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.

6) 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.

7) Induction motors will likely retain a cost advantage over PM machines. 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.

Conclusion

He says that 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. But, 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.