Week-6 Challenge: EV Drivetrain

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

Converter serves more than one function that gives output which is differ than input. Converter is an electrical circuit that changes electrical energy from one to desired form for specific load application.

A device called convereter falls under 2 categories of Step up converter and step down converter

In EVs/HEVs step up and step down converter are combined into single unit

step up converter is used for conversion of 180-300 V to about 650 v to power the traction motor

For eficient high voltage motor less expensive and smaller while increasing the voltage from the battery using step up converter

Step down converter used for conversion of DC (180-300 V) from HEV Battery to low voltage of (12-24v ) DC that can be used to charge the 12 volt auxiliary battery and operate light load devices such as lighting, radio, and windows.

Power electronic converters classified as follows

• Diode Rectifier.
• AC to DC Converter (Controlled Rectifier)
• DC to DC Converter (DC Chopper)
• AC to AC Converter (AC voltage regulator)
• DC to AC Converter (Inverter)
• Static Switches.

advanced DC/DC , AC/DC and DC/AC converters are required to adapt the output voltage and current levels with high power quality.

DC-DC Converters

DC-to-DC converters are used to reduce High voltage DC input to low voltage DC output for some specific appliances. They are also used to isolate some highly sensitive components in a circuit from other components of the circuit to avoid any kind of damage.

Electric vehicles (EV) use two different power systems; a high-voltage battery (200 to 450 VDC) for traction and a low-voltage (12 V) one for supplying all the electric appliances in the vehicle.

Traditionally the low-voltage battery was charged from the alternator, but in today's vehicles it gets its power from the high-voltage battery pack. However, in specific electric car architectures, this low voltage battery should be ready to help recharge the high-voltage battery pack in order to provide energy for cranking the car. This means that the on-board DC-DC converter must be bi-directional and very efficient as well as highly reliable in order to run the complex control algorithms needed to ensure an energy-efficient solution.

AC to DC Converter (Controlled Rectifier)

It's called the "onboard charger" though it really is a converter. It converts power from AC to DC and then feeds it into the car's battery. This is the most common charging method for electric vehicles today and most chargers use AC power.

DC to AC Conveter (Inveter)

The DC to AC conveter is required to convert the dc power to a sutaible ac power for electrical machine the power ouput from the battery pack (DC current) is converted using inverter to be used in electric motor (AC current)

Converts direct current (DC) from the battery to alternating current (AC) to be used by other devices such as the traction motor and coolant pump.

Given:-

The following input parameters are given:-

Battery Pack voltage(V) = 72 V

Rated armature current = 400 A

Armature Resistance(Ra) = 0.5

Motor Constant (K) = 0.7

Chopper Switching frequency = 400 Hz

Duty Cycle = 70%

Solution:-

Vehicle Output Voltage() = Battery Voltage(V)*Duty Cycle

= 72*0.70

= 50.4 V

So, the vehicle voltage is 50.4 V

Now, Equation for Vehicle torque is :

Tv">Tv= Vv">Vv.KϕRa">Kϕ/Ra-(Kϕ)2Ra">(Kϕ)2/2Ra.w">w

Where,

Tv">Tv= Vehicle Torque (N-m)

Vv">Vv= Vehicle Voltage (Volts)

w">w= Steady State speed of Vehicle (rps)

K">Kϕ">ϕ= Motor Constant

Ra">Ra = Armature Resistance

Substituting all the values in equation 1, we get

Tv">Tv= 50.4×0.70.5-(0.7)20.5×w">50.4×0.70/.5−(0.7)2/0.5×w ...(1)

Tv">Tv= 70.56-0.98w2">70.56−0.98w2 ...(2)

Now, the vehicle speed torque characteristics is given as,

Tv=24.7-0.0051w2">Tv=24.7−0.0051w2 ...(3)

At Steady State,

Eq (2) = Eq (3) ... (4)

24.7-0.0051w2=70.56-0.98w2">24.7−0.0051w2=70.56−0.98w2

0.0051w2+0.98w-45.85=0">0.0051w2+0.98w−45.85=0 ... (5)

To solve the above eq we have to apply the QUADRATIC EQUATION formula,

x=-b±b2-4ac2a">x=−b±√b2−4ac/2a

x=-0.98±0.982-4(0.0051)(-45.85)2⋅(0.0051)">x=−0.98±√0.982−4(0.0051)(−45.85)/2⋅(0.0051)

w=">w=38.92 rad/sec

So, The steady-state speed of the vehicle is 38.92 rad/sec.

Answer for question 3

Battery pack voltage is found to be reduced for simulation time of 10 sec from 72 Volt to 62 volt constant flux and armature resistance of the motor

4. Refer to the blog on below topic:

Induction Versus DC brushless motors by Wally Rippel, Tesla

Explain in brief about author’s perspective.

Wally Ripple is a long-time proponent of electric vehicles. Prior to joining Tesla Motors, he was an engineer at AeroVironment, where he helped develop the EV1 for General Motors and was featured in the documentary movie, Who Killed the Electric Car? Wally has also worked for the Jet Propulsion Laboratory on electric vehicle battery research, among other projects. In 1968, as a Caltech undergraduate student, he built an electric car (a converted 1958 Volkswagen microbus) and won the Great Transcontinental Electric Car Race against MIT.

In the beginning, the author describes the dominance of Internal Combustion Engines(ICE) in the world of automobile industry. He explains that all engines were similar and it was up to the user to select according to their requirement. Next, the author highlights topics related to EV technology especially related to DC Brushless Motor and Induction Motor.

According to him, DC Brush Motors is replaced by either DC Brushless Motor or Induction Motor. Also, the Lead Acid Battery is replaced by lithium-ion batteries for EVs.He also quoted that there will be the future of both the technology (DC Brushless Motor and Induction Motor) depends upon their demands and uses.

DC Brushless Motor:

With brushless machines, the rotor includes two or more permanent magnets that generate a DC magnetic field (as seen from the vantage point of the rotor). 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.

The author also said that 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.

Advantage:-

The advantage of DC Brushless Motor is that there is low heat generation in the rotor and its cooling capacity is better as compared to Induction Motor. Also, 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%. 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.

Disadvantage:-

As Machine size grows, the magnetic losses increase proportionately and part load efficiency drops. Permanent magnets are expensive and difficult to handle due to very large forces that come into play when anything ferromagnetic gets close to them.

Induction Motor:

The technology behind the induction motor is that it basically uses a 3-phase induction motor which was invented by Nikola Tesla sometime before 1889. Unlike the DC brushless rotor, the 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 an Induction motor current interacts with the Magnetic field generated by the stator to produce a force which is required for rotor torque.

When a 3-phase induction motor is connected to utility type 3-phase power, torque is produced at the outset. The fact that induction motors are directly compatible with conventional utility power is the main reason for their success.

Advantage:-

The induction motor does not have any magnets and in this the B field is adjustable. The motor has the ability to start under load and there is no need of an inverter. In an induction motor B field is proportionate to V/f (Voltage to frequency). This technology is used to reduce the magnetic losses and conduction losses, due to which efficiency will remain at maximum peak. This all leads to increasing the performance, induction motor is very useful and efficient for high performance.

Disadvantage:-

An induction motor cannot operate from DC Voltage it always requires an AC voltage supply. In an induction motor, the Shaft speed is proportionate to line frequency. Hence, when used with utility power they are constant speed machines. When operated from utility power, they have limited starting torque and somewhat limited running peak torque capabilities, when compared to DC-type machines.