Week-6 Challenge: EV Drivetrain

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

• Power electronics converters are employed in an electric and hybrid electric vehicles depending upon the application requirements,
  • AC-DC Converter (Rectifier)
  • DC-DC Conerter (Chopper)
  • DC-AC Converter (Inverter)
• AC-DC Converter:
  • AC to DC converters is most commonly used in Plug-in Electric Vehicles (PEV). The vehicle gets its propulsion power indirectly from an external energy source (that is charging stations) which is supplying the electrical energy to the battery pack. The battery needs to be recharged using a direct current source, for this purpose an AC-to-DC converter is employed in a PEV.
• DC-DC Converter:
  
  • The DC-DC converter is used in all types of electric vehicles. Battery power is supplied to both the main and auxiliary load by converting the higher voltage levels to appropriate voltage levels by using a chopper circuit.
• DC-AC Converter:
  
  • The DC-AC converter is mainly used to provide power to the ac machine mainly the induction machine, as they required a balanced three-phase supply to produce a rotating magnetic field in order to start an induction motor. The Inverter circuit is used to convert the fixed dc voltage to 3 phase ac supply.

2) An Electric Vehicle's powertrain with a 72V battery pack is shown in the diagram below. The duty ratio for the acceleration operation is 'd1' and for the braking operation the duty ratio is 'd2'.What is EV steady state speed if the duty cycle is 70%?

Given Parameters:

Motor and Controller Parameters:

• Rated Armature voltage= 72 V
• Rated armature current= 400 A
• Ra= 0.5Ω
• KΦ= 0.7 Volt second
• Chopper Switching frequency= 400 Hz

The given vehicle speed-torque characteristics are:

-----------(1)

For d1= 0.7,

The output voltage of the DC motor is= 72*0.7= 50.4 V

We know torque speed characteristics of the DC motor is

T= (V/Ra)*(KΦ)- (w/Ra)*(KΦ)^2 -----------(2)

Where,

T= Developed electromagnetic torque

V= Terminal Voltage

Ra= Armature resistance

K= Machine Constant

Φ= Field Flux

w= Angular speed.

Putting the above values in equation 2 we get

T= 70.56-0.98w-------------------------------(3)

Solving equations 1 and 3 we get

w= 38.915 rad/sec.

Conclusion: Steady state speed of the electric vehicle with a duty cycle of 70% is found to be 38.915 rad/sec.

3) Develop a mathematical model of a DC Motor for the below equation using Simulink.

Simulink Model:

Blocks Used:

• Constant: To apply a constant input
• Ramp: To apply a ramp input (Torque is provided as ramp input)
• Devide: To divide two number.
• Square: To calculate the square of a number
• Product: To multiply two number
• Add: To add two number
• Scope: To display the result

Output:

Conclusion: To plot the mathematical model of the DC motor parameters given in second question are considered and the torque speed characteristics is ploted as shown above. The simulink model file is also attached for further reference.From the graph it is clear that with increase in torque the speed decreases and vice versa also.

4) Induction Motor vs Brushless DC motor.

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 direction 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.

Advantages of BLDC Motor:

• The main advantage of DC Brushless Motor(BLDC) is that there is low heat generation in the rotor and its cooling capacity is better as compared to Induction Motor.
• The peak point efficiency of this motor is very high.
• The DC brushless drive can operate at unity power factor, whereas the best power factor for the induction drive is about 85%.
• So the peak point energy efficiency for a DC brushless drive will typically be a few percentage points higher than for an induction motor drive.

Disadvantages of BLDC Motor:

• With increase in BLDC Motor size, the magnetic losses increase and load efficiency reduces.
• Permanent magnets are expensive and difficult to handle.

Induction Motor:

• The technology behind the induction motor is that it basically uses a 3-phase induction motor is that unlike the BLDC 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 magnatic field is adjustable.
• The motor has the ability to start under load and there is no need of an inverter.
• In an induction motor magnatic field is proportionate to V/f (Voltage to frequency ratio).
• 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.
• Therefore when Induction motor 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 as compared to DC-type machines.