EV Drivetrain

EV DRIVETRAIN

AIM:

• To study the Power Control Techniques adopted in controlling the Torque and Power of Vehicle.
• To study the Duty Ratio of Power Modules.
• Overview of Induction Motors and BLDC Motors.

INTRODUCTION:

EV/HEV Drivetrain/Powertrain Basics

The Powertrain provides Power to Vehicle. It refers to the set of Components that generate the required amount of Power to move the Vehicle.

The Powertrain of EV is way simpler than the Vehicles Powered by IC Engines.

An ICE Vehicle has a greater number of moving and rotating parts out of which Engine, Transmission and Driveshaft are important. Power is generated by Engine and transmitted to Driveshaft and these consists of major Internal Components. Along with these Differentials, Axles, Emission Control, Engine Cooling System and Exhaust are other Parts.

An EV Drivetrain has very less components compared with that of ICE Powertrain.

• Battery Pack: This is made up of multiple Lithium ion Cells and stores the energy required to run the Vehicle and Provide Direct Current (DC) Output.
• DC-AC Converter: The DC supplied by the Battery Pack is converted to AC to energise the Electric Motor. This Power transfer is managed effectively and efficiently by a Motor Controller which controls the frequency of the Voltage with the help of Power Electronic Modules which in turn control the magnitude of the Voltage in order to manage the Speed of the Motor as per the acceleration at Vehicle.
• Electric Motor: This is the Prime of an EV unit which converts the Electrical Energy to Mechanical Energy and delivers to wheels via Direct Transmission or Single ratio Transmission. This motor can act in Recuperation/Regeneration modes.
• On-Board Charger: Converts AC received from Charge Port into DC to charge back the Battery.
• CAN Module: This carries communication between the Control Unit and the Vehicle and Motor.
• Battery Management System: BMS continuously monitors the state of the Battery and is responsible for Cell balancing to deliver maximum efficiency from the Battery Pack.
• DC-DC Converter: A Battery Pack delivers the Fixed Voltage (E.g.; 48V Battery), but the requirement of the different accessories in the EV would vary. The DC-DC Converter helps to Step-down (E.g.; 12V) and distribute Power to different Systems.

Vehicle & Motor characteristics

The Vehicle Torque requirement increases with increase in Speed as shown in the above Plot.

The Torque Characteristics of the Motor depends on the Supply Voltage and Current. Different Torques can be obtained with different Supply Voltage where the top most curve indicated Torque ate Rated Battery Voltage Supply.

To control the motor to match with varying Load/Torque of the vehicle, we have to control the Voltage f the Battery to make sure Motor Curve makes steady state with the Vehicle Torque Curve.

Power electronics is the application of solid-state electronics to the control and conversion of electric power.

In modern systems, the conversion is performed with semiconductor switching devices such as diodes, thyristors, and power transistors such as the power MOSFET and IGBT. In contrast to electronic systems concerned with transmission and processing of signals and data, in power electronics substantial amounts of electrical energy are processed. The power range is typically from tens of watts to several hundred watts. In industry a common application is the variable speed drive (VSD) that is used to control an induction motor. The power range of VSDs start from a few hundred watts and end at tens of megawatts.

Switching Technique is adopted from Power Electronics to control the Voltage and Current for Electric Motor. This avoids high amount of losses by replacing Resistor with and Power Module/Switch. And these Power Modules are operated at high Frequency where the Rated Battery Voltage (Vbat) can be reduced to desired voltage by ON-OFF of Modules.

Switching Frequency will be in such a way that it utilizes Vbat & switches low and High to maintain Average desired Voltage.

The principle behind the Power Modules is not controlling the Magnitude od the Voltage but the duration of High & Low Value of Voltage resulting the desired Voltage level.

This ON-OFF Switching Frequency is referred in terms of Duty Ratio

Duty Ratio (d) = (ON) time/(ON+OFF) time

By changing the Duty Ratio of the Power Modules, we can vary the Voltage from one level to another level.

Vavg = Vbat * d

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1. Types of Power Converter Circuits employed in EV & HEV

Power conversion is converting electric energy from one form to another such as converting between AC and DC; or changing the voltage or frequency; or some combination of these.

A Power Converter is an electrical or electro-mechanical device for converting electrical energy. This could be as simple as a transformer to change the voltage of AC power, but also includes far more complex systems. The term can also refer to a class of electrical machinery that is used to convert one frequency of alternating current into another frequency.

Typical Converters used in EV/HEV are as follows,

• Bidirectional AC-DC Converters (Rectifier & Inverter)
• Bidirectional DC-DC Converters (Choppers)

Functions,

• AC-DC Converters are called as
  • Rectifiers can convert the AC Supply Voltage from the On-Board Charging Point (OBC) Supply to DC Voltage for charging the Battery.
  • Rectifiers can also be used in Alternators of the Vehicle to charge the Battery with DC Voltage from the generated from 3 phase AC Voltage.
  • Rectifiers are also used in the Control Unit of EV/HEV Motors to charge the Battery during Recuperation/Regenerative mode.
• DC-AC Converters are called as
  • Inverters can convert the DC Supply Voltage from the Battery to AC Voltage required to Propel the Motor.
  • Sometimes both Rectifier and Inverter are clubbed with an advanced Semiconductor Modules such as MOSFETs
  • This MOSFET Inverter can enhance the Voltage level based on Frequency Control.
  • This Control Unit function is directly Proportional to the Accelerator of the Vehicle.
• DC-DC Converters
  • They are also called as Choppers and can be Bi-Directional.
  • They can Convert High Voltage to Low Voltage and Vice versa.
  • They are ranged in Operation from 10V to 475V.
  • Typical EV/HEV application has a DC-DC Converter between 48V Battery and a conventional 12V Battery.

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2. An Electric Vehicle's Drivetrain with 72V battery pack in shown in the diagram below. The duty ratio for acceleration operation is 'd1' and for the braking operation the duty ratio is ‘d2'. The other parameters of the electric vehicle are given below,

Motor and Controller Parameters:

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

• The vehicle characteristics are given by the equation

  TTR=24.7+0.0051w2

  What is EV steady state speed if duty cycle is 70%?

The chopper is the control unit in between the Battery and Motor.

The above Diagram shows the Output of the Motor are Torque and Speed.

Controller/Chopper controls the Voltage and in turn the Current required for particular load and speed of the Motor.

This Controlling is based on the Duty Ratio of the Chopper.

We know that,

Torque-Speed Equation for Motor,

T: Torque (Nm)

K: Motor Constant

Ra: Armature Resistance (ohms)

ω: Angular Velocity (rad/s)

Ø: Field Flux

V: Voltage

Also,

Kø = Eb/ ω

Eb: Back Emf

ω = 2Πn/60

duty cycle ratio = 0.7

Motor Voltage = Rated * d = 72*0.7 = 50.4V

Substituting the same in Torque-Speed Equation for Motor,

T = (50.4(0.7/0.5)) - ((0.7^2)/0.5)w

T = 70.56 - 0.98w ...................(1)

Torque equation for Vehicle is given,

T = 24.7 + 0.0051 w^2 ................(2)

@ Steady state, 
70.56-0.98w = 24.7 + 0.0051w^2

0.0051w^2+0.98w-45.86 = 0
solving the above quadratic equation,

w = 38.92, w = -231.07

Therefore,
w = 38.92 rad/s

The EV Steady state Speed @ Duty Cycle = 70% is 38.92 rad/s.

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Induction v/s BLDC Motors

Geometry & Principle

In Brushless Machines, the rotor has 2 or more Permanent Magnets which will generate a DC Magnetic field. In turn, this magnetic field enters the stator core and interacts with currents flowing within the windings to produce a torque interaction between the rotor and stator.

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.

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.

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 turn, the frequency of this magnetic field as “seen” by the rotor is equal to the difference between the applied electrical frequency and the rotational “frequency” of the rotor itself.

An induced voltage exists across the shorted structure that is proportionate to this speed difference between the rotor and electrical frequency. In response to this voltage, currents are produced within the rotor conductors that are approximately proportionate to the voltage, hence the speed difference. Finally, these currents interact with the original magnetic field to produce forces – a component of which is the desired rotor torque.

• The significant difference in the geometry is the Rotor and the Principle of Inverter Control.
• Brushless Machines requires a Position Sensors (Hall Sensors) whereas the Induction Machines require Speed Sensors.

Brushless or Induction

• The Induction Machines are capable to start under load and they don’t require an inverter.
• 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.
• Induction Machines cannot operate from DC; AC is a must. Shaft speed is proportionate to line frequency.
• IMs have limited starting torque and somewhat limited running peak torque capabilities, when compared to DC type machines.
• Adding an Inverter makes it possible to power an induction machine from a battery or other DC source; variable speed also becomes possible simply by adjusting the inverter frequency.
• Torque performance of IMs is low compared with DC machines.
• Add some feedback loops to IMs 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.
• 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.
• 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 favoured approach where high-performance is desired; peak efficiency will be a little less than with DC brushless, but average efficiency may actually be better.
• Cost Effectiveness
  • 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.
  • 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
  • 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.
  • 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.

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