EV Drivetrain

Aim-

• Types of power converter circuits are employed in electric and hybrid electric vehicle.
• What is EV steady state speed if duty cycle is 70%?
• Study brief about Induction Versus DC brushless motors by Wally Rippel, Tesla.

Solution-

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

Fig- Above fig shows converter circuit used in EV/HEV

• Usually AC motors are used in HEVs or EVs for traction and they are fed by inverter and this DC-AC inverter is fed by DC-DC converter
• From above we can conclude that at least 1 DC-DC & 1 DC-AC converter is required in EV/ HEV.

Thus two major power electronic units are required in the general configuration.

• DC-DC converter
• DC-AC inverter

DC-DC converter

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.

• Buck converters - It is used for voltage step-down
• Boost converters - It is is used for voltage step-up
• Buck-Boost converters - It is used for either step-down or step-up

B. 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.

• Fly-back converters,
• Forward converters

Above converters can be used as bi-directional converters and the ratio of stepping down or stepping up the voltage is high.

DC-AC inverter

1. Voltage source type inverters
2. Current source type inverters

DC/DC converter

1. Non-isolated converters

1. Buck converters

The working of a buck converter can be broken down into a few steps.

STEP – 1:

The switch turns on and lets current flow to the output capacitor, charging it up. Since the voltage across the capacitor cannot rise instantly, and since the inductor limits the charging current, the voltage across the cap during the switching cycle is not the full voltage of the power source.

Fig - Above fig shows switch ON condition

STEP – 2:

The switch now turns off. Since the current in an inductor cannot change suddenly, the inductor creates a voltage across it. This voltage is allowed to charge the capacitor and power the load through the diode when the switch is turned off, maintaining current output current throughout the switching cycle.

Fig - Above fig shows switch OFF condition

These two steps keep repeating many thousands of times a second, resulting in continuous output.

2. Boost converters

A boost converter is one of the simplest types of switch mode converter. As the name suggests, it takes an input voltage and boosts or increases it.

STEP – 1

Here, nothing happens. The output capacitor is charged to the input voltage minus one diode drop.

Fig - Above fig shows switch OFF condition

STEP – 2

Now, it’s time to turn the switch on. Our signal source goes high, turning on the MOSFET. All the current is diverted through to the MOSFET through the inductor. Note that the output capacitor stays charged since it can’t discharge through the now back-biased diode.

The power source isn’t immediately short circuited, of course, since the inductor makes the current ramp up relatively slowly. Also, a magnetic field builds up around the inductor. Note the polarity of the voltage applied across the inductor.

Fig - Above fig shows switch ON condition

STEP – 3

The MOSFET is turned off and the current to the inductor is stopped abruptly.

The very nature of an inductor is to maintain smooth current flow; it doesn’t like sudden changes in current. So it does not like the sudden turning off of the current. It responds to this by generating a large voltage with the opposite polarity of the voltage originally supplied to it using the energy stored in the magnetic field to maintain that current flow.

If we forget the rest of the circuit elements and notice only the polarity symbols, we notice that the inductor now acts like a voltage source in series with the supply voltage. This means that the anode of the diode is now at a higher voltage than the cathode (remember, the cap was already charged to supply voltage in the beginning) and is forward biased.

The output capacitor is now charged to a higher voltage than before, which means that we have successfully stepped up a low DC voltage to a higher one!

Fig - Above fig shows switch OFF condition

These steps happen many thousands of times (depending on the frequency of the oscillator) to maintain the output voltage under load.

3. Buck-Boost converters

A Buck-Boost converter is a type of switched mode power supply that combines the principles of the buck converter and the boost converter in a single circuit.

Operation as a Buck Converter

Fig- Above fig shows Operation as a Buck Converter during Tr1 ‘on’ Period

Fig. shows the circuit operating as a Buck Converter. In this mode Tr2 is turned off, and Tr1 is switched on and off by a high frequency square wave from the control unit. When the gate of Tr1 is high, current flows though L, charging its magnetic field, charging C and supplying the load. The Schottky diode D1 is turned off due to the positive voltage on its cathode.

Fig- Above fig shows Operation as a Buck Converter During Tr1 ‘off’ Period

Fig shows the current flow during the buck operation of the circuit when the control unit switches Tr1 off. The initial source of current is now the inductor L. Its magnetic field is collapsing, the back e.m.f. generated by the collapsing field reverses the polarity of the voltage across L, which turns on D1 and current flows through D2 and the load.

As the current due to the discharge of L decreases, the charge accumulated in C during the on period of Tr1 now also adds to the current flowing through the load, keeping V_OUT reasonably constant during the off period. This helps keep the ripple amplitude to a minimum and V_OUT close to the value of V_S.

Operation as a Boost Converter

Fig- Above fig shows Operation as a Boost Converter During Tr2 ‘on’ Period

In Boost Converter mode, Tr1 is turned on continually and the high frequency square wave applied to Tr2 gate. During the on periods when Tr2 is conducting, the input current flows through the inductor L and via Tr2, directly back to the supply negative terminal charging up the magnetic field around L. Whilst this is happening D2 cannot conduct as its anode is being held at ground potential by the heavily conducting Tr2. For the duration of the on period, the load is being supplied entirely by the charge on the capacitor C, built up on previous oscillator cycles. The gradual discharge of C during the on period (and its subsequent recharging) accounts for the amount of high frequency ripple on the output voltage, which is at a potential of approximately V_S + V_L.

The Off Period

Fig- Above fig shows Operation as a Boost Converter during Tr2 ‘off’ Period

At the start of the off period of Tr2, L is charged and C is partially discharged. The inductor L now generates a back e.m.f. and its value that depends on the rate of change of current as Tr2 switches of and on the amount of inductance the coil possesses; therefore the back e.m.f can be any voltage over a wide range, depending on the design of the circuit. Notice particularly that the polarity of the voltage across L has now reversed, and so adds to the input voltage V_S giving an output voltage that is at least equal to or greater than the input voltage. D2 is now forward biased and so the circuit current supplies the load current, and at the same time re-charges the capacitor to V_S + V_L ready for the next on period of Tr2.

B. Isolated converters

• Fly-back converters,

The flyback converter is a buck-boost converter with the inductor split to form a transformer, so that the voltage ratios are multiplied with an additional advantage of isolation. When driving for example a plasma lamp or a voltage multiplier the rectifying diode of the boost converter is left out and the device is called a flyback transformer.

FIg- Above fig shows working circuit of Fly-Back Converter

• When the switch is closed, the primary of the transformer is directly connected to the input voltage source. The primary current and magnetic flux in the transformer increases, storing energy in the transformer. The voltage induced in the secondary winding is negative, so the diode is reverse-biased (i.e., blocked). The output capacitor supplies energy to the output load.
• When the switch is opened, the primary current and magnetic flux drops. The secondary voltage is positive, forward-biasing the diode, allowing current to flow from the transformer. The energy from the transformer core recharges the capacitor and supplies the load.

Fig- Above fig shows ON & OFF state in Fly-Back converter

The operation of storing energy in the transformer before transferring to the output of the converter allows the topology to easily generate multiple outputs with little additional circuitry, although the output voltages have to be able to match each other through the turns ratio. Also there is a need for a controlling rail which has to be loaded before load is applied to the uncontrolled rails, this is to allow the PWM to open up and supply enough energy to the transformer.

• Forward converters

The forward converter is a DC/DC converter that uses a transformer to increase or decrease the output voltage (depending on the transformer ratio) and provide galvanic isolation for the load. With multiple output windings, it is possible to provide both higher and lower voltage outputs simultaneously.

FIg- Above fig shows working circuit of Forward Converter

While it looks superficially like a fly, it operates in a fundamentally different way, and is generally more energy efficient. A flyback converter stores energy in the magnetic field in the inductor air gap during the time the converter switching element (transistor) is conducting. When the switch turns off, the stored magnetic field collapses and the energy is transferred to the output of the flyback converter as electric current. The flyback converter can be viewed as two inductors sharing a common core with opposite polarity windings.

In contrast, the forward converter (which is based on a transformer with same-polarity windings, higher magnetizing inductance, and no air gap) does not store energy during the conduction time of the switching element — transformers cannot store a significant amount of energy, unlike inductors. Instead, energy is passed directly to the output of the forward converter by transformer action during the switch conduction phase.

While the output voltage of a flyback converter is theoretically infinite, the maximum output voltage of the forward converter is constrained by the transformer turns ratio Ns/Np

Vout= D* Ns/Np * Vsupply

D is the pulse width modulator duty cycle.

• DC-AC inverter

Fig- Above fig shows working principle of DC-AC inverter

An inverter is a semiconductor-based power converter. An inverter that converts a direct current into an alternating current is called a DC-AC inverter. However, the term “inverter” generally refers to the equipment that combines an AC-DC converter (that changes an alternating current into a direct current) and a DC-AC inverter so as to be able to generate arbitrary frequencies and voltages. Figure the concept of an inverter. To drive electric motors efficiently, it is necessary to convert the mains supply into the optimal frequency and voltage.

• Voltage source type inverters

Voltage source type inverters control the output voltage. A large-value capacitor is placed on the input DC line of the inverter in parallel. And the inverter acts as a voltage source. The inverter output needs to have characteristics of a current source. In the case of low impedance load, series reactors are needed for each phase. (See L1 to L3 in Figure) In motor applications, the leakage reactance of a motor fulfils the same function as this reactor. Reverse-conducting semiconductor devices that are capable of dealing with the back-EMF are commonly used as switches.

FIg- Above fig shows working circuit of Voltage source type inverter

• Current source type inverters

Current source type inverters control the output current. A large-value inductor is placed on the input DC line of the inverter in series. And the inverter acts as a current source. The inverter output needs to have characteristics of a voltage source. In motor applications, capacitors are required between each phase-to-phase of motor input (See C1 to C3 in Figure). (A large-value snubber is sometimes necessary to suppress surge voltage caused by an inductive load. In such cases, a snubber loss increases.) This capacitor and motor inductance that work as a kind of filter reduce motor ripple current compared with the case of voltage source type inverters Reverse-blocking devices such as thyristors are required as switching devices. In the case of using IGBTs and so on, series diodes are needed as shown in Figure

FIg- Above fig shows working circuit of Current source type inverters

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 is 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%?

SOLUTION:

Given

Rated Armature Voltage (V) = 72 V

Rated Armature current (I) = 400 A

Armature Resistance Ra = 0.5Ω

Motor Constant Kφ = 0.7 V-s

Chopper Switching frequency = 400 Hz

Duty Cycle = 70%

T = 24.72+0.0051*ω^2……………. (Equ 1)

Thus above equation is given in question

Now we know that

Speed-Torque Equation for D.C Motor is

ω = (V/kφ)-(T.R/(kφ)^2)

By rearranging it in form of torque equation

T = ((V.kφ)/R)-(ω.(kφ)^2/R)…………….(Equ 2)

Now we know the duty cycle is 70%

Thus average voltage supplied to motor will be

V = 72*0.7 = 50.4V

Now substitute the values in equ 2

T = ((50.4*0.7)/0.5)-((ω.(0.7)^2)/0.5)

Tm = 70.56-(ω*0.98)…………(Equ 3)

At the steady state condition of EV, characteristic equation of vehicle & motor will be same

Thus,

Equating (1) & (3),

70.56 - 0.98 * (ω) = 24.7 + 0.0051 ω^2.

0.0051(ω^2) + 0.98 (ω) - 45.86 = 0

Now solve this by using quadratic equation to find (ω),

ax^2+bx+c = 0 ,

where a = 0.0051 ; b = 0.98 ; c = - 45.86 ; x = ω

ω = (-b +- √((b^2) - (4ac))) / 2a.

= - 0.98 +- √((0.98^2) - (4 * 0.0051 * -45.86 ))) / 2 * 0.0051.

= - 0.98 +- √( 0.9604 + 0.9355) / 0.0102

= (-0.98 +- 1.377 ) / 0.0102

(-0.98 + 1.377) / 0.0102 = 38.92

Or

(-0.98 - 1.377) / 0.0102 = -231.07

Thus ω will be 38.92 rad/sec as it can't be negative.

3. Refer to the blog on below topic:

Induction Versus DC brushless motors by Wally Rippel, Tesla

Explain in brief about author’s perspective.

DC Brushless motor

Characteristics

• The rotor includes two or more permanent magnets that generate a DC magnetic field
• The magnitude and polarity of the stator currents be continuously varied by inverter.

Advantages-

• 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
• DC drive are easy to control.

Disadvantages

• A brushless DC motor produces no starting torque when directly connected to fixed frequency utility power. They really need the aid of an inverter.
• There is no easy way of changing magnetic field strength with permanent magnets thus sum of the eddy, hysteresis, and I² losses will be more at low torque requirement.
• Permanent magnets are expensive – something like $50 per kilogram.
• Permanent magnet rotors are also difficult to handle due to very large forces that come into play when anything ferromagnetic gets close to them

Induction Motor

Characteristics

• The induction rotor has no magnets – just stacked steel laminations with buried peripheral conductors that form a “shorted structure

Advantages-

• When a 3-phase induction motor is connected to utility type 3-phase power, the motor has the ability to start under load without inverter.
• Induction machines have no magnets and B fields are “adjustable,” This means that at light loads the inverter can reduce voltage such that magnetic losses are reduced and efficiency is maximized.
• 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.
• Due to the field weakening capabilities of induction machines, inverter ratings and costs appear to be lower, especially for high performance drives.
• Spinning induction machines produce little or no voltage when de-excited, thus they are easier to protect.

Disadvantages

• They cannot operate from DC without inverter, AC is a must.
• 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. An inverter can overcome above disadvantages but still performance is low compared to DC motor.
• The best power factor for the induction drive is about 85 percent
• Induction machines are more difficult to control. 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.

Common in both Induction & Brushless DC Motor:

• Both DC brushless and induction drives use motors having similar stators.
• Both drives use 3-phase modulating inverters.

Difference in Induction & Brushless DC Motor:

• 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

Authors view.

• There is no one best motor type, rather there are different types of motors to suit personal requirements, such as price and performance.
• 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
• The author says that there is no clear winner between these two and we have to make the choice based on the requirements.
• In future the hybrid vehicles also will be develop characteristics like the Pure EV's and for this the author stated that, "we will see induction and DC brushless live and work side by side during the coming golden era of hybrid and electric vehicles".

Conclusion

• Performance required is high with low developing cost & with average efficiency, then we need to select the Induction Motor.
• If the High peak point energy efficiency is required with simple controlling of the motor then we need to select the Brushless DC Motor.

Learning-

• Learned different types of power converter circuits employed in electric and hybrid electric vehicle.
• Learned in detail difference between DC brushless motor & induction motor and its application.