Week-6 Challenge: EV Drivetrain

Agenda:

To explain various types of power electronics converters used in Electric vehicles.

To calculate the speed of EV at steady state.

To explain the authors perspective on the differnece between indictionmotor verses Brushless DC motor.

Explanation:

1. Types of power converters used in EVs

Typically, various types of power electronics converters are used to control the EV operation from filling the fuel (Charging) to maintiaining and running the EV(Dischrging).

So the explanation is starts from filling the fuel, i,e EV battery charging.

The battery of EV needs DC to charge form 0% SOC to 100 SoC. But the commercial available power is A.C power. So there is a need to convert the AC supply to DC through rectifiers. Rectifiers are two types, which are half wave rectifier (HWR) and full wave rectifier(FWR). The FWR are also divided into two types, such as bridge rectifier and center tapped transformer rectifier.

Half Wave rectifier (HWR) :

HWR is the rectifier which converts half of the AC supply to DC using a controlled ( BJT, TRIAC, SCR, etc) or uncontrolled power electronic switches. If HWR is designed with controlled switches then it is called controlled HWR other wise uncontrolled HWR. 

Fig.1 Uncontrolled HWR

The fig.1 shows the uncontrolled HWR designed with diode. The output is a pulsating output which has voltage at odd multiples of 180deg. 

In the controlled HWR, the third terminal called control terminal, controls the output voltage. 

Fig.2 single phase controlled HWR.

The Fig.2 is the single phase controlled HWR, which is used to convert 1-ph ac to controlled DC output voltage. ig is the current pulses to switch on the SCR, at desired angle. According to the desired angle the average value of the output voltage also changes. 

Full wave rectifier:

i) Centre tapped FWR:

Fig.3 Uncontrolled FWR

Like in HWR, FWR also has two types according to its operation which is controlled FWR and uncontrolled FWR. The Fig.3 is uncontrolled FWR, which is desined with diodes. during positive pulse of the input signal D1 is in "ON" state, D2 is in "OFF" state and at -ve pulse D2 is "ON" D1 is "OFF". The output resistor has unidirectional current.

 Fig 4. Controlled FWR. 

The Fig.4 shows the controlled FWR designed with thyristor, SCR which is current controlled device which controlled by gate triggered pulses.

Bridge rectifier: 

Fig.5 Bride rectifier

During positive halfwave the D1 anD D2 are "ON" and D3 and D4 are OFF similarly D3 and D4 are ON anf D1 and D2 are OFF which regulate the input ac inout into dc output.

Fig6. Controlled bridge rectifier. 

The major draw back of single phase rectifiers is that it has more ac (ripple content).  So Three phase rectifier, is used to reduce the ripple content.The functional concept is almost similar to single phase.

2. DC-AC inverter:

Fig 7. 1-phase inverter.

The motor drive in the EV is either induction motor and brushless DC motor. The Indiction motor is working with AC supply, but the battery is DC. So, there is a need to 

 convert DC to AC. 

Fig. 8. 3-phase inverter, 

3. DC-DC Chopper:

If the motor drive is BLDC, there is a need to control the speed. For car accesseries, like window glass control dc motor. The DC- DC inverter is of two types again

1. Step down DC-DC chopper (Buck converter)

2. Step- up DC-DC chopper (Booost Converter)

Step-down DC-DC chopper (Buck Converter):

Fig. 9 Buck Converter

V0=dVi

d-duty cycle

v0 is output

v1 is input voltage

vout=vin/(1-d)

vout- output voltage

vin- input voltage

Fig 10 Boost Converter

4. AC-AC converter.

The Ac-AC converter is used to control the output voltage without change the frquency.

5. Cyclo converter

The cyclo converter is used to convert the frequency from input to output, which is used to control the speed the ac machines.

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2. 

Given data

Rated armature voltage or battery voltage  Vb=72V

va=Vb*0.7

Va=72*0.7 =50.4 V

Rated armature current Ia=400A

Armature resistance Ra=0.5 ohm

K phi= 0.7 volt second

Chopper switching frequency=400

W=(Va/k)-(Ra/k^2)Tv

Given torque speed relation is

W=(72*0.7/0.7)-(0.5/0.7*0.7)Tv

W=72-1.0204(24.7+0.0051 W^2)

W=72-25.20-0.0052W^2;

W=46.8-0.0052W^2

0.0052W^2-W+46.8=0.

The above equation is qadratic equation which is solved using the below code.

>> p=[-0.0052 -1 46.8];
>> r=roots(p)

r =

 -231.2300
   38.9223

Steady state speed is 38.92 rad/sec. since it is for motoring operation.

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3. The author explained the differences between BLDC and Induction motor with respect to Electric vehicles. The author first explains the need of EVs with respect to lack of commercial fuels and pollution. He is not fully supported any of the motor drive by giving the statement that "Each will have its loyal proponents and reigious detractors". 

The author starts with  brushless Dc motor (BLDC). The construction of the rotor is with two or more permenent magnets which are very costly that generates the magnetic flux. The magnetic flux is interacted with stator which is short circuited for current flowing that produces torque. An inverter is needed to change the magnetic field for uniform torque. The BLDC is useless without inverter.  The BLDC is used in General Motors  EV1 

The induction motors are designed with electro magnets at stator. The electro magnets are produced rotating magnetic field. The time varying flux cuts the short circuit conductors of rotor which rotates along with rmf with slip synchronous speed. The induction motor almost provides constant speed and need of inverter also avoided. The induction motor is used in Tesla cars