Week-11 Challenge: Braking

1.

The Drive Cycle chosen is US06.The MATLAB code for calculation of energy required for braking for this is given in Brk_Energy.m file attached herewith.

It clear that the calculated value of Brake Energy required is -24017.27 KJ.

2.

The braking torque of an electric motor is not as high as the accelerating torque because of the reasons mentioned below.

I.

At high speeds the motor is operated above base speed (synchronous speed) and in the field weakening region.Above base speed voltage is kept at rated value and frequency is increased which increases the speed,but decreases the torque value.

II.

The peak battery charging rate is not as high as peak battery discharging rate.Therefore regenerative braking torque cannot be as high as accelerating braking torque.

III.

The limitations associated with power converter is another reason.While recharging the power converter may not have the same efficiency as discharging.

Electric and Mechanical brakes are co-ordinated using the following two techniques:

a.Series and b.Parallel.

Since the electric motor does not have the capacity to fully develop the braking torque intially mechanical brake is applied in higher proportion compared with Electric or Regenerative brake.Once the braking torque value starts to decrease Electric or Regenerative braking value is increased in proportion.

In Series braking Mechanical and Electric or Regenerative braking are applied one after another while in Parallel braking both are applied simultaneously.Parallel braking strategy is more efficient because Regenerative braking torque is low at high speeds and so it cannot be coupled with mechanical brake to bring down the speed.At low speeds Regenerative braking will be the dominant braking system.

3.

The script file has been developed and attached herewith vide MotContour.m enclosed.

At first speed and torque inputs are given.Then the output power is found out by taking the product of torque and speed.Next input power is calculated taking into consideration total loss which is based on the following equations:

Copper Loss=Kc X T square

Iron Loss=Ki X w

Wingage Loss=Kw X w cube

Kc,Ki and Kw are constants experimentally determined and in the code their values are 0.2,0.008 and 0.00001 respectively. Another constant value of loss is added along with the above losses.

From the plot it is observed that as the speed and torque of the motor changes the efficiency also changes accordingly.The motor efficiency with the considered losses is higher when speed(W) is between the range 90 to 700 and torque is in between the range 10 to 150.