74 Operating Point Analysis

The following discussion presents the steady state operating point analysis of the vehicle system at the intersection of the motor torque-speed characteristics with the road load characteristics. Four operating points are chosen for analysis in three different quadrants of the motor speed-torque plane representing four chopper operating modes discussed earlier. These modes are acceleration CCM in Quadrant I (Scenario 1), acceleration CCM in Quadrant IV (Scenario 2), acceleration UNCM in Quadrant II (Scenario 3), and braking CCM in Quadrant II (Scenario 4). The four scenarios are shown in Figure 7.24.

Now, recall the tractive force vs. vehicle speed characteristics of Chapter 2, which essentially define the speed-torque characteristics of the EV or HEV load. The tractive force vs. velocity characteristics of Figure 2.11 can be converted into equivalent vehicle load speed-torque characteristics, knowing the transmission gear ratio and the vehicle wheel radius. The steady-state operating point of the vehicle for certain conditions of the motor drive system and road load characteristics can be obtained by overlaying the two speed-torque characteristics on the same plot. The steady-state operating points at the intersection of the motor and load characteristics are shown in Figure 7.25.

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