Chopper

MOTOR

FIGURE 7.16 Series inductance Lf added in series with the chopper.

Adding a series inductance in the armature increases the electric time constant t. The new time-constant is where Lfis the added series inductance (Figure 7.16). As t increases, ia decreases for fixed switching period T. The trade-off is the increase in i2R losses due to Lf, because practical inductance has series resistance. Also, the electrical response time will increase due to the increase in the time-constant.

Increasing the switching frequency of the chopper, i.e., decreasing T, will also reduce the armature current ripple. The upper limit on switching frequency depends on type of switch used. The switching frequency must also be smaller than the controller computational cycle time, which depends on the speed of the processor used and the complexity of the control algorithm. The trade-off of using a higher switching frequency is higher switching losses in the power devices.

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