7324 Acceleration Discontinuous Conduction Mode DCM

When the torque required from the motor in the acceleration mode is not high enough, the chopper may enter the DCM, where the armature current becomes discontinuous, as shown in Figure 7.19.

In the discontinuous conduction mode, Ia0= 0 and Vbat>E (because power is still flowing from the energy source into the motor). The operation is still in Quadrant I. The condition for DCM of operation is

A

r djT├ĆAT

r ! -

/

X

T k

A

0 V

E

t

VW

D2 off

SWS2

Qioff 1:11 off Q2off Diun Sw$l

Q[Of? D ] off Q2o(f Da off swso

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FIGURE 7.19 Voltage and current waveforms during acceleration in the DCM.

The motor armature current cannot go negative without the use of switch Q2 in the DCM. Q1 is used to control the flow of power from the source to the motor only. In the interval between d1T+ Tand T, for ia to become negative, D1 must turn on. However, during this interval v 0=Vbat>E; hence, ia cannot become negative. Also, ia cannot become positive, because that would require v0=Vbat, which would require Q1 to be on, but ib1=0. Hence, ia=0 over this interval.

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