03-04-2012, 12:00 PM
4 Chopper-Controlled DC Motor Drive
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Chopper: The variable dc voltage is controlled
by varying the on- and off-times of a converter.
• Fig. 4.1 is a schematic diagram of the chopper.
• Its frequency of operation is
and its duty cycle is defined as
• Assuming that the switch is ideal, the
average output is
• varying the duty cycle changes the output
voltage.
T
1
(t t )
f 1
on off
c =
+
=
T
d = ton
s s
on
dc V dV
T
V = t =
• The duty cycle d can be changed in two ways:
(i) varying the on-time (constant switching
frequency).
(ii) varying the chopping frequency.
• Constant switching frequency has many
advantages in practice.
4.3 Four-Quadrant Chopper Circuit
• Fig. 4.2 is the chopper circuit with transistor
switches.
• Fig. 4.3 is first-quadrant operation.
• Fig. 4.4 is first-quadrant operation with zero
voltage across the load.
• Fig. 4.5 voltage and current waveforms in
first-quadrant operation.
• Fig. 4.6 second-quadrant operation
• Fig. 4.7 voltage and current waveforms in
second-quadrant operation.
• Fig. 4.8 Modes of operation in the third
quadrant.
• Fig. 4.9 waveform in third-quadrant operation
• Fig. 4.10 Waveforms in fourth-quadrant
operation.
4.4 Chopper for inversion
• Converter: DC ⇒ DC (different voltage)
• The chopper is the building block for that:
AC ⇒ DC ⇒ DC(different voltage)
4.5 Chopper with other power devices
• MOSFETs, IGBTs, GTOs, or SCRs are
used for different power level.
• The MOSFET and transistor choppers are
used at power levels up to 50kW.
4.6 Model of The Chopper
• The transfer function of the chopper is
where Kr = Vs/Vcm, Vs is the source voltage,
and Vcm is the maximum control voltage.
• Increasing the chopping frequency decreases
the delay time, and its becomes a simple gain.
2
1 sT
G (s) Kr
r
+
=
4.7 Input to the chopper
• The chopper input: a battery or a rectified ac
supply.
• Fig. 4.11 is the chopper with rectified circuit.
• Its disadvantage: it cannot transfer power from
dc link into ac mains.
• Fig. 4.12 Chopper with regeneration capability
• The generating converter has to be operated
at triggering angles greater that 90