MEP 1523

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MEP 1523 ELECTRICAL DRIVES
Current ripple in unipolar and bipolar switching
schemes
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Current ripple
Current ripple unavoidable in power electronic
converter systems
Undesirable because

• Zero average increase machine heating

• Ripple in torque can be reflected in speed
  response

• Ratings of devices must consider ripple - higher
  rating of peak current

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Current ripple
H-bridge dc-dc converter with dc machine as load
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Current ripple
H-bridge dc-dc converter with dc machine as load
Approximate dc machine with RL load
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Current ripple

• Load is linear
• Principle of Superposition can be applied

Unipolar
Bipolar
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Current ripple
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Current ripple
v(t) Vave vripple
i(t) Iave iripple

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Current ripple - unipolar
Switching frequency is high
- Impedance of AC component dominated by L
i(t) Iave iripple
- Ripple is calculated based on v-i relation of
L
- L appear as short circuit in DC


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Current ripple - unipolar
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Current ripple - unipolar
vL ?
TdAB
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Current ripple - unipolar
but Vave dabVdc
, maximum when dab 0.5
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It can be shown that for bipolar scheme,
Max current ripple in bipolar scheme is four
times that of unipolar scheme
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Current ripple - bipolar
Bipolar
v(t) Vave vripple
i(t) Iave iripple
-Vdc

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Current ripple - bipolar
vL ?
TtridAB
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Current ripple - bipolar
but Vave (2dab?1)Vdc
, maximum when dab 0.5
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Current ripple - bipolar