MEP 1523ELECTRICAL DRIVES

Current ripple in unipolar and bipolar switching schemes

Current ripple – unavoidable in power electronic converter systems:

Current ripple

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

Current ripple

H-bridge dc-dc converter with dc machine as load

Current ripple

Approximate dc machine with RL load

H-bridge dc-dc converter with dc machine as load

Current ripple

• Load is linear• Principle of Superposition can be applied

Vdc

Vave

Vdc

Vdc

Vave

Unipolar Bipolar

Current ripple

Vdc

Vave

Unipolar

Current ripple

Vdc

Vave

Unipolar

Vave

v(t) = Vave + vripple

+

i(t) = Iave + iripple

Vdc Vave

Current ripple - unipolar

i(t) = Iave + iripple

= +

i(t) Iave iripple

Switching frequency is high

- Impedance of AC component dominated by L

- Ripple is calculated based on v-i relation of L

- L appear as short circuit in DC

Current ripple - unipolar

iripple

Current ripple - unipolar

iripple

t

VdcVdc-Vave

Ttri

T

TdAB

TdL

VVi ab

avedcrr

ti

Lvdtdi

Lv LL

tLv

i L

i +vL

vL

iripple

Current ripple - unipolar

TdL

VVi ab

avedcrr

but Vave = dabVdc

TVL

ddi dc

2abab

rr

, maximum when dab= 0.5

tridc

2

max,rr TVL2

5.05.0i

L8TV

i tridcmax,rr

It can be shown that for bipolar scheme,

L2TV

i tridcmax,rr

Max current ripple in bipolar scheme is four times that of unipolar scheme

Current ripple - bipolar

VdcVave

Bipolar

Vave

v(t) = Vave + vripple

+

i(t) = Iave + iripple

Vdc Vave

-Vdc

Current ripple - bipolar

iripple

t

2VdcVdc-Vave

Ttri

TtridAB

triabavedc

rr TdL

VVi

ti

Lvdtdi

Lv LL

tLv

i L

i +vL

vL

iripple

Current ripple - bipolar

triabavedc

rr TdL

VVi

but Vave = (2dab1)Vdc

tridc

2abab

rr TV2L

ddi

, maximum when dab= 0.5

tridc

2

max,rr TV2L

5.05.0i

L2TV

i tridcmax,rr

Current ripple - bipolar

L2TV

i tridcmax,rr

L8TV

i tridcmax,rr

Max ripple for bipolar Max ripple for unipolar