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Transformer Construction

ECE 441

1

Three-Phase Transformer

ECE 441

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Transformer Action -- DC

ECE 441

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d e1 N 1 dtOpposes battery voltage

d e2 N 2 dtOpposes flux buildup

ECE 441

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Transformer Action -- AC

ECE 441

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EP 4.44 N P f maxOpposes VT

ES 4.44 N S f maxOpposes M

EP 4.44 N P f max N P ES 4.44 N S f max N SECE 441 6

No-Load Condition

ECE 441

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No load condition continued

Io = Ife + IMIo = exciting current Io provides the magnetizing flux and the core loss

Ife = core-loss currentIM = magnetizing currentECE 441

Ife = VT / RfeIM = VT / jXM8

I O I fe I M N P I O N P I fe N P I MNo-Load Excitation mmf Magnetizing mmf No-Load Core Loss mmf

NP IM M RcoreECE 441 9

VT I P RP EP VT EP IP IO RPECE 441 10

Close the load switch

Secondary current will set up an mmf in OPPOSITION to the primary mmf. The core flux will DECREASE to

M

N P iM N S iS

RcoreECE 441 11

The decrease in flux causes a decrease in the counter-emf EP, and the primary current will increase by an amount known as IP,load, the load component of the primary current. Additional mmf due to this current adds to the magnetizing flux.ECE 441 12

M

N P iM N P iP ,load N S iS Rcore

Primary current increases until NPIP,load = NSIS. The flux M and primary emf EP return to the same values as before the switch was closed.ECE 441 13

I P I fe I M I P ,load I P I 0 I P ,load

Final steady state primary current under loaded conditions is

I P I fe I M I P ,load I P I 0 I P ,loadECE 441 14

Component Fluxes Loaded Transformer

ECE 441

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P = net flux in window of primary lp = leakage flux of primary

S = net flux in window of secondary ls = leakage flux of secondary

M = mutual fluxP = M + lp S = M lsECE 441 16