Saturday, February 18, 2012

AC Circuit Phase Sequence

Phase sequence is the order in which the generated voltages in the phase winding of an alternator reach or  attain  their peak or maximum values. It is represented by the letters a, b, and c or the numbers 1, 2, 3 or by using double letter as ab, bc  and ca or  an, bn  and cn.

For instance , The three phase balanced voltages with their common magnitudes as K have sequence of a b c , then in complex form, 

Positive Phase Sequence 
ABC sequence  -  Va = Ka0˚ , Vb = a-120˚ and Vc = Ka-240˚

In this sequence Vb lags Va by 120˚ and Vc lags Vb by 120 or Vc lags Va by 240. The maximum value of Va comes first in the positive direction, next Vb and then Vc. 

AB - BC - CA → BC - CA - AB → CA - AB - BC
AN - BN - CN → BN - CN - AN → CN - AN - BN

Vector Representation

ABC Phase Sequence
 Sequence ABC  

BCA Phase Sequence
Sequence BCA
CAB Phase Sequence
Sequence CAB

Negative Phase sequence 
ACB sequence- Va = Ka0˚ , Vb = a-120˚ and Vc = Ka-240˚

Voltage Vc lags Va by 120˚ and voltage Vb lags Vc by 120˚

AB - CA - BC → CA- BC - AB → BC - AB - BA
AN - CN - BN → CN - BN - AN → BN - AN - CN

Vector representation
BAC Phase Sequence
Sequence BAC
CBA Phase Sequence
Sequence  CBA

ACB Phase Sequence
Sequence ACB

Assume a positive phase sequence if the phase sequence is not given . The three  phase alternators are designed to operate with positive phase sequence voltages.