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.
Voltage Vc lags Va by 120˚ and voltage Vb lags Vc by 120˚.
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.
ABC →BCA →CAB
AB - BC - CA → BC - CA - AB → CA - AB - BC
AN - BN - CN → BN - CN - AN → CN - AN - BN
Vector Representation
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| Sequence ABC |
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| Sequence BCA |
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| Sequence CAB |
Negative Phase sequence
ACB sequence- Va = Ka0˚ , Vb = a-120˚ and Vc = Ka-240˚
ACB →CBA →BAC
AB - CA - BC → CA- BC - AB → BC - AB - BA
AN - CN - BN → CN - BN - AN → BN - AN - CN
Vector representation
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| Sequence BAC |
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| Sequence CBA |
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| 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.