Join 60,000+ competitive exam aspirants
or with email
Your Cart
Subtotal₹0
Total₹0
Examoogle • User • info@examoogle.com • EE-2024-8821
Chapter 1 of 12 • Page 1 of 248🔒 Protected PDF • Watermarked
Join 60,000+ competitive exam aspirants
Unbalance loading in alternator produce
Negative phase sequence current
Positive phase sequence current
Both A and B
None of above
Negative phase sequence current
Quick Summary: Unbalanced loading in an alternator creates an unsymmetrical armature reaction, which can be decomposed into positive, negative, and zero sequence components using symmetrical component analysis. The negative phase sequence currents are specifically induced by unbalanced loads and circulate in the stator winding, creating a magnetic field rotating in the opposite direction to the rotor, causing severe heating in the rotor pole faces and dampers.
Unbalanced loading in an alternator creates an unsymmetrical armature reaction, which can be decomposed into positive, negative, and zero sequence components using symmetrical component analysis. The negative phase sequence currents are specifically induced by unbalanced loads and circulate in the stator winding, creating a magnetic field rotating in the opposite direction to the rotor, causing severe heating in the rotor pole faces and dampers.
Ia1=31(Ia+aIb+a2Ic) — Positive sequence component
Ia2=31(Ia+a2Ib+aIc) — Negative sequence component
When a three-phase alternator experiences an unbalanced load, the phase currents Ia,Ib,Ic are unequal in magnitude and/or phase displacement. These currents produce an armature MMF that is not constant in space or time. This MMF can be resolved into: (1) A positive sequence component (rotating at synchronous speed in the direction of the rotor), (2) A negative sequence component (rotating at synchronous speed in the opposite direction of the rotor), and (3) A zero sequence component (which cancels out in a balanced star connection without neutral). The negative sequence field induces double-frequency currents in the rotor, leading to eddy current losses and overheating.
Negative phase sequence currents circulate at twice the power frequency (100Hz) in the rotor.
These currents induce significant eddy current heating in the rotor body and damper windings.
Alternators have a negative sequence capability limit, typically defined by the constant I2°2t.
Unbalanced load is a common fault condition that triggers negative sequence protection (46 relay).
Protection of large synchronous generators
Power system stability analysis
The negative phase sequence current causes the alternator rotor to experience double-frequency torque pulsations.
Option B (Positive phase sequence) represents the normal operating component of a balanced load.
If the load is perfectly balanced, negative sequence currents are zero.
A is correct — Unbalanced loading in an alternator induces negative phase sequence currents which flow in the stator and produce a magnetic field rotating in the opposite direction of the rotor.
Always remember that while positive sequence current is useful for power transfer, negative sequence current is purely detrimental and used as a threshold for generator de-rating and protection.