Hunting & damper (amortisseur) windings

Reference notes

Synchronous-machine rotors swing about their steady-state load angle δ after disturbances — a phenomenon called hunting. Damper windings (amortisseur) suppress this oscillation through induction-motor action that only activates when the rotor slips away from synchronism. Use Next → to walk through the swing equation, damper construction, and the additional roles dampers play in line-starting synchronous motors and negative-sequence-current absorption.

What is hunting?

Hunting is the oscillation of the rotor load angle δ about its steady-state value δ₀ after a load disturbance. Frequencies are typically 0.5–3 Hz for utility synchronous generators. Without adequate damping, oscillations can persist for many seconds or grow into pole-slip and loss of synchronism.

The swing equation

Rotor dynamics, electrically referenced:

Linearize around the operating point δ₀. Synchronizing-power coefficient:

Adding damping D (largely from damper windings), the linearized hunting equation becomes:

This is a familiar mass-spring-damper. Natural frequency ω_n = √(K_s/J). Damping ratio ζ = D / (2√(J·K_s)). Properly designed dampers give ζ ≈ 0.3–0.5, so oscillations decay to 5 % in 2–4 cycles.

Damper / amortisseur winding — construction

• Salient-pole machine — heavy copper or brass bars run axially through holes in each rotor pole face, short-circuited at the ends (per-pole rings, or a continuous ring connecting all poles). Forms a partial squirrel-cage rotor active only during transients.
• Cylindrical-rotor (turbogenerator) — the solid forged-steel rotor body iron itself + optional copper wedges in the rotor slots serve the same function. Effect is somewhat less per-unit than dedicated dampers on salient-pole machines, but adequate for the very high inertia of turbogenerators.
• Cost: 2–4 % of total machine cost. Stability benefit: enormous. All modern synchronous generators larger than a few MW have damper windings.

How damper windings work

The mechanism is induction-motor action that activates ONLY during transients:

• Steady-state synchronism (ω_r = ω_s): stator field is stationary relative to the rotor. No flux change through damper bars. No induced EMF. No current. Damper invisible.
• During hunting (ω_r ≠ ω_s): slip-frequency relative motion induces currents in the damper bars. Those currents create a Lenz-law force opposing the relative motion — accelerating rotor is pushed back, decelerating rotor is pushed forward.
• Damping torque is approximately proportional to slip speed dδ/dt — exactly the D · dδ/dt term in the swing equation.
• The cleverness: zero loss in steady state, large damping during transients. No control system needed.

Other crucial roles of damper windings

1. Line-starting synchronous motors. A bare synchronous motor cannot start from a 3-φ stator supply — the rotating field is too fast for the inert rotor. With damper windings, the motor starts as an induction motor (cage rotor = the damper bars), producing the starting torque needed to accelerate the load up to near-synchronous speed. Then the DC field winding is energized and the rotor pulls into synchronism. Damper currents drop to zero. This is the universal scheme for medium-to-large synchronous motors (compressors, pumps, mill drives), and the damper's starting torque capability is one of its three core design specifications.
2. Voltage support during faults. Rotor speed deviates during short-circuit transients; dampers induce currents that support terminal voltage and contribute additional fault current to relays.
3. Negative-sequence-current (NSC) suppression. Negative-sequence stator currents from unbalanced loading produce a backward-rotating MMF at 2f relative to the rotor. Dampers conduct heavy 2f currents that partially cancel this MMF and absorb its heating, protecting the field winding and rotor body. NSC tolerance ratings (e.g., I₂² · t = 30 s for a typical turbogenerator) are set by what the dampers can absorb.

Why K_s drops near the stability limit

K_s = (V·E/X_s) · cos(δ₀) goes to zero as δ₀ → 90°. Near the steady-state stability limit, the system has minimal restoring force and is highly susceptible to pole-slip from any perturbation. Operating reserve and good damping are essential — and this is why governor / AVR / PSS tuning matter.

Power System Stabilizer (PSS) — modern damping enhancement

For very large or critical machines, additional damping is provided electronically via a Power System Stabilizer that modulates the field excitation in response to detected speed or power oscillations. PSS supplements (but does not replace) the physical damper windings. PSS tuning is one of the standard tasks in interconnection studies for new generation.

Keyboard shortcuts

• Step 1–3 visualizes undamped hunting; step 4 onward switches in the damper winding so you can compare the decay envelopes side by side.