Rotor-resistance speed control (slip-ring IM)

Reference notes

The slip-ring (wound-rotor) induction motor has a 3-φ rotor winding brought out via slip rings and brushes — enabling external resistance to be inserted in series with the rotor. Use Next → to walk through the construction, the speed-control mechanism (s_max ∝ R_2), the energy-efficiency penalty (slip power wasted as heat), the slip-energy-recovery alternatives (Kramer / Scherbius / DFIG), and where this control method still wins today.

Construction

• Stator: identical to cage IM — 3-φ distributed winding.
• Rotor: 3-φ wound winding (not cage), wye-connected. Three winding ends brought out via three slip rings on the shaft.
• Brushes: carbon brushes ride on the slip rings, giving external access to the rotor circuit. Three external terminals (one per phase) plus optional neutral.
• External 3-phase resistor bank connects to these terminals.

Speed-control mechanism

Recall the induction-motor torque equation:

Two key properties:

• s_max = R_2' / X — slip at peak torque is DIRECTLY PROPORTIONAL to rotor resistance.
• T_max is INDEPENDENT of R_2' — peak torque magnitude unchanged by adding external R.

Adding external resistance shifts the entire T-s curve right (toward higher slip) without lowering T_max. For a fixed load torque T_load, the operating-point slip rises → rotor speed drops. Continuous speed control from ~100% to ~50% sync speed is achievable.

The efficiency penalty

Rotor copper loss = s · P_air-gap ≈ s · P_input. To run the motor at 30% slip (speed reduced from sync to 70% sync), 30% of input power is dissipated as heat in the rotor circuit — mostly in the external resistor bank you added:

• Motor efficiency drops to ~60–65% at significant speed reduction.
• VFD-driven cage motor at the same speed: ~90% efficiency (changes supply frequency, doesn't waste slip power).
• External resistor bank needs forced cooling.
• Brushes and slip rings need periodic maintenance.

This is why slip-ring motors with R_ext have been largely replaced by VFD + cage induction motors over the past 30 years.

Slip-energy recovery — modern variants

• Kramer drive — rectify the rotor AC slip power, smooth to DC, feed back to the grid through a line-commutated thyristor inverter. Subtraction-of-power architecture; net efficiency comparable to VFD. Limited grid-side PF.
• Scherbius drive — bidirectional power converter (typically VSC + VSC) on the rotor circuit. Energy flows in either direction → full speed range above AND below synchronous. Better PF and grid-friendly than Kramer.
• DFIG (doubly-fed induction generator) — Scherbius drive in generator mode, used in megawatt-class wind turbines. Stator direct to grid, rotor via slip rings + bidirectional converter sized for slip power only (~30 % of total). Type 3 wind turbine architecture. Vestas, GE, Siemens-Gamesa products. Dominant in onshore wind installed base.

Where rotor-resistance + slip-ring still wins today

• Very high starting-torque loads — ball mills, kilns, conveyors, crushers, hoists. R_ext tapped in delivers full breakdown torque at standstill (~2-3× rated). A VFD on a cage motor would need oversize ratings to match.
• Weak-grid sites — R_ext limits stator inrush to ~rated current, avoiding the 6× FLA direct-on-line inrush that overloads weak grids.
• Retrofit — replace R_ext starter with Kramer / Scherbius drive on an existing slip-ring motor to avoid replacing the motor itself.
• DFIG wind turbines — the modern application of slip-ring topology, with rotor converter instead of resistor.

Typical industrial applications

500–5000 HP range, medium voltage (2.3–6.6 kV). Cement-plant ball mills, kilns, iron-ore crushers, coal grinders, large conveyors, mine hoists, ship propulsion. Many 1960s–1980s installations remain in service; retrofits with Kramer/Scherbius converters are an active market.

Modern decision matrix