More project photos (3)
The problem
Speed control of a wound-rotor induction motor below synchronous speed is classically done by inserting resistance in the rotor circuit — simple, but every watt of slip power is burned as heat in the resistors. The static Scherbius scheme fixes exactly this waste: the slip-frequency power from the rotor slip rings is rectified and fed back into the AC supply through a line-commutated inverter, so the energy that resistance control would destroy is recovered instead. This project builds that scheme at laboratory scale: a fractional-HP wound-rotor motor, a three-phase diode bridge on the rotor side, a DC-link inductor, and a thyristor bridge operated in inverter mode returning power to the mains. A firing-angle controller sets the inverter angle, giving smooth subsynchronous speed control — and a switchable rotor-resistor bank lets students measure the before/after difference the recovery makes.
Wound-rotor motor whose slip rings expose rotor power.
Follow the working sequence
How it works
- The wound-rotor motor runs from the AC mains; its rotor slip rings carry slip-frequency AC proportional to the slip.
- A three-phase diode bridge rectifies the rotor output; a DC-link inductor smooths the resulting DC.
- A thyristor bridge, synchronized to the mains and fired in inverter mode, converts the DC-link power back to AC and feeds it into the supply.
- The firing-angle controller sets the inverter angle: advancing it raises the counter-EMF, increases the effective rotor resistance electrically and slows the motor — speed control without physical resistors.
- Flipping the mode switch inserts the rotor-resistor bank instead, demonstrating conventional dissipative control for comparison.
- Meters on the DC link and the mains side let students observe the slip power being recovered rather than burned.
Tech stack:
- Fractional-HP wound-rotor (slip-ring) induction motor
- Three-phase diode bridge rectifier (rotor side)
- Line-commutated thyristor inverter bridge (mains side)
- DC-link smoothing inductor
- Microcontroller firing-angle controller with mains synchronization
- Panel meters (DC-link V/I, mains-side indication)
| Parameter | Value |
|---|---|
| Motor | Fractional-HP wound-rotor induction motor with slip rings |
| Rectifier | Three-phase diode bridge on the rotor slip power |
| Inverter | Line-commutated thyristor bridge returning power to the mains |
| Speed range | Below synchronous speed; design target approx. 50–100% of synchronous speed |
| Control | Inverter firing-angle controller with commutation-margin limit |
| Metering | DC-link voltage/current, mains-side power indication |
| Comparison | Switchable rotor-resistor bank for resistance-control mode |
| Protection | Line fuses, DC-link inductor, firing-angle limits |
Project features
- [Wound-rotor motor with slip-ring access] A fractional-HP slip-ring induction motor exposes the rotor circuit so slip power can be tapped, rectified and recovered.
- [Rotor diode-bridge rectifier] A three-phase diode bridge converts the slip-frequency rotor output to DC for the link inductor.
- [Line-commutated inverter] A thyristor bridge operated in inverter mode returns the DC-link power to the AC mains — the heart of the Scherbius recovery.
- [Firing-angle speed control] Varying the inverter firing angle changes the counter-EMF seen by the rotor circuit, giving smooth speed control below synchronous speed.
- [Comparison mode] A switchable rotor-resistor bank lets the same motor run with plain resistance control, so students can compare wasted versus recovered slip energy on the meters.
- [Slip-power metering] DC-link voltage and current meters plus mains-side indication show where the slip energy goes in each mode.
- [Protection built in] Line fuses, a DC-link smoothing inductor and a commutation-margin limit on the inverter firing angle protect the bridges.
What is included
- Working slip-power recovery drive panel with wound-rotor motor rig
- Controller firmware source (firing-angle control, synchronization)
- Circuit and wiring documentation, component list
- Operating procedure: recovery mode vs resistance-control comparison
- Project report PDF (Scherbius theory, power-flow analysis, comparison procedure)
- PPT presentation for final review
- Viva Q&A preparation document (slip power, Scherbius vs Kramer, inverter commutation)
- Setup and safety guide
Advantages
- Recovers slip energy instead of wasting it as heat in rotor resistors, by rectifying the rotor output and returning it to the AC mains through the line-commutated inverter.
- Smooth subsynchronous speed control by varying the inverter firing angle, which changes the counter-EMF seen by the rotor circuit.
- Built-in comparison mode: the switchable rotor-resistor bank lets the same motor run with plain resistance control, so recovered versus wasted slip energy can be compared on the meters.
- Slip-power metering is built in, with DC-link voltage and current meters plus mains-side indication showing where the slip energy goes in each mode.
- Protection is part of the build: line fuses, a DC-link smoothing inductor and a commutation-margin limit on the inverter firing angle protect the bridges.
Disadvantages
- Speed control is subsynchronous only; it cannot run the motor above synchronous speed, which would need a doubly-fed arrangement outside this project's scope.
- The line-commutated inverter draws reactive power and harmonic currents from the mains, so the panel suits short laboratory demonstrations rather than continuous connection without filtering.
- At laboratory scale the recovered power is small; the value of the build is demonstrating the principle and the measurement procedure, not the wattage.
- Wound-rotor motors are less common in the current market, so sourcing a suitable fractional-HP unit is part of the build timeline.
- Efficiency and recovery figures are measured by the student during their own comparison tests; no performance numbers are pre-claimed.
Limitations & prerequisites
- Speed control is subsynchronous only — this scheme cannot run the motor above synchronous speed; that needs a doubly-fed arrangement, outside this project's scope.
- The line-commutated inverter draws reactive power and harmonic currents from the mains; the panel is for short laboratory demonstrations, not continuous connection without filtering.
- At laboratory scale the recovered power is small — the value of the build is demonstrating the principle and the measurement procedure, not the wattage.
- Wound-rotor motors are less common in the current market; sourcing a suitable fractional-HP unit is part of the build timeline.
- Efficiency and recovery figures are measured by the student during their own comparison tests — no performance numbers are pre-claimed.
Frequently Asked Questions
How is this different from rotor-resistance speed control?
Resistance control burns slip power as heat in the rotor resistors. This drive rectifies the slip power and inverts it back into the mains — the same speed control, but the energy is recovered instead of wasted.
Which controller is used?
A microcontroller synchronized to the mains generates the thyristor firing pulses for the inverter bridge, with the firing angle adjustable to set speed.
Can the motor run above synchronous speed?
No — the static Scherbius scheme gives subsynchronous control only. Super-synchronous operation needs a doubly-fed (rotor-side converter) arrangement.
What are the main limitations?
Subsynchronous range only; inverter harmonics limit it to short lab demos; recovered power is small at this scale; efficiency numbers come from the student's own measurements.
What power supply is needed?
A standard 230 V AC lab outlet for the motor and the inverter side; the rotor-side circuitry is derived from the slip rings.
Is this project suitable for a final-year project?
Yes — for Electrical Engineering programs. It demonstrates electrical machines, power electronics, energy recovery and honest experimental comparison in one build. Suitable for B.E./B.Tech final-year projects in Electrical Engineering.
Components & software requirements
- Fractional-HP wound-rotor (slip-ring) induction motor
- Three-phase diode bridge rectifier (rotor side)
- Line-commutated thyristor inverter bridge (mains side)
- DC-link smoothing inductor
- Microcontroller firing-angle controller with mains synchronization
- Panel meters (DC-link V/I, mains-side indication)
Delivery information
Built-to-order project. Delivery timeline is shared after order confirmation based on current queue.
Support terms
Complete documentation, setup guide, and viva preparation included. Support for setup and explanation provided.
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Kumar et al. - Review of Slip Power Recovery Scheme (IJRRA, 2015)