- Inverted Induction Motor Definition: An inverted induction motor is defined as a motor where the rotor has a three-phase winding that receives power supply, creating mechanical revolving characteristics in both the stator and rotor.
- Connection Setup: The stator and rotor both have three-phase windings, with the rotor winding in a star configuration connected to slip rings.
- Operational Principle: The motor operates by setting up rotating magnetic fields in both the stator and rotor, with the rotor’s speed depending on the phase difference between their applied voltages.
- Rotor Frequency: The rotor’s speed is influenced by the difference in frequency between the rotor and the stator.
- Purpose of Use: The motor is used for voltage variation analysis and measurement circuit analysis in both no-load and load conditions.
In a conventional induction motor, the stator receives AC power and currents are induced in the rotor. An inverted or rotor-fed motor swaps these electrical roles: AC power is supplied to a wound rotor, and the stator acts as the short-circuited secondary. A star-connected rotor is common in teaching machines, but it is an implementation choice rather than part of the definition. Feeding power through rotating contacts makes this arrangement uncommon outside research and special machines.
Connection of an Inverted or Rotor-Fed Induction Motor
In the basic rotor-fed arrangement, the stator has a short-circuited three-phase winding. The wound rotor receives a balanced three-phase supply through brushes and slip rings. A laboratory rotor may use a star connection with three line terminals, but other rotor connections and converter-fed open-ended windings are possible.
Some research machines add search coils in the rotor and stator slots. Rotor search coils require additional slip rings so their signals can reach stationary instruments. These coils belong to the test instrumentation and are not required for torque production.
A wound induction machine can be excited from the stator or rotor side, but the unused winding must be connected for the intended operating mode.
In an inverted laboratory machine, placing the cage-equivalent winding on the stator makes its conductors easier to inspect, instrument and modify. The applied rotor current creates changing magnetic fields that induce current in the short-circuited stator.
Operating Principle of a Rotor-Fed Induction Motor
When the rotor winding receives a balanced three-phase supply, it establishes a rotating magnetic field at synchronous speed relative to the rotor. The field crosses the stationary, short-circuited stator conductors and induces current in them. The resulting torque drives the rotor opposite to the field’s direction relative to the rotor, reducing their relative speed.
The supplied rotor field induces EMF and current in the stator by transformer action. The stator current creates its own air-gap field. For steady torque, the stator and rotor magnetomotive-force waves must travel at the same speed in space. Lenz’s law sets the torque direction; the two fields do not simply cancel and stop the rotor.
In a machine whose stator and rotor both receive controlled supplies, mechanical speed depends on each electrical frequency, the pole count and the relative phase sequence. The phase difference between stator and rotor voltage waves affects torque and power flow after their fields are synchronised. A relation written as (fs – fr) applies to its stated sign and pole convention. Supply distortion, saturation and slotting can produce harmonics; the fundamental rotor-fed principle uses the balanced field components.
Uses of an Inverted Induction-Motor Test Stand
- Measure search-coil voltage as rotor speed, slip and supply conditions change.
- Compare measurement-circuit signals during no-load operation of the inverted rotor induction motor.
- Compare the same signals under controlled load and introduced fault conditions.
