- Induction Motor Rotor Definition: The rotor is the rotating part of an electrical machine, where current is induced by a rotating magnetic field.
- Types of Rotors: Induction motor rotors are of two types: Squirrel Cage Rotor and Wound Type Rotor (Slip Ring Rotor).
- Squirrel Cage Rotor Features: This rotor has no definite number of poles, low leakage reactance, and provides good running performance but poor starting performance.
- Wound Type Rotor Features: Includes slip rings and external resistors that improve starting torque and power factor while limiting starting current.
- Performance Comparison: Wound type rotors offer good starting performance but poor running performance, unlike squirrel cage rotors.
A rotor is the rotating part of an electrical machine. In an induction motor, current is induced in that rotor by a rotating magnetic field through transformer action.
An induction motor rotor uses one of two constructions:
- Squirrel Cage Rotor
- Wound Type Rotor or Slip Ring Type Rotor
Squirrel Cage Rotor
In this rotor the winding is conductors in the form of copper or aluminium bars in semi-closed slots of a laminated core. End rings short-circuit both ends of the bars so the cage has a closed path.
Features of Squirrel Cage Rotor
This rotor is not wound for a chosen pole number. The stator field induces the same pole count in the cage. For a squirrel cage rotor the induced poles therefore match the stator.
The squirrel cage rotor has low leakage reactance because the bars and end rings are short and tightly coupled, not because it has no winding. The cage is the winding. Low rotor resistance at supply frequency gives modest starting torque and a high running efficiency once the motor is up to speed.
Starting torque rises if rotor resistance at start is higher. A slip-ring machine can add external resistors in series with the rotor winding. A squirrel cage rotor cannot, because the bars are shorted by the end rings. Deep-bar and double-cage designs raise effective start resistance without external resistors. A plain cage therefore has strong running performance and weaker starting performance unless the bar shape is designed for start.
Drawbacks of Squirrel Cage Rotor
- Low starting torque
- High starting currents
- Poor power factor
Starting of a plain squirrel-cage rotor can be improved by deep-bar or double-cage design, or by a reduced-voltage starter or a variable-frequency drive.
Skewed Rotor Bars
Skewing the rotor bars mainly reduces cogging, crawling and magnetic noise. The bars become a little longer, so their resistance rises slightly. That small rise is a side effect, not the main reason for skew.
Wound Type Rotor or Slip Ring Type Rotor
This rotor is also a laminated silicon-steel core, often non-oriented rather than cold-rolled grain-oriented, to limit eddy-current and hysteresis loss. The rotor winding is distributed and short-pitched so the induced voltage is close to a sine wave.
An induction motor needs the same number of stator and rotor poles. A wound rotor does not pick up a new pole count by itself when the stator pole number changes, so the rotor winding must be built for that pole number.
If the rotor has a three-phase winding, that winding is usually star-connected so only three slip rings are needed, whether the stator is star or delta.
Features of Wound Type Rotor or Slip Ring Type Rotor
The construction difference versus a squirrel cage is the slip rings on a wound type rotor. Plants also name a wound type rotor a slip-ring rotor. The three terminals of the star-connected rotor winding come out to external resistors through those rings.
Slip rings are usually phosphor bronze or brass, chosen for wear and conductivity, not as a high-resistance starting element. The starting resistance sits in the external resistors. Brushes, carbon or copper, connect the rings to that circuit. Carbon is common because it is self-lubricating, so friction is lower.
External resistors raise starting torque and limit the starting current. With the extra rotor resistance, starting power factor is also better than a low-resistance cage on the same supply.
If those extra resistor banks stay in circuit after start, a Wound Type Rotor or Slip Ring Type Rotor induction motor has strong starting performance and extra running loss. After start the rings are normally shorted so running performance can approach a cage machine.
