- Wound Rotor Induction Motor Definition: A wound rotor induction motor (also known as a slip ring motor) is defined as a type of 3-phase AC motor designed for high starting torque by connecting external resistance to the rotor circuit.
- Rotor and Stator Design: The rotor has three-phase insulated windings connected to slip rings via brushes, while the stator is similar to a squirrel cage motor.
- Speed Control: Speed is controlled by varying the resistance in the rotor circuit, which changes the motor’s speed and torque characteristics.
- Comparison to Squirrel Cage Motors: Wound rotor motors have high starting torque and adjustable speed, whereas squirrel cage motors are simpler, cheaper, and require less maintenance.
- Common Uses: Wound rotor motors are used in high-power industrial applications requiring high starting torque and adjustable speed, such as cranes, hoists, and elevators.
What is a Wound Rotor Induction Motor?
A wound-rotor induction motor (also called a slip-ring motor, not a “would round” motor) is a three-phase induction machine. External resistance in the rotor circuit raises starting torque and limits starting current. Other names are phase-wound motor and slip-ring induction motor.
Like any induction motor it runs below the synchronous speed of the stator field (it has slip). Asynchronous motor is the family name for that behaviour, not a label unique to the slip-ring type.
Wound Rotor Motor Diagram
The stator is built like a squirrel cage induction motor. The rotor winding must have the same pole count as the stator.
Three insulated rotor phases come out to slip rings. Brushes take the induced rotor current to an external circuit.
Classically those brushes feed a three-phase star rheostat. Electronic or liquid-rheostat starters exist too. The stored figure is the rheostat layout.
At start, extra rotor resistance improves rotor power factor so starting torque rises, while the larger impedance cuts starting current.
The rheostat is cut out as the motor comes up. Extra impedance at start reduces current magnitude. Once running with rings shorted, that extra R is gone.
Starting of Wound Rotor Induction Motor
Rotor Resistance/Rheostat Starting
The usual method is full stator voltage, with starting current set in the rotor. Reduced-voltage stator starting is less common on this type, not forbidden.
A star rheostat (or equivalent) sets starting current. Cut the resistance out as speed rises so you do not leave I²R loss in the starter at run.
More rotor R at start: lower rotor and stator current, higher rotor power factor, so starting torque can still be high.
That is the point of the method: high starting torque at a moderate line current, compared with a squirrel-cage motor started DOL on the same supply.
It can start into a load if the rheostat is set for enough torque. At run, many machines short the rings; some also lift the brushes. Others leave the brushes on shorted rings. Follow the machine in front of you.
How to Control Speed of a Wound Rotor Motor
Rotor Resistance Control
With the motor running, extra rotor R increases slip, so speed falls for a given load. This rotor-resistance speed control exists only on wound-rotor machines, not on a cage rotor.
Inserting the full rheostat while running drops speed (higher slip) and wastes power in the resistors.
Induced rotor voltage is proportional to slip. At the new, higher slip the motor develops the load torque again. It does not “create extra torque” beyond what the load demands.
Cutting rotor R out raises speed toward the light-resistance curve. The stored figure is that family of torque-speed curves.
On the stored plot, rotor resistance R1 gives speed N1. The R1 curve is the blue line.
A larger R2 shifts the peak toward lower speed N2. The R2 curve is the green line.
What is the Purpose of the Slip Rings Located on the Rotor Shaft of a Wound Rotor Motor?
The rings let you connect an external three-phase circuit (rheostat, liquid starter or power-electronics recovery) while the rotor turns.
A slip ring is an electromechanical rotary joint: current or signals between a stator and a rotor.
Other names: rotary joint, rotary interface, collector ring.
Wound Rotor Induction Motor vs Squirrel Cage
What is the Difference Between a Squirrel-Cage Motor and a Wound-Rotor Motor?
The stator is similar. The rotor is not.
Cage bars are closed at the end rings, so you cannot insert external R. A wound rotor brings the windings out so you can.
The stored table is a teaching comparison. Treat the 90%/5–10% industry split and “explosion-proof” cage claim as rough catalogue wording, not measured facts. NEMA design C/D cages can also raise starting torque.
| Specifications | Wound-rotor or slip ring induction motor | Squirrel cage induction motor |
| Construction | Complicated due to the presence of slip rings and brushes | Simple due to the absence of slip rings and brushes |
| Starting Method | The motor requires slip rings, brush gear, short-circuiting device and starting resistance, etc.… | The motor can be started with a star-delta starter. |
| Starting Torque | High starting torque can be obtained due to the presence of external resistance in the rotor circuit. | Poor starting torque and cannot be improved. |
| Space factor in slots | Better | Poor |
| Rotor | The rotor is wound rotor type with its terminal ends connected to 3 slip rings on the output shaft. | The rotor is skewed rotor type with its terminals are short-circuited at end rings. |
| No. of turns on the rotor | More | Less |
| The induced voltage in the rotor | Higher | Less |
| Speed Control | Speed control by rotor resistance method is possible | Speed cannot be controlled by the rotor resistance method |
| Maintenance | Frequent maintenance is required due to the presence of brushes and slip rings | Less maintenance is required |
| Copper Losses | High | Less |
| Efficiency | Low due to power loss in external resistance | High |
| Cost | High Cost | Cheaper in cost |
| Applications | Used where high starting torque required such as cranes, hoist, elevator, etc.… Rarely used about 5%-10% of the industry uses slip ring induction motor. | Used in lathe machines, drilling machines, blowers, fan, etc.… Widely used about 90% of the industry uses squirrel cage induction motor. |
Advantages of Wound Rotor Motor
Wound-rotor strengths, with the usual limits:
- High starting torque when the external rotor R is in.
- Smooth acceleration into a heavy load if the rheostat is staged. Overload capacity still follows heating and the nameplate.
- Lower starting current than a same-size cage motor started DOL, because of that extra rotor impedance.
- Speed below synchronous can be set with rotor R, at the cost of I²R in the rheostat. It is a limited variable-speed method, not a modern VFD.
- Better rotor (and thus starting) power factor with R in at start. Running pf with rings shorted is similar to a cage machine of the same rating.
Advantages of Squirrel Cage Motor
Cage-motor strengths:
- Simple and rugged in construction
- Cheaper in cost
- Maintenance cost is low
- Nearly constant speed on a fixed supply (small slip with load)
- High overload capacity within thermal limits (not unique to this type)
- Simple starting arrangement
- Often a high breakdown torque for a given frame; compare nameplates, do not assume it always beats a wound rotor
- Available in explosion-proof listed enclosures; a cage rotor is not automatically explosion-proof
- End rings can help as a fan; cooling still depends on the enclosure
What Are Common Uses of a Wound Rotor Induction Motor
Common uses of a wound-rotor induction motor:
- High-power industrial electrical drives that need high starting torque: lifts, line shafts, cranes, hoists, elevators, conveyors, winding machines, mills. VFDs now cover many of these jobs instead.
- Older adjustable-speed drives used rotor resistance or recovery systems (Kramer, Scherbius). Interval speed change is possible. A VFD on a cage motor is the usual new choice.
- Example: coal-handling conveyors at a power plant that need a hard start.
- Use one where DOL cage starting would pull the supply voltage down too far.
- Punch presses and shears with large flywheels are a historic application (high starting torque, then run near full speed).
