What is a Wound Rotor Induction Motor?
A wound rotor induction motor (also known as a would round motor or slip ring induction motor) is defined as a special type of 3 phase AC induction motor designed to provide high starting torque by connecting an external resistance to the rotor circuit. The motor’s rotor is a type of wound rotor. Hence why it is also known as a wound rotor or phase wound induction motor.
The speed at which a slip ring induction motor operates is not equal to the synchronous speed of the rotor. Hence, it is also known as an asynchronous motor.
Wound Rotor Motor Diagram
The stator of the wound rotor induction motor is the same as a squirrel cage induction motor. The rotor of the motor is wound for the same number of poles as that of the stator.
The rotor has three-phase insulated windings with each winding connected to slip rings via brushes. Here, the function of the brush is to collect current to and from the rotor winding.
These brushes are further connected to the three-phase star-connected rheostat. The below figure shows the wound rotor induction motor diagram.
In a slip ring or wound rotor induction motor, the torque is increased by adding external resistance in the rotor circuit from a star-connected rheostat.
This rheostat resistance is gradually cut out as the speed of the motor increases. This additional resistance increases the rotor impedance hence also reduces the rotor current.
Starting of Wound Rotor Induction Motor
Rotor Resistance/Rheostat Starting
The slip ring induction motor is practically always started with full line voltage applied across the stator terminals.
The value of starting current is adjusted by introducing a variable resistance in the rotor circuit. The controlling resistance is in the form of star-connected rheostat; the resistance is being gradually cut out as the motor gains speed.
By increasing the rotor resistance, the rotor current is reduced at starting, and hence stator current is also reduced but at the same time torque is increased due to improvement in power factor.
As discussed, the additional resistance in the rotor circuit enables the slip-ring motor to develop a high starting torque with a moderate starting current.
Hence, a wound rotor or slip ring motor can always be started with some load. When the motor runs under normal conditions the slip rings are short-circuited and brushes are removed.
How to Control Speed of a Wound Rotor Motor
Rotor Resistance Control
The speed of the wound rotor or slip ring induction motor can be controlled by varying the resistance in the rotor circuit. This method is applicable to slip ring induction motor only.
When the motor is running and if the full resistance is connected in the rotor circuit the speed of the motor decreases.
Now if the speed of the motor decreased then more voltage is induced in the rotor circuit to produce the necessary torque. Thus, torque increases.
Similarly, when the rotor resistance decreased the speed of the motor increased. The below figure shows the speed-torque characteristics of the slip ring induction motor.
As shown, when rotor resistance per phase is R1 the speed of the motor becomes N1. The torque-speed characteristics of the motor at R1 are represented as a blue line.
Now if rotor resistance per phase is increased to R2 the speed of the motor is reduced to N2. The torque-speed characteristics of the motor at R2 are represented as a green line.
What is the Purpose of the Slip Rings Located on the Rotor Shaft of a Wound Rotor Motor?
The purpose of the slip rings located on the rotor shaft of a wound rotor motor is to connect external resistance in the rotor circuit in the form of a star-connected rheostat.
Note that slip rings is an electro-mechanical device that is used to transmit power or electrical signals from stationary to rotating parts.
Slip rings are also known as electrical rotary joints, electrical rotary interfaces, or collector rings.
Wound Rotor Induction Motor vs Squirrel Cage
What is the Difference Between a Squirrel-Cage Motor and a Wound-Rotor Motor?
The major difference between squirrel cage induction motor and wound rotor induction motor is in the design of rotor.
In squirrel cage induction motors, the rotor bars are permanently short-circuited at the end rings hence external resistance cannot be connected whereas, in slip ring induction motor external star connected resistance is connected in the rotor circuit.
Other differences between the two motors are discussed in the table below.
| 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
Some of the advantages of wound rotor induction motor are discussed below.
- High starting torque – Slip ring induction motor can provide high starting torque due to the presence of external resistance in the rotor circuit.
- High overload capacity – Slip ring induction motors have high overload capacity and smooth acceleration during heavy loads.
- Low starting current in comparison with squirrel cage motor – The additional resistance in the rotor circuit increases the rotor impedance hence reduces the starting current.
- Adjustable speed – speed can be adjusted by varying the rotor circuit resistance. So that it is considered as a “variable speed motor”.
- Improved power factor
Advantages of Squirrel Cage Motor
Some of the advantages of squirrel cage induction motor are discussed below.
- Simple and rugged in construction
- Cheaper in cost
- Maintenance cost is low
- Nearly constant speed
- High overload capacity
- Simple starting arrangement
- More pull-out torque and greater maximum power output
- Explosion-proof
- Can be cooled better because of base end rings
What Are Common Uses of a Wound Rotor Induction Motor
Some of the common applications of a Would Rotor induction motor includes:
- A slip ring induction motor or wound rotor induction motor is suitable for most high-power industrial electrical drives where high starting torque is required. It is used to drive a heavy load such as: 1. Lifts 2. Line shafts 3. Cranes 4. Hoist 5. Elevators 6. Conveyor 7. Winding Machines 8. Mills etc….
- A wound rotor or slip ring induction motor can be used in various forms of adjustable speed drive. Some speed drives require to vary their speed with a certain interval of time. Thus, a slip ring induction motor is suitable for this application.
- A slip ring induction motor or wound rotor induction motor is used to provide high starting torque to unloading the coal in a coal handling power plant.
- A wound rotor induction motor is used where starting current is too high as compared to the capacity of the power system.
- A wound rotor or slip ring induction motor is used for driving machines that use large flywheels to carry peak loads such as punch presses and shears.
