A wound-rotor synchronous motor, also known as a slip-ring synchronous motor, or externally excited synchronous motor (EESM) is a type of electric motor that combines features of both synchronous motors and wound-rotor induction motors.

How does it work

A wound-rotor synchronous motor has a stator similar to a synchronous motor, which consists of a laminated iron core with stator windings. However, the rotor of a wound-rotor synchronous motor is different from a conventional synchronous motor.

BMW 5th generation eDRive with WRSM

The rotor winding of a wound-rotor synchronous motor is typically made up of multiple coils of wire, which are connected to the rotor via slip rings and brushes. The slip rings allow electrical connection to the rotor winding, while the brushes provide a means of transferring electrical power to and from the rotor.

BMW 5th generation eDRive with WRSM. Brush /slip-ring at the end

The wound-rotor synchronous motor operates based on the principles of electromagnetism and synchronous motor operation. The stator windings are connected to an AC power supply, which creates a rotating magnetic field. The rotor winding is also connected to the power supply through the slip rings and brushes. The slip rings allow for variable resistance or impedance to be connected to the rotor winding, which can be used to control motor performance.

BMW 5th generation eDRive stator

The rotor winding is excited with DC current, creating a magnetic field in the rotor. The interaction between the stator’s rotating magnetic field and the rotor’s magnetic field causes the rotor to rotate in synchronism with the stator’s magnetic field, hence the name “synchronous motor.”

Currently BMW, Nissan and Renault uses WRSM motors.


Wound-rotor synchronous motors offer several advantages, including:

Variable Speed and Torque Control: The use of slip rings and brushes in the rotor circuit allows for variable resistance or impedance to be connected, which enables control over the motor’s speed and torque characteristics. This makes wound-rotor synchronous motors suitable for applications that require precise speed and torque control, such as industrial processes or traction systems in electric trains.

High Efficiency: Wound-rotor synchronous motors can achieve high efficiency due to their ability to control the rotor circuit impedance, which allows for optimized motor performance under different load conditions.

Higher Power Factor: Wound-rotor synchronous motors typically have a higher power factor compared to induction motors, which can help improve the overall efficiency of the motor and reduce reactive power demand from the power grid.

No need for rare-earth materials WRSM have no need for rare-earth materials.


Wound-rotor synchronous motors also have some limitations, including:

Complexity: Wound-rotor synchronous motors are more complex compared to other types of motors, such as induction motors or permanent magnet motors, due to the presence of slip rings, brushes, and rotor windings. This can increase the complexity of motor control and maintenance requirements.

Higher Cost: The additional components, such as slip rings and brushes, can increase the cost of wound-rotor synchronous motors compared to other motor technologies.

Maintenance Requirements: The presence of slip rings and brushes in the rotor circuit can require regular maintenance, such as cleaning, inspection, and replacement, which can increase operating costs and maintenance efforts.

Renault WRSM motor

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