Electrically Excited Synchronous Motors (EESMs) are a type of AC synchronous motor where the rotor’s magnetic field is generated by an external electrical current supplied to windings on the rotor, rather than by permanent magnets or induction. Unlike permanent magnet synchronous motors (PMSMs), which use rare earth permanent magnets in the rotor to create a magnetic field, EESMs use an electromagnet, created by passing current through windings on the rotor. This excitation current is supplied through slip rings and brushes or contactless systems.

Construction and Working of EESM

An Electrically Excited Synchronous Motor (EESM) consists of a stator and a rotor. The stator is made of laminated steel and houses three-phase AC windings arranged in slots, similar to those in an induction motor. The rotor, typically of salient-pole or cylindrical design, contains windings made of copper wire. These rotor windings are supplied with direct current (DC) via slip rings and brushes, or sometimes through a brushless excitation system. An external DC source or an exciter provides the necessary field current to the rotor windings, generating a controllable magnetic field.

When three-phase AC power is applied to the stator windings, it produces a rotating magnetic field. Simultaneously, the DC-excited rotor creates its own magnetic field. The interaction between the stator’s rotating field and the rotor’s stationary field causes the rotor to be pulled into synchronism with the stator’s field, resulting in rotation at synchronous speed. By adjusting the DC excitation, the strength of the rotor’s magnetic field can be controlled, allowing for optimization of torque, efficiency, and power factor. Once synchronized, the rotor runs at a constant speed determined by the supply frequency and the number of poles.

The speed of an EESM is strictly determined by the supply frequency and the number of poles, given by:

Where   N_s - synchronous speed (rpm)

              f   - supply frequency (Hz)

              P  - number of poles

EESMs typically require auxiliary means (like an external motor) to bring the rotor close to synchronous speed before excitation is applied, as they cannot start by themselves. Once synchronized, the rotor runs exactly at synchronous speed, and there is no slip as in induction motors. Modern EESMs in EVs use electronic controllers to precisely manage both the AC supply to the stator and the DC excitation to the rotor for optimal performance.

Key Features of EESMs in EVs

Externally Excited Rotor (No Permanent Magnets): The rotor’s magnetic field is generated by supplying DC to windings on the rotor, rather than using rare earth permanent magnets. This eliminates dependence on rare earth materials, reducing cost and supply chain risks while enhancing environmental sustainability.

Adjustable Magnetic Field Strength: The excitation current to the rotor can be dynamically adjusted, allowing real-time control over the magnetic field strength. This enables optimization of torque and efficiency across various driving conditions, such as acceleration, cruising, or regenerative braking.

Precise Speed and Torque Control: EESMs allow for highly accurate control of motor speed and torque by adjusting the frequency and amplitude of the AC supply to the stator and the excitation current to the rotor. This results in smooth, responsive driving and efficient adaptation to changing load demands.

High Power Density and Performance: The ability to modulate the rotor field enables EESMs to deliver high torque and power density, making them suitable for high-performance electric vehicle applications. Innovative rotor cooling solutions further enhance their ability to sustain high performance during demanding cycles.

Field Weakening Capability: EESMs can effectively perform field weakening (reducing the magnetic field at high speeds), which is essential for extending the motor’s speed range and improving high-speed efficiency, an advantage over some permanent magnet designs.

Sustainability and Cost Advantages: By avoiding rare earth magnets, EESMs contribute to a lower environmental footprint and are less susceptible to raw material price fluctuations and supply constraints.

Advanced Cooling and Power Supply Solutions: EESMs feature innovative rotor cooling (such as slot coolers) and contactless or space-neutral rotor power supply systems, which improve reliability and performance while reducing maintenance needs.

Maintenance Considerations: Traditional EESMs use slip rings and brushes to supply current to the rotor, which can require maintenance due to wear. However, modern designs increasingly use protected or contactless systems to enhance durability and reduce maintenance demands.

Scalability and Versatility: EESMs are scalable and can be designed for a wide range of power outputs, making them suitable for various applications from compact passenger EVs to commercial vehicles

Application of EESMs in EVs

Main Traction Motors in Passenger and Commercial EVs: EESMs are used as the primary drive motors in both passenger cars and commercial vehicles. Their ability to deliver high torque and power density across a wide speed range makes them suitable for propelling vehicles under various driving conditions. 

Alternative to Permanent Magnet Synchronous Motors (PMSMs): EESMs are being developed as an alternative to PMSMs, eliminating the need for rare earth magnets. This reduces environmental impact and supply chain risks, while offering similar or better efficiency at high speeds and long-distance driving. Various automakers are actively developing new EESM-based drive platforms for upcoming EVs.

High-Speed and Highway Efficiency: EESMs excel in highway and long-distance driving, where their efficiency can surpass that of PMSMs, especially at higher speeds due to their field weakening capability and absence of permanent magnetic drag.

Flexible Control for Energy Optimization: The real-time adjustability of the rotor’s magnetic field allows EESMs to optimize energy consumption and performance based on driving demands, contributing to improved overall vehicle efficiency and range.

Hybrid and Multi-Motor Architectures: EESMs are integrated into hybrid powertrains or used in combination with other motor types (such as induction or permanent magnet motors) to balance cost, efficiency, and performance across different vehicle operating modes.

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