EESM Test Bench: Rotor Winding, Field Excitation, and Validation for Electrically Excited Synchronous Motors

Why EESM Is Returning to the EV Powertrain Conversation

Permanent magnet motors have dominated electric vehicle traction for over a decade, but electrically excited synchronous motors are staging a significant comeback. Renault’s Megane E-Tech uses an EESM with a wound rotor and brushless exciter. Mercedes-Benz specifies EESM for the eSprinter commercial platform. The reason is straightforward: no rare-earth magnets, controllable field, and inherently safe demagnetization behavior in high-temperature or fault conditions. As automotive OEMs seek to reduce dependency on neodymium and dysprosium supply chains, EESM qualifications are accelerating.

Testing an EESM is not the same as testing a PMSM. The wound rotor adds a second electrical circuit — the field winding — with its own insulation class, thermal limit, and fault modes. The brushless exciter (a small generator integral to the rotor shaft) adds mechanical complexity that PMSM qualification procedures simply ignore. The validation engineer needs a test protocol that explicitly covers field current control, rotor winding insulation, exciter performance, and the interaction between field excitation level and torque output — none of which appear in standard AC induction motor or PMSM test plans.

How EESM Differs from PMSM and Induction Motor Electrically

• Field winding on the rotor: Unlike PMSM (permanent magnets on rotor) or induction motor (short-circuit squirrel cage rotor), the EESM has a DC-energized coil wound around the rotor core. Field current level directly controls air-gap flux — and therefore the torque-speed curve — in real time.
• Brushless exciter: Most automotive EESMs use a brushless excitation system: a small AC generator on the rotor shaft (reversed stator/rotor polarity compared to the main machine) provides DC to the field winding via an onboard rectifier rotating with the rotor. This eliminates brushes and slip rings but requires the exciter to be validated as a subsystem.
• Maximum Torque Per Ampere (MTPA) with field weakening: In a PMSM, MTPA and field weakening are pure d-q current angle optimization. In an EESM, field weakening is achieved by reducing field current — a different control axis. The EESM can achieve deeper field weakening than a PMSM without current limit constraints, enabling better high-speed constant-power range.
• Zero cogging torque: With no permanent magnets, cogging torque is eliminated by design. This is an advantage for high-efficiency cruise operation and low-noise NVH but removes a self-diagnostic indicator that PMSM test engineers rely on for magnet health assessment.

Core Test Items for EESM Validation

1. Field Current vs. Open-Circuit Voltage Mapping

With the rotor spinning at rated speed and the stator open-circuited, field current is swept from zero to maximum rated value (typically 5–25 A DC depending on machine size). The open-circuit voltage at the stator terminals is recorded at each current step. This magnetization curve (air-gap flux linkage vs. field current) characterizes the magnetic circuit’s saturation behavior and confirms that the exciter delivers adequate field current at all speeds.

Key metrics: open-circuit voltage at rated speed and rated field current (V), knee-point saturation field current (A), exciter current demand at rated field (A from inverter).

2. Rotor Winding Insulation and Resistance

The rotor field winding is a high-risk insulation fault location: it spins, experiences centrifugal stress, sees differential thermal expansion between copper and iron, and is exposed to rectified DC rather than sinusoidal AC. Rotor winding resistance is measured at cold and at thermal steady state (calculating temperature coefficient). Insulation resistance is measured between the field winding terminals and the rotor core at 500 VDC, with acceptance criterion typically 10 MΩ minimum. Winding inductance is measured to confirm absence of shorted turns.

Key metrics: cold DC resistance (Ω), hot resistance at thermal steady state (Ω), insulation resistance (MΩ at 500 VDC), field winding inductance (mH).

3. Torque Production vs. Field Current at Multiple Speed Points

At each of three speed setpoints (30%, 70%, and 100% rated speed), stator current is held at rated value while field current is swept from 20% to 120% of rated. Output torque is recorded at each combination point. This generates the fundamental MTPA surface — the field current level that produces maximum torque for a given stator current — and reveals any asymmetry in the control surface that indicates rotor winding imbalance or exciter limitation.

Key metrics: MTPA field current at rated speed (A), torque at MTPA normalized to rated torque (%), field current at which torque saturates.

4. Field Weakening Range and High-Speed Performance

The motor is accelerated from rated base speed to maximum test speed (typically 1.5–2× base speed) at rated stator current while field current is progressively reduced. Torque and power are recorded as a function of speed in the field weakening region. The constant-power speed ratio — the ratio of maximum speed to base speed — is a key specification for traction motors and must be verified experimentally against the design target.

Key metrics: constant-power speed ratio, torque at maximum speed, stator current required for maximum speed field weakening (A), exciter back-EMF at maximum speed (V).

5. Brushless Exciter Performance and Rectifier Validation

The exciter subsystem — the small generator wound in reverse on the shaft plus the rotating rectifier — is tested independently by connecting a known resistive load (simulating the field winding) to the rectifier output while sweeping exciter stator input current and rotor speed. Output DC voltage and ripple are measured. This test identifies rectifier diode failures (detectable as increased ripple) and exciter winding asymmetry before full machine assembly.

Key metrics: DC output voltage at rated exciter input current (V), AC ripple on rectified output (mV), rectifier output at minimum cranking speed (V at 200 rpm).

6. Thermal Management of the Rotor Field Winding

The rotor winding has no direct cooling path — heat must conduct through the rotor iron to the shaft and air-gap. Rotor winding temperature is measured using resistance rise (ΔR method per IEC 60034-1) because direct sensors are difficult to mount on rotating components. The test runs the motor at rated load for the defined thermal test duration and confirms that rotor winding temperature rise remains within the insulation class limit (Class F: 100 K rise above 40°C ambient, Class H: 125 K rise).

Key metrics: rotor winding temperature rise at thermal steady state (K), stator winding temperature rise (K), bearing temperature at thermal steady state (°C).

Integrating EESM Testing into Your EV Development Program

EESM qualification does not replace PMSM test experience — it extends it. The same four-quadrant dynamometer, the same torque transducer accuracy requirements (0.1% FS or better), and the same thermal instrumentation apply. What changes is the control interface: the test bench must supply and precisely control a DC field current source independent of the main stator inverter, synchronized to the rotor position for accurate MTPA mapping. The rotor winding diagnostics — insulation resistance, resistance rise, inductance — require a separate low-voltage DC measurement channel that can safely access the exciter output terminals through a slip ring fixture or connection to the rotor winding terminals before assembly.

If your EV development program is moving toward EESM traction motors — whether for rare-earth reduction, field-weakening range extension, or thermal fault tolerance — the test bench specification needs to be updated before the first prototype arrives. Contact our engineering team to review your EESM validation requirements and configure the right test solution.