A synchronous motor is an alternating current (AC) machine where the rotor turns at the exact same speed as the stator's rotating magnetic field. Unlike induction motors, which rely on 'slip' to induce rotor current, a synchronous motor's rotor is either a permanent magnet or an electromagnet fed by direct current (DC). This locks the rotor to the stator field, yielding zero slip and a constant speed dictated strictly by the supply frequency and the number of poles ($N_s = 120f / P$).
In 2026, with IE4 and IE5 premium efficiency mandates reshaping industrial drive selections, synchronous motors—particularly Permanent Magnet Synchronous Motors (PMSM) and externally excited Wound Rotor Synchronous Motors (WRSM)—are the default choice for high-torque, low-speed, and high-precision applications. But selecting one requires understanding its unique torque curve, excitation requirements, and failure modes.
Synchronous vs. Induction vs. BLDC: The Selection Matrix
Before specifying a drive, you must match the motor type to the mechanical load profile. Treating a stepper as a continuous-duty servo, or dropping a standard induction motor onto a high-inertia compressor without a soft-start, will result in immediate mechanical or electrical failure. Use the matrix below to identify which motor architecture fits your application.
| Motor Type | Torque Curve Profile | Control / Drive Needs | Relative Cost | Best Load Profile |
|---|---|---|---|---|
| Synchronous (WRSM) | Constant speed, high pull-out torque, zero slip | AC Stator + DC Rotor Exciter, LCI or Cycloconverter VFD | High ($$$) | High-inertia, continuous heavy loads (compressors, ball mills, ship propulsion) |
| Induction (TEFC) | Speed drops with load (slip), high starting torque (Design E) | DOL, Star-Delta, or standard V/Hz VFD | Low ($) | Pumps, fans, conveyors, general manufacturing |
| PMSM / BLDC | High torque density, flat torque curve up to base speed | FOC (Field Oriented Control) VFD with rotor position feedback | Medium ($$) | EV traction, HVAC compressors, robotics, aerospace |
| Stepper | Discrete steps, high holding torque, drops off at high RPM | Step/Direction chopper driver (open-loop) | Low ($) | 3D printers, CNC positioning, low-speed indexing |
Wiring, Terminals, and Drive Requirements
Wiring a synchronous motor is fundamentally different from a standard 3-phase induction motor because you are managing two independent electrical circuits: the AC stator and the DC rotor field.
Terminal Identification (Wound Rotor Synchronous Motor)
- U, V, W (or T1, T2, T3): The main 3-phase AC stator terminals. These connect to the grid or the output of the VFD.
- F+ and F- (or EXC+, EXC-): The DC field excitation terminals. These connect to the exciter cabinet via slip rings and carbon brushes. Voltage is typically 120V to 250V DC, depending on the motor frame size.
- Amortisseur (Damper) Cage: Not externally accessible. These are copper or brass bars embedded in the rotor pole faces, shorted by end rings, acting exactly like a squirrel-cage induction rotor to get the motor up to near-synchronous speed before the DC field is applied.
What Driver or Controller Does It Demand?
A WRSM cannot be started by simply throwing a 3-phase contactor. The inrush current would be catastrophic, and the rotor would violently vibrate without locking to the field. It demands a specific starting sequence:
- Reduced Voltage Start: An autotransformer or soft-starter applies reduced AC voltage to the stator (U, V, W). The motor accelerates as an induction motor via the amortisseur cage.
- Field Application: At roughly 95% of synchronous speed, the exciter applies DC current to F+ and F-. The rotor poles 'catch' the stator field and pull into synchronism.
- Variable Frequency Drives: For variable speed applications, large WRSMs use Load Commutated Inverters (LCI). Unlike standard PWM VFDs, LCIs use the motor's own back-EMF to commutate the thyristors, making them highly efficient for multi-megawatt drives.
Sizing Rule of Thumb: A Worked Compressor Load Example
The most common mistake when sizing synchronous motors is looking only at continuous horsepower. Synchronous motors are actually sized by their pull-out torque—the maximum torque the motor can produce before it breaks synchronism and stalls. According to NEMA MG 1 standards, pull-out torque typically ranges from 150% to 225% of full-load torque.
Worked Example: Sizing for a Reciprocating Air Compressor
The Load: A 4-pole, 60 Hz reciprocating compressor running at 1800 RPM. The continuous running torque is measured at 160 Nm. The compressor experiences cyclic torque spikes of up to 240 Nm every revolution.
- Calculate Continuous HP:
Power (kW) = (Torque × RPM) / 9550 = (160 × 1800) / 9550 = 30.1 kW.
Power (HP) = 30.1 kW / 0.746 = 40.3 HP.
Select a standard 40 HP (30 kW) motor frame. - Check the Pull-Out Torque Requirement:
Full Load Torque = 160 Nm.
The cyclic spike is 240 Nm. This spike is 150% of the full load torque (240 / 160 = 1.5).
To prevent the motor from pulling out of step during the spike, the motor's pull-out torque must be at least 175% to 200% of full load.
Required Pull-Out = 160 Nm × 1.75 = 280 Nm. - Verify the Spec Sheet:
When ordering, you must specify a 40 HP, 1800 RPM, 200% pull-out torque synchronous motor. If you accidentally order a standard 150% pull-out design, the motor will stall every time the compressor piston hits top-dead-center.
| Parameter | Target Value | Why It Matters |
|---|---|---|
| Continuous Rating | 40 HP (30 kW) | Handles the baseline thermal load of the compressor. |
| Pull-Out Torque | 200% (320 Nm) | Absorbs the 240 Nm cyclic spikes without stalling. |
| Power Factor (PF) | 0.8 Leading | Over-excited rotor supplies VARs to the plant, correcting grid PF. |
| Amortisseur Design | Heavy-duty copper | Dissipates the massive heat generated during the 10-second start cycle. |
Failure Signatures: Hum, Heat, and Hunting
Synchronous motors fail differently than induction motors. Because the rotor is magnetically locked to the stator, mechanical and electrical faults manifest in highly specific ways. Here is how to diagnose the three most common field failures.
1. The 'Hunting' Oscillation
Symptom: The motor shaft physically surges forward and backward slightly, accompanied by a rhythmic fluctuation in the stator current meters.
Cause: Hunting occurs when a sudden load change causes the rotor angle to oscillate around its new equilibrium point. While the amortisseur cage is designed to damp this, hunting often indicates that the load's cyclic frequency is resonating with the motor's natural electromechanical frequency, or the DC field excitation is too weak.
Fix: Increase the DC excitation current (over-excite the motor). This stiffens the magnetic coupling between the stator and rotor, pulling the rotor angle tighter and damping the oscillation.
2. Pulling Out of Step (Stall)
Symptom: A loud, violent mechanical shudder, followed by an immediate drop to zero RPM and a massive spike in stator current (often tripping the upstream breaker).
Cause: The mechanical load torque exceeded the motor's maximum pull-out torque. The magnetic lock breaks. Once synchronism is lost, the motor acts as a heavily overloaded induction motor and stalls instantly.
Fix: This is a sizing or process fault. You must either reduce the mechanical load (e.g., unload the compressor valves during startup) or replace the motor with a higher pull-out torque rating. Do not simply increase the VFD current limit; the physics of the magnetic lock cannot be bypassed by software.
3. Amortisseur Overheating and Rotor Hum
Symptom: A deep, loud 120Hz electrical hum during startup that does not resolve, accompanied by a burning smell from the rotor enclosure.
Cause: The DC field excitation failed to engage, or engaged too late. The motor is stuck at 95% slip speed, and the amortisseur (damper) bars are carrying continuous induction current. These bars are not rated for continuous duty; they will melt and braze themselves to the pole faces within seconds.
Fix: Immediately trip the stator breaker. Inspect the exciter cabinet, the slip ring brushes, and the field contactor. As detailed in All About Circuits' motor theory guides, the timing of the field application relay is critical—it must trigger precisely when the slip frequency drops below 2 Hz.
