Select a synchronous motor when your application demands strict speed regulation (zero slip) under varying mechanical loads, high power factor correction, or premium efficiency at low RPMs. Unlike standard induction motors that slip 2-5% below synchronous speed, a synchronous motor's rotor locks exactly to the AC stator's rotating magnetic field (e.g., exactly 1200 RPM on a 60Hz, 6-pole system). It requires a DC excitation source or permanent magnets (PMSM) to establish the rotor field. If your load fluctuates but speed cannot drop, or if you need to correct a lagging plant power factor, this is your machine.
The Core Decision: Synchronous vs. Induction vs. Servo
Choosing the right motor topology requires matching the motor's inherent torque curve to the mechanical load profile. A common mistake on the bench or in the plant is treating AC synchronous motors and AC servos as interchangeable. They are not. Servos are optimized for rapid acceleration, precise positioning, and peak torque at zero RPM. Synchronous motors are optimized for continuous, constant-speed rotation and high-inertia starting.
| Motor Type | Torque Curve & Slip | Control / Drive Needs | Relative Cost | Best Load Profile |
|---|---|---|---|---|
| Induction (NEMA Design B) | Breakdown torque at ~80% speed; 2-5% slip at full load. | Direct-on-line (DOL), Soft Starter, or standard VFD. | Low ($) | Fans, pumps, conveyors where slight speed drop under load is acceptable. |
| Synchronous (Wound Rotor / PMSM) | Pull-in torque for starting; zero slip at running speed. High pull-out torque. | Reduced-voltage starter + DC exciter, or specialized Sync VFD. | High ($$$) | Compressors, large mills, generators, constant-speed high-inertia loads. |
| AC Servo | Peak torque at 0 RPM; continuous torque up to base speed. Zero slip (closed-loop). | Dedicated servo drive with high-resolution encoder feedback. | Very High ($$$$) | CNC spindles, robotics, pick-and-place, precise angular positioning. |
Sizing Rule of Thumb and Worked Load Example
Sizing a synchronous motor is not just about matching continuous running horsepower. You must account for pull-in torque (the motor's ability to accelerate the load and pull it into synchronism) and pull-out torque (the maximum load torque before the rotor drops out of sync). According to NEMA MG 1 standards, synchronous motors are typically sized at 1.15 to 1.25 times the continuous running horsepower to handle transient pull-in requirements for high-inertia loads.
- Load Requirement: 400 HP continuous at 1200 RPM (6-pole, 60Hz system).
- Load Inertia: High (compressor flywheel effect).
- Grid Condition: Plant suffers from 0.85 lagging power factor; utility penalizes reactive power.
Calculation: Because of the high inertia, we apply the 1.25x pull-in sizing multiplier.
400 HP × 1.25 = 500 HP.
The Pick: Select a 500 HP, 1200 RPM, 4000V, 60Hz synchronous motor. To solve the grid penalty, specify a 0.8 leading power factor excitation rating. The motor will act as a synchronous condenser, injecting reactive power (VARs) back into the plant grid while driving the compressor.
Wiring, Terminals, and Drive Requirements
Unlike a standard 3-phase induction motor where you simply land three AC lines and a ground, a wound-rotor synchronous motor requires managing two distinct electrical circuits: the AC stator and the DC rotor field.
Terminal Identification
- Stator (AC Power): Labeled T1, T2, T3 (or U, V, W). These connect to the 3-phase AC mains or the output of the VFD.
- Field / Excitation (DC): Labeled F1, F2 (or E1, E2). These connect to the DC excitation supply or the brushless exciter rectifier.
- Field Discharge Resistor: Often internally or externally wired across F1 and F2. This is critical. When the motor is starting (acting as an induction motor via the damper winding), the rotating stator field induces a massive, potentially destructive voltage in the DC field winding. The discharge resistor absorbs this energy.
Controller and Drive Demands
For fixed-speed applications, you need a reduced-voltage starter (like an autotransformer or reactor starter) paired with a static excitation panel. The panel monitors the rotor slip frequency and applies the DC field current at the exact right moment to pull the rotor into sync.
If variable speed is required, you cannot use a standard V/Hz VFD. You must use a specialized synchronous drive, such as the ABB ACS880 or Siemens SINAMICS G120 configured with a synchronous motor module. These drives use closed-loop vector control to manage the stator angle relative to the rotor field position, preventing the motor from falling out of synchronism at low speeds.
Failure Signatures: Diagnosing Hum, Overheat, and Stall
When a synchronous motor fails, the symptoms are distinct from induction motors. Recognizing these signatures on the floor saves weeks of downtime.
| Symptom | Probable Cause | Measurement / Verification | Corrective Action |
|---|---|---|---|
| Loud 120Hz Hum & High Stator Current | Loss of DC excitation. The motor has lost its rotor field and is operating as an unloaded induction generator/motor, drawing massive reactive current. | Measure DC voltage at F1-F2. Read 0V or severe ripple. Check exciter fuses. | Replace exciter thyristors or repair the rotating rectifier diodes on a brushless exciter. |
| Overheating (Stator or Rotor) | Over-excitation (too much DC field current) causing high stator reactive current, OR damper (amortisseur) winding burnout from too many consecutive starts. | Check power factor meter. If heavily leading (>0.95), reduce DC field. Check start-count interlocks. | Adjust excitation panel limits. Enforce a 15-minute cool-down timer between start attempts to protect the damper bars. |
| Stall (Pull-Out) | Load torque transiently exceeded the motor's pull-out torque limit, or AC line voltage dipped below 80% nominal. | Motor abruptly drops speed and trips the stator overload. Voltage dip logged on power analyzer. | Install a fast-acting excitation forcing relay that boosts DC field current during voltage sags to maintain pull-out torque. |
The Final Decision Tree: Which Motor to Buy
Use this decision path to terminate your selection process. Do not default to a synchronous motor if an induction motor will suffice; the excitation maintenance and drive complexity are not justified for simple loads.
| IF your application requires... | AND your load profile is... | THEN select this motor type |
|---|---|---|
| Variable speed + precise position holding | Low inertia, high dynamic response | AC Servo Motor (e.g., Yaskawa Sigma-7) |
| Variable speed + continuous running | High inertia, speed can slip 3% | Induction Motor + VFD (e.g., WEG W22 + CFW11) |
| Strict constant speed (zero slip) | Low to medium inertia, no grid PF correction needed | Permanent Magnet Synchronous (PMSM) (e.g., WEG W22 Magnet) |
| Strict constant speed + Grid PF correction | High inertia, >200HP, utility PF penalties apply | Wound Rotor Synchronous Motor (Default Pick: WEG W22 Synchronous series, 0.8 PF leading rating) |
The Default Pick for Heavy Industrial Constant-Speed: If your load exceeds 200HP, demands absolute zero-slip speed regulation under fluctuating torque, and your facility suffers from utility power factor penalties, the WEG W22 Synchronous (Wound Rotor) line is the benchmark. Specify it with a brushless exciter to eliminate slip-ring maintenance, and pair it with an autotransformer reduced-voltage starter to manage the high inrush current without collapsing the local bus voltage. For sub-200HP applications where you want zero slip but lack the space for an excitation panel, step down to a WEG W22 Magnet (PMSM) driven by a dedicated synchronous VFD.
For deeper engineering parameters regarding pull-in and pull-out torque calculations, refer to the DOE Motor Systems Sourcebook and always verify your final selection against the specific inertia (WK²) data provided by the driven equipment manufacturer.
