If you need absolute speed precision under varying loads without the cost and complexity of a full servo system, the synchronous motor (frequently mistyped in search queries as a synchronos motor) is your workhorse. Unlike standard induction motors, which always operate with a slight "slip" (the rotor spins slightly slower than the stator's rotating magnetic field), a synchronous motor's rotor locks exactly to the stator field frequency. Zero slip means zero speed deviation, making it the definitive choice for applications like reciprocating compressors, precision conveyors, and paper mills.
But selecting the right motor and drive isn't as simple as matching a nameplate horsepower. You must match the motor's torque curve to the load's inertia, size the drive for vector control, and wire the feedback loops correctly. Here is the bench-to-jobsite guide to specifying, sizing, and troubleshooting synchronous motors.
The Synchronous Motor Advantage: Where It Fits
The most common mistake in motor selection is treating all AC motors as interchangeable, or worse, assuming steppers and servos can fill the same high-torque continuous-duty roles. A stepper motor will violently lose torque and stall at high RPMs, while a servo motor is vastly over-engineered (and overpriced) for a simple constant-speed conveyor.
To determine which motor type fits your specific load profile, compare their torque delivery, control architecture, and cost against the application demands.
| Motor Type | Torque Curve | Control Needs | Relative Cost | Best Load Profile |
|---|---|---|---|---|
| AC Induction (TEFC) | High starting torque, drops slightly near sync speed | Simple V/Hz VFD or DOL starter | Low | Variable torque (pumps, fans, blowers) |
| AC Synchronous (PMSM/Wound) | Constant torque to base speed, zero slip | Flux Vector VFD with encoder feedback | Medium-High | High-inertia constant speed (compressors, extruders) |
| AC Servo | Peak torque at zero speed, highly dynamic | Dedicated closed-loop servo drive | High | Rapid acceleration/deceleration, positioning (CNC, robotics) |
| Stepper | High holding torque, severe drop-off at speed | Open-loop step/direction driver | Low | Low-speed, low-inertia positioning (3D printers, small actuators) |
Sizing Rule of Thumb and Worked Load Example
The golden rule for sizing any continuous-duty motor is to select a frame that delivers 110% to 125% of the continuous running torque, while explicitly verifying that the motor's pull-in torque exceeds the load's breakaway (starting) torque. Converting HP to kW without looking at the load's torque curve is a guaranteed way to burn out a stator.
Worked Load Example: Reciprocating Compressor
- Continuous Running Torque: 15 Nm at 1800 RPM
- Breakaway (Starting) Torque: 25 Nm (due to cylinder compression)
Step 1: Calculate Continuous Power Requirement
Using the mechanical power formula: P(kW) = (Torque(Nm) × Speed(RPM)) / 9550
P = (15 × 1800) / 9550 = 2.82 kW
Step 2: Apply the Sizing Safety Factor
2.82 kW × 1.25 = 3.52 kW
We round up to the next standard IEC/NEMA frame size: 4.0 kW (approx. 5 HP).
Step 3: Verify Breakaway Torque
A standard 4.0 kW, 1800 RPM synchronous motor has a rated continuous torque of:
(4000 × 9550) / 1800 = 21.2 Nm
Most industrial synchronous motors are rated for a 150% pull-in torque capability for short durations.
21.2 Nm × 1.50 = 31.8 Nm
Since 31.8 Nm > 25 Nm (the load's breakaway requirement), the 4.0 kW motor will successfully pull the load into synchronism without stalling.
Drive Requirements and Terminal Wiring
An AC synchronous motor—particularly a Permanent Magnet Synchronous Motor (PMSM)—cannot run on a basic Volts-per-Hertz (V/Hz) VFD. If you apply a standard V/Hz profile to a PMSM, the rotor will violently chatter, fail to synchronize, and the drive will trip on overcurrent.
What Driver It Demands:
You must use a VFD capable of Flux Vector Control (FVC) or a dedicated sensorless vector algorithm specifically tuned for PMSM (e.g., setting a Yaskawa drive to Control Method A1-02 = 4). For high-inertia loads or applications requiring zero-speed holding torque, you must use Closed-Loop Flux Vector Control, which requires an encoder physically mounted to the motor shaft.
Wiring and Terminal Identification:
When opening the peckerhead (terminal box) of a wound-rotor or PMSM synchronous motor, you will typically find three distinct wiring groups. Miswiring these will instantly destroy the drive or the motor.
| Terminal Group | Standard Markings | Function & Wiring Rules |
|---|---|---|
| Stator Power | U, V, W (or T1, T2, T3) | 3-Phase AC from VFD output. Use proper torque on lugs; loose connections cause single-phasing. |
| Field Excitation (Wound Rotor only) | F1, F2 (or E1, E2) | DC field voltage. Supplied by a separate exciter or brushless exciter circuit. Never connect to VFD. |
| Encoder / Feedback | A, B, Z (Incremental) or U, V, W (Halls) | Low-voltage feedback to VFD. Must use shielded, twisted-pair cable. Shield grounded at drive end only. |
| Thermal Protection | T1, T2 (or PTC1, PTC2) | Embedded thermistors. Wire to the VFD's analog/digital input for over-temperature shutoff. |
Failure Signatures: Hum, Overheat, and Stall
When a synchronous motor system fails, it rarely does so silently. Reading the physical and electrical signatures will save you from swapping out perfectly good hardware.
1. The 120Hz Electrical Hum vs. Mechanical Clatter
If the motor emits a deep, steady 120Hz hum while stationary or running, you likely have single-phasing (a blown fuse or a failed IGBT in the VFD output stage). The motor is trying to run on two phases. Conversely, if you hear a rhythmic mechanical clatter or "cogging" sound during startup, the motor is failing to achieve pull-in torque. The rotor is slipping poles. This means the load inertia is too high, the VFD acceleration ramp is too aggressive, or the motor is under-sized.
2. Overheating at No-Load
If the stator casing is too hot to touch but the mechanical load is light, check your V/Hz ratio or vector control tuning. Over-excitation (supplying too much voltage for the given frequency) forces excess magnetizing current through the stator windings, generating massive I²R heat without producing additional mechanical work. Additionally, if you are running a TEFC (Totally Enclosed Fan Cooled) motor below 50% of its base speed for extended periods, the shaft-mounted fan cannot move enough air. You must add a forced-cooling blower or switch to an inverter-duty motor with an independent cooling fan.
3. Stall (Pull-Out)
If a sudden mechanical jam or load spike exceeds the motor's pull-out torque, the rotor will physically break synchronism with the stator field. When this happens, the motor instantly draws Locked Rotor Amps (LRA)—often 500% to 600% of full load current. If the VFD's overcurrent protection does not trip within milliseconds, the stator windings will melt. According to the DOE Advanced Manufacturing Office, ensuring your drive's current limit parameters are correctly scaled to the motor's nameplate FLA is the primary defense against catastrophic stall burnouts.
Synchronous Motor FAQ
Can I run a permanent magnet synchronous motor on a standard V/Hz VFD?
No. A standard V/Hz drive assumes the slip characteristics of an induction motor. If connected to a PMSM, the drive will not correctly align the stator magnetic field with the rotor's permanent magnets. The motor will violently vibrate, draw massive current, and fail to rotate. You must use a VFD with a dedicated PMSM control mode (Sensorless Vector or Closed-Loop Flux Vector) that can measure or estimate the rotor position to commutate the stator fields correctly.
Why does my synchronous motor stall and trip the breaker under sudden load spikes?
Synchronous motors have a hard physical limit called the "pull-out torque" (typically 150% to 225% of rated torque). Unlike induction motors, which will simply slow down (increase slip) and continue pulling when overloaded, a synchronous motor will instantly lose magnetic lock and stall if the load spike exceeds this limit. To fix this, you must either increase the motor frame size to raise the pull-out torque threshold, add a mechanical flywheel to absorb transient kinetic spikes, or tune the VFD to aggressively limit torque before the physical pull-out angle is reached.
What is the wiring difference between a PMSM and a wound-rotor synchronous motor?
A Permanent Magnet Synchronous Motor (PMSM) has magnets embedded in the rotor, meaning it only requires 3-phase AC wiring to the stator (U, V, W) and low-voltage encoder feedback. A wound-rotor synchronous motor, typically used in massive industrial applications (500 HP+), uses copper windings on the rotor instead of magnets. This requires a separate DC excitation voltage applied to the rotor via slip rings and brushes (terminals F1/F2) or a brushless exciter circuit to create the rotor's magnetic field. You will not find DC excitation terminals on a standard PMSM.
