The Core Definition: What Is the Synchronous Motor?
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 require 'slip' (the rotor spinning slightly slower than the magnetic field) to generate torque, a synchronous motor operates at zero slip. The rotor locks magnetically to the stator field and maintains a rigid, constant speed determined strictly by the line frequency and the number of motor poles.
The synchronous speed ($N_s$) in RPM is calculated using the standard formula:
$N_s = (120 \times f) / P$
Where f is the AC frequency in Hertz, and P is the number of poles. For a 4-pole motor on a 60Hz grid, the speed is exactly 1800 RPM. It will spin at 1800 RPM at no-load, and it will spin at 1800 RPM at full-load. If the mechanical load exceeds the motor's magnetic locking force (pull-out torque), the motor does not slow down; it violently snaps out of synchronization and stalls.
The rotor achieves this magnetic lock via one of two methods: permanent magnets (PMSM) or an electromagnet (wound rotor) fed by a DC excitation current. According to the NEMA MG-1 standard, large industrial synchronous motors are heavily favored for high-horsepower, constant-speed applications like compressors and pumps, and for their unique ability to correct power factor on a plant's electrical grid when over-excited.
Synchronous vs. Induction vs. BLDC: Matching the Load Profile
Choosing the right motor requires matching the torque curve to the load profile. A common mistake on the bench is treating all 'brushless' or 'magnet' motors as interchangeable. Stepper motors (open-loop, high holding torque, low speed) and servo motors (closed-loop, high dynamic acceleration) serve entirely different motion-control niches than continuous-rotation synchronous drives.
| Motor Type | Torque Curve Characteristic | Control / Drive Needs | Relative Cost | Ideal Load Profile |
|---|---|---|---|---|
| Induction (TEFC) | Speed drops slightly as load increases (slip). High starting torque. | Across-the-line (DOL) or standard V/Hz VFD. | Low | Fans, centrifugal pumps, general conveyors. |
| Synchronous (Line-Start) | Flat speed up to pull-out torque limit. Zero speed regulation error. | DOL with damper winding, or reduced-voltage soft start. | Medium-High | Reciprocating compressors, large ID fans, power factor correction. |
| PMSM (AC Synchronous) | High torque density, flat speed, high efficiency at partial loads. | Dedicated FOC (Field Oriented Control) drive required. | High | Precision extruders, HVAC chillers, marine propulsion. |
| BLDC (Trapezoidal) | Torque ripple present, high torque at low RPM. | Six-step commutation via Hall sensors. | Medium | Drones, small cooling fans, RC vehicles. |
Which motor type fits this load profile? If your load demands absolute speed accuracy regardless of torque fluctuations (like a timing conveyor or a reciprocating compressor where speed droop causes pressure issues), the synchronous motor is the correct choice. If the load is a simple centrifugal pump where a 3% speed drop under load is irrelevant, an induction motor is cheaper and easier to drive.
Terminal Identification and Drive Requirements
Wiring a synchronous motor involves two distinct circuits: the AC stator and the DC rotor (if wound).
| Terminal Label | Circuit | Function & Notes |
|---|---|---|
| T1, T2, T3 (or U, V, W) | AC Stator | Main 3-phase AC power input. Connect to line or VFD output. |
| F1, F2 | DC Rotor | Field winding input. Requires a DC exciter or brushless excitation module. |
| Amortisseur (Damper) | Internal | Squirrel-cage bars embedded in the rotor face. No external terminals; acts only during startup. |
What driver/controller does it demand? A line-start synchronous motor can be started across-the-line just like an induction motor because the embedded amortisseur (damper) winding acts as a squirrel cage during acceleration. Once the rotor nears synchronous speed, the DC excitation is applied to F1/F2, and the rotor 'pulls in' and locks.
However, modern Permanent Magnet Synchronous Motors (PMSM) lack a damper winding and cannot be started across the line. They require a dedicated synchronous drive utilizing Field Oriented Control (FOC). A standard V/Hz Variable Frequency Drive (VFD) will cause a PMSM to stall, overheat, and trip on overcurrent because it cannot track the rotor's magnetic angle. Always verify your drive supports 'Sensorless Vector' or 'Closed-Loop FOC' for PM synchronous machines.
Bench Tip: When wiring the F1/F2 DC excitation circuit on a wound rotor, always include a discharge resistor across the terminals. If the DC supply is interrupted while the motor is spinning, the collapsing magnetic field will induce a massive voltage spike that can arc across the slip rings and destroy the exciter.
Sizing Rule of Thumb and Worked Load Example
A critical error in motor sizing is performing a raw HP to kW conversion (1 HP = 0.746 kW) without accounting for the load's inertia and transient torque spikes. Synchronous motors are sized based on continuous thermal capacity and pull-out torque.
The Sizing Rule of Thumb:
1. Size the motor's continuous HP rating for 125% of the continuous running load.
2. Verify the motor's Pull-Out Torque (typically 150% to 225% of full-load torque) exceeds the absolute peak transient torque of the driven equipment.
Worked Load Example: Reciprocating Air Compressor
You are driving a 2-stage reciprocating compressor. The nameplate says it requires 15 HP continuous to maintain 120 PSI. However, because it is a reciprocating load, the compression strokes create severe torque pulsations, spiking to 22 HP for milliseconds per revolution.
- Step 1 (Continuous Sizing): 15 HP × 1.25 = 18.75 HP. You select the next standard NEMA frame size: 20 HP.
- Step 2 (Pull-Out Verification): A standard 20 HP synchronous motor has a pull-out torque rating of 150%. 20 HP × 1.50 = 30 HP pull-out capacity.
- Step 3 (Compare to Peak): The 30 HP pull-out capacity safely exceeds the 22 HP compression spike. The motor will remain locked in synchronization.
If you had blindly chosen a 15 HP motor based on the continuous nameplate, the 22 HP spike would exceed the 22.5 HP pull-out limit (15 × 1.5), causing the motor to pull out of sync and trip the main breaker on every compression stroke.
Failure Signatures: Diagnosing Hum, Overheat, and Stall
When a synchronous motor fails, the symptoms are distinct from induction motors. Use your multimeter and clamp meter to diagnose these three primary failure modes.
1. Hum Without Rotation (Failure to Start)
Cause: Single-phasing on the AC stator, or a dead DC exciter on a wound rotor.
Diagnosis: If the rotor lacks DC excitation, it cannot lock to the field and will simply vibrate at line frequency. Check the DC voltage across F1 and F2. If it reads 0V, the exciter or brushes are faulty. If excitation is present, check the AC line-to-line voltages at T1-T2, T2-T3, and T1-T3. A missing phase will result in a loud 120Hz hum and zero starting torque.
2. Overheat (Stator Casing > 80°C)
Cause: Incorrect power factor operation (VAR mismatch) or continuous operation near pull-out torque.
Diagnosis: Synchronous motors can be 'over-excited' to supply reactive power (leading PF) to the grid. However, if the DC excitation current is set too high or too low for the current mechanical load, the stator draws excessive reactive current, causing $I^2R$ heating without doing extra mechanical work. Use a power analyzer to check the power factor; adjust the DC field rheostat to bring the PF as close to 1.0 (unity) as possible for the current load.
3. Stall (Pull-Out Event)
Cause: Mechanical load exceeded pull-out torque, or a severe voltage sag on the AC supply.
Diagnosis: This is a violent event. The rotor physically slips poles. You will hear a loud mechanical 'clunk' or chatter, followed immediately by the breaker tripping. A clamp meter will show the current spiking to Locked Rotor Amps (LRA)—often 500% to 600% of full-load current. Check the driven load for mechanical binding (e.g., a seized compressor bearing) or check the utility supply for brownouts, as pull-out torque drops with the square of the applied voltage.
Frequently Asked Questions
What is the difference between a synchronous motor and an induction motor?
The fundamental difference is 'slip'. An induction motor relies on the rotor spinning slightly slower than the stator's magnetic field to induce current and create torque; this speed difference is slip. A synchronous motor's rotor spins at the exact same speed as the magnetic field (zero slip), achieving this via permanent magnets or a separately excited DC field. Consequently, synchronous motors maintain exact constant speed from no-load to full-load, while induction motors slow down slightly as load increases.
Why does a synchronous motor need a damper winding?
A pure synchronous motor has zero net starting torque when connected to an AC line because the stator field rotates too fast for the heavy rotor to catch and lock. The damper (amortisseur) winding consists of copper or brass bars embedded in the rotor face, shorted at the ends like a squirrel cage. During startup, this winding acts exactly like an induction motor, accelerating the rotor to near-synchronous speed. Once close, the DC field is applied, and the rotor pulls into synchronization.
Can I run a PMSM synchronous motor on a standard VFD?
No. Standard V/Hz (Volts per Hertz) VFDs are designed for induction motors and assume a slip frequency. If you connect a Permanent Magnet Synchronous Motor (PMSM) to a standard V/Hz drive, the drive will not know the rotor's physical position, resulting in severe torque ripple, stalling, and immediate overcurrent trips. You must use a drive specifically programmed for PM motors, utilizing Sensorless Vector Control or closed-loop Field Oriented Control (FOC) with an encoder.
What is the synchronous speed of a 6-pole motor at 50Hz?
Using the formula $N_s = (120 \times f) / P$, we plug in 50 for frequency and 6 for poles: $(120 \times 50) / 6 = 1000$ RPM. A 6-pole synchronous motor on a 50Hz European or Asian grid will spin at exactly 1000 RPM under any load condition up to its pull-out torque limit.
