An ac synchronous electric motor is the definitive choice when your application demands absolute speed precision locked to the AC line frequency (or VFD output) without slip. Unlike induction motors that require slip to generate torque, a synchronous motor's rotor locks magnetically to the stator's rotating field. This makes it ideal for continuous-duty, high-inertia loads like large conveyors, compressors, and precision timing systems where speed droop under load is unacceptable.
The Core Principle: Zero Slip and Magnetic Lock
The defining characteristic of an ac synchronous electric motor is its speed equation: Ns = 120f / P, where Ns is synchronous speed in RPM, f is supply frequency in Hz, and P is the number of stator poles. If you supply 60 Hz to a 4-pole motor, it spins at exactly 1800 RPM—whether it is unloaded or pushing its maximum rated torque.
Modern industrial applications predominantly use Permanent Magnet Synchronous Motors (PMSM). By embedding high-energy neodymium magnets in the rotor, PMSMs eliminate rotor I²R (copper) losses, pushing efficiency into the IE4 and IE5 classifications defined by Motor Systems Resource Facility guidelines. However, this magnetic lock requires precise electronic commutation; you cannot simply throw a contactor and apply raw grid power to a PMSM without risking severe mechanical shock or demagnetization.
Motor Type Comparison: Synchronous vs. Induction vs. Stepper
Selecting the right motor requires understanding how torque curves and control architectures differ across motor families. Steppers and servos are not interchangeable, and induction motors will always exhibit slip.
| Criteria | AC Synchronous (PMSM) | AC Induction (TEFC) | Stepper Motor |
|---|---|---|---|
| Torque Curve | Flat continuous torque up to base speed; constant power above base speed. | Peaks at breakdown torque (approx. 85% of sync speed); zero torque at sync speed. | Highest at standstill; drops off rapidly as speed increases due to back-EMF. |
| Control Needs | Requires Vector Drive (FOC) with rotor position feedback (encoder/resolver). | Runs on raw AC grid or basic V/Hz VFD; no feedback required for open-loop. | Requires dedicated step/direction pulse generator and microstepping driver. |
| Cost (per HP) | High ($300–$600+ per HP for motor + drive). | Low ($50–$150 per HP for motor only). | Very High for high-torque frames ($500+ per HP). |
| Best Load Profile | High-inertia, continuous duty, strict speed regulation, high efficiency. | Variable torque (pumps/fans), high starting torque, rugged environments. | Low-inertia, precise open-loop positioning, low-speed holding. |
Wiring, Terminals, and Controller Demands
A standard 3-phase PMSM will have a main power terminal box and a separate feedback connector. Miswiring either will instantly fault the drive or destroy the encoder.
- U, V, W: Main 3-phase power inputs. Phase sequence must match the drive's expected rotation. Swapping any two leads on a PMSM without updating the drive's commutation angle will cause a violent shudder and an overcurrent fault.
- PE (Protective Earth): Mandatory safety ground. Must be bonded to the drive's earth bar to bleed off high-frequency common-mode currents from the VFD's PWM switching.
- Feedback (M12 or 15-pin D-sub): Carries the encoder signals (A, B, Z, U, V, W hall traces, and 5V/GND). This cable must be shielded, with the shield grounded at the drive end only to prevent ground loops.
Controller Demands: An ac synchronous electric motor demands a drive capable of Field Oriented Control (FOC), often marketed as a Vector Drive or Servo Drive. Basic Volts-per-Hertz (V/Hz) drives cannot track the rotor's magnetic pole position and will fail to commutate the motor. For high-dynamic loads, you need a closed-loop vector drive that reads the encoder to continuously adjust the stator current vector, keeping it exactly 90 degrees ahead of the rotor flux for maximum torque.
Sizing Rule of Thumb: A Worked Conveyor Load Example
Never size a motor based purely on horsepower or kilowatt conversions without calculating the actual load torque profile. The golden rule for continuous-duty synchronous sizing is: Calculate steady-state torque, add 20% for static breakaway friction, then select a motor where the rated continuous torque exceeds this total by a 1.15 service factor.
Worked Example: Heavy Flat Belt Conveyor
- Load Mass: 500 kg (belt + product)
- Belt Speed: 0.5 m/s
- Drive Pulley Radius: 0.2 m
- Rolling Friction Coefficient (μ): 0.1
Step 1: Calculate Steady-State Force and Torque
Force (F) = Mass × Gravity × μ = 500 kg × 9.81 m/s² × 0.1 = 490.5 N.
Steady Torque (T) = F × Radius = 490.5 N × 0.2 m = 98.1 Nm.
Step 2: Apply Breakaway and Service Factors
Breakaway Torque = 98.1 Nm × 1.20 = 117.7 Nm.
Required Motor Rated Torque = 117.7 Nm × 1.15 = 135.3 Nm.
Step 3: Determine Power and Pick the Frame
Assuming a target speed of 150 RPM at the pulley (15.7 rad/s):
Power = Torque × Angular Velocity = 135.3 Nm × 15.7 rad/s = 2,124 Watts.
You must select a motor rated for at least 2.2 kW (approx. 3 HP) with a continuous torque rating exceeding 135.3 Nm at 150 RPM. According to IEC motor efficiency standards, an IE4/IE5 2.2 kW 8-pole PMSM fits this exact profile, eliminating the slip you would suffer with a standard induction motor.
Failure Signatures: Diagnosing Hum, Overheat, and Stall
When an ac synchronous electric motor fails, the physical symptoms point directly to the electrical or mechanical root cause. Do not ignore these signatures; they precede catastrophic insulation or magnet failure.
| Symptom | Physical Signature | Root Cause & Diagnostic Fix |
|---|---|---|
| Hum / Cogging | Audible low-frequency vibration at low speeds; shaft feels 'notchy' when turned by hand while unpowered. | Cause: Drive commutation angle is misaligned, or encoder is slipping on the shaft. Fix: Run the drive's auto-tune/rotor identification routine. Check encoder set-screws. |
| Overheat | Stator casing exceeds 80°C; thermal PTC thermistors trip the drive fault. | Cause: Running a TEFC (Totally Enclosed Fan Cooled) motor at low RPM for long periods. The shaft-mounted fan cannot move enough air. Fix: Add an external forced-cooling blower or switch to a liquid-cooled frame. |
| Stall (Pull-out) | Motor abruptly stops or drops out of sync under load spike; drive reports 'Overcurrent' or 'Loss of Sync'. | Cause: Load torque exceeded the motor's peak pull-out torque (usually 150-200% of rated). The rotor physically breaks the magnetic lock. Fix: Increase gear reduction ratio to multiply torque, or upsize to the next motor frame. |
Decision Path: Selecting Your Exact Motor and Drive
Use this decision tree to lock in your hardware. This path eliminates 'analysis paralysis' and terminates in a specific, orderable bill of materials for a standard industrial constant-speed application.
| If your application requires... | Then select this architecture... |
|---|---|
| Speed accuracy > 0.1% AND high continuous torque at low RPM. | Closed-Loop PMSM with absolute encoder. |
| Speed accuracy > 1% AND high starting torque, but budget is tight. | Standard AC Induction Motor with V/Hz VFD. |
| Precision positioning (moves < 1mm) AND holding torque at zero speed. | AC Synchronous Servo Motor (High pole count). |
The Concrete Pick for High-Inertia Constant-Speed Loads:
If your load profile matches the conveyor example above (high inertia, strict speed regulation, continuous duty), stop evaluating and specify the Siemens 1FK7042-5AF21 Compact Synchronous Motor (rated 1.2 kW, 11.4 Nm continuous at 1000 RPM, scalable via gearbox for higher torque/lower speed) paired with the Siemens SINAMICS V90 servo drive. The V90 natively supports FOC, features an integrated safety STO (Safe Torque Off) function, and auto-tunes the commutation angle for the 1FK7 series out of the box. For higher power requirements (like the 2.2 kW conveyor calculation), step up to the Siemens 1FK7063 frame. This combination guarantees zero slip, IE5-class efficiency, and eliminates the speed droop inherent in induction alternatives.
