A synchronous motor is an AC electric motor that rotates at a speed exactly synchronized with the frequency of the supply current — meaning its rotor turns at the same speed as the rotating magnetic field of the stator. Unlike induction motors, it operates at a constant speed regardless of load (within its torque limits), making it ideal for precision industrial applications.
The synchronous motor belongs to the family of doubly-excited AC motors. It is supplied with alternating current on the stator windings, which creates a rotating magnetic field. The rotor — excited by a DC source — locks into this rotating field and spins at exactly the synchronous speed (Ns), defined by:
Where f is the supply frequency (Hz) and P is the number of poles. For a 4-pole motor on a 60 Hz supply, this gives Ns = 1800 RPM — a fixed, unwavering speed.
This characteristic is fundamentally different from an induction motor, which always operates below synchronous speed (called "slip"). In a synchronous motor, there is zero slip under steady-state operation.
Understanding the working principle requires examining two key phenomena: the creation of the rotating magnetic field and the locking mechanism of the rotor.
When three-phase AC is applied to the stator windings, it produces a rotating magnetic field (RMF) that sweeps around the stator at synchronous speed. The speed and direction of the RMF depend entirely on the supply frequency and winding configuration.
The rotor poles are energized by a DC excitation source (either brushes and slip rings, or a brushless exciter). This creates a fixed magnetic field on the rotor, giving it distinct North and South poles.
The stator's rotating field "pulls" the rotor poles along with it through magnetic attraction. Once the rotor achieves synchronous speed, the North pole of the rotor locks with the South pole of the rotating stator field. This is called magnetic locking or "pull-in." From this point, the rotor rotates at exactly synchronous speed.
A synchronous motor is not self-starting. At standstill, the inertia of the rotor prevents it from following the rapidly rotating stator field. Common starting methods include:
Synchronous motors are classified based on rotor construction, excitation method, and size:
The classical design. The rotor has wound coils fed by DC through slip rings. Offers precise control of excitation current, making it ideal for power factor correction. Common in large industrial drives (compressors, mills, pumps).
Uses permanent magnets on the rotor instead of wound coils. Eliminates the need for DC excitation and slip rings. Delivers high efficiency, high power density, and compact size. Widely used in electric vehicles, servo drives, HVAC compressors, and robotics.
Has a salient-pole rotor with no windings or magnets. Torque is produced purely by magnetic reluctance variation. Simple, robust, and low-maintenance, though generally lower in torque density.
Uses the hysteresis properties of a special rotor material. Notable for smooth, quiet operation and inherent self-starting capability. Common in timing devices, clocks, and precision instruments.
The most common comparison in the industry is between synchronous motors and induction motors (asynchronous motors). Here is a detailed breakdown:
| Feature | Synchronous Motor | Induction Motor |
| Speed | Exactly synchronous (constant) | Slightly below synchronous (slip) |
| Slip | Zero slip | 2–8% slip at full load |
| Excitation | Requires DC excitation (or PM) | No separate excitation needed |
| Power Factor | Controllable (unity or leading) | Always lagging (0.7–0.9 typical) |
| Self-Starting | Not self-starting (requires aid) | Self-starting |
| Efficiency | Higher (especially PMSM) | Moderate |
| Cost | Higher initial cost | Lower initial cost |
| Maintenance | Higher (brushes/slip rings in wound type) | Lower (robust, simple) |
| Speed Control | Via VFD (frequency change) | Via VFD or pole changing |
| Best For | Precision speed, PF correction, high power | General industrial drives |
The unique properties of synchronous motors make them the preferred choice in a wide range of demanding applications:
| Application Sector | Specific Use | Motor Type Preferred |
| Oil & Gas | Compressors, pipeline pumps | Wound-field, large frame |
| Steel & Mining | Rolling mills, ball mills, crushers | Wound-field, high torque |
| Electric Vehicles | Traction drives, e-axles | PMSM (permanent magnet) |
| HVAC & Refrigeration | Scroll and centrifugal compressors | PMSM, reluctance |
| Robotics & CNC | Servo axes, precision positioning | PMSM servo motors |
| Power Utilities | Synchronous condensers (PF correction) | Wound-field, no-load |
| Textile & Paper | Speed-critical processing lines | Wound-field or PMSM |
| Consumer Electronics | Clocks, timers, turntables | Hysteresis, small PM |
For engineers selecting a synchronous motor, the choice between permanent magnet and wound-field types is critical:
Because synchronous speed is directly governed by supply frequency, speed control of a synchronous motor is achieved by changing the frequency of the AC supply. This is done through:
Modern synchronous motors, particularly PMSMs, are leading the adoption of IEC 60034-30 efficiency classes IE4 (Super Premium) and IE5 (Ultra Premium). In contrast, most squirrel-cage induction motors max out at IE3.
For a 37 kW motor operating 6,000 hours/year, the efficiency difference between IE3 (induction) and IE5 (synchronous) can save hundreds of kilowatt-hours annually — translating to significant cost and carbon savings over a motor's 15–20 year service life.
When AC is first applied, the stator creates a rotating field that spins at synchronous speed immediately. The stationary rotor, due to inertia, cannot instantly follow. The field reverses direction before the rotor moves, resulting in zero average starting torque. Starting aids (damper windings, VFD, pony motor) are required to bring the rotor to near-synchronous speed first.
Mechanically, they are identical machines. When mechanical energy is input to rotate the shaft, it operates as a generator (alternator). When electrical energy is input to the stator, it operates as a motor. The distinction is purely about the direction of energy conversion.
A synchronous condenser is a synchronous motor running at no mechanical load (no connected shaft load). By adjusting its DC excitation, it absorbs or generates reactive power (VAR), acting like a large variable capacitor. Utilities use it extensively for power factor correction and voltage regulation on the grid.
Yes. Many large wound-field synchronous motors are started via damper windings and run directly on-line at fixed speed. However, a VFD is required for variable speed operation and is the preferred modern starting method for PMSM types.
If the mechanical load torque exceeds the motor's pull-out torque (maximum synchronous torque), the rotor loses magnetic lock with the rotating stator field and decelerates. This is called "losing synchronism" or "pulling out." The motor must be stopped, the overload removed, and restarted. Over-excitation increases pull-out torque, improving stability margins.
This is the unique and powerful feature of wound-field synchronous motors:
— Normal excitation: Unity power factor (motor draws only active power)
— Over-excitation: Leading power factor (motor generates reactive power, helping other lagging loads)
— Under-excitation: Lagging power factor (motor absorbs reactive power)
Both are permanent magnet synchronous motors, but they differ in back-EMF shape. PMSM has a sinusoidal back-EMF and is driven by sinusoidal currents (via FOC), resulting in smooth torque output. BLDC (Brushless DC) has a trapezoidal back-EMF and uses rectangular commutation, simpler but with higher torque ripple. PMSM is preferred for precision servo applications.
The synchronous motor stands as one of the most sophisticated and versatile machines in electrical engineering. Its defining characteristic — operating at exactly synchronous speed — delivers benefits that induction motors simply cannot match: zero slip, controllable power factor, and superior efficiency at high duty cycles.
For high-power industrial applications (compressors, mills, pumps) where both speed precision and power factor correction matter, the wound-field synchronous motor remains unmatched. For compact, high-efficiency drives (EVs, servo systems, HVAC), the permanent magnet synchronous motor (PMSM) leads the way, pushing efficiency to IE5 levels that represent the future of electric motor technology.
As global energy efficiency standards tighten and variable-speed drive costs continue to fall, synchronous motors — particularly PMSM types — are rapidly expanding their share of the industrial motor market, displacing conventional induction motors in an ever-growing range of applications.
Copyright © 2018 Cixi Wayad Motor Manufacturing Co., Ltd.Todos os direitos reservados.
Conecte-se
Fabricantes de motor AC atacado
