A synchronous motor is an AC electric motor whose rotor spins at exactly the same speed as the rotating magnetic field produced by its stator. That locked-step relationship is what gives the motor its name: the rotor and the magnetic field are perfectly synchronized, with zero slip between them. This makes synchronous motors fundamentally different from the more common induction motor, where the rotor always lags slightly behind.
How the Rotor Locks to the Magnetic Field
Every AC motor creates a rotating magnetic field inside its stator, the stationary outer shell. In an induction motor, the rotor has to fall behind that field slightly so that current is induced in it, generating the torque that keeps it turning. That lag is called “slip,” and it’s essential for the induction motor to work at all.
A synchronous motor eliminates slip entirely. Instead of relying on induced current, the rotor carries its own magnetic field, either from permanent magnets or from an external DC power supply fed through slip rings. Because the rotor is already magnetized, its poles lock directly onto the rotating stator field and follow it at precisely the same speed. Think of it like two gears meshing: once the rotor’s magnetic poles align with the stator’s rotating poles, they stay locked together and rotate in unison.
The Synchronous Speed Formula
The speed of the rotating magnetic field, and therefore the speed of the rotor, depends on only two things: the frequency of the AC power supply and the number of magnetic poles in the motor. The formula is:
Synchronous Speed (RPM) = 120 × Frequency (Hz) ÷ Number of Poles
On a standard 60 Hz power supply, a 2-pole motor spins at 3,600 RPM. A 4-pole motor spins at 1,800 RPM. A 6-pole motor spins at 1,200 RPM. As long as the motor stays within its load limits, the rotor holds that speed exactly, regardless of how much work it’s doing. That precision is one of the motor’s biggest advantages.
Types of Synchronous Motors
Synchronous motors fall into two broad categories based on how the rotor gets its magnetic field.
Permanent Magnet
Permanent magnet synchronous motors (PMSMs) use magnets embedded in the rotor to create a constant magnetic field with no external power needed. They’re compact, highly efficient, and common in applications ranging from electric vehicles to industrial servo drives. Modern PMSMs routinely achieve efficiencies between 92% and 97%, with laboratory tests recording peaks as high as 98.7%. That puts them in the top tier of motor efficiency classes.
DC Excited (Electromagnetic)
Larger synchronous motors, typically above about 1 horsepower, use electromagnets on the rotor instead of permanent magnets. A separate DC current is fed to the rotor’s field windings, usually through slip rings or through a brushless exciter mounted on the shaft. Because the strength of the rotor’s magnetic field is adjustable, these motors offer a unique capability: you can tune their power factor by changing the DC excitation current.
Power Factor Correction
This is arguably the most distinctive feature of DC-excited synchronous motors. The DC field current on the rotor can replace, partially or completely, the magnetizing current that would otherwise be drawn from the AC supply. Since the power factor of a motor is determined by that magnetizing current, adjusting the DC field current lets you control the power factor.
When the field excitation is set so the rotor’s internal voltage matches the supply voltage, the motor runs at unity power factor, drawing the minimum possible current for its load. Increase the excitation beyond that point and the motor becomes “over-excited,” supplying leading reactive current back to the power system. This is useful because many industrial loads (induction motors, transformers, fluorescent lighting) pull lagging reactive current, which wastes capacity on the electrical grid. An over-excited synchronous motor can offset that, improving the power factor of the entire facility.
Decrease the excitation and the motor becomes “under-excited,” drawing lagging current just like an induction motor would. In practice, most facilities run their synchronous motors slightly over-excited to correct for the lagging power factor of other equipment on the same supply.
The Starting Problem
A synchronous motor cannot start on its own. When power is first applied, the stator’s magnetic field immediately begins rotating at full synchronous speed. The stationary rotor, heavy and inert, gets pulled in one direction by one passing stator pole, then the opposite direction by the next. The field sweeps past so quickly that the rotor cannot accelerate fast enough to catch it. The net starting torque is zero.
Engineers solve this with a damper winding, a set of copper or aluminum bars embedded in the rotor face in a pattern similar to the squirrel cage of an induction motor. At startup, the stator field is energized while the rotor’s DC field winding is left disconnected. The rotating field induces currents in the damper bars, creating torque the same way an induction motor does, and the rotor accelerates. Once the rotor reaches a speed close to synchronous, the DC excitation is applied to the field winding, and the rotor’s magnetic poles snap into lock-step with the stator field. From that point on, the motor runs at synchronous speed and the damper winding goes idle.
Permanent magnet synchronous motors used in variable-speed drives sidestep this problem entirely. Their electronic controllers ramp the supply frequency up gradually from zero, so the rotating field never outruns the rotor.
Synchronous vs. Induction Motors
- Speed regulation: A synchronous motor holds a constant speed set by the supply frequency. An induction motor’s speed drops slightly as load increases, because it needs slip to generate torque.
- Efficiency: Synchronous motors, especially permanent magnet types, are more efficient at rated load. The absence of slip means less energy is wasted as heat in the rotor.
- Power factor: Induction motors always operate at a lagging power factor. Synchronous motors can operate at unity or leading power factor, making them valuable for power factor correction.
- Starting: Induction motors start easily on their own. Synchronous motors need damper windings or electronic drives to get up to speed.
- Cost and complexity: Synchronous motors are more expensive and require either permanent magnets or a DC excitation system. Induction motors are simpler and cheaper, which is why they dominate general-purpose applications.
Common Applications
Synchronous motors show up wherever precise speed, high efficiency, or power factor correction justifies their added cost. In heavy industry, they drive compressors, large pumps, conveyor belts, and grinding mills. These are often multi-thousand-horsepower DC-excited machines chosen as much for their ability to correct a plant’s power factor as for the mechanical work they do.
Permanent magnet synchronous motors dominate the world of precision motion control: CNC machines, robotics, and automated manufacturing lines rely on them for exact positioning and smooth speed regulation. They’re also the motor of choice in most modern electric vehicles, where their high power density and efficiency directly translate into longer driving range. Smaller versions appear in appliances, HVAC compressors, and industrial fans wherever energy efficiency standards push manufacturers beyond what induction motors can deliver.