No, a synchronous motor is not an induction motor. They are two distinct types of AC motor with different operating principles, different rotor designs, and different speed behavior. Both plug into an AC power supply and both use a rotating magnetic field in the stator, but that’s where the similarities end. The confusion is understandable because the two are closely related branches of the same family tree, and some synchronous motors even borrow induction-style components to get started.
The Core Difference: How Each Motor Produces Torque
An induction motor (also called an asynchronous motor) generates torque through electromagnetic induction. The stator creates a rotating magnetic field that sweeps past conductive bars embedded in the rotor. As the field passes these bars, it induces electric currents in them, and the interaction between those currents and the magnetic field pushes the rotor to spin. This only works if the rotor is turning slower than the magnetic field. If the rotor ever caught up to the field’s speed, there would be no relative motion, no induced current, and no torque. That speed gap is called “slip,” and it’s a defining feature of every induction motor.
A synchronous motor works on a completely different principle. Instead of relying on induced currents, the rotor has its own magnetic field, created either by permanent magnets or by a separate DC power source feeding electromagnets on the rotor. The rotor’s magnetic poles lock onto the stator’s rotating magnetic field like two magnets snapping together, and the rotor spins at exactly the same speed as the field. There is zero slip. The rotor’s south pole locks to the rotating field’s north pole and vice versa, and the two stay locked together as long as the motor isn’t overloaded.
Why Speed Matters So Much
The rotating magnetic field in any AC motor spins at a speed determined by the power supply’s frequency and the number of poles in the motor. This is called synchronous speed. On a standard 60 Hz supply, a two-pole motor has a synchronous speed of 3,600 RPM, a four-pole motor runs at 1,800 RPM, and so on.
A synchronous motor’s rotor matches that speed exactly. A four-pole synchronous motor on 60 Hz power always turns at precisely 1,800 RPM regardless of load (up to its maximum capacity). An induction motor with the same pole count might turn at 1,750 or 1,770 RPM, with the exact speed dipping slightly as more load is applied. That small difference, typically 2% to 5% of synchronous speed, is the slip that makes induction possible.
This distinction matters in applications where precise, constant speed is critical. Synchronous motors are the choice when timing accuracy is essential. Induction motors are preferred when slight speed variation is acceptable and simplicity is the priority.
Rotor Construction Looks Nothing Alike
The easiest way to tell the two apart is to look at the rotor. Most induction motors use a “squirrel cage” rotor: a cylinder of stacked steel sheets with aluminum or copper bars running through it, connected at both ends by metal rings. There are no magnets, no external power connections, and no moving electrical contacts. It’s an elegantly simple design with very few parts that can wear out.
Synchronous motor rotors come in several varieties, but all of them carry their own magnetic field. Permanent magnet rotors contain a series of magnets either on or inside the rotor surface. Wound-rotor designs use coils fed with DC current, typically delivered through slip rings and brushes on the motor shaft. Some use a reluctance design where the rotor’s shape creates preferred magnetic pathways that lock to the rotating field.
The Starting Problem
Here’s where the two motor types intersect in an interesting way. A synchronous motor cannot start on its own. At standstill, the stator’s magnetic field whips past the stationary rotor poles so fast that it alternately attracts and repels them, producing no net torque. The rotor just sits there vibrating.
The most common solution is to embed a partial squirrel cage winding, called a damper winding, into the rotor’s pole faces. These are heavy copper bars with their ends shorted together, exactly like a miniature version of an induction motor’s rotor. When the motor is first energized, the damper winding lets it start as an induction motor, accelerating the rotor to near-synchronous speed. Once the rotor gets within roughly 50 RPM of the field’s speed, the DC excitation is applied (or the permanent magnets take over), the poles lock in, and the motor transitions to true synchronous operation. At that point, induction is no longer needed.
So while a synchronous motor may use induction to start, it does not operate as an induction motor once it reaches running speed. Calling it an induction motor would be like calling a car a bicycle because you pedaled to get it rolling.
Efficiency at Different Loads
Synchronous motors generally achieve higher efficiency than induction motors. In one comparative study of 2.2 kW, four-pole motors, a line-start synchronous motor hit 94.4% efficiency at full load, enough to qualify for the IE4 “super premium” efficiency class under international standards. Comparable induction motors in the same power range reached 86.7% to 88.4% at full load, placing them in the IE3 “premium” category.
Induction motors also lose efficiency more noticeably at partial loads. A premium-class induction motor operating at 50% load dropped to around 85% to 86% efficiency, while its full-load rating was closer to 88%. Synchronous motors maintain more consistent performance across varying loads because they don’t lose energy to slip. The energy that an induction motor “spends” on slip dissipates as heat in the rotor, which is energy that never becomes useful shaft power.
Maintenance and Cost Tradeoffs
Induction motors are cheaper to buy, simpler to install, and easier to maintain. With no brushes, slip rings, or separate excitation system, there are fewer components to inspect and replace. This rugged simplicity is why induction motors dominate industrial applications worldwide, running everything from pumps and fans to conveyors and compressors. They can operate for years in harsh environments with minimal attention.
Synchronous motors with wound rotors require regular inspection of their brushes and slip rings, plus maintenance of the DC excitation system. Permanent magnet synchronous motors eliminate brushes but cost more upfront because of the magnet materials. Both types carry higher purchase prices than equivalent induction motors. The payback comes through lower energy costs over the motor’s lifetime, which can be substantial for large motors running continuously.
Both Work With Variable Frequency Drives
Modern variable frequency drives (VFDs) can control either motor type by adjusting the frequency of the power supply, which directly changes the speed of the rotating magnetic field. Induction motors are highly compatible with VFDs and have been paired with them for decades. Synchronous motors, particularly permanent magnet designs, are increasingly used with VFDs in applications like electric vehicles and high-performance industrial equipment where their superior efficiency and precise speed control justify the added cost.
The combination of a permanent magnet synchronous motor with a VFD has become the dominant drivetrain in electric cars, while induction motors with VFDs remain the workhorse of factory automation. Each type has carved out the applications where its strengths matter most, but neither one is the other.