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In a three-phase induction motor, the stator’s rotating magnetic field induces current in the rotor, and the interaction between those fields produces torque. In a permanent-magnet (PM) synchronous motor, magnets on or inside the rotor provide its magnetic field; the rotor follows the stator’s rotating field at synchronous speed. The key difference is that an induction motor needs slip to induce rotor current, while a PM synchronous motor does not.
How an induction motor produces torque
When three-phase power energizes the stator windings, they create a rotating magnetic field. That changing field induces voltage and current in the rotor. In a squirrel-cage motor, bars joined by end rings provide the path for this current. The rotor current creates a magnetic field of its own, and its interaction with the stator field develops torque. The U.S. Department of Energy explains this operating principle in its 2014 motor and drive system sourcebook.
Why the rotor runs below synchronous speed
The stator field rotates at synchronous speed, determined by the supply frequency and the number of motor poles. The induction rotor must turn more slowly than that field for the field to keep inducing rotor current. The difference between the field’s speed and the rotor’s speed is called slip. As mechanical load increases, the rotor slows slightly, slip rises, and more rotor current is induced to produce the additional torque.
This does not mean the rotor is stationary or that the motor is inherently inefficient; it describes how an induction motor develops torque. DOE describes induction motors as available in squirrel-cage and wound-rotor designs, with low cost, low maintenance, high reliability, and a range of torque and slip characteristics.
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How a PM synchronous motor produces torque
A PM synchronous motor also has a stator that creates a rotating magnetic field. Its rotor has permanent magnets mounted on the surface or embedded within it, supplying the rotor’s magnetic field without needing induced rotor current. The magnetic fields interact to produce torque, and the rotor tracks the stator field at synchronous speed. The DOE’s Premium Efficiency Motor Selection and Application Guide describes PM motors as intended for variable-speed operation and says a suitably developed inverter or variable-speed drive is needed to start and synchronize them.
Because the PM rotor does not need induced current to create its field, the design avoids the induction rotor’s secondary-circuit I²R losses. That is a design-level distinction, not a guarantee that every PM motor system will use less total energy than every induction motor system: the motor, drive, load, and operating profile all matter.
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- Reversible: electric motor is a reversible gear motor, capable of rotating in both directions, ideal for applications that require reversing the direction of rotation, such as conveyor belts or winches
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Induction vs. PM synchronous motors
| Comparison | Induction motor | PM synchronous motor |
| Source of rotor field | Current induced in the rotor creates its magnetic field. | Permanent magnets on or within the rotor provide its magnetic field. |
| Speed relative to stator field | Runs below synchronous speed; the difference is slip, which rises with load. | Rotor follows the rotating stator field at synchronous speed. |
| Starting and control | Drive choice depends on the required speed and control; induction motors are also used with direct-on-line operation in some applications. | DOE says a variable-speed inverter or drive designed for the motor is needed for proper starting and synchronization. |
| Design consideration | Induced rotor current entails secondary-circuit losses; DOE cites low cost, low maintenance, and reliability as advantages. | Avoids induced rotor-current losses; drive requirements and magnet materials and cost are relevant selection considerations. |
| Examples described by manufacturers | ABB describes flexible direct-on-line and variable-speed-drive operation in some food-processing and pharmaceutical compressor environments. | ABB describes PM motors for low-speed, high-torque uses, including refrigeration and process compressors. |
The examples in the final row are ABB product and application descriptions, not universal rules. ABB also lists mining, pulp and paper, and water treatment for its low-voltage PM motors.
What the efficiency evidence does—and does not—show
The U.S. Department of Energy’s Building Technologies Office Motor Energy Savings Potential Report says PM motors can be more efficient than induction motors by “up to 10 percentage points,” especially during part-load operation, attributing that comparison to Advanced Design Technology Ltd. (ADL, 1999). This is a historical, qualified figure cited by the DOE report, not a current universal efficiency gap or a result that applies to every motor and drive pairing.
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ABB says that omitting rotor windings and slip speed in PM synchronous and synchronous-reluctance motors can extend efficiency gains over a wider torque-speed range compared with induction motors. That is ABB’s control and product-context claim, not a guarantee for every application. A meaningful comparison needs matched motor ratings, compatible drives, and the same load and speed duty; the cited figures alone do not establish a winner for a particular installation.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to choose between the motor types
Choose for the complete duty, not the motor label. A motor and drive should be evaluated against the load and operating profile, including how the system starts, how much torque it needs, and how often speed changes.
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- Load and torque: Establish the required torque over the full operating range, including startup and any low-speed, high-torque periods.
- Speed profile: Define the required speeds and how long the motor will run at each. A fixed-speed duty and a frequently varying-speed duty can favor different system designs.
- Starting and drive control: Confirm whether direct-on-line starting or a variable-speed drive is appropriate. For a PM synchronous motor, verify that the inverter or drive is designed for its starting and synchronization requirements.
- Efficiency across the duty: Compare motor-and-drive performance at the actual load and speed points, rather than assuming a motor type always wins.
- Cost and maintenance: Assess initial system cost, maintenance needs, and reliability for the specific equipment and installation; the DOE’s general advantages do not establish a universal lifecycle-cost winner.
- Product and installation details: Check voltage, phase, power, speed, frame, duty rating, and drive compatibility, along with product-specific magnet and service considerations.
PM designs can combine low speed and high torque and may avoid a gearbox in some applications, according to DOE, but they still require an appropriate controller. Conversely, induction motors’ broad industrial use and cited low-cost, low-maintenance, reliable design do not by themselves make them the best choice for every load. The final decision depends on the specified motor-drive system and operating conditions.
Quick Recap
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