Two fundamentally different concepts
The induction motor operates without external electronics or feedback—it is robust, low-maintenance, and has a mean time between failures (MTBF) of over 40,000 hours. The servo motor is usually a permanent magnet synchronous motor (PMSM): An encoder measures the rotor position, and an electronic controller adjusts the position, speed, or torque in real time. This enables positioning accuracies of up to ±0.001°, but makes the system more complex and expensive.
The main differences
| Criterion | Induction motor | Servo motor |
|---|---|---|
| Positioning accuracy | ±5–10° | ±0.01–0.001° |
| Response time | 100–500 ms | < 5 ms |
| Short-term overload | 1.5–2 × rated torque | 3–5 × rated torque |
| Costs (Motor + Electronics) | Reference (low) | significantly higher (3–5 ×) |
| Reliability (MTBF) | > 40,000 h | 15,000–25,000 h |
In summary: Induction motors offer advantages in terms of cost, reliability, and robustness—servo motors offer advantages in terms of precision, dynamics, and control accuracy. Choose an induction motor for continuous operation with a constant or adjustable load and no positioning requirements (pumps, fans, conveyors); choose servo motors when accuracy of ±0.1° or better is required, response times are under 5 ms, or frequent start-stop cycles are involved (CNC, robotics, automation).
Inertia ratio as an additional criterion
For dynamic servo axes, it is not the continuous power rating that matters, but rather the ratio of load inertia to motor inertia J_red/J_Motor (load inertia reduced to the motor shaft) determines the control quality: up to 3 is ideal for highly dynamic positioning; 3 to 10 is still acceptable with a rigid coupling and tuned control; above 10, resonance and stability problems are likely. A gearbox reduces the reflected load inertia by a factor of 1/i²—the same relationship that was already introduced in Module 1 for the reduction to the motor shaft. In contrast, grid-connected induction motors do not have a position control loop, which is why the inertia ratio is not a critical factor in this case.
The synchronous reluctance motor as a third option
The synchronous reluctance motor (SyRM) bridges the gap between asynchronous and permanent magnet servo motors: Like the induction motor, it does not require permanent magnets, but achieves IE4/IE5 efficiency levels when driven by an inverter and has a lower rotor moment of inertia. It is suitable for energy-efficient, speed-controlled drives that do not have the most demanding positioning requirements—an intermediate solution when neither the sheer robustness of a standard induction motor nor the full precision of a servo motor is required.
Mnemonic
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