Side-by-side comparison
| Parameter | AC | DC Servomotor |
|---|---|---|
| Construction | Wound armature with commutator and brushes | Squirrel cage or PM rotor, no brushes |
| Maintenance | Regular brush replacement (every 2000–5000 hrs) | Minimal — no brushes or commutator |
| Speed range | Wide (0–3000 RPM) with linear torque-speed | Very wide with vector control (0–6000+ RPM) |
| Torque at low speed | High and constant — good for direct drive | High with FOC/vector drive (e.g., Yaskawa Sigma-7) |
| Drive/controller complexity | Simple — H-bridge PWM driver (e.g., L298N) | Complex — vector control DSP, current sensors needed |
| Power density | Lower | Higher — PM synchronous motors especially |
| EMI / sparking | Brush sparking causes EMI; not suitable for explosive atmospheres | No sparking — safe for all environments |
| Typical application | Small robots, educational kits, legacy CNC axes | Modern CNC, industrial robots, EV traction drives |
| Cost (motor only) | Lower for small sizes | Higher motor cost, higher drive cost |
Key differences
DC servomotors use a mechanical commutator to switch armature current — this gives excellent torque linearity and simple voltage-to-speed control with just an H-bridge, but brushes wear out and spark. AC servomotors (PMSM type, like Mitsubishi HG series, or induction type) require field-oriented control (FOC) implemented on a DSP-based drive, which is more expensive but eliminates maintenance and allows higher power density. Modern PMSM servos deliver torque ripple below 1% with encoder feedback at 2500 pulses/revolution, far outperforming brushed DC at equivalent sizes.
When to use AC
Use a DC servomotor in a low-budget, low-duty-cycle application such as a laboratory pick-and-place robot or an educational CNC router where brush maintenance is acceptable and the simple L298N H-bridge driver keeps system cost low.
When to use DC Servomotor
Use an AC PMSM servomotor whenever the application demands high reliability, high duty cycle, or a controlled environment is not guaranteed — industrial robot joints (e.g., Fanuc αi series), CNC machine tool axes, and semiconductor wafer handlers all use PMSM servos with absolute encoders.
Recommendation
For placement interviews and modern embedded motor control projects, choose AC PMSM servomotors — they dominate new designs. Understand FOC conceptually: it decouples torque-producing and flux-producing current components, making AC motor control as straightforward as separately excited DC.
Exam tip: Examiners ask you to compare torque-speed characteristics of AC and DC servomotors and to explain why the DC motor has a linear characteristic while the AC induction motor droops at high slip.
Interview tip: Interviewers at robotics and automation companies expect you to explain field-oriented control at a conceptual level — specifically, the role of the d-axis and q-axis current components and why controlling Id = 0 maximises torque per ampere in a PMSM.