Motors & Drives 101 — Servo, BLDC, Synchronous & Asynchronous
A quick, practical guide to picking the right motor and drive. Learn how servo vs. BLDC vs. AC (synchronous & asynchronous) compare, what a drive actually does, and how controls (I/O, fieldbus, safety) tie it all together.
Overview
Servo Motor (PM AC Servo)
Permanent-magnet synchronous motor with high-resolution feedback and a **servo drive**. Delivers **precise position, speed, and torque** with high dynamics.
Highest accuracyClosed loopEtherCAT/EIP/PN
[Drive] ⇄ [Encoder/Resolver]
│ ↑ feedback
└── 3ϕ PWM → Servo Motor (PM)
Brushless DC (BLDC)
Electronically commutated **DC** motor. Uses **Hall sensors** for simple control; add an encoder for closed-loop positioning. Great efficiency & power density.
24–60 VDCCompactHigh efficiency
[BLDC Drive] → 3-phase inverter → BLDC
Hall sensors → basic commutation
+ Encoder → precise control
Synchronous AC (PM AC)
Electrically the same machine as a servo motor, but run with lower feedback resolution or simpler control when **full servo features** aren’t required.
High efficiencyFixed speed with VFDAsynchronous AC (Induction)
Workhorse **induction** motor driven by a **VFD**. Ideal for **speed control** and rugged duty. Add encoder/vector control for basic positioning.
IEC/NEMA framesCost-effectiveVFDCompare Motor Technologies
| Criteria | Servo (PM) | BLDC | Synchronous AC | Asynchronous AC (Induction) |
|---|---|---|---|---|
| Control & feedback | Servo drive + high-res encoder/absolute | Halls (basic) · Encoder (closed-loop) | VFD/servo-class drive · optional encoder | VFD (scalar/vector) · optional encoder |
| Precision / dynamics | Highest—camming, contouring, robotics | Good—applications needing compact power | Good—fixed speed or basic closed loop | Low for position; excellent for speed control |
| Voltage | 200/400/480 VAC | 24–60 VDC | 230/400/480 VAC | 120/230/480 VAC |
| Best for | Precision motion, indexing, multi-axis | Mobile/compact drives, pumps, fans, AGV | High-efficiency constant speed / sync apps | Conveyors, pumps, fans, general purpose |
| Pros | High accuracy, torque density, bandwidth | Efficient, compact, low voltage options | High efficiency, smooth, simple VFD control | Rugged, low cost, widely available |
| Considerations | Higher cost; tuning; feedback alignment | EMI, need encoder for precision, gear options | Less dynamic than full servo | Positioning needs encoder/vector; low torque density |
Drives & Controls
Servo Drives
- Position/velocity/torque modes; camming & gearing.
- Fieldbuses: EtherCAT, EtherNet/IP, PROFINET.
- Safety: STO, and options like SLS, SSM, SS1/2.
VFDs (AC Drives)
- Scalar (V/f) for simple speed; **vector** for torque control.
- Encoder feedback enables basic positioning / tension.
- Harmonics, braking chopper/resistor sizing.
Controllers & I/O
- PLC/PAC or motion controller issues trajectories.
- I/O: home/limits, enable, E-Stop, safety relays.
- HMI/SCADA for recipes, trends, and alarms.
Rule of thumb: start from the mechanics and throughput → compute force/speed/accel → choose motor/gear → size the drive → confirm power, safety, and fieldbus.
Selection Tips
Key Inputs
- Load, move profile (distance, time, dwell), duty cycle
- Required accuracy & repeatability
- Environment (IP rating, temperature, washdown, EMI)
- Power available (VAC/VDC), fieldbus, safety requirements
Quick Choices
- High precision / dynamic: Servo (PM) + encoder
- Long travel / high speed: Belt + servo or induction + VFD
- Low-voltage compact: BLDC drive + encoder
- Rugged speed control: Induction + VFD
FAQ
Is a servo motor the same as a synchronous motor?
A modern AC servo is a **permanent-magnet synchronous motor** paired with a high-performance servo drive and high-resolution feedback. All servos are synchronous, but not all synchronous motors are equipped or tuned as servos.
When is BLDC better than a servo?
For **low-voltage**, compact systems (AGV/AMR, pumps, fans) where you need efficiency and speed control, BLDC is ideal. Add an **encoder** if you need precise positioning.
Can an induction motor do positioning?
Yes—use a **vector VFD** with an encoder (closed loop). It won’t match servo dynamics, but it’s good for **indexing**, winding, and tension control at lower cost.
What feedback should I choose?
For servos: **absolute encoders** (BiSS/EnDat) simplify homing. For BLDC/induction: a **quadrature encoder** is common. Resolver is rugged but lower resolution.