Difference Between Synchronous Motor And Induction Motor: Formler & Selection (2026 Guide)
Quick Answer
A synkronmotor runs at exactly the supply’s synchronous speed (Ns = 120f/P) med zero slip, while an asynkronmotor always turns slightly slower than Ns—that slip s = (Ns−Nr)/Ns is what induces rotor current and creates torque. The decisive practical differences are starting and power factor: induction motors are self-starting, rugged, and always draw lagging reactive current; synchronous motors need a starting aid (damper winding or VFD) but can be over-excited to a leading power factor and even supply reactive power like a capacitor.
Efficiency is comparable—IE3/IE4 induction reaches ~89–96%, PMSM synchronous ~93–98%—but synchronous machines cost 30–50% more and are specified only where constant speed or power-factor correction justifies them. Both are built to IEC 60034 och NEMA MG 1; choose induction for general-purpose variable-load drives and synchronous for precision, constant-speed, or reactive-compensation duties.
What Are Synchronous and Induction Motors?
Both are three-phase AC-motorer that use a stator-generated rotating magnetic field (RMF). They diverge in how the rotor interacts with that field—and that single difference drives every comparison below.
Synkronmotor (Definition)
A synchronous motor is a doubly-excited machine: the stator gets three-phase AC, and the rotor gets a separate DC excitation (via slip rings, a brushless exciter, or permanent magnets). The rotor field locks magnetically to the stator RMF and turns at exactly synchronous speed, Ns = 120f/P, regardless of load. Speed is fixed by supply frequency and pole count; load only changes the torque (ladda) angle, not the rpm. Key sub-types: PMSM (permanent-magnet), wound-field (salient/cylindrical rotor), reluctance, and hysteresis motors.
Asynkronmotor (Definition)
An induction motor is a singly-excited machine: only the stator receives AC. The rotor current is induced by electromagnetic induction from the slip between rotor speed and the RMF. Because torque needs relative motion, the rotor must run slower than Ns—this speed gap is glida. Sub-types: squirrel-cage (≥80% of industrial units, simplest) and wound-rotor (slip-ring, for high starting torque or speed control).
| Aspect | Synkron | Induktion |
|---|---|---|
| Excitation | Doubly excited (AC stator + DC rotor) | Singly excited (AC stator only) |
| Rotor speed | = Ns (zero slip) | < Ns (slip 2–5%) |
| Torque source | Magnetic locking | Induced rotor current from slip |
| Self-starting | Nej | Ja |
How Do Synchronous and Induction Motors Work?
The stator is identical in both: three-phase windings create a rotating magnetic field at Ns. What happens at the rotor is the whole story.
Steg 1 — The Rotating Magnetic Field
Applying balanced three-phase AC to the stator produces a field that sweeps around the bore at Ns = 120f/P (TILL EXEMPEL., 1,500 RPM for a 4-pole machine on 50 Hz; 1,800 RPM on 60 Hz).
Steg 2 — Synchronous Motor: Magnetisk låsning
The rotor carries its own DC or permanent-magnet field. Once pulled up to speed (by a damper winding, pony motor, or VFD ramp), its poles lock to the stator field and rotate in step—zero slip at all loads. If load exceeds the pull-out torque, the machine loses synchronism and stalls. Load changes shift the torque (ladda) angle but not the speed.
Steg 3 — Induction Motor: Slip Creates Torque
The rotor is a shorted cage. The RMF cuts the stationary (or slower) rotor bars, inducing current (Faraday’s law). That current makes a rotor field which is dragged along by the stator field, producerar vridmoment. Torque exists endast because the rotor lags—no slip, no induced current, no torque. Under load, slip grows, induced current rises, and torque increases until it balances the load.
Steg 4 — Power Factor Behavior
An induction motor always draws magnetizing current from the supply, so it runs at a lagging effektfaktor (≈0.8–0.9 at full load, as poor as 0.3–0.4 at light load). A synchronous motor’s field current is adjustable: under-excited = lagging, normal = unity, over-excited = leading—it can then export VARs like a capacitor bank (a “synchronous condenser”).
Synchronous vs Induction Motor: Feature Comparison
This side-by-side is the decision table buyers actually paste into spec sheets. Every row reflects IEC 60034 / NEMA MG 1 behavior.
| Parameter | Synkronmotor | Asynkronmotor |
|---|---|---|
| Speed vs Ns | Exactly Ns | Below Ns (slip 2–5%) |
| Slip | Zero | Non-zero (essential for torque) |
| Excitation | Doubly excited (AC + DC) | Singly excited (AC only) |
| Self-starting | Nej (needs damper/VFD/pony) | Ja |
| Power factor | Adjustable: lagging / unity / leading | Always lagging (0.8–0.9 FL) |
| Effektivitet | PMSM 93–98%; large up to 98% | IE3 89–94%; IE4 91–96% |
| Speed stability | Konstant (load-independent) | Varies slightly with load |
| Startmoment | None inherent; needs aid | Moderate–high (DOL/star-delta) |
| Kosta | 30–50% higher | Lägre |
| Underhåll | Måttlig (exciter/slip rings) | Mycket låg (cage = rugged) |
| Hastighets kontroll | VFD / field only | VFD, pole-change, rotor resistance |
| Hunting | Possible under load steps (damper reduces) | Ingen |
| Typical use | Constant-speed, PF correction, >1 MW | General-purpose, variable load |
Rule of thumb: for any self-starting, cost-sensitive, variable-load drive (pumps, fans, transportörer) the induction motor wins. Reach for synchronous only when you need exact speed, leading power factor, or a large continuous load where the 30–50% premium pays back through efficiency and VAR support.
Engineering Data & Formler
Use these equations to predict speed, glida, and torque without a datasheet.
Key Formulas
| Quantity | Formula | Meaning |
|---|---|---|
| Synchronous speed | Ns = 120·f / P | f = supply Hz, P = poles. Same for both motor types. |
| Slip (fraction) | s = (Ns − Nr) / Ns | Zero for synchronous; 0.02–0.05 for induction at full load. |
| Rotor speed (induction) | Nr = Ns·(1 − s) | Induction motor actual speed. |
| Slip frequency | fglida = s·f | Rotor current frequency; sets induced EMF. |
| Pull-out torque | Tmax ∝ V² (induction) | Beyond this, induction stalls; synchronous loses synchronism. |
| Power factor (sync.) | Adjust via If (field current) | V-curve: min armature current = unity PF. |
Worked Example — 4-Pole, 50 Hz
- Synchronous speed: Ns = 120·50 / 4 = 1,500 RPM.
- Induction motor at 4% full-load slip: Nr = 1,500·(1 − 0.04) = 1,440 RPM.
- Slip speed: Ns − Nr = 60 RPM; slip frequency = 0.04·50 = 2 Hz.
- A synchronous motor on the same supply holds 1,500 RPM from no-load to pull-out—useful where ±60 RPM drift (2%) is unacceptable.
Efficiency Classes (IEC 60034-30-1)
| IE class | Induktion (typical) | Synkron / PMSM (typical) |
|---|---|---|
| IE2 | Standard | — |
| IE3 | 89–94% | — |
| IE4 | 91–96% | 93–97% |
| IE5 | Super-premium (rare cage) | 96–98% (axial-flux, PMSM) |
Insulation Temperature Limits (IEC 60034-1)
| Klass | Max winding temp | Use |
|---|---|---|
| B | 130 °C | General-purpose, lower duty |
| F | 155 °C | Most industrial induction/synchronous |
| H | 180 °C | Dragning, high-ambient, enclosed |
For long life run Class F insulation at a Class B rise (≈40 °C margin) and de-rate above 40 °C ambient per IEC 60034-1. According to the IEA, motor efficiency upgrades are among the most cost-effective industrial energy-saving measures, och DOE notes motor systems consume ~70% of industrial electricity.
Best Applications for Each Motor
| Ansökan | Recommended type | Why |
|---|---|---|
| Pumps, fans, blåsare, transportörer | Induktion (IE4 cage) | Self-starting, cheap, rugged, VFD-ready |
| Domestic appliances, kompressorer | Induktion | Low cost, underhållsfri |
| EV traction (Tesla-type) | Induction or PMSM | Högt vridmoment, simple, no exciter |
| Constant-speed mills, krossar, >1 MW | Synkron | Exact speed, high efficiency at full load |
| Power-factor correction (plant VARs) | Synkron (over-excited) | Leading PF, acts as condenser |
| CNC, robotik, textile/paper precision | PMSM (synchronous servo) | Constant speed, high dynamic accuracy |
Note the overlap in EVs: a Tesla Model S uses an induction rear unit, while most modern EVs use a permanent-magnet synchronous (PMSM) machine—each trades off cost, rare-earth reliance, och effektivitet.
AC Motor Selection Guide (Step by Step)
- Fix the speed requirement. If speed must be exactly Ns (or locked to a VFD setpoint) and load-independent, plan for synchronous/PMSM; if ±2–5% drift is fine, induction is simpler.
- Check power factor needs. If the plant suffers lagging PF, an over-excited synchronous motor corrects it while driving the load—two jobs, one machine.
- Define duty. Continuous, constant load >1 MW favors synchronous efficiency; frequent start/stop or variable load favors induction.
- Compute synchronous speed. Ns = 120f/P; pick poles for the needed base speed, then add a VFD for control.
- Size torque, not just power. Induction starting torque depends on rotor design (DOL vs star-delta vs rotor resistance); synchronous needs a damper winding or VFD ramp to reach Ns.
- Confirm efficiency class. Specify IE4 (or IE5 PMSM) per IEC 60034-30-1; verify the premium pays back over run hours.
- Mechanical interface. Select IEC B5/B14 or NEMA C-face flange and confirm shaft/brake/encoder options.
- Verify standards. Quote to NEMA MG 1 och IEC 60034; test per IEEE 112.
Common AC Motor Mistakes
| Mistake | Consequence | Fix |
|---|---|---|
| Assuming a synchronous motor self-starts | Rotor never reaches Ns; no net torque | Add damper winding, pony motor, or VFD ramp |
| Forgetting induction needs slip | Believing it can hit Ns; mis-sized coupling | Design for Nr = Ns(1−s), not Ns |
| Over-exciting synchronous without VAR budget | Leading PF trips protection / over-voltage | Set field current at V-curve unity point |
| Sizing on nameplate power only | Thermal or starting-torque failure | Size on duty-cycle torque + start method |
| Ignoring light-load PF on induction | Poor 0.3–0.4 PF, penalty charges | Add caps or a synchronous condenser |
| Skipping VFD on constant-speed need | Wasted efficiency / surge currents | Use VFD for soft start + exakt kontroll |
AC Motor Troubleshooting Table
| Problem | Likely cause | Lösning |
|---|---|---|
| Induction overheats, draws high current | Överbelastning, låg spänning, high slip | Reduce load; check V; verify cooling; de-rate |
| Induction won’t start (hum, no spin) | Single-phasing, open rotor bar | Check supply balance; megger kropp; replace rotor |
| Synchronous won’t pull into step | No start aid / field off at run-up | Use damper winding or VFD ramp; energize field |
| Synchronous loses synchronism under load | Ladda > pull-out torque | Upsize machine or reduce load; check V |
| Synchronous hunting / oscillation | Sudden load step, weak damper | Strengthen damper bars; soften load ramp |
| Poor power factor on induction | Light load, no compensation | Add caps or switch to synchronous (over-excited) |
| Excessive vibration | Unbalance, bearing wear, felinställning | Balance rotor; replace bearing; align coupling |
| Insulation failure | Over-temp, fukt, spänningsspikar | Verify Class rating; dry/varnish; use dv/dt filter on VFD |
Vanliga frågor
What is the main difference between a synchronous and an induction motor?
A synchronous motor rotates at exactly the synchronous speed Ns = 120f/P with zero slip, while an induction motor always runs slightly slower—its slip s = (Ns−Nr)/Ns induces the rotor current that produces torque. Synchronous machines are doubly excited (AC + DC rotor); induction machines are singly excited (AC only).
Why is a synchronous motor not self-starting?
At standstill the rotor cannot instantly follow the fast-rotating stator field, so no steady torque develops. It needs a damper (squirrel-cage) winding for asynchronous run-up, a pony motor, or a VFD that ramps frequency from zero up to Ns.
Which motor has better efficiency?
Comparable at the top end: IE3/IE4 induction reaches ~89–96%, while PMSM synchronous reaches ~93–98% and large synchronous machines up to 98%. The synchronous premium (30–50% cost) only pays back on continuous high-power or constant-speed duties.
Can an induction motor improve power factor?
Nej. An induction motor always draws lagging magnetizing current (0.8–0.9 at full load, as low as 0.3–0.4 light). Only a synchronous motor—when over-excited—can run at a leading power factor and supply VARs like a capacitor bank.
Can an induction motor ever reach synchronous speed?
Nej. Without slip there is no relative motion, no induced rotor EMF, and no torque. It asymptotically approaches Ns only at zero load and zero losses—never in practice.
Which motor is used more in industry?
Induction motors dominate (över 90% of installed AC machines) because they are self-starting, rugged, and cheap. Synchronous motors are reserved for constant-speed, power-factor-correction, or large (>>1 MW) continuous loads.
Why Choose Greensky for AC & Precision Motor Solutions?
Greensky is a China-based B2B motor manufacturer supplying IE3/IE4 high-efficiency three-phase induction motors, permanent-magnet synchronous (PMSM) machines, and integrated BLDC servo & gear-motor units to OEMs worldwide. Every unit is built to IEC 60034 och NEMA MG 1 dimensions, efficiency classes, and insulation limits (Class F/B rise).
- Full AC portfolio—squirrel-cage induction (0.18–315 kW) and PMSM synchronous, with VFD-ready designs for soft start and precise speed control.
- Efficiency compliance—IE4 as standard, IE5 PMSM options, full test data per IEEE 112 on request.
- Ingenjörsstöd—we run the Ns = 120f/P, glida, and power-factor checks above so your unit is sized on duty cycle and start method, not just nameplate power.
- Flexible MOQ & customization—IEC B5/B14 or NEMA C-face flanges, bromsar, kodare, and matched gear-motors for constant-speed or precision loads.
Whether you need a cost-optimized induction drive or a constant-speed PMSM with leading power factor, our team delivers prototypes in weeks and production at scale. Request a quote with your speed, vridmoment, and power-factor requirements.
Related Reading
- Vad är skillnaden mellan AC- och DC-motorer?
- What Is a DC Motor? Typer & Formler
- Disadvantages of Brushless DC Motors (and When They Still Win)
- What Is a Motor Flange? IEC vs NEMA Mounting
- Gearbox vs Gear-Motor: Urvalsguide
- Why Robotic Arms Need Speed Reducers
- Everything to Know About Worm Gears
- Topp 12 Coreless Motor Manufacturers in the World
Referenser
- IEC 60034-1: Rotating electrical machines — Rating and performance (fart, insulation, ambient). https://webstore.iec.ch/publication/56936
- IEC 60034-30-1: Efficiency classes (IE1–IE5) for low-voltage motors. https://webstore.iec.ch/publication/63493
- NEMA MG 1: Motors and Generators — dimensions, prestanda, glida & testning. https://www.nema.org/standards/view/mg-1
- IEEE Std 112: Standard Test Procedure for Polyphase Induction & Synchronous Motors. https://standards.ieee.org/ieee/112/590/
- U.S. DOE: Improving Motor and Drive System Performance (motor systems ≈70% of industrial electricity). https://www.energy.gov/eere/amo/improving-motor-and-drive-system-performance
- Internationella energibyrån (IEA): Motor efficiency as a top industrial energy-saving measure. https://www.iea.org/
- Siemens: Low-voltage motor portfolio & synchronous/induction selection notes. https://www.siemens.com/global/en/products/drives/motors.html
- ABB: Electric motor and drive efficiency guides (IEC/NEMA). https://new.abb.com/motors-generators
- SKF: Bearing selection & L10 life for motor shafts. https://www.skf.com/group/products/bearings-units-housings
- Maxon / Faulhaber technical library: synchronous (BLDC/PMSM) vs induction fundamentals. https://www.maxongroup.com/maxon/view/content/design-in
