16.1 Synchronous & Induction Motors
Key Takeaways
- Synchronous motors lock rotor speed exactly to synchronous speed ns = 120f/P (P = number of poles); induction motors always run slightly slower under load because of slip
- Slip s = (ns − n)/ns; rotor frequency and induced EMF both scale with slip; at standstill s = 1, at synchronous speed s = 0
- Polyphase (especially three-phase) induction motors are self-starting from a rotating stator field; single-phase induction motors need an auxiliary starting method
- Synchronous motors need DC rotor excitation (or permanent magnets) and a means to bring the rotor near ns before locking; they can run at unity or leading power factor
- Aircraft AC motors appear in fans, pumps, actuators, and ground equipment — Module 3 expects construction, slip, and ns formula fluency, not specific part numbers
16.1 Synchronous & Induction Motors
Quick Answer: Synchronous motors lock to ns = 120f/P (exact speed). Induction motors run below ns with slip s = (ns − n)/ns. Three-phase induction motors start from a rotating stator field; single-phase need help to start. Sync motors need DC field (or PM) and pull-in near ns.
CAAS SAR-66 Module 3 topic 3.18 AC Motors follows AC generators (3.17). Generation created a rotating magnetic field at electrical frequency f. Motors reverse the story: a rotating (or oscillating) field interacts with a rotor to produce torque. Two families dominate Module 3: synchronous and induction motors, in single-phase and polyphase forms.
Shared Vocabulary Before the Split
| Term | Meaning |
|---|---|
| Stator | Stationary core and windings connected to the AC supply |
| Rotor | Rotating member that develops torque |
| Poles (P) | Number of magnetic poles of the machine (must be even: 2, 4, 6, …) |
| Synchronous speed ns | Speed of the rotating magnetic field in rev/min |
| Rotor speed n | Actual shaft speed in rev/min |
| Slip s | Fractional lag of rotor behind the field (induction motors) |
Synchronous speed formula (memorise):
ns = 120 f / P
| Symbol | Unit | Notes |
|---|---|---|
| ns | rev/min | Field (and sync-motor) speed |
| f | Hz | Supply frequency |
| P | — | Number of poles (not pole-pairs; Module 3 / Part-66 style uses total poles) |
Worked example 1 — aircraft 400 Hz. Four-pole motor on 400 Hz:
ns = 120 × 400 / 4 = 12 000 rev/min.
Worked example 2 — utility 50 Hz. Same four-pole machine on 50 Hz:
ns = 120 × 50 / 4 = 1500 rev/min.
Worked example 3 — two-pole 400 Hz. ns = 120 × 400 / 2 = 24 000 rev/min — very high shaft speed; multipole designs lower ns for the same f.
Synchronous Motors — Construction and Principle
A synchronous motor runs at exactly ns in steady state (zero average slip). The stator is wound like an alternator stator (often three-phase). The rotor carries a DC-excited field (slip rings or brushless exciter) or permanent magnets. Once the rotor magnetic poles lock with the rotating stator field, the shaft turns in step with the field.
| Part | Role |
|---|---|
| Stator AC windings | Produce rotating (polyphase) or pulsating (single-phase) field at frequency f |
| Rotor field | DC or PM poles — magnetic “lock” to stator field |
| Excitation (wound rotor) | Sets field strength → affects power factor and pull-out torque |
| Damper / amortisseur windings (often) | Help starting and damp hunting |
Operating principle. After the rotor is brought near ns (by damper winding induction action, pony motor, or frequency ramping), rotor poles lock to stator poles. Torque then maintains synchronism against load up to pull-out torque. Overload beyond pull-out → loss of sync (“pull-out”) — rotor slips poles and must be restarted properly.
| Characteristic | Synchronous motor |
|---|---|
| Steady speed | Exactly ns (independent of load, within sync) |
| Starting | Not inherently self-starting from standstill without help |
| Excitation | DC field or PM required |
| Power factor | Can be lagging, unity, or leading (over-excited) |
| Slip in sync | Zero average |
Aircraft / hangar link. Pure classical sync motors are less common as everyday cabin fans than induction motors, but the ns formula and lock-to-field idea underpin constant-speed AC drives, some actuator concepts, and understanding why frequency and poles fix speed. Permanent-magnet sync machines appear in modern compact drives.
Worked concept — lock. A 6-pole sync motor on 400 Hz: ns = 120 × 400 / 6 = 8000 rev/min. At 50% rated torque it still runs 8000 rev/min if it remains in sync. Speed does not sag with load the way an induction motor does.
Induction Motors — Construction and Principle
An induction motor (asynchronous motor) has a stator that creates a rotating field at ns. The rotor has no DC excitation. Rotor currents are induced by relative motion (slip) between field and rotor conductors — Faraday + Lenz: induced currents create torque that tries to catch the field but never quite reaches ns under load (if it did, induction would cease).
Rotor constructions
| Type | Construction | Notes |
|---|---|---|
| Squirrel-cage | Shorted bars in laminated core, end rings | Rugged, common, low maintenance |
| Wound-rotor (slip-ring) | Three-phase winding brought out via slip rings | External resistance for starting/torque shaping |
| Part | Role |
|---|---|
| Stator | Polyphase or single-phase windings on laminated core |
| Rotor (cage or wound) | Induced currents → torque |
| Air gap | Magnetic coupling path — keep clean and within limits |
| End rings / slip rings | Close cage circuit or connect external resistors |
Slip
s = (ns − n) / ns (often expressed as a percentage: s × 100%)
| Condition | Slip |
|---|---|
| Standstill (start) | s = 1 (100%) |
| No-load (ideal frictionless) | s ≈ 0 |
| Rated load | Small positive s (e.g. 2–5% typical teaching figures) |
| Synchronous speed | s = 0 — no induction torque in a pure induction machine |
Rotor electrical frequency fr ≈ s × f. At start, fr = f; near sync, fr is low.
Worked example 4 — slip calculation. Four-pole, 400 Hz → ns = 12 000 rev/min. Shaft runs at 11 640 rev/min.
s = (12 000 − 11 640) / 12 000 = 360 / 12 000 = 0.03 = 3%.
Worked example 5 — find n from slip. Same machine, s = 4% = 0.04.
n = ns (1 − s) = 12 000 × 0.96 = 11 520 rev/min.
Worked example 6 — 50 Hz check. Four-pole, 50 Hz, n = 1440 rev/min.
ns = 1500; s = (1500 − 1440)/1500 = 0.04 = 4%.
Polyphase vs Single-Phase Induction Motors
Polyphase (especially three-phase)
Three stator windings 120° apart produce a true rotating magnetic field of nearly constant magnitude (topic 3.17 rotating-field idea). The rotor starts and runs without an auxiliary phase under normal conditions.
| Trait | Three-phase induction |
|---|---|
| Starting | Self-starting |
| Torque smoothness | Good |
| Direction | Swap any two supply lines → reverse rotation (§16.2) |
| Aircraft / industry | Fans, pumps, compressors, ground equipment |
Single-phase induction
A single stator winding produces a pulsating field, equivalent to two counter-rotating fields. Net starting torque is zero without an auxiliary method (capacitor, shaded pole, etc. — §16.2). Once running, the motor continues on the main winding with slip.
| Trait | Single-phase induction |
|---|---|
| Starting | Needs auxiliary winding / shaded pole / etc. |
| Running | Possible after start; lower performance than polyphase of same size |
| Use | Small fans, pumps, appliances, some aircraft accessories |
Synchronous vs Induction — Exam Comparison
| Feature | Synchronous | Induction |
|---|---|---|
| Steady speed | Exactly ns | Below ns (slip) |
| Rotor excitation | DC or PM | None (induced) |
| Self-starting (polyphase) | Generally needs help | Yes (3-phase) |
| Slip | Zero in sync | Essential for torque |
| Power factor control | Yes (excitation) | Usually lagging |
| Typical Module 3 calc | ns = 120f/P | ns and s = (ns−n)/ns |
Torque and Load Behaviour (Qualitative)
Induction: As load rises, rotor slows slightly → slip rises → induced rotor EMF/current rise → more torque until the peak (breakdown) torque. Beyond that, motor stalls. Efficiency and heating worsen at high slip (start and heavy overload).
Synchronous: Speed fixed; load angle between rotor and stator fields increases with load. Too much load → pull-out. Over-excitation can make the machine draw leading current (acts like a capacitor to the bus — power-factor correction idea).
Section Synthesis
- ns = 120f/P fixes field speed for both families.
- Sync motors lock to ns; induction motors need slip and never stay at ns under load.
- Cage vs wound rotor; three-phase self-start vs single-phase needs help.
- Drill slip arithmetic both ways (n from s, s from n).
Next (§16.2): how to control speed and direction, and how capacitor, inductor, and shaded/split-pole methods create a rotating field for single-phase starting.
A 6-pole AC machine is supplied at 400 Hz. What is its synchronous speed ns?
Why can a loaded induction motor never run continuously at exactly synchronous speed?
A 4-pole, 50 Hz induction motor runs at 1440 rev/min. What is the slip?
Which comparison between synchronous and squirrel-cage induction motors is correct?