16.2 Speed Control, Direction & Rotating Field Methods

16.2 Speed Control, Direction & Rotating Field Methods

Quick Answer: Speed ≈ follow ns = 120f/P (and slip). Change f, P, or slip-related controls. Reverse 3-phase by swapping two lines. Single-phase needs a phase-shifted second field: capacitor, inductor, or shaded/split pole.

Section 16.1 fixed what sync and induction motors are. This section answers three technician questions: How do we change speed? How do we reverse rotation? How do we make a rotating field when only single-phase power is available?

Speed Control — Principles Tied to ns and Slip

For an induction motor:

n = ns (1 − s) = (120 f / P) (1 − s)

So every speed-control method attacks f, P, or s (or the torque curve that sets the operating slip).

Worked example 1 — frequency control. Four-pole induction motor. At 400 Hz, ns = 12 000 rev/min. If an inverter feeds 200 Hz (V reduced roughly in proportion):

ns = 120 × 200 / 4 = 6000 rev/min. With 3% slip, n ≈ 5820 rev/min — about half speed.

Worked example 2 — pole changing. Same frame, 400 Hz, reconnect from 4-pole to 8-pole:

ns₄ = 12 000; ns₈ = 120 × 400 / 8 = 6000 rev/min. Discrete half-speed field without changing f.

Worked example 3 — slip increase (wound rotor). At ns = 1500 rev/min (50 Hz, 4-pole), inserting rotor resistance might set operating point at s = 0.10 under a given load → n = 1350 rev/min. Useful for starting torque shaping; continuous high slip wastes I²R heat in the resistors.

Synchronous-motor speed control

Steady sync speed is rigidly ns = 120f/P. To vary speed you must vary supply frequency (and manage excitation) or change pole count if the design allows. You do not “add slip” for normal speed control — slip means loss of synchronism.

Aircraft link. Fixed-frequency 400 Hz buses fix ns for a given pole count — accessory motors often run at essentially fixed design speed. Variable-frequency generators or dedicated inverters enable controlled-speed AC loads. Wrong frequency on a motor (ground cart mismatch, failed converter) → wrong speed and possible overheating.

Direction of Rotation

Three-phase motors

The rotating-field direction follows the phase sequence (ABC vs ACB). Reverse any two of the three supply leads → reverse sequence → reverse rotation.

Worked concept. A hydraulic pump motor runs the wrong way after maintenance. Likely cause: two phase leads swapped at the connector. Correct by restoring original sequence — do not randomly swap until nameplates and plumbing allow safe bump-test.

Single-phase motors

Direction depends on the relative phase of the auxiliary winding (or shaded-pole geometry). Reverse by reversing the auxiliary winding leads relative to the main winding (or reversing capacitor connections per manufacturer diagram) — not by a simple “swap two of three” rule, because there are not three supply phases.

Producing a Rotating Field on Single-Phase — Why It Matters

A single main winding alone gives a pulsating field → no starting torque. Create a second magnetic axis displaced in space, fed with current displaced in time (phase), and the resultant approximates a rotating field for start (and sometimes run).

Capacitor Methods

A capacitor in series with the auxiliary winding advances auxiliary current (current leads voltage in a capacitive branch), creating a large phase shift relative to the main (inductive) winding.

Worked qualitative example. Main winding current lags supply voltage. Auxiliary + capacitor current leads. Phase difference approaches quadrature → effective rotating field → motor starts toward the designed direction.

Failure mode (exam/hangar). Open start capacitor or failed start switch → motor hums, draws high current, does not start → overheating. Shorted cap → wrong phase shift / blown protection.

Inductor (Reactor) Methods

An inductor (reactor) or a highly inductive auxiliary winding produces phase shift by making auxiliary current lag differently from the main winding. Classic split-phase motors use a high-resistance start winding (more resistive phase angle) rather than a large capacitor; some texts group inductive/reactive auxiliary arrangements with “inductor” phase-shift methods for Module 3.

Comparison for exams: capacitor methods usually give better starting torque than plain split-phase; inductors/reactors alone are less common today than capacitors but appear in syllabus language as a phase-shift tool.

Shaded Pole and Split Pole

Shaded-pole motor

A shading ring (copper loop) encircles part of each pole face. Induced currents in the ring delay flux in the shaded portion. The flux “sweeps” from unshaded to shaded region → weak rotating field → low starting torque, one preferred direction.

Split-pole (related idea)

Split-pole constructions divide the pole or use auxiliary magnetic paths so flux in one portion lags the other — same goal as shading: time-displaced flux across the pole arc to nudge a rotating field for start. Syllabus wording “shaded or split pole” groups these salient-pole flux-delay starters.

Worked concept — troubleshooting. A shaded-pole cabin blower fails to start but spins freely by hand and then runs: look for seized bearings or debris first; if it never develops even a twitch of torque, shading ring damage or open winding is plausible. A capacitor-start motor with the same symptom more often has a bad capacitor or start switch.

Direction + Rotating-Field Summary Table

Section Synthesis

1. Speed follows n = (120f/P)(1 − s) — control f, P, or slip-related means.
2. Reverse three-phase by two-line swap; single-phase by auxiliary/capacitor lead reversal.
3. Capacitor, inductor/split-phase, and shaded/split-pole methods all create time-shifted flux for starting torque.
4. Match symptom to method: failed start capacitor vs inherently weak shaded-pole torque.

Topic 3.18 closes Module 3’s rotating-machine story: generators made the rotating field; motors use it — with slip, sync lock, and single-phase starting tricks every CAAS SAR-66 candidate must recognise.