AC-machines

Vocabulary flashcards covering the operation, construction, characteristics, power relationships, starting methods, and protection schemes of three-phase synchronous and polyphase induction motors.

Synchronous Motor

A three-phase AC motor whose rotor rotates at the exact same speed as the stator rotating magnetic field ($\text{slip} = 0$), requiring an external direct current source to energise its rotor winding.

Synchronous Speed ($n$ or $N_s$)

The speed of the rotating magnetic field produced by the stator windings, mathematically defined as $n = \frac{f}{p}\,\text{revs/s}$ or $N_s = \frac{60f}{p}\,\text{revs/min}$, where $f$ is the supply frequency in hertz and $p$ is the number of stator pole pairs.

Angle of Retard (Load Angle, $\alpha$)

The angular displacement by which the rotor magnetic poles drop behind the stator rotating magnetic field when a mechanical load is applied to a synchronous motor.

Capacitor Action (Over-Excited Synchronous Motor)

The operational property of an over-excited synchronous motor where the generated EMF exceeds the supply voltage, causing the machine to draw a leading current and behave like a capacitor for power factor improvement.

"Vee" Curves

Characteristic V-shaped curves obtained by plotting armature current against field excitation current for various mechanical loads on a synchronous motor, identifying the unity power factor point at the minimum armature current.

Synchronous Capacitor

A synchronous motor operated without a mechanical shaft load (on no-load) and over-excited, used specifically to generate leading reactive power for power factor correction or transmission line voltage regulation.

Pull-Out Torque (Synchronous Motor)

The critical maximum load torque on the shaft beyond which the magnetic flux lines between the rotor and stator pull apart, causing the rotor to fall out of synchronism and stall.

Damping Grids

Short-circuited copper or aluminium conducting bars embedded in the rotor pole shoes of a synchronous motor that act as a cage rotor during startup to accelerate the machine to near-synchronous speed.

Squirrel-Cage Rotor

A rugged induction motor rotor consisting of conducting bars (copper or aluminium) placed into longitudinal slots of a laminated steel core and permanently short-circuited at both ends by heavy end rings.

Rotor Bar Skewing

The practice of angling rotor slots relative to the motor shaft rather than aligning them parallel, which produces a more uniform driving torque and reduces magnetic humming noise during operation.

Double-Cage Rotor

An induction motor rotor design comprising a high-resistance, low-reactance outer cage for elevated starting torque and a low-resistance, high-reactance inner cage that carries the bulk of the running current for high operational efficiency.

Wound-Rotor (Slip-Ring) Motor

An induction motor with an insulated three-phase star-connected winding on the rotor connected to shaft-mounted slip rings, allowing connection of external starting or speed-controlling resistors.

Rotor Slip ($s$)

The fractional or percentage difference between the synchronous speed ($n$) and the actual rotor mechanical speed ($n_r$), expressed as $s = \frac{n - n_r}{n}$, where $s = 1$ represents standstill and $s = 0$ represents synchronous speed.

Rotor Current Frequency ($f_r$)

The frequency of the EMF and current induced in the rotor conductors by relative motion, governed by the formula $f_r = s f$, matching mains frequency at standstill and reducing toward zero as the motor accelerates.

Induction Motor Equivalent Circuit

A transformer-based electrical model of an induction motor where stator resistance ($R_1$), stator reactance ($X_1$), core components ($R_m, X_m$), and slip-dependent secondary components ($sE_2, R_2, sX_2$) represent motor operational behaviour.

Rotor Copper Loss

The electrical $I^2 R$ heating loss within the rotor circuit of an induction motor, calculated as the product of fractional slip and rotor input power ($\text{Rotor Copper Loss} = s \times P_2$).

Induction Motor Power Flow

The step-by-step conversion of electrical input power ($P_1$) down to shaft mechanical power ($P_M$): stator input loses stator losses to become rotor input ($P_2$), which loses rotor copper losses to develop internal mechanical power ($P_r$), and finally loses friction and windage losses.

Breakdown Torque (Pull-Out Torque of Induction Motor)

The maximum operating torque developed by an induction motor at rated voltage and frequency without stalling, occurring when rotor resistance equals the running rotor reactance ($R = s X_0$), typically at around $20\%$ slip.

NEMA Design A Motor

An induction motor classification characterized by higher breakdown torque than Design B, high starting currents, and a full-load slip of $5\%$ or less.

NEMA Design B Motor

The standard general-purpose polyphase induction motor featuring normal starting torque (around $150\%$ of rated), acceptable starting current, and a slip of $5\%$ or less.

NEMA Design C Motor

An induction motor with high starting torque (around $150\%$ of rated), normal starting current, and low slip ($\le 5\%$), specially suited for high breakaway load applications like conveyors, reciprocating pumps, and compressors.

NEMA Design D Motor

An induction motor engineered with very high starting torque (around $300\%$ of rated), low starting current, and high slip ($5\%$ to $13\%$), resulting in noticeable speed decreases under fluctuating mechanical loads.

Direct-On-Line (DOL) Starting

The basic starting method where a squirrel-cage motor connects directly across full line voltage, resulting in a large surge current of $5$ to $7$ times rated current and restricted by supply authorities to small motors (typically below $3.73\,\text{kW}$).

Star-Delta Starting

A starting method for delta-wound motors where the stator windings are initially configured in star to reduce the phase voltage to $\frac{1}{\sqrt{3}}$ and both starting current and starting torque to $\frac{1}{3}$ of their delta values, switching to delta after run-up.

Auto-Transformer Starting

A reduced-voltage method utilizing polyphase autotransformer taps ($75\%$ or $80\%$) to limit initial starting voltage and line current, resulting in starting torques of $56\%$ or $64\%$ of rated starting torque.

Rotor Resistance Starting

A starting method applied to wound-rotor induction motors where external star-connected resistance is introduced across the slip rings to boost starting torque up to $2$ to $2.5$ times full-load torque while holding starting current between $1$ and $1.5$ times rated current.

AC Magnetic Contactor

An electromechanical switching device consisting of an operating coil, magnetic yoke, armature, and return spring, held closed by continuous coil energisation to disconnect the motor automatically if voltage falls or fails.

Utilisation Category AC-3

The standard contactor operational duty category defined under IEC 60158 and BS EN 60947-4-1 for switching squirrel-cage induction motors during starting and normal running.

Thermal Overload Relay

A motor overload protection unit consisting of three bi-metallic strips heated directly or indirectly by phase currents, which deflect upon sustained excessive current to open the contactor coil circuit.

Magnetic Overload Relay

An instantaneous-reset overload relay using electromagnetic phase coils to lift a plunger against gravity, damped by a silicon-filled oil dashpot with an adjustable bleed hole to ignore normal starting current surges.