Synchronous Motors

Flashcards covering key definitions, construction details, starting methods, excitation modes, power flow, and torque equations for synchronous motors based on the lecture module.

Synchronous Motor

An alternating-current (AC) machine that converts electrical energy into mechanical energy while its rotor rotates at the same average speed as the rotating magnetic field produced by the stator.

Damper Winding

Conductive bars embedded in the rotor pole faces and short-circuited at the ends that assist in starting the motor via induction-motor action and damp out rotor oscillations known as hunting.

Hunting

Rotor oscillations around synchronous speed that occur in synchronous motors and are reduced by the damper winding.

Salient-Pole Synchronous Motor

A synchronous motor featuring projecting rotor poles, a relatively large diameter, and a short axial length, typically used in low- and medium-speed applications.

Cylindrical-Rotor Synchronous Motor

A synchronous motor featuring a smooth rotor with field windings embedded in slots, suitable for high-speed applications and exhibiting nearly uniform air-gap reluctance.

Permanent-Magnet Synchronous Motor (PMSM)

A synchronous motor that uses permanent magnets instead of a DC field winding, widely applied in servo drives, electric vehicles, and high-efficiency variable-speed systems.

Synchronous Speed

The speed of the stator's rotating magnetic field, given by $N = \frac{120f}{P}$ in revolutions per minute, or $\omega = \frac{2\pi N}{60} = \frac{4\pi f}{P}$ in radians per second.

Torque Angle ($\delta$)

The angular displacement between the stator rotating magnetic field and the rotor magnetic field, representing the angle by which the internal generated voltage $E_a$ lags the terminal phase voltage $V_\varphi$ during motor operation.

Damper-Winding Starting

A starting method where damper bars function as a squirrel-cage winding to produce induction-motor torque to accelerate the rotor close to synchronous speed before DC excitation is applied.

Pony Motor

A separate auxiliary motor used to mechanically accelerate a synchronous motor close to synchronous speed before establishing DC field excitation and AC supply synchronization.

Synchronous Reactance ($X$)

A per-phase quantity representing the combined total of the armature leakage reactance ($X_\ell$) and the armature-reaction reactance ($X_{ar}$), expressed as $X = X_\ell + X_{ar}$.

Synchronous Impedance ($Z$)

The per-phase impedance of the armature winding, given by $Z = R_a + jX$, with a total magnitude of $|Z| = \sqrt{R_a^2 + X^2}$.

Internal Generated Voltage ($E_a$)

The per-phase induced voltage developed inside the synchronous motor that acts as counter EMF opposing the applied terminal phase voltage $V_\varphi$.

Underexcitation

The condition where the rotor field current is lower than the value required for unity power factor at a given real load, resulting in a lagging armature current and absorption of reactive power.

Normal Excitation

The condition where the field current is adjusted such that the motor operates at approximately unity power factor, resulting in zero net reactive power exchange at the terminals.

Overexcitation

The condition where the rotor field current is higher than the value required for unity power factor at a given real load, resulting in a leading armature current and the supply of reactive power to the AC system.

V-Curve

A graph plotting armature current against rotor field current for a constant mechanical load, showing a minimum current point near unity power factor.

Inverted V-Curve

A graph plotting power factor against rotor field current for a constant mechanical load, which peaks near unity power factor.

Synchronous Condenser

An overexcited synchronous motor operated with little or no mechanical load to supply controllable reactive power to an electrical power system for power-factor correction and voltage support.

Converted Power ($P_{conv}$)

The electromagnetic power transformed from electrical to mechanical form inside the motor, equal to $P_{in} - P_{Cu,a} - P_{core}$, or $\frac{3V_\varphi E_a}{X}\sin(\delta)$ for an idealized cylindrical rotor.

Shaft Output Power ($P_{out}$)

The useful mechanical power available at the motor shaft, obtained by subtracting rotational losses ($P_{rot}$) and stray-load losses ($P_{stray}$) from converted electromagnetic power ($P_{conv}$).

Pull-Out

The loss of synchronism that occurs when the mechanical load applied to the rotor exceeds the maximum stable electromagnetic torque capability of the synchronous motor.

Reluctance-Power Component

The second term of the salient-pole power-angle equation, given by $\frac{3V_\varphi^2}{2}\left(\frac{1}{X_q} - \frac{1}{X_d}\right)\sin(2\delta)$, which results from unequal direct-axis ($X_d$) and quadrature-axis ($X_q$) reactances.