Basics of Electric Machines Practice Flashcards

Comprehensive practice vocabulary flashcards covering transformers, DC machines, induction machines, synchronous machines, and general electric machine concepts based on the lecture notes.

Ideal transformer

A hypothetical lossless transformer characterized by perfect magnetic coupling, zero winding resistance, and infinite core permeability.

Excitation branch

A parallel branch in the transformer equivalent circuit where the shunt resistance ($R_{Fe}$) models power lost to iron losses and the magnetizing reactance ($X_m$) represents the reactive network sustaining core flux.

No-load current ($I_0$)

The small three-phase current (typically $2\%$ to $6\%$ of rated current) drawn by a primary winding when secondary terminals are open-circuit, characterized by a low power factor.

Open-circuit test

A measurement performed by applying rated voltage to the low-voltage ($LV$) side of a transformer while the high-voltage ($HV$) side is isolated to determine core losses.

Short-circuit test

A test conducted by shorting the low-voltage side and applying a reduced voltage ($V_{SC}$) to the high-voltage side until rated current is reached, used to measure copper losses.

Armature reaction

A phenomenon in DC machines where the current flowing through armature conductors creates a secondary magnetic field that distorts the main magnetic field.

Compensating poles (Interpoles)

Small auxiliary poles placed between the main poles of a DC machine, connected in series with the armature to improve commutation and reduce sparking.

Compensating windings

Conductors embedded in the pole faces of the main poles in a DC machine that counteract armature reaction by carrying current in the opposite direction to adjacent armature conductors.

Back electromotive force (Back EMF)

An induced voltage produced during the rotation of a DC motor that opposes the applied supply voltage.

Shunt DC motor

A motor configuration where the current splits into parallel paths for the field winding and armature, allowing for nearly constant speed under fluctuating loads.

Series DC motor

A motor where the field winding and armature are connected in a single loop, producing immense starting torque but poor speed regulation.

Separately excited DC motor

A motor where the field winding and armature are connected to independent power supplies, allowing for precise and flexible speed control.

Universal motor

A modified DC series motor with a laminated magnetic core designed to operate on both DC and single-phase AC power.

Synchronous speed ($n_s$)

The speed of the rotating magnetic field determined by the formula $n_s = \frac{120f}{P}$, where $f$ is the supply frequency and $P$ is the number of magnetic poles.

Variable Frequency Drive ($VFD$)

A device used to control motor speed by scaling input frequency ($f$) and voltage ($V$) proportionally to maintain a constant $V/f$ ratio.

Self-excitation

A process in islanded induction generators where a capacitor bank provides reactive power to amplify the magnetic field initiated by residual rotor magnetism.

Salient pole rotor

A rotor topology with poles that physically project outward, used in low-speed applications such as hydroelectric generators.

Cylindrical (Smooth) rotor

A rotor with a uniform air gap used in high-speed turbo alternators driven by steam or nuclear power.

Permanent Magnet Synchronous Motor ($PMSM$)

A synchronous machine using permanent magnets on the rotor to produce a locked magnetic field, eliminating rotor copper heat losses.

Reluctance torque

Torque produced by the tendency of a rotor to align with the minimum reluctance magnetic path in machines with non-uniform air gaps.

Excitation torque

Electromagnetic torque produced by the interaction between the stator's rotating magnetic field and the rotor's excitation field.

Switched Reluctance Motor ($SRM$)

A motor with a laminated steel rotor and no windings or magnets, producing torque as the rotor moves toward the position of minimum magnetic reluctance.

Stepper motor

A type of electric motor that rotates in discrete angular steps in response to electrical pulses, governed by the step angle formula $\theta_s = \frac{360^{\text{o}}}{N_r \times m}$.

Iron losses (Core losses)

Losses occurring in the magnetic core consisting of hysteresis losses (realignment of magnetic domains) and eddy current losses (induced circulating currents).

Laminations

Thin insulated sheets of silicon steel used to construct the magnetic core to reduce eddy current losses by increasing electrical resistance.