Electrical fundamentals — DC/AC, motors, distribution

46 CFR §111.51-3

§ 111.51-3 -3 Short-circuit calculations for systems below 1500 kilowatts. The following short-circuit assumptions must be made for a system with an aggregate generating capacity below 1500 kilowatts, unless detailed computations in accordance with § 111.51-4 are submitted: (a) The maximum short-circuit current of a direct current system must be assumed to be 10 times the aggregate normal rated generator currents plus 6 times the aggregate normal rated currents of all motors that may be in operation. (b) The maximum asymmetrical short-circuit current for an alternating current system must be assumed to be 10 times the aggregate normal rated generator currents plus 4 times the aggregate normal rated currents of all motors that may be in operation. (c) The average asymmetrical short circ…

DOE-HDBK-1011 Vol.4 §12-1

DOE-HDBK-1011 Vol.4 §12-1 — AC motor types, nameplate data, and protection AC motors drive the great majority of shipboard auxiliaries. The induction motor dominates: a three-phase stator sets up a rotating field at synchronous speed (Ns = 120f/poles) that induces rotor currents and drags the squirrel-cage rotor along at a slight slip. It is chosen for ruggedness, low cost, and needing no rotor connections. The synchronous motor runs at exactly synchronous speed with a DC-excited rotor and, when overexcited, supplies leading reactive power to correct plant power factor, but it needs a separate excitation source and a starting means. Wound-rotor induction motors allow external rotor resistance for high starting torque and speed control. A motor's nameplate fixes its rated voltage, full-loa…

DOE-HDBK-1011 Vol.4 §15-1

DOE-HDBK-1011 Vol.4 §15-1 — Distribution system layout and components An electrical distribution system carries power from the generators to the loads through switchboards, feeders, panels, and branch circuits. It begins at the generator and main switchboard, runs through feeder breakers to distribution panels and load centers, and finally to branch circuits serving individual motors, lighting, and equipment. Systems are arranged as radial (one path to each load — simple and cheap but a fault or outage upstream kills everything downstream), or as more reliable ring/loop and selective (secondary-network) layouts that feed critical buses from two directions so a single failure need not cause a blackout. Shipboard practice provides a normal and an emergency switchboard, with an emergency gen…

NEETS Mod. 2 §1-1

NEETS Mod. 2 §1-1 — AC vs DC, generation, and the sine wave Alternating current (AC) periodically reverses direction and continuously changes in amplitude, unlike direct current (DC) which flows in one direction at a steady value. AC is the shipboard standard for generation and distribution because its voltage is easily raised or lowered by transformers, allowing efficient transmission and simple motor design. AC is produced by electromagnetic induction: when a conductor loop rotates in a magnetic field, the induced voltage varies as the sine of the angle between the conductor's motion and the flux, tracing a sine wave. One complete positive-and-negative excursion is a cycle; the number of cycles per second is the frequency in hertz (Hz). Common shipboard frequencies are 60 Hz (US) and 50…

NEETS Mod. 5 §1-1

NEETS Mod. 5 §1-1 — The elementary DC generator and commutation A generator converts mechanical energy into electrical energy by electromagnetic induction: when a conductor cuts magnetic flux, a voltage is induced in it whose magnitude depends on flux strength, the number of conductors, and the speed of cutting, and whose direction is given by the left-hand rule for generators. In the elementary generator a wire loop rotates between field poles; the induced voltage is inherently alternating because each side of the loop passes alternately under a north and a south pole. A DC generator makes this output unidirectional with a commutator — a split ring on the shaft whose segments, contacted by carbon brushes, reverse the external connections to the loop at the instant the induced voltage wou…

NEETS Mod. 5 §2-1

NEETS Mod. 5 §2-1 — Motor principle, counter-EMF, and speed/torque A DC motor is essentially a DC generator run in reverse: supplying current to armature conductors sitting in the field flux produces a force on them (the motor, or right-hand, rule), and the resulting torque turns the armature. As the armature rotates, its conductors also cut flux and generate a voltage that opposes the applied voltage — this counter-EMF (back-EMF) limits armature current. At standstill there is no counter-EMF, so starting current is very high and must be limited by a starting resistance or controller; as the motor speeds up, counter-EMF rises and current falls to the value needed for the load. Speed is therefore governed by counter-EMF, which depends on field flux and applied voltage: reducing field flux …