TL;DR — A three-phase squirrel-cage induction motor is the dominant shipboard prime mover, running at synchronous speed minus 2–5% slip; a DC series motor must never run unloaded or it will overspeed dangerously. Generators rated 50 V or more require a voltmeter and ammeter, AC generators also require a frequency meter, and overcurrent protection must not exceed 115% of full-load rating.
What the Rule Says
Federal Installation Requirements
Every generator and motor installed aboard a vessel subject to 46 CFR Part 183 must be located where it is accessible, adequately ventilated, and as dry as practicable, and must be mounted above the bilges to prevent splash damage and contact with low-lying vapors. 46 CFR §183.320
The standard design ambient temperature is 50 °C (122 °F). If the space will not exceed 40 °C (104 °F) under normal operating conditions, a machine rated for 40 °C may be installed. If a 40 °C-rated machine is placed in a 50 °C space, it must be derated to 80% of its full-load rating, and the overcurrent device rating or setting must be reduced accordingly.
For any generator rated at 50 volts or more, a voltmeter and ammeter capable of measuring operating voltage and current must be provided. For AC generators, a means of measuring frequency must also be provided.
Each generator must carry a nameplate meeting NFPA 70 Article 445 requirements; each motor must carry a nameplate meeting NFPA 70 Article 430 requirements. Where a machine has been derated, the derated capacity must appear on the nameplate.
Generator overcurrent protection must be set at a value not exceeding 115% of the generator's full-load rating.
The requirements applicable to generators under 46 CFR §111.12-5 apply equally to motors. 46 CFR §111.25-1
DC Generators — Construction and Types
A DC generator converts mechanical energy to electrical energy by electromagnetic induction: a conductor cutting magnetic flux has a voltage induced in it proportional to flux strength, conductor count, and cutting speed. The elementary machine produces an inherently alternating voltage; the commutator — a split ring contacted by carbon brushes — reverses the external circuit connections at the moment the induced voltage would reverse, making the output unidirectional. Multiple armature coils and commutator segments smooth the pulsating output into nearly steady DC. Output voltage is regulated by varying DC field current, not shaft speed. NEETS Mod. 5 §1-1
DC generators are classified by field-winding connection:
- Series-wound: field in series with armature and load; voltage rises with load current — unstable, rarely used for ship's service power. NEETS Mod. 5 §1-3
- Shunt-wound: field in parallel with armature; terminal voltage drops only moderately with load; adjusted by field rheostat.
- Compound-wound: combines shunt and series fields; the series field compensates for voltage droop under load. A flat-compounded machine holds nearly constant voltage from no load to full load; an over-compounded machine allows voltage to rise with load to offset line drop.
Self-excited generators build voltage from residual magnetism. Failure to build up voltage is caused by loss of residual magnetism, an open field circuit, or reversed field connections. Residual magnetism is restored by briefly flashing the field from a separate DC source.
DC Motors — Types, Starting, and Speed Control
DC motors share the same winding classifications as DC generators, and the connection determines speed-torque behavior:
- Series motor: very high starting torque, heavy current at low speed, speed varies widely with load. It will overspeed and destroy itself if run without a mechanical load. Used for starters, hoists, and traction; always directly coupled or geared — never through a clutch that could release. NEETS Mod. 5 §2-2
- Shunt motor: nearly constant speed from no load to full load, moderate starting torque. Suited to pumps, fans, and machine tools.
- Compound motor: good starting torque from the series field, better speed regulation from the shunt field. Used for compressors and winches with sudden torque demands.
All DC motors require a starter to limit inrush current. Manual face-plate starters and automatic controllers insert resistance in the armature circuit at start and cut it out in steps as counter-EMF builds. Direction of rotation is reversed by reversing either the armature connections or the field connections — not both simultaneously. Speed is controlled above base speed by field rheostat and below base speed by armature voltage.
Three-Phase AC Induction Motors
The squirrel-cage induction motor is the engine room workhorse — rugged, simple, no brushes or commutator, no external rotor connections. Three-phase stator currents, displaced 120° in time and space, create a magnetic field rotating at synchronous speed:
Ns = 120f / poles NEETS Mod. 5 §4-1
This rotating field induces currents in the shorted rotor bars, which produce a rotor field dragged along by the stator field to generate torque. The rotor cannot reach synchronous speed — at synchronism there would be no relative motion, no induced current, and no torque. The difference between synchronous and rotor speed is slip, typically 2–5% at full load. Torque increases with slip up to a breakdown point.
At standstill the motor behaves like a short-circuited transformer, drawing starting inrush of 5–7 times full-load current at poor power factor. Larger motors use reduced-voltage or soft starters to limit the surge and the resulting bus-voltage dip. Direction of rotation is reversed by swapping any two of the three supply leads.
The induction motor is chosen for ruggedness, low cost, and requiring no rotor connections. Wound-rotor induction motors allow external rotor resistance for high starting torque and speed control. DOE-HDBK-1011 Vol.4 §12-1
Synchronous and Single-Phase Motors
A synchronous motor runs at exactly synchronous speed with zero slip. Its DC-excited rotor poles lock into step with the rotating stator field. Because it has no inherent starting torque, it is started as an induction motor using damper (amortisseur) windings; once near synchronous speed, DC excitation is applied to pull the rotor into step. NEETS Mod. 5 §4-2
The key shipboard application of the synchronous motor is power-factor correction: when overexcited, it draws leading reactive current, offsetting the lagging power factor of induction-motor loads. Run without shaft load purely for this purpose, it is called a synchronous condenser. A synchronous motor loaded beyond its pull-out torque falls out of step and stalls.
Single-phase induction motors have no starting torque from a single winding alone (it produces only a pulsating field). An auxiliary start winding phase-shifted by a capacitor (capacitor-start) or resistance (split-phase) creates a temporary rotating field for starting; a centrifugal switch disconnects the start winding once the motor reaches operating speed. Shaded-pole motors provide low starting torque for small fans.
Motor Protection
Motors face three distinct hazard categories: short circuits (cleared by fuses or the magnetic trip of a breaker, sized well above inrush), sustained overload (cleared by thermal overload relays set near full-load current, which trip before insulation is damaged), and abnormal conditions including single-phasing, undervoltage, and locked rotor.
Single-phasing — loss of one of the three supply phases — is especially destructive because the motor continues to run on two phases at greatly increased current. Phase-loss protection is provided on important machines. Insulation life is roughly halved for each approximately 10 °C of sustained over-temperature, making correct overload relay sizing central to long-term reliability.
A motor's nameplate specifies rated voltage, full-load current, horsepower, frequency, speed, service factor, insulation class, and duty — all data required to size cables, overload relays, and starters.
Why It Matters on the Exam
Exam questions on this topic cluster around four areas:
1. Regulatory thresholds: The 115% overcurrent limit for generators, the 80% derating rule for 40 °C machines in 50 °C spaces, and the metering requirements (voltmeter + ammeter for generators ≥ 50 V; frequency meter for AC generators) are direct recall items drawn from 46 CFR §183.320. 46 CFR §183.320
2. DC motor hazards: The series motor overspeed-on-no-load rule is a classic exam trap. Expect a scenario question asking what happens if a series motor's load is suddenly disconnected — the answer is uncontrolled acceleration. NEETS Mod. 5 §2-2
3. Induction motor starting: Questions ask why large motors use reduced-voltage starters — the answer is the 5–7× inrush current and resulting bus-voltage dip. NEETS Mod. 5 §4-1
4. Self-excited generator failure to build voltage: The three causes (lost residual magnetism, open field, reversed field) and the remedy (flash the field from an external DC source) are frequently tested. NEETS Mod. 5 §1-3
Common Pitfalls
- Confusing the 80% derating rule: it applies when a 40 °C-rated machine is installed in a 50 °C ambient — not the reverse. The overcurrent device must also be reduced, not just the machine rating. 46 CFR §183.320
- Believing a DC series motor can be safely run unloaded. It cannot — unloaded speed is uncontrolled. NEETS Mod. 5 §2-2
- Reversing both armature and field connections on a DC motor to change direction. Reversing both cancels out and the motor runs in the same direction. Only one set of connections is reversed.
- Assuming a squirrel-cage induction motor reaches synchronous speed. It never does — slip is always present at load. NEETS Mod. 5 §4-1
- Forgetting that a synchronous motor has no self-starting torque and must be started on its damper windings before DC excitation is applied. NEETS Mod. 5 §4-2
- Overlooking the frequency meter requirement for AC generators. The voltmeter and ammeter requirement applies to all generators ≥ 50 V, but the frequency meter is an additional requirement specific to AC machines.
- Treating single-phasing as a minor fault. It is especially destructive because the motor continues to run at greatly elevated current on two phases. DOE-HDBK-1011 Vol.4 §12-1
Quick Check
Q1 — A generator rated at 50 V or more must be equipped with what instruments? What additional instrument is required if the generator is AC?
A voltmeter and an ammeter capable of measuring operating voltage and current must be provided. For an AC generator, a means of measuring frequency must also be provided. 46 CFR §183.320
Q2 — A motor rated for 40 °C ambient is installed in a space where ambient temperature reaches 50 °C. What must be done?
The motor must be derated to 80% of its full-load rating, and the rating or setting of the overcurrent devices must be reduced accordingly.
Q3 — What is the maximum overcurrent device setting permitted for a generator's protection?
The overcurrent device must be set at a value not exceeding 115% of the generator's full-load rating.
Q4 — A DC series motor is running a hoist. The load is suddenly released and the hook runs free. What is the danger, and why?
The motor will accelerate uncontrollably because a series motor's speed varies inversely with load current — with no load there is no back-EMF limit on speed. Series motors must never be run without a mechanical load. NEETS Mod. 5 §2-2
Q5 — A self-excited DC shunt generator fails to build up voltage when brought online. Name three possible causes and the corrective action for the most common one.
Causes: (1) loss of residual magnetism in the field poles, (2) open field circuit, (3) reversed field connections. The most common cause is loss of residual magnetism; it is corrected by briefly flashing the field from a separate DC source. NEETS Mod. 5 §1-3
Q6 — Why does a three-phase squirrel-cage induction motor draw 5–7 times full-load current at starting, and how is this managed on large motors?
At standstill the motor behaves like a short-circuited transformer, producing high inrush at poor power factor. Large motors use reduced-voltage or soft starters to limit the current surge and the resulting bus-voltage dip. NEETS Mod. 5 §4-1