Motors and Generators: Protection, Ratings, and Operating Characteristics

What the rule requires

The generator circuit breaker

Every ship's service and emergency generator gets its own trip-free air circuit breaker whose tripping characteristics can be set or adjusted to match the generator's capability and to coordinate with the rest of the plant. Propulsion generators are outside this section entirely 46 CFR §111.12-11.

Learn the breaker by its triggers, because that is how the question is written — a condition on the plant, and one required response:

The prime mover shuts down, and the breaker opens.
Longtime overcurrent trips or relays are set to coordinate with the feeder breaker settings, with pickup no larger than 115 percent of the generator rating for a continuous rated machine, or 115 percent of the overload rating on a machine having a 2-hour or greater overload rating.
An instantaneous trip is prohibited, with two exceptions where it becomes required: three or more AC generators can be paralleled, or the breaker serves a DC generator. Where fitted, it is set above but as close as practicable to the maximum asymmetrical short circuit available from any one of the generators that can be paralleled.
The generator is arranged for parallel operation, so it must have reverse-power or reverse-current trips.
After a trip, the breaker must not close automatically.
There are poles for each generator lead, except in the neutral lead.

Location is also fixed by rule: the ship's service generator overcurrent protective device goes on the ship's service generator switchboard, and generator and switchboard share the same space. A control room inside the machinery casing is not treated as a separate space, nor is a dedicated switchgear and converter compartment directly adjacent to and on the same level as the generator room on a MODU.

The available short-circuit current those settings are chosen against is computed from the aggregate contribution of all generators that can simultaneously operate in parallel, plus the largest probable motor load, with a three-phase fault on the load terminals of the protective device 46 CFR §111.51-2.

Three-wire DC generators carry extra requirements: separate poles for the positive and negative leads and for each equalizer lead unless the main poles protect it, algebraic-type overload trips where there are equalizer poles, and no overload trip element on a neutral pole — that duty falls to a neutral overcurrent relay and alarm set no higher than the neutral rating .

Motor circuits and duty rating

When a question hands you a motor rather than a generator, the protection requirements are in a different subpart. Overcurrent protection routes by equipment type: generators to subpart 111.12, motors and motor circuits and controllers to subpart 111.70 46 CFR §111.50-1.

Two motor circuits are carved out of the general standard. A steering gear motor circuit and its protection go to part 58, subpart 58.25; a propulsion motor circuit goes to subpart 111.35. In an ungrounded three-phase system, only two motor-running protective devices are needed, in any two ungrounded conductors — unless a wye-delta or delta-wye transformer is in the circuit. The disconnecting means must be an externally operable switch or circuit breaker 46 CFR §111.70-1. Motor construction itself follows the generator requirements of §111.12-5 46 CFR §111.25-1.

Motors are rated for continuous duty by default. The exceptions are the intermittent deck machinery in Table 111.25-15: a direct acting windlass needs only a one-fourth-hour short-time rating, a deck winch or direct acting capstan a half hour, direct acting steering gear one hour, and indirect drive steering gear continuous operation at 15 percent load followed by one hour at full load 46 CFR §111.25-15.

Installation, ambient temperature and instruments

Generators and motors are installed where they are accessible, adequately ventilated, and as dry as practicable, and mounted above the bilges clear of splash and low-lying vapors. The design ambient is 50 °C (122 °F), reduced to 40 °C (104 °F) only where the space will not exceed 40 °C under normal operating conditions. A 40 °C machine may still be used in a 50 °C space if it is derated to 80 percent of full load rating with the rating or setting of the overcurrent devices reduced accordingly 46 CFR §183.320.

Instrumentation follows from the rating: a voltmeter and an ammeter for any generator rated 50 volts or more, plus a means of measuring frequency on every AC generator. Nameplates carry the NFPA 70 information — Article 445 for generators, Article 430 for motors — and the derated capacity where the machine has been derated. The generator overcurrent device is set no higher than 115 percent of full load rating.

Machine characteristics behind the troubleshooting questions

In a large AC generator the field revolves and the armature is wound on the stator, and the rotating member is called the rotor rather than the armature. That construction is chosen for three reasons worth holding: stator load current connects directly to the external circuit without a commutator, only two slip rings are needed to feed excitation to the revolving field, and the stator winding escapes centrifugal stress. In a small AC generator the arrangement inverts — the armature revolves, the field is stationary, and there is still no commutator NAVEDTRA 14104 Ch. 12 — Ac Generators. Practically all ships now run 450-volt, 60 Hz AC ship's service and emergency generators; the DC machines that remain serve ship's service or exciter duty at 120 or 240 volts NAVEDTRA 14104 Ch. 12 — Dc Generators and Exciters.

The induction motor drives most auxiliaries: a three-phase stator sets up a rotating field at synchronous speed Ns = 120f / poles, inducing rotor currents that drag the squirrel-cage rotor along at a slight slip. The synchronous motor runs at exactly synchronous speed with a DC-excited rotor, and when overexcited it supplies leading reactive power to correct plant power factor — at the cost of a separate excitation source and a starting means. Wound-rotor machines take external rotor resistance for high starting torque and speed control DOE-HDBK-1011 Vol.4 §12-1.

Single-phasing is the failure mode to recognise on sight. Lose one supply phase and the motor keeps running on two at greatly increased current, which is why phase-loss protection is fitted to important machines. Insulation life is roughly halved for each 10 °C of sustained over-temperature, so an overload relay set loosely does not buy margin — it buys windings.

On the DC side, everything follows from counter-EMF. At standstill there is none, so starting current is very high and must be limited by starting resistance or a controller; as speed builds, counter-EMF rises and current falls to what the load needs. Torque is armature current times field flux, and speed rises when field flux is reduced, because the machine must speed up to restore counter-EMF. That is the mechanism behind the runaway: a shunt motor that loses its field can race to destructive overspeed, which is what field-loss relays guard against NEETS Mod. 5 §2-1.

A 60 Hz, four-pole induction motor is tagged 1,725 rpm on the nameplate. Is the nameplate wrong?

No. Synchronous speed is 120 × 60 ÷ 4 = 1,800 rpm, and the rotor of an induction motor always runs slower than the rotating field by the amount of slip — otherwise no voltage would be induced in the rotor at all. 1,725 rpm is that slip. Only a synchronous motor would show 1,800 rpm on the plate , NEETS Module 5 — Glossary ¶2.

Telling it apart

Armature reaction against motor reaction

Both are caused by armature current. The separating criterion is what the current acts against — the magnetic field, or the shaft.

Armature reaction — magnetic. Armature current sets up a field almost perpendicular to the main field, distorting it and shifting the neutral plane in the direction of rotation. Because the shift varies with load current, the fix in a large machine is not brush shifting but compensating windings in slots in the pole faces, or interpoles between the main poles, both connected in series with the armature so their opposing field tracks armature current NEETS Module 5 — Learning Objectives ¶3.
Motor reaction — mechanical. Armature current produces a force opposing rotation of the armature, multiplied by the number of conductors. As load current rises, the prime mover must supply more turning force to hold speed NEETS Module 5 — Learning Objectives ¶4.
Most often misfiled: sparking at the brushes. Sparking indicates improper commutation, and the main cause is improper brush placement — not motor reaction.

Voltage regulation against voltage control

Voltage regulation — internal, and it happens to you. It is the voltage change from no-load to full-load, expressed as a percentage of full load; a machine going 462 V no-load to 440 V full-load regulates at 5 percent, and lower is better NEETS Module 5 — Learning Objectives ¶6.
Voltage control — imposed, and you do it. Field circuit resistance is changed, which changes field current, which changes terminal voltage. On a hand-operated field rheostat, rotating toward RAISE decreases resistance and raises output.

Slip rings against a commutator

Slip rings — conduct the induced armature currents out to the external load. The output stays alternating NEETS Module 5 — Learning Objectives ¶1.
Commutator — insulated segments that mechanically reverse the armature connections at the instant the induced polarity reverses, so brush current stays unidirectional. It is the one component that makes a generator's output DC NEETS Mod. 5 §1-1.

Working a question

A 450-volt AC generator is nameplated for a 40 °C (104 °F) ambient with a full load rating of 200 amperes. Its space reaches 50 °C (122 °F) under normal operating conditions. What is the highest permitted setting of its overcurrent device?

Identify the governing paragraph. The machine is designed for a lower ambient than its installed location, so §183.320(b) controls whether it may stay at all.
Apply the derating condition. A 40 °C machine may be used in a 50 °C location provided it is derated to 80 percent of full load rating and the rating or setting of the overcurrent devices is reduced accordingly. Derated capacity: 200 × 0.80 = 160 A.
Apply the 115 percent ceiling to the derated figure, not the nameplate figure. The overcurrent device set value must not exceed 115 percent of the generator full load rating: 160 × 1.15 = 184 A. The distractor on this question is 230 A — 115 percent of the undertated 200 A — and it is wrong because paragraph (b)(2) has already reduced the operative rating 46 CFR §183.320.
Check what else the derating drags with it. The nameplate must show the derated capacity.
Confirm the instrument fit. Rated over 50 volts, so a voltmeter and ammeter capable of reading the machine in operation, and being AC, a means of measuring frequency.

The same 115 percent ceiling governs the longtime overcurrent trip pickup under §111.12-11(d), so if the question puts you on a vessel under part 111 instead, the arithmetic does not change — only the trip you are setting does 46 CFR §111.12-11.

Where candidates lose the point

Answering that every generator breaker needs an instantaneous trip. It sounds like more protection, so it reads as the safe answer. The rule prohibits it except on a DC generator breaker or where three or more AC generators can be paralleled. Two AC generators in parallel is not the trigger.
Answering that the breaker recloses to restore the bus. Generator circuit breakers must not automatically close after tripping. Restoration is a manual act.
Fitting reverse-power trips to a machine that never parallels. The trigger in paragraph (f) is arranged for parallel operation.
Taking 115 percent of the nameplate on a derated machine. Once §183.320(b)(2) derating applies, the overcurrent rating or setting comes down with it.
Reading "each motor must be rated for continuous duty" and applying it to the windlass. Table 111.25-15 gives that machine a one-fourth-hour short-time rating, and the deck winch a half hour 46 CFR §111.25-15.
Putting three running-overload devices in an ungrounded three-phase motor circuit. Two, in any two ungrounded conductors, satisfy §111.70-1(b) — unless there is a wye-delta or delta-wye transformer.
Answering that a DC shunt motor stops when its field circuit opens. It does the opposite: flux collapses, counter-EMF falls, and the machine accelerates to restore it, potentially to destruction.
Calling the rotating member of a large AC generator the armature. In AC machines the rotating member is the rotor, and it carries the field.

Check yourself

You are lighting off a third AC ship's service generator so that three can be paralleled on the bus. What does that change about the generator circuit breakers?

An instantaneous trip is now required, where with two paralleled machines it would be prohibited. It is set above, but as close as practicable to, the maximum asymmetrical short circuit available from any one of the generators that can be paralleled 46 CFR §111.12-11.

The engine driving a paralleled ship's service generator trips on low lube oil pressure. What must the generator breaker do, and what protects the plant if the driven end keeps turning?

The breaker must open upon the shutting down of the prime mover. If the machine motorizes, the reverse-power or reverse-current trips required on any generator arranged for parallel operation take it off the bus.

You are asked to compute available short-circuit current at a feeder breaker with two of three generators on the bus and the third in standby, all three capable of paralleling. What generators do you count?

All generators that can simultaneously operate in parallel — all three — plus the largest probable motor load, with a three-phase fault assumed on the load terminals of that protective device 46 CFR §111.51-2.

A 440-volt three-phase pump motor is found running hot with badly unbalanced line currents and a growl. One line fuse is open. Why is this urgent?

The motor is single-phasing: with one supply phase lost it continues on two phases at greatly increased current. Insulation life is roughly halved for every 10 °C of sustained over-temperature, so the windings are being consumed while it runs DOE-HDBK-1011 Vol.4 §12-1.

A DC generator is arcing at the brushes under load. Which condition does that indicate first, and what corrects it in a large machine?

Sparking indicates improper commutation, most often from improper brush placement — the brushes are short-circuiting coils that have voltage induced in them because the neutral plane has shifted with armature reaction. In a large machine the correction is compensating windings or interpoles, not repeated brush shifting, because the shift varies every time load current changes NEETS Module 5 — Learning Objectives ¶3.

A DC generator reads 462 V at no load and 440 V at full load. Is that regulation or control, and what is the figure?

Regulation, 5 percent. It is the internal voltage change from no-load to full-load expressed as a percentage of full load, and it happens on its own as load changes. Control would be your deliberate adjustment of field circuit resistance to move terminal voltage NEETS Module 5 — Learning Objectives ¶6.

A generator designed for 40 °C is to be installed in a machinery space that will not exceed 40 °C in normal operation, and the yard proposes mounting it low on the deck plates near the bilge for access. What is the objection?

The ambient design is acceptable at 40 °C where the space will not exceed it. The mounting is not: each generator and motor must be mounted above the bilges to avoid damage by splash and contact with low lying vapors, in a location accessible, adequately ventilated and as dry as practicable 46 CFR §183.320.