[Chromatix]

First, some electrical laws:

Power = volts * amps (as above)
Volts = amps * impedance
Conductance = 1 ÷ impedance

Hence:
Power = volts * volts ÷ impedance
Power = amps * amps * impedance
Impedance = volts ÷ amps
Conductance = amps ÷ volts

Impedances in series sum, as do voltages.
Conductances in parallel also sum, as do currents.
The voltage sum around any loop in a circuit is always zero.
The current sum into any point on a circuit is always zero (currents in always balanced by currents out).

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The electric motors in both types of propulsion machinery are similar: dual-armature and separately-wound, designed for DC power supply. Their basic characteristics are remarkably similar to those of a Bo-Bo locomotive, so I can base this description closely on practical work I've previously done on these.

A separately-wound motor is typically used in four configurations:

1: With the field connected in parallel with the armature (emulating a "shunt wound" machine), it acts either as a constant-speed motor (with torque varying with voltage & current) or as a generator (when it is turned in the opposite direction to its "motoring" tendency). The latter is normally more useful - this is how a motor can be used to recharge the batteries, using their residual charge to "bootstrap" the field.

2: With the field connected in series with the armature (emulating a "series wound" machine), it acts as a powerful and flexible motor whose torque varies with the square of the armature current, and impedance linearly with the motor speed. The torque direction does not depend on the polarity of the applied power.

3: As 2, but with the field reversed in polarity relative to the armature, the motor acts in the opposite direction with the same characteristics. This would be used to go astern.

4: The field can also be driven and controlled directly and independently of the armature. This is the normal configuration of a dedicated generator (as the armature voltage varies with the product of speed and field strength) or a traction motor being used as a rheostatic brake, but some relatively modern motor applications also make use of this feature for more precise control.

The two armatures on each motor may be connected in series or in parallel, and the two motors may also be connected separately (effectively in parallel) or in series. The slowest but most economical configuration is with armatures and motors in series. The fastest is with armatures and motors in parallel. With motors in parallel and armatures in series, an intermediate speed range is available.

Submarines usually have two main batteries of equal capacity, one mounted forward of the control room and the other aft. These may also be connected in series ("group up" for more voltage) or in parallel ("group down" for longer endurance). They may also instead be connected separately to each motor, or used individually with both motors. Ideally the batteries should be kept at the same state of charge as each other, but this is not always possible; care must be taken to avoid reverse-charging any cells, as this can damage them. The batteries, being almost universally of the basic wet-cell lead-acid type, require significant amounts of distilled water for topping up, and product hydrogen gas when charging which must immediately be vented overboard to prevent an explosion.

The combination for slowest speed and longest endurance is thus "group down, motors and armatures in series"; this is what the engine room selects when "dead slow" or "silent speed" is rung up on the telegraph. Conversely, for flank speed submerged, the configuration would be "group up, motors and armatures in parallel".

However, it is dangerous to start a heavy-duty motor by simply connecting it to power, especially in a high-power configuration; the impedance of a motor at rest is nearly zero, ie. a short-circuit, and can even be negative if the motor is being hot-reversed (as in a "crash stop"). There is thus a set of starting resistances incorporated into the armature series/parallel control switch, limiting the starting current until the motor comes up to speed. The starting resistances are built only for intermittent duty - they cannot safely be used for continuous running at reduced speed.

The engineer must select "Series 1" (armatures in series, all starting resistances in) and wait for the ammeter needle to drop to a safe current, then advance to "Series 2" (cutting out one resistance bank) and wait again, then "Series 3" and finally "Full Series". If in fact parallel operation is required, he then continues in the same form to "Parallel 1", "Parallel 2" and "Full Parallel". On American boats at least, the Parallel positions are on the opposite side of the "neutral" position to the Series positions, so he must go back through the Series starter positions and the neutral while the motor is still spinning.

Once "Full Series" or "Full Parallel" are selected, motor power and speed can be fine-tuned if required by means of the field-weakening control. This shorts out part of the field winding in the motor, resulting in a lower overall impedance and thus higher armature current for a given voltage. Selecting a degree of "weak field" is likely when flank speed is called for, or when trying to match the motors to the engines on a diesel-electric boat at high power settings.

(NB: all of the above is automatically controlled on a railway locomotive. Marine applications, especially naval, tend to do it the hard way instead.)

While when running on batteries, the "group up" and "group down" configurations are the only two supply voltages available, when running on engines the diesel-electric boat has a continuous series of voltages available by varying the field strength of the generators and the speed of the engines. In practice the engine speed is normally set for the maximum continuous rating, and the field strength is then adjusted until the engine torque (measured by the fuel injectors' rack position, which is controlled by the speed governor) required to maintain that speed also corresponds to the maximum continuous rating. If the maximum field strength is reached without sufficiently loading the engine, the field weakening control is used on the motors.

An increase of speed is then normally effected by bringing another engine "on line". To do this, it must first be secured from battery charging (if that is in progress) or started (otherwise). It is then brought up to speed and the field strength adjusted so that the voltage is slightly higher than the bus it is attaching to. Only then is it tied to the bus. At this point it is still taking very little of the load, so the field strength is increased further. The other engines will now be unloaded somewhat and must have *their* field strengths increased to compensate, or the motor field weakened, or even the motors reconfigured to a higher-power combination.

Obviously the reverse process must occur to bring an engine "off line" for any reason.

By contrast, a boat with direct-drive machinery is quite easy to run on diesels. Disengage the engine clutch, start the engine, engage the tail clutch, carefully let in the engine clutch, then adjust the engine speed or power setting until the desired propeller RPM is achieved.

Incidentally, when charging batteries, they are always connected either individually or in "group down", never in "group up". The latter would require twice the generator voltage to effect a charge.