Batteries don't charge at flank speed?

[Gutted]

Recently noticed this on patrol in a type VIIC. Is this intended or a bug?

[dbond1]

Intended. A U-boat has two engines, and when charging, one of those must be on line, limiting top speed. When you call for flank speed it requires both engines, leaving none free for recharging your batteries. Working as intended.

[Gutted]

Roger That!

[Noctover]

common misconception that going faster charges quicker, other way around. Quickest way to charge is the charge battery option, which will have the engines chugging at flank without you moving.

[dbond1]

Good point ^

In Hitler's U-boat War, Clay Blair makes the argument that the boats should have been designed with three engines. This would leave one free for charging and, would have lengthened the boat, increasing speed and habitability, and made the frequent engine failures less of a risk and burden to the boat. A reasonable argument.

[Noctover]

i'll take a third engine :D

[Kornstalx]

I played for dozens of hours before I realized this is intended.
It's actually one of the loading screen tips.

[Dazzed]

Originally posted by dbond1:

Intended. A U-boat has two engines, and when charging, one of those must be on line, limiting top speed. When you call for flank speed it requires both engines, leaving none free for recharging your batteries. Working as intended.

that's not quite how that works.

the electric engines sit on the main propulsion shaft between the diesels and the part that goes out the propellers. there is a clutch to disconnect the diesels, but the electric engines are always spinning with the shaft. A 2nd clutch can be used to disconnect the propeller, so the battery can be charged without spinning the propeller.

These electric engines have different modes of operation, depending how their magnetic field coils are energized. When those coils are disconnected (no current -> no magnetic field), the engine can spin freely - it's basicly in idle.

When the coils are energized, the engine can operate either as an engine, taking power from the battery to spin the shaft, or as a generator, taking power from the shaft and charging the battery.

In generator mode, the amount of electrical power output mainly depends on 2 things - RPM and field strength. This power has to be provided by the diesel as mechanical power first and the electric engine converts it, which in turn produces a force (torque) on the shaft, effectively acting as a brake.

In order to reach high speeds on the propellers, this braking force from the electric engines needs to be limited. This can be achieved by lowering the strength of the magnetic field, that's why at GF (speed setting 4) there is a reduced amount of battery charging, and at AK (speed 5) the field is shut down completely, so all the power from the diesels can be used to drive the propellers.

In real life there was also a special mode where the electric engines would be set to "help" the diesels drive the sub, in order to reach the absolute top speed possible (about 18 kts on a type VII) - but this would drain the battery very fast due to the high RPM.

[DECAFBAD]

A beautiful explanation!
The motors can be controlled by altering the magnetic field strength of the poles using variable resistance. This effect is briefly explained here:
https://www.youtube.com/watch?v=ih6HXOVDkQE

(see if you can find the small mistake in this video)

[DECAFBAD]

Originally posted by dbond1:

Good point ^

In Hitler's U-boat War, Clay Blair makes the argument that the boats should have been designed with three engines. This would leave one free for charging and, would have lengthened the boat, increasing speed and habitability, and made the frequent engine failures less of a risk and burden to the boat. A reasonable argument.

If they had any more space, they'd just make the two engines that much bigger to drive the boat even faster with all power going to the propellers and none to charge the batteries while doing so. Call her a Type IX or something.

[VipreRX]

Originally posted by dbond1:

Good point ^

In Hitler's U-boat War, Clay Blair makes the argument that the boats should have been designed with three engines. This would leave one free for charging and, would have lengthened the boat, increasing speed and habitability, and made the frequent engine failures less of a risk and burden to the boat. A reasonable argument.

Is that why some US boats in the Pacific had four engines?

[Dazzed]

Originally posted by DECAFBAD:

(see if you can find the small mistake in this video)

1) the field current won't happily form a step function if you increase field circuit resistance, self-inductance in the field coil leads to a gradual decrease in current as the field weakens
(this may even fry the current controller if resistance is increased too quickly, since the field coils will dump their excess energy into that resistor)

2) As a result of the field current dropping, armature CEMF drops proportionally, leading to an increase in armature current (since CEMF counteracts the external voltage applied to the armature, which can be assumed to be constant here)

3) Motor torque is proportional to the product of armature current AND field current.

Whether this product and therefore torque increases or decreases depends on the current RPM of the motor, generally at low RPM torque would decrease while at high RPM torque would increase.

Essentially, running with a weakened field "flattens" the torque - RPM curve of the motor - initial torque (RPM = 0) is lower but max RPM (torque = 0) is higher.

Whether RPM increases or decreases as a result of lowering the field strength depends on the load - if there is minimal load on the motor then RPM would increase, but with a higher load the motor would actually slow down because it is already running in a high torque - low RPM configuration.

Overall reducing field strength lowers efficiency and reduces maximum power due to a significant increase in copper losses (and therefore thermal load, which is the limiting factor for armature current), and it only makes sense when running at a high RPM but low power.

[Gutted]

Well this thread took aa nerdy turn.

[DECAFBAD]

All true!
But you've missed it: there's a discontinuity in the speed graph is what I wanted to point out. A slightly more serious blunder than having a crude model without self inductance. But it's a valid point anyway.

I made a model of the Type VIIC U-boat propulsion a while back. Before that I was also a bit concerned about an operator causing the field being weakened too fast with a quick hand, thus leading to less torque overall when trying to increase speed. I wasn't able to replicate that under propeller load. The shaft RPM follows the input almost instantly. So much torque!

[Dazzed]

Originally posted by DECAFBAD:

All true!
But you've missed it: there's a discontinuity in the speed graph is what I wanted to point out. A slightly more serious blunder than having a crude model without self inductance. But it's a valid point anyway.

Pretty much the same thing. Both have stored energy, the rotating mass has kinetic energy stored while the magnetic field has electromagnetic energy stored, and either way that energy can't instantaneuously change because that would require an infinite force or an infinite voltage.

IMO the big mistake in the video is having torque proportional to I_a while ignoring that it's also proportional to I_f

[DECAFBAD]

Of course, but there is a momentary amplification due to the change of the field current - a small change in the field current will always lead to a much greater change in armature current before the new speed is attained. The torque gained by the increased armature current dwarfs the torque lost due to the field weakening. (This is true in case of U-boats, maybe not true for elevators or locomotives?)

Maybe it's okay to ignore the small effect and focus on the bigger effect in a 5 minute video! But not when modelling. After all torque does depend on both current as well as flux.