Looking for Our Missing Pieces, Part 1A: Power Supply

I left off far too long ago explaining that the ROV had been converted to a drop camera.  This meant that most of the equipment needed to actually make the propellers spin was missing.

The three main parts of the propulsion system are the power modules, motor controllers, and the thruster motors.  Over this and the following two posts, we’ll outline them one by one with explanations of what they’re for, how they work, and what we came up with to replace them.

Here goes:

The ROV's two power modules take the high voltage from the umbilical tether (240 volts AC), and convert it to a safe supply that can be used by the motors (12 volts DC).

Why does the tether run at such high voltage?
When electricity runs through a long cable at high current, a lot of energy is lost as heat. Current (amperage)and voltage are at a sort of trade-off when it comes to delivering a given amount of power.

As the formula has it:

P = I•E_MF
where P is power in Watts, I  is current in amps, and E_MF is voltage.

Thus, since we want about 2000 watts at the vehicle, it makes sense to make the umbilical voltage as high as is convenient and keep I nice and low, so we don’t lose too many of those watts on the way down.

Four component types keep the modules humming: transformers, rectifiers, capacitors, and circuit breakers.

The transformer is the part of the power supply that trades voltage for current.  We’re using the GE model pictured here, which takes in 240V at 1A and puts out 12V at 20A.  There’s one of these for each of the four thrusters.

 

Out of the transformer's
low-voltage coil, we get 12V AC

 This and the following three figures are plots of voltage (y-axis) over time in seconds (x-axis), as typically displayed by an oscilloscope.

The rectifier is an arrangement of schottky diodes (one-way valves for electricity) that turns this AC into DC.

For the mathematically inclined, a full-wave rectifier like ours outputs the absolute value of the input signal.

We get 12V "bump waves" (OK, the standard name is "rectified sine wave") out:

The bumps would adversely impact motor performance, so the  reservoir capacitor smooths the signal out, reducing the bumps to an acceptable "ripple" of only a few volts:

ripple function on its own

1.2V (10%) ripple riding the rectified sine

Want some of the theory behind this? Your stop is here.

image credit: C&G Technical Solutions

Calculations say that at full 20A load, this application would take a 140 mF capacitor to achieve the 1.2V ripple shown.  For a capacitor able to withstand 20+ volts, that's big (see right). Some performance trials will determine the most practical capacitance.

Last but certainly not least, there’s the circuit breaker. On the vehicle, the breaker serves to shut off the motor automatically if a) its prop gets stuck or b) a short circuit occurs due to seawater ingress or a faulty isolator.  In either of these cases, the current through the motor circuit would skyrocket, and components would literally fry (high current dissipates heat!), if not for the breaker.

Here's the inside of the port side power module with rectifiers and breakers fitted.



Finally, here's a parting shot of one of the awesome anodized-aluminum power module housings, made to protect the above-mentioned electronics under 1000 feet of seawater. To cool the equipment, it employs the largest heatsink in the world: the Pacific Ocean.

Stay tuned for part 1B of this mammoth multipost, featuring motor controllers. Goodnight!

->|-Jacob->|-