Electric boat conversion - drive electronics

This photo shows the electronic control unit for the drive motor - as supplied by Rupert Latham. It's a "standard" unit made by Electrofit Zapi, cost (ex VAT, 2006, £1,300, 72V 500A unit.) As shown elsewhere it is mounted in our engine space on the rear cabin bulkhead entirely under our gas box, and so entirely protected from the elements.

Note engineer Alan's neat wiring of the heavy duty (400A) connecting cables - but the runs are never more than 1m long with the drive motor only just out of sight below this photo and the batteries nearly as near on either side!

The four-barred copper device in the middle of the photo is the 300A shunt used to detect drive current. The wandering cable running across the top is a typical example of my untidy wiring - it's a multicore connecting my detector of the direction of current flow through the batteries to the control panel. The "pipe" on the right is where all connections go through the bulkhead to just inside the cabin to connect to the control panel and the other devices just inside the cabin (see other postings.) Note that ALL high currents (drive and recharging) flow in cables within the engine space with very short lead lengths.

The unit is in fact a highly complex electronic device where the details don't matter in terms of operation, but I think one should be aware the motor is fed high frequency, high current, pulses (at a rate of about 16kHZ - 16 thousands times per sec) to the armature coils and about 1kHZ to the field coils. The combination (in detail) produces speed and direction control. In many ways the speed control is much like a cordless drill ... and in the same way the combination of this unit and the motor can produce "singing" tones - especially at low speeds... although less pronounced that drills I have used!!

I have looked at the actual pulses being sent on my oscilloscope (via suitable reduction devices!)... and it is truly startling how huge currents are zinging around with "ringing" effects because of the high inductance properties of the motor windings - it is quite essential the motor is powered from the generator via the batteries which effectively "smooth" everything (like a giant electrolytic capacitor) or... electronics would be going "pop" in all directions! I haven't tried, but I very much doubt any voltage conversion devices could cope with such high-power complex signals were it not that that the generator, batteries and motor all work at the same nominal voltage (72V.)

A word on this "nominal" value at this point - I know it confuses people! The actual (average) voltage of the system in fact varies quite widely - with the generator on and the batteries near full charge, the VCS limits the generator output so that actual voltage is about 87.5V (and the batteries are charging.) If the batteries are fully charged and not in use, voltage available is about 80V. Once you start using them this drops to about 72V.... if the generator is off and you are running along and the voltage is about 60V, it's time to seriously think about recharging (although you'd have at least another hour in normal use before things became desperate!)... my point is, this "nominal voltage" is in fact just that - it varies according to what's happening!

As you can see from the picture, this unit is cased in heavy Aluminium designed to assist cooling - what you can't see is a very heavy Aluminium heat sink attached at the back - only its front plate. In our case the whole is bolted to the (steel) cabin bulkhead, itself not that far from the (steel) bottom of the boat, so cooling is quite effective.

In normal use, the unit has proved to behave nearly impeccably - but see below! It is incredibly complex and self-testing - when you turn on it runs through a self-test and only connects the high power feed via a heavy duty contactor (just visible in the photo as a black blob down left.)

Herein lies one operating snag - if the control lever is not set at zero at turn-on, the test will not complete... which can be quite alarming in that you turn on the key and there is no re-assuring "clunk" (about a second later) that the contactor has gone over and all is well... it is easy enough for the lever to have been knocked off zero fitting ropes, etc.!!

Another oddity is that after use "driving" the boat, the unit gives some bleeps (control to zero)... which then shut down... there's a lot going on in there!!

Incidentally, having seen the handbook which was clearly written for those know how it works exactly (not me!), it appears to have protective devices against almost any possibility, so, for example, I would gather (although I've not tested this and have no intention of trying) that if the propellor was suddenly stopped by an obstruction the resulting surge in current demand would cut power so fast no mechanical or electrical damage should result (other than the original "clunk"!)

Interestingly, we have observed another protection device cut in - unexpected at the time. Boating against the stream on the Thames under "Yellow" stream conditions I deliberately tested how well we could manage maintaining shore speed despite a blatantly strong river current.

To do this I was drawing 110A for more than 2 hours almost continuously, with the generator happily providing 70A and the batteries 40A. I expected to make up the battery deficit over lunch easily (as we did.) The boat didn't vibrate at all although the water noise of the propellor fighting the current was loud... but suddenly power cut by about 50%. For about a second, then came back on... and this repeated....

When I cut driving current (a bit) the boat ran longer at that full power before about two seconds of reduced power...

To cut the long story of my deductions short... then verified (I think!) by looking at this unit's handbook... this unit had overheated. Even more validated at the time by the observation it was seriously hot!

It would seem the unit has an overheat protection that is wonderful - unlike the generator, it didn't cut out entirely, merely reduced output by about 50% to allow itself to cool. And cut back in until it got too hot again.....

Given our 1 hour lunch break (that time) after which the batteries were back to full charge (generator running) I then drove on limiting my drive current to 90A - and this "cut out" effect vanished...

Obviously, this is not an ultimately validated test, but I think it was pretty clear that if you ran continously with a current in excess of 100A for enough time, this unit overheats... but not catastrophically because of its built-in safeguards.

Alan has suggested we might either (a) fit a fan to assist its heat sink or (b) even think of water-cooling its heatsink. Actually, it was quite unnecessary to run the drive that fast and I only did so to see how the system coped!....

And I got my answer. This unit overheats as we had fitted if you run drawing in excess of 100 Amps continuously for more than two hours. Interesting to find this limit. Way beyond what we actually need normally. And if you DO get the thing too hot, it self-protects without leaving you stranded... quite enough power left to "manage"... and re-sets to full power the moment it's cooled off enough! Brilliant.