I have been reading this thread (and related one) now and want to put in my two cents worth.
First of all, I am 100% behind the idea that we all need to be using systems that are safe and reliable. To that end, Electric Yacht systems are designed with a number of built in safety features. These include a class T fuse and holder, manual disconnect switch, thermal current foldback in the controller, finger guards, active currrent limiting and shut down in the case propeller rotation stops. Similar features are incorporated into well designed competitive products, as well.
One area of debate here is around the issue of a controller failure and a run away motor. It is important to understand how this can happen and design safety features to deal with it.
In order to understand this, it is important to understand the common failure modes of the MOSFET switching elements used in most of the controllers on the market.
Without getting into detail, it is fair to say that they can either fail open (no current flow) or shorted (high current flow). In practice, it is most common that they fail shorted, at least initially.
For the most part, the run away condition are only likely in systems using the simple low side switching controllers often used in golf cars, such as the Curtis controllers many are using. Reverse is obtained by mechanically reversing the current flow direction to the motor. Typically, these controllers don't regulate current or have much in the way of shut down features (don't flame me if the one you are using does).
In these systems, one side of the motor is connected to the battery bank (+) connection. The controller switches one or more MOSFETs on and off to control the voltage applied to the motor. If the MOSFET(s) fail, they create a short circuit such that full battery voltage is applied to the motor. Since the current is limited by the motors internal impedance, the device will often remain in this state, hence the run away motor condition.
In these systems is is wise to have a manual disconnect that is reasonably within reach of the helm. Also, a high quality circuit breaker may save fuse replacement, should a locked rotor condition cause it to trip. BTW, the same condition can occur if a short circuit bypasses the controller. Again, the manual disconnect is your most readily available way to shut down the motor.
More sophisticatd systems use a 4 quadrant controller. This is the case with virtually all brushless motor controllers and the brush motor controllers that electronically reverse direction. Internally, there are MOSFETs in an H-bridge arrangement (three half bridges in the brushless motor case), meaning there is a switch from both + and - to each motor lead. In normal operation, the high side and low side switch in each leg are NEVER turned on at the same time. To do so causes a direct short circuit across the battery. Since the batteries can supply very high current under this condition, the fuse or breaker should trip.
In almost any failure mode, the internal MOSFETs will quickly fail open. Initially, they may fail short, but the high current surge will usually cause catastrophic failure that ends in an open circuit. In many cases, this may occur before the fuse or breaker trips. Typically, when we have tested this condition with our systems (not always on purpose), the MOSFETs fail open. On our smaller systems with 200A fuses, the fuse also trips. On our large systems with 350A fuses, the fuse does not trip.
It is much less likely that a run away motor condition will occur. In fact, it is nearly impossible with an electronically commutated (brushless) motor. For this reason, these systems are safer in failure. Of course, the same fire and burn hazards are possible. A manual disconnect and fuse are still mandatory. Also, care must be excercised when working with battery cabling.
On the subject of fuse vs circuit breaker:
I am in the fuse camp. Proper circuit breakers designed to interrupt high Dc currents are expensive. The inexpensive kind available at West Marine, etc WILL weld, if called on to interrupt high dc currents. A blown fuse should be a very rare event in a well designed system. Fuses are reliable and will not weld. It is easy to carry a spare or two. If you are having issues with your fuse or breaker tripping, you need to address the root cause, rather than relying on reseting a breaker.
Cheers,
Scott McMillan
Electric Yacht
--- In electricboats@
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> I sense that this discussion is beginning to scare some folks away from converting their own boats---folks who don’t want complication, but do want safe and simple.
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> And given the 2 of you are both “in the electric boat biz†I can’t help but sense your point here is a bit like converting boats to electric is a job for the experts.
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> But I’m probably wrong.
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> A couple points come to mind with what I’ve read from you guys:
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> 1) SAE: Society of Automotive Engineers ----- folks, SAE standards are not something that concerns electric boats. ABYC, sure, but not SAE.
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> 2) Boat fires: sure, electrically-
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> 3) Fuse vs Resettable circuit breaker: I’m with Arby on this. With a resettable breaker, when it blows, 99% of us would likely look around, check a few things, then just flip that breaker back ON. And if the controller or something else developed a short circuit, there’s a very high risk that the resettable breaker could weld shut. Sure, include an accessible main manual power disconnect in the system and put in at least one quick acting fuse per battery pack, attached very near or on the battery post. Do not get a fuse rated for much more than you expect to operate at max throttle. You don’t need to spend anywhere near $100 for such a fuse as the class T fuses I use can be had for under $20 each, some with holes that will slip right on a battery post. Worst case, if you blow a fuse and don’t have a replacement, in a pinch you can jump the fuse to get to safe harbor. But carry extras---I carry extra fuses, extra motor brush set and an extra controller--
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> 4) Charging Interlocks: I dunno….sure, I guess. But be careful what you wish for in this regard. Imagine going to your boat and only wanting to demonstrate to your friend the power of the electric outboard. You don’t want to go anywhere and you leave all the lines and shore powered tied. You just want to demonstrate the pulling power and push a lot of water at the dock. You flip the switch---nothing. Scratching your head, you start checking or pulling fuses, checking wiring, cussing, etc. After about 10 minutes you finally figure out, oh yeah, I need to disconnect the AC power to do this. There are alternatives to charge interlocks including some that were proven a hundred years ago with electric car charging. My 1920 Milburn Light Electric car’s charge plug is a cylindrical coaxial plug designed to simply slide out the back if the car accidentally is driven away. I’m not advocating this here, but it’s funny how many modern electric cars choose the complicated (interlocks) over the easy solutions. Do what makes sense in your situation---
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> 5) Worst Case: So your controller fries shut while you’re on the water and your circuit breaker also welds on---motor is now running full bore and you forgot to include a manual disconnect. This is a really bad thing for a car---but how bad is it really with our electric boats? I mean, really…these are boats, and in boats we generally stay far away from other boats and objects except when docking or traveling thru narrows or some sort. And instead of the capability of 60mph top speeds and 3000# as with perhaps the typical electric car, we’re talking maybe 6mph and 4000#, with the typical electric boat. This is 75x less kinetic energy we’re talking about here. Sure, you’ll likely have to think fast and decide a course of action, but unless you’re heading towards the dock or cannot maneuver around boats or people at that moment, you can likely run the boat like this until you either (a) burn thru the batteries’ energy in less than an hour, (b) manually remove one of the power cables from a battery and eliminate the hazard. Be aware of the possibilities here, develop a contingency plan and be ready to act.
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> I’m all for coming up with standards and adopting safety measures that make sense. But there are many options to achieving safety. A controller charging interlock sounds good, but without providing feedback to the captain (visual or audio) as to why the freakin’ boat motor won’t go when it’s locked out, the interlock could be almost as embarrassing and even tragic as forgetting to pull the plug in the first place. Some folks would start cutting wires and replacing motors before figuring out the interlock was the cause. Our new fancy electric clothes dryer has a freakin’ lockout----a child protection lockout. Well, having no children, we never paid attention to that feature until one day we accidentally got it in that mode. The manual didn’t make clear we were in that mode. Online resources didn’t shed light. Finally, a service call was made. The service man arrived and said for me to go get the operators manual, and when I came back he had the dryer running and pointed out the lockout feature. He wanted me to pay $80 for a service call. I refused and eventually got them to waive the fee, blaming the company for putting a lockout feature that didn’t adequately explain how to know you were in lockout. Bottom line: If you add a lockout, be sure to add a visual or audio indicator that lets you know what is going on.
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> -Myles Twete, Portland, Or.
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> Just another electric boater: www.evalbum.
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> From: electricboats@
> Sent: Wednesday, February 10, 2010 6:47 AM
> To: electricboats@
> Subject: Re: [Electric Boats] (A Little) More On Electrical Safety...
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> Ironicaly, SAE just released their new standard for electric vehicle charging protocol. The plug has heavy conductors for charging the battery string, and several light conductors to energize a relay in the charger, and signal the correct current and voltage. I own a copy of the standard (60+ pages) but it's locked, and I can only take screen shots. The plug and recepticle are being manufactured by several vendors, I'll try and list them.
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> A GFI breaker might be a solution. If over 50mA bleeds into the water, it'll trip.
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> Also of note: A 45ft yacht burned to the water last week, the result of an electrical fire. Most of the boats being converted these daze are 30 to 40 years old, and sport ancient shore power cords and connectors get even hotter with battery chargers refilling the acumulators. I now include replacing old crusty power components with new stainless ones. Freshness counts.
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> Be Well,
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> Arby
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> On Feb 9, 2010, at 5:55 AM, "Monte Gisborne" <emotive@...
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> Another point worth making about electrical safety is the extension cord used to tap into the shorepower outlet. If your boat does not have an interlock to immobilize the boat while under charge, it is quite possible for someone (never us; usually the dumb uncle or nephew who borrowed the boat without asking first) to leave the dock with the boat still on charge. The worst-case scenario here is that the live end of the still-plugged-
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> Few of our boats have the proper interlock which tells the motor controller not to energize as long as the batteries are under charge, so the simplest way to add safety here is to create the scenario that if the boat were to leave to dock whilst under charge, and the cord became taut as the boat moved along, the disconnect would happen at the dock side of the equation and not the boat side. This is usually done by simply attaching the short charger AC input cord to the extension cord by knotting them together or similarly. In this event, the cord falling into the water would not be live, a trick which is long-understood by many e-boaters and intuitive to many on this list but if one more person learns of this safety procedure it’s worth posting.
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> One of the features I like about the Delta-Q QuiQ chargers is that unlike almost all other chargers they provide the facility to disable the vessel during battery charge. However, this feature is only of benefit if the motor’s controller is set up to allow the input from the charger, which isn’t the case with most electric outboards. I have discussed this with Torqeedo, hopefully they will make the necessary modification to their products to take advantage of this interlock feature.
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> Off my safety soapbox for now, let’s enjoy some good, safe boating out there!
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> Monte Gisborne
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