Moving towards an AC boat

[pbiJim]

Thanks

[Adelie]

Originally Posted by goboatingnow

most invertors are isolated, so teh batteryis not in the return path

You are correct the electrical energy comes out of the battery thru the inverter, then shorts back to via some path to the inverter. The point, however, is that the energy is much less likely to short back thru the water the boat rides in, whereas shorepower shorts are much more likely to.

[Oceanride007]

I'm the OP
On this thread I'm trying to fathom if we can use AC for more circuits on a boat and please discuss what you found.

Lake-effest you said

"Where an electrical machine can be lower-cost and more reliable with AC drive, the best efficiency comes from a dedicated variable frequency drive paired with the motor, and itself powered by DC."

Can you elaborate, have you seen a variable frequency drive used on a small boat (<65') was it 1 ph or 3ph.

I commented'

"I don't know cost well enough other than a 24Volt motor I'm contemplating for Autopilot costs a couple of thousand and the control gear for it another 1.5 Thou, I bet the equivalent AC motor is a couple hundred. Its only I feel that the Marine Market has this momentum towards DC so there is no AC unit to compare, DC is unique and pricey."

I think on that statement that there is plenty of motivation to move towards AC.
I commented on safety implications already, how it was solved on ship, I don’t believe generating AC on a ship is dangerous to anybody, and with regard to safety and redundancy all small boat operators could learn much from there.

Oceanrider

[RaymondR]

They really knew how to do the mad scientists thing in those days. Tesla's idea was to transmit electrical power without wires like radio waves making it available to the public for free. With the robber baron business mindset of the day that was enough to have you branded as a dangerous lunatic. However, when you look at the massive modern electricity distribution systems largely based upon his more lucid discoveries and ideas your obliged to the opinion that he was a pretty cluey guy anyway.

There is a:

Boatowner's Illustrated Handbook of Wiring by Charlie Wing

which gives a pretty extensive but understandable explanation of AC wiring also.

[goboatingnow]

Originally Posted by Oceanride007

I'm the OP
I think on that statement that there is plenty of motivation to move towards AC.
I commented on safety implications already, how it was solved on ship, I don&rsquo;t believe generating AC on a ship is dangerous to anybody, and with regard to safety and redundancy all small boat operators could learn much from there.

Oceanrider

I see no change in the immediate future. AC primarily exists on board to power connivence items. DC powers the boats systems. Given the costs , AC generation issues , safety etc, this is unlikely to change and anyway such change would be unlikely to lower costs or improve inefficiencies in any practical way

Dave

[pbiJim]

DC has traditionally been more pricy than AC. 20 years ago, The performance of DC drives was so much better than that of AC drives that it was often worth the difference in price. Modern AC drives are so much better that I have not wanted to use a DC drive in a new installation for more than 10 years now. As a general rule, AC now tends to be much cheaper & much better in performance compared to DC. The argument about drives only matters if you are in need of a variable speed application or you want to soft start a motor.

If you want to compare the power efficiency of the motor it self, then you need to take it on a case by case basis. Poorly wound AC motors will heat more than good quality DC motors & well wound AC motors will heat less than poor quality DC motors. Heat = energy loss.

In my experience, a VFD does not need to be matched to a specific motor to give good power usage efficiency. It may need to be matched to the motor if you will need precise position control in an application where you are using a vector VFD drive to approximate servo performance, but I see no application like that likely on a boat, except maybe on an autopilot & then you could have a small stand alone unit.

I have never seen a VFD that puts out single phase myself, but I am told that Minirik (sp?) does make them.

AC inverters in general, may have a safety concern on boats.

As I sit & think about it now, the common deign of VFD with the capacitor bank in the middle, will isolate a GFCI on it's primary side & may allow ground fault currents to flow from it's secondary that the primary side will never know about.

Some inverters that I have seen are, internally, the direct coupled type & lack the intermediate filtering. These MAY translate a ground fault to the safety device on the primary side. I'm not sure.

I don't know what type of circuitry is currently used in the marine inverters that are commonly used to make 120vac out of 12dc these days. I assume that most of them are PWM output types, but I don't know that for a fact. I also don't know if they translate secondary side ground faults to their primary side or if GFCI devices even exist that will work correctly when installed on the secondary of a PWM output.

Static 3-phase inverters do seem to share common flow from primary to secondary, so they are likely to trip a safety device as needed.

A ronk box has 1 leg common to both the primary & secondary, so if a fault occurs through that leg, then a proper trip should occur. This may triple the current flow required to make a trip though as the secondary current through the common leg is 1/3 the total current of the secondary.

Can anybody fill in the blanks on this one?

[Lake-Effect]

Originally Posted by Oceanride007

Lake-effect you said

Can you elaborate, have you seen a variable frequency drive used on a small boat (<65') was it 1 ph or 3ph.

Well, what I was really trying to say by bringing up variable-frequency drive was:

• For most motor applications on a small boat, you won't find that a "standard" 50 or 60 Hz single-phase AC motor is better/cheaper than a DC motor, and
• Variable frequency drive gives the best efficiency out of AC motors, obviously this requires a dedicated drive controller for each motor, and that controller can just as easily be powered by DC as AC.

I commented'

I think on that statement that there is plenty of motivation to move towards AC.

Can you post the specifics of that comparison? Sounds too good to be true. It's not that DC is pricy, it's that MARINE is pricy. Stick an anchor on the label and triple the price.

In any event that's just ONE motor. If what you're saying is true, then you could simply put in an inverter just for that one motor and save some $$$. It's not an argument for abandoning 12v DC and converting the whole boat to AC.

[pbiJim]

Originally Posted by Lake-Effect

• Variable frequency drive gives the best efficiency out of AC motors, obviously this requires a dedicated drive controller for each motor,

I have multiple motors off of a single VFD many times. The hang up here is that the motors all need to start & stop together as a bank. This works well when you have 4 different motors on a single conveyor belt system that all need to run the same speed. This does not work if the motors need to operate independently. Then you do need a separate drive for each motor.

If you want to run an AC motor at half speed so that you will use half the energy, perhaps on a fan, A VFD will allow you to do this & save a lot of power.

[pbiJim]

Originally Posted by Oceanride007

I commented'

Quote:
"I don't know cost well enough other than a 24Volt motor I'm contemplating for Autopilot costs a couple of thousand and the control gear for it another 1.5 Thou, I bet the equivalent AC motor is a couple hundred. Its only I feel that the Marine Market has this momentum towards DC so there is no AC unit to compare, DC is unique and pricey."

I think on that statement that there is plenty of motivation to move towards AC.

In the case of an autopilot, we are likely talking about a servo motor with closed loop position feedback & a servo amplifier. Servos are always very expensive, AC or DC.