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?