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Soft Start Device for AC Motor, to Reduce Start Up Surge?

PostPosted: Tue Oct 27, 2009 6:58 pm
by PaulD
We are building a lightweight, 38 ft. improved efficiency powerboat. When being used in the field this boat's engine will likely be run for traveling almost daily. Therefore I would prefer to take advantage of the 12V DC power provided by the main engine's alternator & 600 Amp Hour (approx) house battery bank. I am also considering either a Fischer Panda 3.5kw DC generator or a DC Whispergen device for further DC power generation. Most all other appliances on board will be 12V DC. The exception could be a built in 17 cu. ft. freezer system that could use a 1/2 HP 115 volt AC compressor & direct evaporator type plates. I'm told the AC compressors have higher efficiency compared to DC motor & compressor types. I would like to power this refer unit thru an inverter. I'm told there is a potential problem in that the start up surge for the 1/2 HP compressor is around 57 amps & there are few inverters that could handle this load so I'm inquiring about a "soft start" device to reduce this load. I'm also learning that the compressor might dead head trying to compress upon start up without full start up power.
I seek advice on this entire scheme if it is viable or not.
Additionally has anyone had experience with Phenolic foam insulation? This apparently has about 10% better thermal properties over Poly Isocyanurate, Urethane & Styrofoam, etc.
Thank you. PD

Re: Soft Start Device for AC Motor, to Reduce Start Up Surge?

PostPosted: Tue Oct 27, 2009 11:40 pm
by Coulomb
Let's start with the DC Whispergen: If your biggest need is heat, then this may be the device for you, but it's electrical output is what we'd call trivial at best. Even if heat is what you want, there's better ways to generate it. Webasto and Espar come to mind.

If you love complexity, and its associated price, Panda may be your best choice -- most definitely so if smart battery regulation isn't a consideration. If simplicity and battery performance and longevity is important then you might want to consider an Ample Power Genie.

Next up, refrigeration: AC motors are less efficient that DC motors, and overall efficiency takes a big dive when an inverter enters the system. AC motors are cheaper, mostly because they are made in greater volume. Efficiency isn't normally very high on the list of what a powerboat should have, so most of them have household refrigerators with long hours expected on the genset.

To answer you question, however, there are few inverters rated at 2 kW and higher which can't start a 1/2 HP motor. Inverter manufacturers aren't too willing to make that claim because some people want to start that relic they bought surplus from the 1930s closet. If your refrigeration controls are functional they won't be trying to start the compressor while there is still full head pressure.

12V? If efficiency is a goal, then 24 or 48 Volts should be considered.

Have fun!