Locked roto amps?

[5B Steve]

  I haven't a clue as to the term I was asked about the formentioned.  What does it take to run  2 roof to AC'S from a 24v

  inverter?  One is 15,000 BTU (14.3) amps the other is 13,500 BTU (8.6) amps.  I called Don Rowe inverters to size one that I

  could use to run without using the generator.  The person that I talked to ask me the question about the roto amps.


   Steve 5B......


BTW, My bus has the large alternator 50D-270 amps I believe! Do you need a large battery bank?

[bevans6]

An electric motor and an electric generator are really very similar.  When an electric motor is spinning it develops an electrical force called counter electromotive force, or CEMF.  It's an opposite and not quite equal opposing  electrical force to what the motor draws from the supply.  If the motor is stalled, or not turning, there is no CEMF so the motor draws all the current it can based on it's internal resistance.  When a motor is started, it's obviously not turning so there is no CEMF so it draws all the current.  That is called the locked rotor current - just what the motor draws when it isn't turning.  Like when you stall your  table saw and blow the breaker.

Here is  my take on your real question.  A typical 15Kbtu AC  has two motors, the fan motor and the compressor motor.  The fan motor starts up (on mine) as soon as I  turn it on, and the compressor cycles on and off.  The compressor on mine draws about 13 - 14 amps any time it is on, but draws up to 3 to 5 times that amount when it starts, depending on how fast the motor spools up, the temperature, how much load it happens to be under, etc.  So you plan for 39 to 65 amps for a few milliseconds.  That is around 4600 watts to 7800 watts.  Now, I personally have never observed 5 times, but 3 times I have no troubles with.  You need an instant reading meter with a peak save feature to capture the burst, or an oscilloscope or similar.  So you  look for an inverter that can carry your load continuously, and that has a surge capability to allow the very short burst when starting.  The point of the battery in addition to the bus alternator is so that the inverter can pull the surge current out of the battery instantly, then the alternator takes up the load.  This is also why you size the wires and fuses from the battery to the inverter bigger than your anticipated constant load - the bigger the wires, the better that surge current is handled, with less voltage drop.

I chose a 3000 watt continuous, 6000 watt surge inverter for one AC.  If I was to try for two, I would either get a second similar inverter or a 4000 watt unit like the Trace everyone recommends which has a very high surge capability.  I am planning to use the bus start batteries as the only batteries connected to it, but I will only run the inverter when the bus engine is running, so they will never actually power the inverter on their own.

Hope this helps a bit, it's in layman's terms to a great degree and kind of glosses over some of the details.

Brian

[5B Steve]

   Brian,

  Thanks for the info, is you 5C 24V?  My 5B is 24V.  Is there a way to put in a 12V system as a 24V inverter is rather expensive?

   
   Steve 5B......
 

[bevans6]

My bus is 24v same as yours, and the last thing you want to do is go to 12v for your inverter.  It doubles the current draw, so you need to double the ampacity of the cables, the fuses, it's all bad compared to 24 volt.  The inverters cost virtually the same unless you are looking at real cheap ones.  I chose, after a lot of time deciding, that a Samlex pure sine inverter would suit me and my requirement just fine.  this one:  http://www.samlexamerica.com/products/productdescription.asp?ProductsID=19023

I chose it after talking to their engineers about the application, talking to a local mobile AC engineer who spec's them for installation in ambulances and our Hydro trucks, Cable TV fiber splice trucks, that sort of thing.  I should get it on the weekend, along with the cables, auto-transfer switch, fuses and all that required for the install, so maybe by next week I will be able to give a proper review of it.  Samlex America (I believe, but not 100% sure) spec's them, designs them, has the manufactured offshore, and I know that they OEM them to some other brands.  The engineer sure knew exact answers to the questions I had.  Another plus, for me anyway, is that Samlex America is a Canadian company out of Vancouver   

Brian

[Sean]

Steve,

Brain has given you some excellent advise.  What has yet to be mentioned is that you need a true sine wave model to run air conditioners.

To run two units you will need a 4,000-watt continuous, 8,000-watt surge model, and those are not even available in 12-volt.  As Brian suggested, 24 volts is the way to go.  Besides which, with your 24-volt alternator, you'd need a massive voltage converter to run a 12-volt inverter, and that would far overshadow the cost difference in the inverters.

There is a ton of information on this topic in the archives.  You should read those threads before proceeding.

-Sean
http://OurOdyssey.BlogSpot.com