From: dhky@shaw.ca   
      
   On 05/10/2013 3:36 PM, Tom Biasi wrote:   
   > On 10/5/2013 4:50 PM, Salmon Egg wrote:   
   >> In article <87a9inog8h.fld@apaflo.com>,   
   >>   floyd@apaflo.com (Floyd L. Davidson) wrote:   
   >>   
   >>> Salmon Egg  wrote:   
   >>>> In article ,   
   >>>> "Kissi Asiedu"  wrote:   
   >>>>   
   >>>>> Hello,   
   >>>>> I was wondering if an American Made Referigrator rated 120V 60Hz   
   >>>>> could work   
   >>>>> efficiently with the use of a step down transformer in Africa where   
   >>>>> the   
   >>>>> electricity is rated at 240V 50Hz.   
   >>>>> Thanks,   
   >>>>   
   >>>> As usual, the answer is: It depends.   
   >>>>   
   >>>> If you can, contact the manufacturer to get more information.   
   >>>>   
   >>>> You can try monitoring the current to see if it becomes excessive.   
   >>>>   
   >>>> If the refrigerator does not have to work hard, You might want to   
   >>>> get an   
   >>>> transformer or autotransformer to change to 100V. That would mean that   
   >>>> as the transformer rotates at the lower speed, current for the same   
   >>>> output torque will be close to what it would be for 60Hz. Bit do not   
   >>>> expect the refrigerator to work at full capacity.   
   >>>   
   >>> Good way to burn your house down...   
   >>>   
   >>> The change from 60Hz to 50Hz is an issue with heat.   
   >>> Running a synchronous electric motor at low voltage is   
   >>> also an issue with heat.   
   >>>   
   >>> The transformer does not rotate, and the lower voltage   
   >>> would not change the speed of either the transformer   
   >>> current or the motor rotation.   
   >>   
   >> I do not think you know much about modern refrigerators. They usually   
   >> use induction motors lubricated by the refrigerant. There are no slip   
   >> rings or other electrical connections with rubbing parts.   
   >>   
   >> If the refrigerator manufacturer is not cutting corners too much, there   
   >> should be enough iron and copper to prevent such catastrophe. Even if   
   >> the motor cannot run at 50Hz and 120V. it should be able to run no load   
   >> at 50Hz/100V at about the same CURRENT as at the 60Hz/120V. The only   
   >> problem comes when you add the load of the compressor. If you get   
   >> satisfactory performance without exceeding the the rated current for   
   >> 60Hz/120V, you should be ok. That is, for 50Hz operation, do not exceed   
   >> a VA input more than 5/6 of the VA allowed at 60Hz operation. Just take   
   >> the resultant loss in performance.   
   >>   
   > There is merit in what you say, however, the efficiency loss will   
   > manifest as heat since the reactance is not optimal and the starting   
   > capacitor will not be doing it's job properly. And manufacturers do cut   
   > corners considerably to compete in the US market.   
   > I still would not do it.   
      
   Salmon egg has it right- Motor control generally follows a constant V/f   
   ratio. The efficiency hit is not significant as the load current content   
   at  rated torque  will be roughly 10% higher but the magnetizing current   
   (roughly half the total current)  will be lower.   
   Heating and efficiency will be about normal but the motor cooling will   
   be less effective due to the lower speed. Starting transients are not  a   
   consideration in terms of losses and efficiencies. Will the motor run   
   hotter- maybe - because cooling isn't as good at the lower speed   
      
   Slip will be a bit higher so speed will be a bit less than 5/6 the rated   
   full load speed. Yes, starting current will be higher and the starting   
   capacitor will not be optimum and if starting is marginal (However after   
   the days of brownouts killing fridge motors in the past, I expect that   
   newer fridges have a bit more capability in this respect).   
      
      
   --   
   Don Kelly   
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