sizing a buck boost transformer

[gar]

101216-1008 EST

Smart $:

The heating effect on the supply conductors is not significant in relationship to the very large difference of motor inrush current to steady state current.

When I verified quinn77's voltage drop calculation at 24 A I used 1 ohm/1000 ft which is a miniscule amount over the 0.9989 value at 20 deg C for #10 annealed copper wire. Since the loop is 500 ft the result is 0.5 ohms.

I can expect an induction motor to have maybe 4 to 6 times full load current as an inrush current. Suppose the air conditioner is 15 A steady state, then at 4 times we have 60 A. The incremental change in voltage from this at a 4 times ratio is 30 V. If the ratio is 6 times, then the drop is 45 V. These are only important in that one does not want the motor to be close to its stall point.

If there is an additional load of 9 A at this time, then the drops respectively become 34.5 and 49.5 .

There is also a source impedance looking back toward the source from the main panel. This might be another 1% drop at 25 A, just a wild guess.

One should also consider that loads at the house at the main panel may produce 5 to 10 V drops at that point.

Thus, worst case may be worse than one initially assumes.

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[Smart $]

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Thus, worst case may be worse than one initially assumes.

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...and may is the key word here. Depending on who's making and the premise underlying the assumptions, the worst case scenario may not be as bad as initially assumed.

In the case of the OP, the value engineering D) approach would be to install without the xfmr, test the system, and thus know what will happen, eliminating the assumptions. Adding the xfmr is a minor installation change, and the potential need for it can be explained to the customer at the onset.

 

[gar]

101216-1643 EST

Smart $:

I agree that some testing would be very useful. A lot of questions presented on this forum could be answered with a little experimentation.

There is nothing like real world tests to answer questions you can not get otherwise.

I am somewhat related to one of those problems now. There are field failures in cold climates at the rate of less than 0.1%. Like -40 deg F. These are infant mortality type. Prior to production extensive field testing was done under severe duty, and cold and hot climate conditions. No failures or predictive indication of failures were discovered. Now in production a few failures are occurring. So far lab tests are not providing much help. This is a tough problem to solve when the number of failures is small, and current knowledge does not point in a specific direction. In the production process there are many internal tests performed at various stages of the process. From the information I have so far these do not point to a root cause. But I have not seen the production equipment.

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