The abbreviation for a specific word or phrase takes the same case (upper or lower case) as the word or phrase
Prof. Morgenroth (Professor Morgenroth)
Westinghouse Corp. (Westinghouse Corporation)
When an abbreviation is formed from the first or first few letters of each major word of a compound term, it is called an acronym. Usually acronyms are written in uppercase letters
VAR (value-added reseller)
VHF (very high frequency)
AM (amplitude modulation)
Acronyms that stand for units of measure are written in lowercase
But if the word is a unit of measure, than omit the final period
lb (pound)
yd (yard)
mi (mile)