Air conditioners and refrigeration units have a compressor that functions in a way that cools space-products.
Sometimes the compressor is 'tight', defective, or worn to the point that it sometimes, or eventually will not start up.
There is a good chance that you can save the compressor, and entire unit, just by installing a hard start kit. This is a start capacitor with a drop out device in it to take it out of the circuit.
I will post links I found that will explain it better and clearer than I could.
My neighbor across the street has a very old A/C unit that I have saved twice with this method. The first time his start capacitor failed, actually blew up. The second time the compressor was worn to the point that I had to install a much larger kit.
I have saved 20 or so compressors and units using this method.
Some trailer rooftops come with them, but if your rooftop unit fails to start, I would consider installing a hard start kit, may save you replacing the unit. The links will give instruction and additional information.
I put one on my home unit when I installed it to prolong the life of the compressor.
A one ton, 12,000 BTU's, unit will be one horsepower. It's fine to oversize the kit. (To a point)
Thhhhat's all folks!!!!!
Good explanation:
http://www.kickstartoem.com/faqs/homeowner.html
If you want a headache:
http://www.longviewweb.com/startkit.htm
Good Information:
http://www.myhvacparts.com/Catalogue...it%20Order.htm
http://en.allexperts.com/q/Heating-A...-start-kit.htm
I like the Supco;
http://arnoldservice.com/compressor_...t_boosters.htm