On Sun, 25 Jul 2010 05:46:47 -0700, Jon Danniken wrote:
Find out if your compressor expects the third terminal to be a ground;
the third wire on your dryer is likely a neutral. If such is the case, you
will need to install a properly grounded outlet for your compressor.

Isn't a ground for the compressor NEMA 6-20P plug the same as the neutral
for the dryer NEMA 10-30R receptacle?

As to sharing the receptacle between a dryer and a compressor

Actually, I should have mentioned, there is no sharing. There is no dryer
on this spare receptacle in the garage. The dryer is on a separate NEMA
10-30R in the house. I do agree that unplugging a dryer once every six
months to use the compressor would be a pain.