15% Rent Increase in 21 months?

I live in NC and moved ito my current apartment in September of 2010 with a 9 month lease at about 730.  In June 2011 my rent went up to  773 and June 2012 my rent is going up again to 830.  The basis for each rent raise is that they compared their offerings/rates to other complexes around them.  They concluded they were undervaluaing the rent and raised the rates.  The majority of the arpartments around ours are infact newer facilities.  Nothing in my apartment has been upgraded since I moved in at the 723 rate. 

Is there a maximum rate increase per year limit or can the property manager raise rent for any reason for any amount?
If they are basing the rent increase on surveys of surrounding properties is there any basis by which they can or must justify the rent increase, or disclose which facilities they are comparing themselves to?
Is my only recourse moving out or is there another action available to me?
I had a neighbor move in November at my origional rate which was not a move in special, but my rate in June will be 15% higher than hers, can my landlord legally justify this discrepancy?

Any information would be helpful to understand the legality of the situation.