The purpose of this subchapter is to supplement the existing systems of mortgage insurance for rental housing under this chapter by a special system of insurance designed to encourage equity investment in rental housing at rents within the capacity of families of moderate income. To effectuate this purpose, the Secretary is authorized, upon application by the investor, to insure as hereinafter provided, and, prior to the execution of insurance contracts and upon such terms as the Secretary shall prescribe, to make commitments to insure, the minimum annual amortization charge and an annual return on the outstanding investment of such investor in any project which is eligible for insurance as hereinafter provided in an amount (herein called the “insured annual return”) equal to such rate of return, not exceeding 2¾ per centum per annum, on such outstanding investment as shall, after consultation with the Secretary of the Treasury, be fixed in the insurance contract or in the commitment to insure: Provided, That any insurance contract made pursuant to this subchapter shall expire as of the first day of the operating year for which the outstanding investment amounts to not more than 10 per centum of the established investment.