The increasing penetration of renewable energy leads to more demand for ramping capabilities. As a result, the ramp rate constraints of generators are binding more often than before. Ramp rate constraints cause uplift in some market designs, which distorts the price signal. This paper studies the relationship between ramp rate constraints and uplift from two aspects: market design and the convexity of ramp rate constraints. The market designs compared are real-time market with single-period model and price clearing, multi-interval realtime market (MIRTM) with only the first interval financially binding, and MIRTM with all intervals financially binding. Using a simplified multi-interval economic dispatch model, it was proved that with the last market design, constant ramp rates neither cause lost opportunity cost nor make-whole payments. It was shown that dispatch-dependent ramp rates could cause uplift with a numerical example.