Kinematic constraints define how the structural system is held together geometrically. From a physical standpoint, a sufficient number of kinematic constraints including multipoint constraints (MPC), constraint equations (CE), coupling (CP) and boundary conditions (BC) are necessary for the system to be in stable equilibrium.

Providing sufficient kinematic constraints for a finite element model would lead to a full rank system of equations which would give a unique solution. Lack of sufficient kinematic constraints would make the system unstable. A finite element solution for such a system would fail to converge.

If more than sufficient kinematic constraints are specified for the structural system, the system may remain stable or become unstable. If the extra constraints conflict with the basic constraints necessary to keep the system in stable equilibrium, the system becomes unstable and the finite element solution fails with convergence problems. If the extra constraints do not conflict with the basic constraints, the system is consistently overconstrained and the extra constraints become redundant constraints. The system remains stable; however, there is no unique solution. Depending on how the equations for the finite element model are solved, the solution may or may not converge.

To ensure convergence of the finite element solution, the system must not be underconstrained or overconstrained. Checking for either lack of sufficient constraints or overconstraints can be difficult for complex systems. Ansys, Inc. recommends performing a modal analysis on the system. If the modal analysis yields more zero eigenvalues than the rigid body modes of the system, the system lacks sufficient constraints; if there are fewer eigenvalues than rigid body modes, the system is overconstrained. A closer look at the unwanted eigenmodes can point to the missing or extra constraints.