  In 1976 Larkin and Ovchinnikov [Sov. Phys. JETP 41, 960 (1976)] predicted
that vortex matter in superconductors driven by an electrical current can
undergo an abrupt dynamic transition from a flux-flow regime to a more
dissipative state at sufficiently high vortex velocities. Typically this
transition manifests itself as a large voltage jump at a particular current
density, so-called instability current density $J^*$, which is smaller than the
depairing current. By tuning the effective pinning strength in Al films, using
an artificial periodic pinning array of triangular holes, we show that a unique
and well defined instability current density exists if the pinning is strong,
whereas a series of multiple voltage transitions appear in the relatively
weaker pinning regime. This behavior is consistent with time-dependent
Ginzburg-Landau simulations, where the multiple-step transition can be
unambiguously attributed to the progressive development of vortex chains and
subsequently phase-slip lines. In addition, we explore experimentally the
magnetic braking effects, caused by a thick Cu layer deposited on top of the
superconductor, on the instabilities and the vortex ratchet effect
