How come pumps for water can work for 30 seconds to an hour without burning up but if they work for 6 hours they burn up? Shouldnt they achieve heat equilibrium within a few minutes of working?
This is usually due to the coil windings in the motor that drives the pump. As current is run through them, they will get hot. That heat will build up and may exceed what the insulation on the wires can tolerate, casing the insulation to start melting. The melting insulation can then cause the windings to short out which draws even more current, making even more heat, and burning up even more insulation.