The primary answer is response time. For instance, a typical overload relay (Figure 1) can take up to 20 seconds to respond to a single-phase condition. Imagine the heat build-up in the one remaining winding in a single-phased motor having to carry the entire load by itself. If it doesn't fail immediately, its life will have been substantially shortened by the damage to the winding insulation caused by overheating. A conventional overload relay responds slowly to single-phase conditions because it is reacting to the effect of the single-phase condition, which is heat, not to the actual occurrence of the single-phase condition.