Why can't arbitrary starting motor in cold environment?

Motor in the low temperature environment is too long,will be:(1) the motor insulation cracking;(2) bearing grease freeze.(3) wire connector solder powder.Motor in a cold electric fan motor parts environment,therefore,should be kept heating,winding and bearing check before operation.What are the cause of the unbalanced three-phase current motor?(1) three-phase voltage imbalance;(2) motor phase within a pumps certain branch welding is bad or contact;(3) the motor winding interturn short circuit or to ground,interphase short circuit;(4) connection errors.Why can't a 60 hz motor with pick up to 50 hz power supply?Generally make the silicon steel sheet when motor design work in the saturated area of the magnetization curve,when the supply voltage must be reduced frequency will increase magnetic flux,exciting current Flow increase,lead to motor current is added,copper loss increase,eventually led to electricalmotor1.blogspot.com the motor temperature rise up,serious when may also be due to the coil overheating and burning motor.