Stalling protection motors

When the motor overheats, the thermostat opens, interrupting motorline current. Pilot thermostats mounted on the windings of larger motors trip the motor starter rather than interrupt line current. This method gives good protection for sustained overloads, but because of the thermal time lag between the copper winding and the thermostat it may not provide adequate protec tion for stalled conditions or severe overloads. 

To ensure a time long enough to disconnect the motor from its power supply when the motor is stalled, fE shall exceed 5 s. When current dependent protective devices are applied to initiate the disconnection, fE shall increase with decreasing ratios of starting current to rated current. 

Overload protection is recommended for all motors, especially coolant pump motors, and shall be provided for each motor rated at more than 0.5 kW. Continuously operating motors over 1 kW shall be protected against overloads and against a stalled rotor state with built-in thermal sensors typically employed (EN 60034-11). The use of appropriate protection devices for special-duty motors (e.g., rapid traverse, locking, rapid reversal) is recommended. For motors with ratings less than or equal to 2 kW, overload protection may not be required. 

If safety programs have stalled on a plateau of acceptance, professionals should zero in on the causes of most work fatalities motor vehicle safety, truck fleet safety, workplace violence prevention, and fire and explosion protection (fatal work injuries resulting from fires and explosions rose from 113 in 2009 to 187 in 2010, an increase of 65 percent, according to the BLS. 

Are disconnecting switches for electrical motors in excess of 2hp capable of opening the circuit when the motor is in a stalled condition, without exploding (Switches must be horsepower rated equal to or in excess of the motor hp rating.) Is low-voltage protection provided in the control device of motors driving machines or equipment that could cause probable injury from inadvertent starting ... 

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