It can also be caused by an open circuit of one of the supply phases. Stalling can be caused by the drive shaft being seized, for example due to a loss of lubricating oil, corrosion of bearing surfaces, fluid in the driven machine becoming very thick or even solidifying. Therefore the rate of rise of surface temperature is bound to be faster in a stalling situation. The same kind of damage that can occur during prolonged run-up times will be caused by a stalling condition, but the time taken will be less because the rotor remains stationary and so no air can be circulated to remove the heat. It is also known as locked rotor torque.ĭuring which the starting (or stalling) current will be much higher than the normal current. Stall torque is the load torque at which the motor shaft stops rotating. A motor can withstand a stall condition for a limited period of time, This will enable the relay protection for stalling to be correctly set. If stall current is drawn by a motor for a longer time, motor gets overheated causing damage of winding. It is the highest current a motor can draw and is proportional to its rotor resistance. Stall current is the current drawn by the motor at locked rotor condition. Mechanical reason: Rotor jam, overload or load unable to move. Electrical reason: Missing out of one phase (two phasing) or single phasing of input supplyĢ. At this condition, the motor drains the maximum current and the speed comes zero.ġ. This condition occurs when the torque required by the load is more than the maximum torque (Breakdown torque) that can be generated by the motor. Stalling is a condition at which a motor stops rotating even when there is sufficient voltage at it’s terminals. The stalling time that can be tolerated needs to be known. This process could also cause the windings to become loose in their slots and such damage would be followed by vibrational wear of the insulation.) (In that event the insulation temperature would creep up and the material would eventually fail. When considering the run-up time it is also necessary to know how many times the motor needs to be started in, say, one hour because successive starting would not permit the conductors or the insulation time to cool down before the next start takes place. T1 to T6 as defined for example in IEC60079 part 8. Attention should be given to the temperature classification, e.g. With hazardous area applications this temperature rise can be very significant for some types of enclosures, especially Ex(e) motors. The surface temperature of these conductors can reach values high enough to cause damage to the winding insulation and slot wedges. When these currents exist for, say, 20 seconds, the amount of heat created in the stator windings and the rotor bar conductors is considerable. Most induction motors in the oil industry are started direct-on-line and the starting and run-up currents drawn by the motor can be in the range between about 4 and 7 times the rated current. However, a long run-up time in itself is not usually a problem for the driven machine. High inertia loads can cause very long run-up times. The run-up time is determined by the static torque versus speed characteristic, and the moment of inertia of the load. One is the allowable run-up or starting time and the other is the maximum stalling time. There are two important time periods that are critical in the application of induction motors.
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