CRITICAL TIMES FOR INDUCTION MOTORS

There are two important time periods that are critical in the application of induction motors. One is the allowable run-up or starting time and the other is the maximum stalling time.

The run-up time is determined by the static torque versus speed characteristic, and the moment of inertia of the load. High inertia loads can cause very long run-up times. However, a long runup time in itself is not usually a problem for the driven machine. 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. When these currents exist for, say, 20 seconds, the amount of heat created in the stator windings and the rotor bar conductors is considerable.

The surface temperature of these conductors can reach values high enough to cause damage to the winding insulation and slot wedges. With hazardous area applications this temperature rise can be very significant for some types of enclosures.

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. (In that event the insulation temperature would creep up and the material would eventually fail. 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.)

The stalling time that can be tolerated needs to be known. This will enable the relay protection for stalling to be correctly set. A motor can withstand a stall condition for a limited period of time, during which the starting (or stalling) current will be much higher than the normal current. 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. Therefore the rate of rise of surface temperature is bound to be faster in a stalling situation. 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. It can also be caused by an open circuit of one of the supply phases. Modern protective relays are available for detecting a stalling condition and a loss of one phase of the supply.