It is observed that induction motors, particularly the squirrel-cage type, some-times exhibits a tendency to run stably at speeds as low as one-seventh of their synchronous speed. This phenomenon is called the crawling.
A-3 phase winding carrying sinusoidal currents produces harmonics of the order n = 6N + 1 where N is an integer (+ means with the rotation and – means against the rotation). The synchronous speed of nth order of harmonic is 1/n th of the synchronous speed of the fundamental. For N=1, a 3-phase winding produces a forward rotating 7th harmonic and backward rotating 5th harmonic.
Considering 7th harmonic, the interaction between the fictitious stator and rotor 7thharmonic poles will produce a positive torque and if the torque is sufficiently pronounced it may prevent the motor speed to exceed one-seventh of normal speed. Thus the motor crawls at about oneseventh the normal speed.
What is Cogging?
When the stator slots and rotor slots are equal in number, the speeds of all the harmonics developed by the stator slots coincide with the speed of corresponding rotor harmonics. This harmonics of every order try to exert synchronous torques at their synchronous speeds and so the machine fails to start. This is known as cogging or magnetic locking.
Effects of various operating conditions of Induction Motor
Effect of Loading
With the increase in load, speed of Motor falls and consequently torque increases. A point will reach when the maximum torque will be developed. A further increase in load causes further drop in speed, consequently the driving torque decreases and the rotor stops ultimately.
Effect of Unbalanced Supply Voltage
If the 3-phase supply to the motor is not balanced, the rotating magnetic field will move at a nonuniform rate and in a non-uniform strength. This will cause more unbalanced currents in the stator windings and, therefore, produce an unequal heating.
Effect of Break in One Phase
If one phase of Polyphase induction motor breaks, may be due to any reason, when the motor is in operation, the motor will continue to operate from single phase, provided that the load does not exceed 57.7% of the normal rating, with about the same temperature rise as when carrying rated load as a 3- phase induction motor. It will not, however, start from single phase but if brought up to speed by some external means, such as pulling on the belt, it will continue to operate satisfactorily on load below 57.7% of rated one.
A break in one phase of the rotor of a wound-rotor induction motor may prevent it from starting but if it is brought up to speed before the circuit is opened, it will operate under reduced load but usually with considerable vibrations.
Effects of Variations in Line Frequency and Line Voltage
It is usual for manufacturers to guarantee satisfactory operation of motors with only slight altered characteristics-with voltage or frequency variations amounting to not more than 10% above or below normal, provided, however, that when both voltage and frequency vary at the same time the combined variations shall not exceed 10% above or below normal.
The variations in supply frequency and supply voltage will not affect only the speed and torque of the motor but also the starting current, full load current, starting torque, maximum torque (as given below) and operating temperature of various parts of the machine and thus make their operation unsatisfactory.