Post a Comment. Motor Power System Power Quality. What is Cogging and Crawling? The cogging phenomenon is also called the magnetic locking. If the load torque is less the motor may keep on operating at lower speed. The adverse effects of crawling and cogging of induction motor can be eradicated or minimized by taking care while designing the motor and also by taking care when the motor is operated through variable frequency drive.
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Post a Comment. Motor Power System Power Quality. What is Cogging and Crawling? The cogging phenomenon is also called the magnetic locking. If the load torque is less the motor may keep on operating at lower speed. The adverse effects of crawling and cogging of induction motor can be eradicated or minimized by taking care while designing the motor and also by taking care when the motor is operated through variable frequency drive. Cogging of Induction Motor If the number of stator slots is equal to or an integral multiple of the rotor slots, the motor may refuse to deliver the torque because of the magnetic locking between the stator teeth and rotor teeth caused by the minimum reluctance.
The reluctance is minimum when the stator slots are equal to or an integral multiple of the rotor slots. The phenomenon of magnetic locking created between the stator and the rotor teeth is called the cogging. The phenomenon of cogging can be avoided by taking appropriate combination of the stator and the rotor slots while designing the motor.
The cogging in induction motor is undesired phenomenon. Conditions that lead to cogging phenomenon. If the harmonic frequency coincides with slot frequency it causes torque modulation and it can create the condition of magnetic locking.
How to reduce Cogging Phenomenon? T here are following ways to eradicate the problem of cogging. Th e rotor and stator slot harmonic order: The stator slot harmonics depends on the number of the stator slots and number of the poles. If the number of stator and the rotor slots are equal for a particular number of poles machine, the harmonic order produced will be the same. If the stator and rotor slot harmonic order is 11th and 13th.
The 11th slot harmonic order will produce backward rotating field and the th 13th harmonic order produces the forward rotating field. If the 11 the harmonic order produces the same backward rotating field only if the rotor speed is zero. The above points are taken into consideration while designing the motor to avoid the cogging.
Example A three phase volts,6 poles,50Hz squirrel cage induction motor have following design data. To avoid cogging,. Sr Ss. The rotor slots must be less than To avoid synchronous hooks and cups in slip torque characteristics.
To avoid noisy operation. When the induction motor is operated with VF drive, the harmonics of even and odd orders are generated in the motor. The harmonics current produce rotating magnetic field in the stator and the flux gets linked to the rotor. As a result, the current starts flowing in the rotor which produces the positive and negative torque with respect to fundamental. The positive and negative torque produced by the various orders of harmonic current increase or decreases the net torque of the motor.
The reduction in the net torque deteriorates the efficiency of the motor. The order of ha rmonics and the phase sequence of harmonic current are as given below. Harmonic Order. Phase Sequence. The phase sequence of the 5 th order harmonic is opposite to the phase sequence of the fundamental current. The fundamental current produces the positive torque and the 5 th order harmonic current produce the negative torque. The net torque of the motor is always less than the torque produced by the fundamental current if the voltage fed to the stator is distorted.
The phenomenon of running of motor at slow speed is known as the crawling. The slip of the motor at 5 th and 7 th order harmonic frequency is as given below. The synchronous speed of 5th order harmonics;. As the slip is more than unity, the 5 th order harmonics will exert negative torque on the rotor. The slip at the 7 th order harmonics. The slip at the 13 th order harmonics. If the torque requirement is less, the motor can keep on operating at 0. A RPM motor can get its stable point of operation at and RPM for 7 th and 13 th order harmonic frequency respectively.
The phenomenon of operation of induction motor at low RPM is known as crawling. The torque slip characteristics of the induction motor for harmonics order frequencies are as given below. The overall efficiency of the motor operating under harmonic current gets deteriorated because the harmonic current not only reduces the torque of the motor but it also increases the heating in the motor on account of higher copper losses.
If the sum total of the negative torques produced by the various negative phase sequence harmonic current is more than the positive torque producing current, the net torque may not be sufficient to drive the equipment and motor may refuse to start.
The crawling and cogging is not predominant in slip ring induction motors. The cogging and crawling is almost absent in wound rotor or slip ring induction motors. Why is the Cogging absent in a slip ring induction motor? In an induction motor the rotor bars are spaced apart and the conductor is also have more spacing between them. The torque produced is not even throughout the revolution. By skewing the rotor bars the cogging phenomenon can be reduced to greater extent.
In slip ring or wound rotor induction motor, the rotor winding is distributed through slots. The wound rotor has more number of slots and the slots are closer to each other as compared to the slots of the squirrel cage induction motor. This arrangement of wound rotor produces even magnetic field and more torque throughout the revolution and cogging is eliminated. Why is the Crawling absent in slip ring induction motor?
The slip ring or wound rotor induction motor produces much more torque compared to squirrel cage induction motor. Labels: cogging , crawling , Induction Motor. No comments:. Newer Post Older Post Home. Subscribe to: Post Comments Atom.
Cogging and Crawling of Induction Motor
The phenomenon of Magnetic Locking between the stator and the rotor teeth is called Cogging or Teeth Locking. Even after applying full voltage to the stator winding, the rotor of a 3 phase induction motor fails to start. This condition arises when the number of stator and rotor slots are either equal or have an integral ratio. The number of the stator slots equal to or an integral multiple of the rotor slots, strong alignment forces is produced between the stator and the rotor. As a result of these forces an alignment torque greater than the accelerating torque with consequent failure of the motor to start. Thus, a locking is created between the stator and rotor teeth.
Induction Motor Cogging and Crawling
Induction motors that cog or crawl will not accelerate to full speed. Cogging motors do not accelerate at all, and crawling induction motors stop accelerating at part speed. Acceleration can also be limited by the torque output of the motor relative to the load torque at that speed. Induction motors have a series of slots in the stator and in the rotor. These slots should not be equal in number because if they are, there is a good chance that the motor will not start at all due to a characteristic known as cogging. The slots will align like a stepper motor.
What Is Crawling and Cogging ?