Presence of resistance causes more losses. Large speed change requires a large value of resistance, and if such large value of resistance is added in the circuit, it will cause large copper loss and hence reduction in efficiency. In Plugging or Braking of Induction motor, slip more than 1 is achieved to quickly bring the rotor at rest. The speed above the normal value is not possible. Therefore, the relative speed between them will be (N s + N r). Note that: the value of the slip is: s 1 such that s1 in case of motor is. This means if magnetic flux is rotating in clockwise direction, then rotor is rotating in anticlockwise direction or vice versa. magnetic flux in the stator is said to rotate at synchronous speed. Slip more than 1 implies that, rotor is rotating in a direction opposite to the direction of rotation of magnetic flux. Read Torque Slip Characteristics for detail. Under this condition, the machine operates as an Induction Generator. This is only possible, when the rotor is rotated in the direction of rotating magnetic flux by some prime mover. Negative value of slip in induction motor can be achieved when the rotor speed is more than the synchronously rotating magnetic flux. Slip = 1, means that rotor is stationary. This is the reason slip is never zero in an induction motor. Therefore, it very important for induction motor to have a positive value of slip. This means no electromagnetic torque will be produced. Hence, no emf will be generated in rotor coils to produce rotor current. Therefore, there will not be any flux cutting action of rotor coils.
#POSSIBL VALUES OF SYNCHRONOUS SPEED NS SERIES#
series motors induction motors The crawling in the induction motor is caused by low voltage supply high loads harmonics develped in the motor improper design of the machine An induction motor has a rated speed of 720 r.p.m.
Under this condition, there will not be any relative motion between the rotor coils and rotating magnetic flux. s Ns - N/N s N - Ns/Ns The term cogging is associated with three phase transformers compound generators D.C. Explanation: From Formula of synchoronous speed of induction motors magnetic fields: sync 120f /P. f frequency of electrical power supply (Hz, cycles/sec, 1/s). The synchronous speed can be calculated as: n f (2 / p) 60 (1). the power supply frequency, and the number of poles in the motor winding. Zero slip means that rotor speed is equal to synchronously rotating magnetic flux. The synchronous speed of 60 Hz, 4 Pole Induction Motor is: 9 RPM 36 RPM Correct answer: 2. The synchronous speed for an electric induction motor is determined by. The behavior of induction machine is quite dependent on the value of slip. The significance of slip in induction machine is tabulated below. With these machines the output can be in megawatts a value too large to be. Different Values of Slip and Significance: The principle of operation of a synchronous motor can be understood by considering the stator windings to be connected to a three-phase alternating-current. supply and run at synchronous speed is referred to as a synchronous machine. Therefore, slip is never zero in induction motor. Thus rotor of 3 phase Induction Motor can never attain synchronous speed. If rotor is rotating at synchronous speed in the direction of rotating magnetic field the, there will be no flux cutting action, no emf in the rotor conductors, no current in rotor bar conductor and hence no development of electromagnetic torque. Zero slip means that rotor speed is equal to synchronous speed. Why Slip is Never Zero in an Induction Motor? Consequently a relative speed equal to (N s – N r) is set up between them and gives rise to slip in induction motor. The interaction of this rotor current with rotating magnetic flux produces a torque and hence the rotor begins to rotate in the direction of rotating magnetic field as per lenz’s law. Here, both the rotors speed and synchronous speed are not equivalent (Nr < Ns) and the slip value is constantly < 1. As rotor circuit is short circuited, this generated emf gives rise to rotor current. B.A.This rotating magnetic field cuts the stationary rotor conductors to generate emf.energized and the rotor torque increases to value V and as the rotor speed increases. Electronics and Telecommunication Engineering As defined and well understood in the art, synchronous speed, Ns.
Hence RMS value of emf induced E 1.11 x Eph, avg.
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