Therefore smooth speed control can be obtained. In this method, air gap flux remains constant. Thus we get the constant V/f speed control of induction motor. This supply is applied to the stator winding of the motor. The inverter converts the DC voltage into a 3 phase variable voltage variable frequency supply. The output of the rectifier is filtered by a capacitor bank and given to an inverter circuit. In this 3 phase motor speed control method, the AC input of constant voltage and constant frequency is given to a rectifier which converts AC into DC. So this method is also known as constant (V/f) control. This is necessary to keep the air gap flux constant. Hence the ratio of supply voltage and supply frequency is kept constant by changing the stator voltage and frequency simultaneously. Because air gap flux is proportional to the ratio of supply voltage and supply frequency. But only change in supply frequency, keeping supply voltage constant has an adverse effect on the air gap flux. The synchronous speed can be changed by changing the stator supply frequency (f). Therefore, speed control of 3 phase induction motor can be achieved by following methods: Speed Control of Induction Motor by Stator frequency So by changing the supply frequency, the number of stator poles and slip we can change the speed of 3 phase induction motor. The synchronous speed of induction motor depends upon supply frequency and the number of stator poles. This shows the speed of three phase induction motor depends upon synchronous speed (N s) and slip(s). The value of the induction motor slip is adjusted according to the requirement of the driving torque at the normal working condition.The speed of a three phase induction motor is given by, The small slip produces a small torque on the motor. The slip of the motor is kept low when the induction motor is running at no-load. This can be achieved by increasing the amount of the slip and reducing the speed of the rotor. For full-load, a high value of torque is required. The value of the slip is adjusted by considering the load on the motor. This EMF develops the heavy torque on the rotor conductors. The high value of slip induces the emf in the rotor. The above equation shows that the torque induces on the rotor is directly proportional to the slip of the induction motor. Therefore, Hence, torque is directly proportional to slip. The torque is directly proportional to the rotor current. The rotor current is directly proportional to the induced emf. The emf induces in the rotor because of the relative motion, or we can say the slip speed of the motor. As we know, the slip speed is the difference between the synchronous and rotor speed of the induction motor. Slip plays an essential role in the induction motor. The slip of the induction motor varies from 5 percent for small motors to 2 percent for large motors. The percentage slip in revolution per second is given as shown below. n r is the actual rotor speed in revolution per second.n s is the synchronous speed in revolution per second.Therefore, the rotor speed is given by the equation shown below: The fraction part of the synchronous speed is called the Per Unit Slip or Fractional Slip. The slip speed of the induction motor is given as, N r is the actual rotor speed in revolution per minute.If N s is the synchronous speed in revolution per minute.Thus, the slip speed expresses the speed of the rotor relative to the field. The speed of the rotor is slightly less than the synchronous speed. In other words, the slip speed shows the relative speed of the rotor concerning the speed of the field. The difference between the synchronous speed and the actual speed of the rotor is known as the slip speed. The speed at which the induction motor work is known as the slip speed. Thus, the speed of the rotor is always kept slightly less than the synchronous speed. The electromagnetic torque is also not induced. Then no EMF induces in the rotor and zero current generates on the rotor conductors. If the speed of the rotor is equal to the synchronous speed, no relative motion occurs between the stationary rotor conductors and the main field. The speed of the rotor is always less than that of the synchronous speed. The induction motor never runs at synchronous speed. The value of slip at full load varies from 6% in the case of a small motor and 2% in a large motor.
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