when only motor B works, corresponding speed = Ns2 = 120f / P 2ģ. when only motor A works, corresponding speed =. With this method, four different speeds can be obtainedġ.
![synkron injection synkron injection](https://cplvetnova.co.in/images/axone-forte.jpg)
The arrangement is as shown in following figure. One motor is fed from a 3phase supply and the other motor is fed from the induced emf in first motor via slip-rings. Both are mounted on a same shaft so that both run at same speed. In this method of speed control, two motors are used. But this method is only applicable to slip ring motors, as addition of external resistance in the rotor of squirrel cage motors is not possible. This method is similar to that of armature rheostat control of DC shunt motor. Ii) synchronous speed when 6 pole winding is connected, Ns = 120*50/6 = 1000 RPM I) synchronous speed when 4 pole winding is connected, Ns = 120*50/4 = 1500 RPM Change in stator poles is achieved by two or more independent stator windings wound for different number of poles in same slots.įor example, a stator is wound with two 3phase windings, one for 4 poles and other for 6 poles. This method is generally used for squirrel cage induction motors, as squirrel cage rotor adapts itself for any number of stator poles. Along with wide range of speed control, this method also offers 'soft start' capability.įrom the above equation of synchronous speed, it can be seen that synchronous speed (and hence, running speed) can be changed by changing the number of stator poles. Therefore, majority of AC speed drives employ constant V/F method (or variable voltage, variable frequency method) for the speed control. This method gives higher run-time efficiency. Also, by keeping V/F constant, the developed torque remains approximately constant. Hence, if the ratio of voltage to frequency is kept constant, the flux remains constant. The magnitude of the stator flux is proportional to the ratio of the stator voltage and the frequency. Therefore, the stator voltage should also be reduced in proportional to the frequency so as to maintain the air-gap flux constant. This will cause excessive stator current and distortion of the stator flux wave. As in above method, if the supply frequency is reduced keeping the rated supply voltage, the air gap flux will tend to saturate. This is the most popular method for controlling the speed of an induction motor. And if the frequency is increased beyond the rated value, the maximum torque developed falls while the speed rises.ģ.
![synkron injection synkron injection](https://vse-sekrety.ru/uploads/posts/2012-01/1326196526_v1500.gif)
Also, at lower frequency, the motor current may become too high due to decreased reactance.
SYNKRON INJECTION GENERATOR
It may be used where, the induction motor is supplied by a dedicated generator (so that frequency can be easily varied by changing the speed of prime mover). Actual speed of an induction motor is given as N = Ns (1 - s). Hence, the synchronous speed changes with change in supply frequency.
![synkron injection synkron injection](http://cplvetnova.co.in/images/enrotreat-s.jpg)
Where, f = frequency of the supply and P = number of stator poles.