How to adjust motor speed? How many methods to adjust  motor speed ?

How to adjust motor speed? How many methods to adjust  motor speed ? Many problems have been studied about adjusting  asynchronous motor speed, but generally, there is advantages and disadvantages in  asynchronous motor speed adjustment methods  and can not   solve all problems such as adjustment scope, power consumption, flatness in adjustment and devices 
The main methods can be used:
 - On stator: change the voltage suppling  stator , change the pole number of stator or change the source frequency.
 - On the rotor: change the rotor resistor or connect serially on the rotor circuit one or many connectors.

1. Adjust the speed by changing the number of pole pairs
The speed levels correspond to the number of polar pairs that the stator winding can be connected. Therefore, the speed can only be changed unevenly one by one. There are many ways to change the number of poles of the stator winding
- Change the wiring to have a different number of pole pairs. Used in two-speed electric motors in a 2: 1 ratio
- On the stator groove, place 2 independent windings with different number of pole pairs, usually to achieve 2 speeds in a 4: 3 or 6: 5 ratio
- On the stator groove, there are 2 independent windings with different number of pole pairs, the connection of each winding can be changed to have different number of poles.
The rotor winding in a winding asynchronous rotor has the number of pole pairs equal to the number of poles of the stator winding. Hencen, when reconnecting the stator winding to have different number of poles, the rotor winding must also be re-matched, which is inconvenient
In contrast, the squirrel cage rotor winding adapts to any single pole length of the stator winding, thus, it is suitable for electric motors to change the number of poles to adjust the speed.
Although the speed adjustment level is unstable, it helps to keep the stiffness of the mechanical properties.
2. Adjust the pitch by varying the frequency
Speed ​​of motor KĐB n = n1 (1-s) = (60f / p) (1-s)
When the slip coefficient changes little, the speed is proportional to the frequency.
On the other hand, from the expression E1 = 4.44f1W1KdqØmax we find that max is proportional to E1 / f1.
It is desirable to keep Ømax = const
E / f is adjusted for this purpose at the same time, which means it is required to use a special power source which is the industrial frequency converters.
Due to the strong development of microelectronics and power electronics, the inverters has opened up a great prospect in the field of controlling AC motors by frequency method. Using inverters to control the motor according to different rules (U / f rule, vector control ..) has created a motor speed control system with outstanding features.
3. Adjust the speed by varying the stator supply voltage.
It is known that the limit slip coefficient Sth does not depend on voltage. If R’2 is constant, when reducing the source voltage U, the limit slip coefficient Sth will no longer decrease Mmax proportional to U2.
 This method is only implemented when the machine is loaded. When the machine is not unloaded, and the power supply is reduced, the speed remains roughly stable.
4. Adjust the speed by changing the rotor circuit resistance of the winding rotor motor.
Through the slip ring, an adjustable 3-phase resistor is connected to the rotor winding
For a given load torque, the larger the auxiliary resistance, the greater the slip at the operating point (from a to b then c) leading to the lower the speed. Since the torque is proportional to the resistance power Pdt, we have: (r2 / s2) = ((r2 + rf) / s)
Since Pdt itself is constant, I2 is also constant so a part of the former mechanical power has turned into loss I2 x Rf. Since the input capacity is constant at that time, the efficiency is reduced, This is the disadvantage of this method. On the other hand, whether the speed of adjustment is more or less depends on the large or small load.