What is the methods used to increase motor speed?

Either though VFD or by changing pole of motor

Speed of a motor can be increased by

 1 - increasing the frequency of the power supply

 2- decreasing number of poles 

Speed of a motor can be increased by increasing the frequency of the power supply as Ns = 60F/P. This equation means that, Synchronous speed (Ns) which is the speed of the rotating magnetic field is proportional to supply frequency (F) and inversily proportional to per pole (P) of the motor.

Star-delta start, and variable speed drive

Speed of a DC Motor can be increased by increasing Armeture Voltage or reducing Field current.

Speed of an AC Motor can be increased by either by V/f upto base speed, or by increasing frequency beyond base speed.

practically speaking only by driving the motor through a VFD. no of poles cannot be changed in a running motor nor the rated voltage can be increased to a maximum defined level ( as per nameplate information)

Frequency is directly related to the motor’s speed that the faster the frequency, the faster the rotation speed and vice versa sa

VFDs can be used to increase and decrease the speed.

VFDs vary the voltage or frequency to increase or decrease speed according to this formula N=(120*f)/Poles.

The speed of AC motor is given by:

N=(120*f)/Poles

So, speed of motor can be increased by

1) Increasing the frequency

2) Decreasing the no. of poles.

Also,

3) Increasing the line voltage.

4) By decreasing the rotor resistance in case of wound rotor machine.