how can increase the speed of an ac motor?
You can't really increase the speed of an AC motor other than putting gears or a hydrostatic transmission on the output shaft. DC motors you can increase speed by upping voltage, but with AC the speed of the motor is determined by the frequency of the AC power supply and also the number of poles in the motor. Two poles is generally around 3450 rpm and four poles is 1750 or so. This is because the electromagnets that make the motor a motor can only operate as quickly as the poles are turned on and off in synchronous speed. Your only options are to increase the frequency of the AC with your own generator set and then you have to match the poles to the frequency. In other words the reason a 60Hz motor goes 3450 rpm is because the electricity cycles back and forth 3600 times each minute (60 cycles x 60 sec) so the highest theoretical rpm for the motor is 3600 rpm. It usually goes about 3500 rpm without a load due to friction forces and hysteresis eddy currents and so forth that use up some of the energy. So if you would up to a 100Hz incoming power supply you could put in two poles but not three since 3 doesn't divide into 100 equally. But say you did run two poles at 100Hz the theoretical speed of the motor would be 100 cycles * 60 sec = 6,000 rpm
You can use a VFD or any other method that alters the Line Frequency. The speed of your motor is dependant upon the Frequency, because the alternating of the magnetic poles is what creates the motor action. The faster you alternate from Positive to Negative and back, the faster the motor will turn. If you are using a 3-phase motor, simply purchase a VFD that is large enough for your motor, and then you can increase the frequency which will increase the speed of your motor. Be careful not to exceed the ratings of your bearings on the motor/gearbox and if you run the motor at a slow speed, there's a chance you may overheat the motor.
A variable-frequency drive (V F D) is a system for controlling the rotational speed of an alternating current (AC) electric motor by controlling the frequency of the electrical power supplied to the motor.
