Developments in power electronics over the last 10–15 years has made it possible to control not only the speed of the AC induction motors but also the torque. Modern ACVSDs, with flux-vector control, can now meet all the performance requirements of even the most demanding applications.
The methods of speed control are listed below:
1. Stator voltage control
2. Supply frequency control
3. Rotor resistance control
4. Pole changing.
Usually, the AC motor speed control is achieved by varying its supply frequency. In order to keep the losses minimum, the terminal voltage frequency is changed to keep the v/f ratio constant. The frequency control method of changing the speed of AC motors is a well-known technique for decades. Only recently, however, it has become a technically viable and economical method of VSD control.
AC drives have become a more cost-effective method of speed control, in comparison to DC drives, for most VSD applications of up to 1000 kW. It is also the technically preferred solution, for many industrial environments, where the reliability and the low maintenance, associated with the AC squirrel-cage induction motor are important.
The mains AC supply voltage is converted into a DC voltage and current through a rectifier. The DC voltage and current are filtered to smooth out the peaks before being fed into an inverter, where they are converted into a variable AC voltage and frequency.
The output voltage is controlled, so that the ratio between the voltage and frequency remains constant in order to avoid over-fluxing the motor. The AC motor is able to provide its rated torque over the speed range of up to 50 Hz, without a significant increase in losses.
The motor can be run at speeds above the rated frequency, but with a reduced output torque. The torque is reduced because of the reduction in the air-gap flux, which depends on the V/f ratio. At frequencies below 50 Hz, a constant torque output from the motor is possible. At frequencies above the base frequency of 50 Hz, the torque is reduced in proportion to the reduction in speed.
One of the main advantages of VVVF (variable voltage variable frequency) speed control system is that, while the controls are necessarily complex, the motors themselves can be of a squirrel-cage construction, which is probably the most robust, and maintenance-free form of electric motor yet devised. This is particularly useful where the motors are mounted in hazardous locations, or in inaccessible positions, making routine cleaning and maintenance difficult.
In locations that require machines to have flameproof or even waterproof enclosures, a squirrel-cage AC induction motor would be cheaper than a DC motor. On the other hand, an additional problem with the standard AC squirrel-cage motors when used for variable speed applications is that they are cooled by means of a shaft mounted fan.
At low speeds, cooling is reduced, which affects the load ability of the drive. The continuous output torque of the drive must be de-rated for lower speeds, unless a separately powered auxiliary fan is used to cool the motor. This is similar to the cooling requirements of DC motors, which require a separately powered auxiliary cooling fan.