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.
No comments:
Post a Comment