What is an AC Drive?
AC Drives are also known as Inverters,
Variable Speed Drives (VSDs), Variable
Frequency Drives (VFDs) and Frequency
Convertors.
AC Drives are used to control AC Induction motors; the most
common motor technology used in industry. The motors are
rugged, require very little maintenance and can easily
be manufactured with high environmental protection.
If an AC motor is connected directly
to the mains supply the motor will accelerate quickly and then
rotate at a fixed speed which is dependant upon the electrical
supply frequency and the motor design.
For some applications fixed speed is acceptable and, by
carefully selecting gearbox ratios, a desired final output
speed can be achieved. However, for most applications, hard
acceleration, unbraked deceleration and the inability
to vary the speed and torque represent
a serious problem. AC Drives provide a solution
by modifying the voltage and frequency of the AC supply
to the motor. Control is achieved by either estimating
the motor speed, known as open loop control, or by
measuring the speed, known as closed loop
control. Both techniques have advantages and
disadvantages:
Open Loop Control
Open loop control provides the simplest means of
controlling AC induction motors, as the speed of the motor
is dependant upon the frequency of the supply, a basic means
of control is simply to vary the voltage and frequency to
achieve the desired motor speed. This method is known as V/Hz
control. The major disadvantages of this method of
control are poor precision and poor low speed torque.
A huge amount of research has been undertaken to improve
the open loop performance of AC Drives. Open Loop Vector and Direct
Torque Control technologies attempt to model the motor in real
time to compensate for a wide range of factors that affect the
speed and torque delivered at the output shaft. The result of
these methods is a large performance and accuracy improvement,
however, at low speed performance may still be a
problem.
Rotor Flux Control (RFC) is a new method of control for
open loop drives which offers improved stability and greater
dynamic performance.
Click here for more information.
Open loop drives are used where the motor is never required to
hold a load at zero or low speed, and where the torque and speed
accuracy are not important, typical applications include:
- Pumps
- Fans
- Conveyors
- Mixers
- Centrifuges
Closed Loop Control
Closed loop control is achieved by using a speed and
position sensor connected to the motor shaft.
The result is stable, accurate, dynamic speed and
torque control from standstill through to maximum
speed. Typical application include:
- Cranes and hoists
- Lifts
- Winding
- Material handling
- Automotive testing
- Extruding
Intelligent Drives
Control Techniques are market leaders in intelligent drives;
this refers to drives that have programmable automation or
motion controllers within the product as standard or as an
option. Intelligent drives offer significant
advantages; because the controller is located within the
drive itself, communication delays are eliminated
therefore improving performance in automation and motion
applications and removing the need for external controllers, saving
panel space. Unidrive SP, Commander SK, Digitax ST and Mentor
II are all considered to be intelligent.
Energy Saving
It is estimated that energy saving of up to 20% can be
achieved through installing and operating motors in a more
efficient way. Installing AC drives in certain applications
such as fans and pumps can have a dramatic effect on
the energy consumed. With motors consuming an
estimated two thirds of electrical energy within
industry, drives must form a key part of any programme to reduce
carbon emissions.