Electrical drives are integral part of industrial and automation processes particularly where precise control of speed of the motor is the prime requirement. In addition , all modern electric trains or locomotive system have been powered by electrical drives . Robotics is another major area where adjustable speed drives offer precise speed and position control.
What is an electric drive ? why it is needed ?
A drive operates and controls the speed , torque , direction of moving objects. Drives are generally employed for speed or motion control applications such as machine tools, transportation, robots , fans , etc. The drives used for controlling electric motors are known as electrical drives.
Why electric drives are needed ?
- To achieve high efficiency ; electrical drives enable to use wide range of power , from milliwatts to megawatts for various speed and hence the overall cost of operating the system is reduced.
- To increase the speed of accuracy of stopping or reversing operation of motor.
- To control the starting current.
- To provide the protection .
- To establish advance control with variation of parameters like temperature , pressure , level , etc.
Basic block diagram of AC drive
In the above block diagram of an electric drive system , electric motor , power processor , controller , sensors and actual load or apparatus are known as the major components included in the drive.
Power processors is also called as power modulator which is basically a power electronic converter and is responsible for controlling the power flow to the motor so as to achieve variable speed, reverse and brake operations of the motor . The power electronic converters include AC-AC , AC-DC, DC-AC, converters.
The controller tells the power processor how much power it has to generate by providing the reference signal to it after considering the input command and sensor inputs. The controller could be micro controller , a microprocessor or a DSP processor.
A variable speed drive used to control DC motors are known as DC drives and the variable speed drives used to control AC motors are called as AC drives.
Classification of A.C. Drives
AC drive is also called as variable frequency drive (VFD), variable speed drive (VSD), or adjustable speed drive (ASD).
Figure shows its basic working principal of drive.
speed of an induction motor is proportional to the frequency of the supply
(N = 120f/p).
where N = No. of turn of motor
F = frequency
P= Pole no.