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 systems have been powered by electrical drives. Robotics is another major area where adjustable speed drives offer precise speed and position control.

1. ELECTRIC DRIVE
FOR BOTH AC AND DC MOTORSBy
M.ASHWIN
DEPARTMENT OF
ELECTRICAL AND ELECTRONICS
ENGINEERING
JEPPIAAR INSTITUTE OF
TECHNOLOGY

2. •
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 systems have
been powered by electrical drives. Robotics is another major area where adjustable speed drives offer precise
speed and position control.
Even in our day-to-day lives, we can find so many applications where variable speed drives (or adjustable
speed drives) have been using for fulfilling a wide range of functions including control of electric shavers,
computer peripheral control, automatic operation of washing machines, and so on.
INTRODUCTION

4. •
A drive operates and controls the speed, torque and 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.
The drives can be of constant or variable type. The constant speed drives are inefficient for variable speed
operations; in such cases variable speed drives are used to operate the loads at any one of a wide range of
speeds.
WHAT IS ELECTRIC DRIVES

5. The adjustable speed drives are necessary for precise and continuous control of speed, position, or torque of different loads.
Along with this major function, there are many reasons to use adjustable speed drives. Some of these include
• To achieve high efficiency: Electrical drives enable to use wide range of power, from milliwatts to megawatts for various
speeds and hence the overall cost of operating the system is reduced
• To increase the speed of accuracy of stopping or reversing operations of motor
• To control the starting current
• To provide the protection
• To establish advanced control with variation of parameters like temperature, pressure, level, etc.
The advancement of power electronic devices, microprocessors and digital electronics led to the development of modern
electric drives which are more compact, efficient, cheaper and have higher performance than bulky, inflexible and
expensive conventional electric drive system that employs multi-machine system for producing the variable speed.
WHY ELECTRICAL DRIVES ARE
NEEDED?

6. BLOCK DIAGRAM OF AN AC ELECTRIC
DRIVE

7. In the above block diagram of an electric drive system, electric motor, power
processor (power electronic converter), controller, sensors (e.g PID
Controller) and the actual load or apparatus are shown as the major
components included in the drive.
The electric motor is the core component of an electrical drive that converts
electrical energy into mechanical energy . The motor can be DC motor or AC
motor depends on the type of load.

8. Power processor 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
and DC-DC 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 a microcontroller, a
microprocessor, or a DSP processor.

9. • AC drives are used to drive the AC motor especially three phase induction motors because
these are predominant over other motors in most of the industries. In industrial terms, AC drive
is also called as variable frequency drive (VFD), variable speed drive (VSD), or
adjustable speed drive (ASD).
• Though there are different types of VFDs (or AC drives), all of them are works on same
principle that converting fixed incoming voltage and frequency into variable voltage and
frequency output. The frequency of the drive determines the how fast motor should run while
the combination of voltage and frequency decides the amount of torque that the motor to
generate.
• A VFD is made up of power electronic converters, filter, a central control unit (a microprocessor
or microcontroller) and other sensing devices. The block diagram of a typical VFD
CLASSIFICATION OF AC DRIVES

10. BLOCK DIAGRAM OF AC DRIVE
(TYPICAL VFD)

11. • Rectifier and Filter section converts the AC power into DC power with negligible ripples. Mostly, the rectifier section is
made with diodes that produce uncontrollable DC output. The filter section then removes ripples and produces the fixed
DC from pulsating DC. Depends on the type of supply number of diodes is decided in the rectifier. For example, if it is
three phase supply, a minimum of 6 diodes are required and hence it is called as six pulse converter.
• The inverter takes the DC power from the rectifier section and then converts back to the AC power of variable voltage
and variable frequency under the control of microprocessor or microcontroller. This section is made with series of
transistors, IGBTs,SCRs, or MOSFETs and these are turned ON/OFF by the signals from the controller. Depends on the
turn ON of these power electronic components, the output and eventually the speed of the motor is determined.
• The controller is made with microprocessor or microcontroller and it takes the input from sensor (as speed reference)
and speed reference from the user and accordingly triggers the power electronic components in order to vary the
frequency of the supply. It also performs overvoltage and under voltage trip,power factor corrections, temperature control
and PC connectivity for real time monitoring.
CONSTRUCTION AND PARTS OF A
TYPICAL VFD AC DRIVE

12. We know that the speed of an induction motor is proportional to the frequency of the supply (N = 120f/p) and
by varying the frequency we can obtain the variable speed. But, when the frequency is decreased, the torque
increases and thereby motor draw a heavy current. This in turn increases the flux in the motor. Also the magnetic
field may reach to the saturation level, if the voltage of the supply is not reduced.
Therefore, both the voltage and frequency have to be changed in a constant ratio in order to maintain the flux within
the working range. Since the torque is proportional to the magnetic flux, the torque remains constant throughout the
operating range of v/f.
•
PRINCIPLE OPERATION OF VARIABLE
FREQUENCY DRIVE (VFD)

13. • There are different speed control techniques implemented for variable frequency
drives. The major classification of control techniques used in modern VFDs is given
below.
• Scalar Control
• Vector Control
• Direct Torque Control
CONTROL SCHEMES OF VFD

15. DC Drives AC Drives
The Power circuit and control
circuits are simple & inexpensive
The power circuit and control
circuits are complex
It requires frequent maintenance Less maintenance required
The commutation makes the motor
bulky, costly and heavy.
These problems are not there.
Motors are inexpensive, particularly
the squirrel cage motor.
Speed and design rating are limited
due to commutation.
Speed and design rating have no
upper limits.
It is used in certain locations only. It can be used in all locations.
Fast response and wide speed
range are smoothly achieved by
conventional method and solid state
control.
In solid state control the speed
range is wide and in conventional
method it is stepped and limited.
The line conditions are very poor.
ie, Poor power factor, harmonic
distortion of the current.
For regenerative drives the line
power factor is poor. For non-
regenerative drives the line power
factor is better.
Power/Weight ratio is small. Power/Weight ratio is large
COMPARISON

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