Speed And Frequency Control Of Ac Induction Motor Using Variable Frequency Drive

Energy saving is of great importance to any country such as in Nigeria. In order to overcome energy crisis the only possible solution with minimum cost is to save energy.

Original price was: ₦ 3,000.00.Current price is: ₦ 2,999.00.



Energy saving is of great importance to any country such as in Nigeria. In order to overcome energy crisis the only possible solution with minimum cost is to save energy. It has been stated that 25% of the world’s electricity is consumed by AC motors. And there is a big problem associated with these motors that is, large starting inrush current. As the motor starts a large current is drawn by the motor which is of no use until motor reaches it synchronous speed. This high current not only produces heat but also reduces the life time of electrical equipment and power consumption also increased. Thus there is a need to reduce this current somehow or the other .This current can be reduced by making use of variable frequency drive. Variable frequency drive is a technique used to control the speed and frequency of AC induction motors thus it is also known as adjustable speed drive or variable speed this phenomenon voltage and frequency of the motor is controlled using a technique named as PWM (Pulse Width Modulation). There are so many other techniques used to reduce the current of motor like soft starters. But the benefits of variable frequency drive are more than soft starters. Like it provide energy to electrical appliance according to demand, It enhances the life time of equipment but the biggest advantage is, It is energy  saving device, and such device is greatly needed in a country like Pakistan where energy crisis has halt the economic wheel of the country.


There are several terms used to describe devices that control speed. While the acronyms are often used interchangeably, the terms have different meanings.

Variable Frequency Drive (VFD)

This device uses power electronics to vary the frequency of input power to the motor, thereby controlling motor speed.

Variable Speed Drive (VSD)

This more generic term applies to devices that control the speed of either the motor or the equipment driven by the motor (fan, pump, compressor, etc.). This device can be either electronic or mechanical.

Adjustable Speed Drive (ASD)

Again, a more generic term applying to both mechanical and electrical means of controlling speed.

























1.0                                                        INTRODUCTION

Electric AC motor works at fixed speeds and are ideally suited to applications where a constant output speed is required. However around half of all motor applications have some kind of varying speed demand and this includes processes such as moving air and liquids (fans and pumps), winding reels and precision tools.

Historically in applications requiring precise speed control such as paper winding reels expensive direct current (DC) motors or hydraulic couplings were used to regulate the machine speed, whereas in other applications the processes have been controlled by opening and closing dampers and valves, or changing output speeds with gears, pulleys, and similar devices whilst the motor works at constant speed.

In the 1980’s and 1990’s, variable frequency drives started appearing on the electric market offering an alternative method of control. A variable frequency drive, also called frequency inverter, adjustable speed drive, the basic working principle is adjusting the electrical supply to an AC motor with a corresponding frequency and voltage change in the motor’s speed and torque output.

By implementing this type of control a very close match between motor speed and the process requirements of the machine it is driving may be achieved.

Variable frequency drive technology is now mature and enjoying widespread adoption and use with AC motors; Variable frequency drives are extremely versatile and offer a high degree of motor control where motor speeds can be accurately varied from zero rpm through over 100% of the rated speed, whilst the torque is also adjusted to suit.

Large part of electrical energy production is used to be converted into mechanical energy via different kinds of electric motors –DC Motors, Synchronous Motors and Induction Motors. This is because it is one of the most widely used motors in the world. The major reason behind the popularity of the Induction Motors are[4]:

  1. Induction Motors are cheap compared to DC and Synchronous In this age of competition, this is prime requirement for any machine. Due to its economy of procurement, installation and use, the Induction Motors is usually the first choice for an operation.
  2. Squirrel cage Induction Motors are very rugged in There robustness enables them to be used in all kinds of environment and for long duration of time.
  3. Induction Motors have high efficiency of energy Also they are very reliable.
  4. Owing to their simplicity of construction, Induction Motors have very low maintenance

The Variable Frequency Drive (VFD) industry is growing rapidly and it is now more important than ever for technicians and maintenance personnel to keep VFD installations running smoothly. Variable Frequency Drives (VFD) change the speed of motor by changing voltage and frequency of the power supplied to the motor. In order to maintain proper power factor and reduce excessive heating of the motor, the name plate volts/hertz ratio must be maintained. This is the main task of Variable Frequency Drive.

Variable Frequency Drive (AC drives) are used to stepless speed control of squirrel cage induction motors mostly used in process plants due to its ruggedness and maintenance free long life.

When varying the speed of an induction motor, there is a direct relation between speed of motor and the frequency of motor operation. Therefore by varying the frequency of ac voltage the motor speed can be adjusted according to desirable value.

N= f*120/P         (1)

N = Speed of motor (RPM)

F = Electrical Frequency of motor P = No of poles of motor

These variable frequency drives are very important for HVAC systems where a very large power is consumed before motor reaches at its full speed and  a very huge amount of inrush current is being drawn by motor causes this great loss of energy.[11],[5] This starting current can be reduced by making use of variable frequency drive and thus it saves energy up to a large extent. There are various application of variable frequency drive in different appliances like fans,[16],[17] pumps,[17] tower cooling systems,[18] micro wave ovens,[19] air conditioners and ship propulsion systems.[20] it has been said that from the energy consumed by ac motors 10% goes idle and 12% – 15% is lost when motor does not run at full load.[21] So there is a great desire of user to reduce this energy wastage and this can be possible only by making use of device like variable frequency drive because its biggest advantage is its energy saving.[11]

Variable frequency drive is basically comprises of three portions. These are as follow.[22]

  • AC to DC Converter
  • DC Bus
  • Invertor (DC to AC convertor)

When a fixed AC voltages are fed into AC to DC rectifier the AC voltages are converted in DC voltages. Which are further directed towards DC bus which comprises of capacitors and used to store voltages and removes ripples in the DC voltage thus it smooth out the waveform.[23] Inverter is the last section which is the most important one because it performs the DC to AC conversion by approximate the square waveform with that of sine wave form whose pulse is adjusted in order to control the voltages and frequency of motors.[24]

1.1                                               BACKGROUND OF THE STUDY

In electronics, Variable frequency drive is typically a tool of power electronics without having a great understanding of power electronics we can never get the depth of variable frequency drive.

The history of power electronics tells us that its evolution started in the very beginning of 20th century. And the first invention that was brought before the world by Peter Cooper was mercury-arc rectifier.[1] Then with the passage of time power electronics had under gone many changes and in the 3rd and 4th decade of 20th century it was evolved to gas tube electronics [1],[2] and saturated core magnetic the mid of 20th century many advancement were seen in field of power electronics and the work which came forward was silicon controlled rectifier (SCR) and thyristor.[1] After that power electronics has shown countless advancement in research and development field till today. The history of variable frequency drive is not much older as that of power electronics. Rather its evolution was very rapid and, in a very short period of time it has gain a very important place in industry. The first variable frequency drive was based on mechanical principals because power electronics had not made great advancements till that day. It was consist of adjustable pitch diameter pulleys.[3] AC induction motors were firstly designed in 1924 [3]and the speed of motors were dependent upon frequency and poles of motors. After the invention of motors it was thought that motors could be run on variable speed, for that matter the only possible solution was to vary the frequency, In order to run the motor on a variable speed, because frequency has direct relation with motor speed. Variable frequency dive based on pulse width modulation was firstly invented in early 60’s in Finland.[4] But on commercial scale a great achievement was made by Martti Harmoinen at Helsinki Metro in 1972.[4] Initially variable frequency drives were based on 6 step voltage design.[3] But after some time a modified form was introduced. This was designed and presented by Phillips in mid-80’s it consisted of sine coded PWM chip set.[3],[4] now there arises a point why variable frequency drive is used only for ac induction motors. The answer is as follow. Universally for induction motor’s optimum working a slip is required and this slip produces required utilizable torque. Different techniques and methods used to improve the efficiency and working of ac induction motors like soft starters[5], voltage reduction method [6], slip compensation [7], and vector control.[8] But all these methods are not proved feasible for avoiding slip already present in the motor. Because this slip in the motor signifies the amount of energy that is dissipated in the form of heat and may harm many other sensitive component and windings of the motor thus the life time of motor is reduced.[9] Therefore a need of variable frequency drive based on pulse width modulation arises in application which does not require high torque and dynamic response.[10] Price is a very important factor for which this adjustable speed drive is being used because it has comparatively less cost associated with the installation and maintenance of the systems.[11] In last 10 years a great advancements has been made in variable frequency drives that are undoubtedly dependent upon the progress in power electronics field.[1] Now the drives coming in market have several better features unlike the previous ones. In the drives high efficiency capacitors are being used which are used to remove ripples in the voltage coming in to the DC bus this advancement in capacitors is leading the drive towards more energy savings as newly made capacitors offer 80% less power loss as compared to the older technology.[12] Furthermore the new drives comprises of DC choke which are used to prevent from unwanted harmonics.[2],[10] An extra feature added in the ac motor drives is heat management system by using this system heat is monitored properly and autonomous actions are performed accordingly.[13] The advancements in semiconductor devices have improved the efficiency of drives up to a greater extent.[14] Power management control which deals with sleep functionality of the device is a very key feature it save energy and unwanted power loss are prevented.[15] Many new advancements in layout of these drives like speed adjustment, keypad for input, and meters for providing information about maintenance and communication with other devices. These characteristics incorporated in the drive have not only beautified the product but also enhanced its ability and utilization.

1.2                                                     PROBLEM STATEMENT

Motors are being used worldwide on industrial or domestic level. Electricity is the most power full tool in order to run any motor. Among all the motors ac induction motors are used most commonly and extensively due to their large no of applications. But there is a need to eliminate the problem associated with ac induction motor and to run it in a very efficient way. For that matter many devices are used but the best among all the devices is Variable Frequency Drive which is used to control the speed and frequency of the motor and by reducing speed motors can be run at various loads. There is a direct relation between speed of motor and the frequency of motor operation. Therefore by varying the frequency of ac voltage the motor speed can be adjusted according to desirable value.

1.3                                                   AIM OF THE PROJECT

The main aim of this work is write on the device that is used in electro-mechanical drive systems to control AC or DC motor speed and torque by varying motor input frequency and voltage.

1.4                                             OBJECTIVE OF THE PROJECT

The objective of this work is to provides variable speed to an electric motor

1.5                                              PURPOSE OF THE PROJECT

The purpose of the work is to convert the incoming electrical supply of fixed frequency and voltage into a variable frequency and variable voltage that is output to the motor with a corresponding change in the motor speed and torque.

1.6                                                 SCOPE OF THE PROJECT

The only really satisfactory way of obtaining the variable-voltage electric supply needed for speed control of an industrial electric motor was to generate it with a electric generator. The generator was driven at fixed speed by an induction motor, and the field of the generator was varied in order to vary the generated voltage.

Low power control circuits are used to monitor the principal variables of interest (usually motor current and speed), and to generate appropriate firing pulses so that the motor maintains constant speed despite variations in the load.

1.7                                                      BENEFITS OF VFD

As VFD usage in HVAC applications has increased, fans, pumps, air handlers, and chillers can benefit from speed control. Variable frequency drives provide the following advantages:

  • energy savings
  • low motor starting current
  • reduction of thermal and mechanical stresses on motors and belts during starts
  • simple installation
  • high power factor
  • lower KVA

Understanding the basis for these benefits will allow engineers and operators to apply VFDs with confidence and achieve the greatest operational savings.

1.8                                              PROBLEM OF THE PROJECT

Drawback of this method of an electric motor speed control is that they are not able to control the motor speed smoothly at lower levels, and as the desired speed is decreased, the torque of the motor also decreases proportionately. Due to this, at any unpredictable point the motor may just halt very abruptly. Also, during power ON, the motor may just not start up at lower speed settings and may require an initial boost by increasing the setting. Such situations are pretty undesirable and do not constitute an ideal speed control.


2.0                                                    LITERATURE REVIEW


Electric motor systems consume 20% of all energy generated in the United States, 57% of all electrical energy, and 70% of electrical energy consumed by industry. Over 1.1 billion motors, of all types, are presently in use in the United States at this time.

Induction motors were invented by Nikola Tesla in 1888 while he was a college student. In the present day, induction motors consume between 90 to 95 percent of the motor energy used in industry…


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