Mar 23, 2021
Speed and frequency control of AC induction motor by frequency converter
In a country like Pakistan, energy saving is very important. In order to overcome the energy crisis, the only solution that can be achieved at the lowest cost is to save energy. It is said that 25% of the world's electricity is consumed by AC motors. These motors have a big problem, that is, the start-up surge current is very large. When the motor starts, the motor absorbs a large current until it reaches the synchronous speed. This high current not only generates heat, but also shortens the service life of electrical equipment and increases power consumption. Therefore, the current needs to be reduced in one way or another. This current can be reduced by using a variable frequency drive. Variable frequency drive is a technology used to control the speed and frequency of an AC induction motor, so it is also called an adjustable speed drive or a variable speed drive. In this phenomenon, the voltage and frequency of the motor are controlled using a technique called PWM (pulse technique). Width modulation). There are many other techniques for reducing motor current, such as soft starters. But the benefit of variable frequency drives is not just a soft starter. Just like it provides energy for electrical appliances according to demand, it increases the service life of the equipment, but the biggest advantage is that it is an energy-saving device, which is very needed in countries like Pakistan where the energy crisis has stopped economic development.
The variable frequency drive is usually a tool for power electronic equipment. We can never get the depth of the variable frequency drive without knowing the power electronic equipment. The history of power electronics tells us that its evolution began in the early 20th century. The first invention Peter Cooper put forward before the world was the mercury arc rectifier. Over time, power electronic products have undergone many changes. In the third and fourth decades of the 20th century, it gradually evolved into gas tube electronics and saturation core amplifiers. In the middle of the 20th century, many advances were made in the field of power electronics, and the jobs that emerged were silicon controlled rectifiers (SCR) and thyristors. Since then, power electronics technology has made countless progress in the field of research and development until today. The history of variable frequency drives is not as old as power electronics technology. On the contrary, its development is very rapid, and in a short period of time it has gained a very important position in the industry. The first frequency converter was based on mechanical principles, because power electronics technology did not make great progress until that day. It consists of pulleys with adjustable diameter. AC induction motors were first designed in 1924 , and the speed of the motor depends on the frequency and poles of the motor. After the invention of the electric motor, it was thought that the electric motor could run at a variable speed because the only possible solution was to change the frequency in order to run the motor at a variable speed, because the frequency is directly related to the motor speed. Frequency conversion diving based on pulse width modulation was first invented in Finland in the early 1960s. But on a commercial scale, Martti Harmoinen made great achievements in the Helsinki Metro in 1972. The original variable frequency drive was based on a 6-step voltage design. But after some time, the revised form was introduced. This was designed and displayed by Phillips in the mid-80s, and it consists of a sinusoidal-encoded PWM chipset. Now there is a problem that variable frequency drives are only used for AC induction motors. The answer is as follows. Generally, for optimal operation of an induction motor, slip is required, and this slip produces the required available torque. Different technologies and methods used to improve the efficiency and operation of AC induction motors, such as soft starters, voltage reduction methods, slip compensation and vector control. However, none of these methods have been proven to avoid the slip that already exists in the motor. Because this slippage in the motor represents energy dissipated in the form of heat and may damage many other sensitive parts and windings of the motor, it reduces the life of the motor. Therefore, a variable frequency drive based on pulse width modulation is required in the application, which does not require high torque and dynamic response. Price is a very important factor in the use of this adjustable speed drive, because it has relatively low costs associated with the installation and maintenance of the system. In the past 10 years, variable frequency drives have made great progress, which undoubtedly depends on the progress in the field of power electronics. The drives on the market now have several better features than before. In the drive, high-efficiency capacitors are being used to eliminate ripples in the voltage entering the DC bus. The advancement of this capacitor makes the drive more energy-efficient, because the power loss of the newly manufactured capacitors is higher than that of the old technology. In addition, the new drive includes a DC reactor to prevent unnecessary harmonics. The additional function added in the AC motor drive is a thermal management system, by using this system, the heat can be monitored correctly and automatic operation can be performed accordingly. The advancement of semiconductor devices has increased the efficiency of the driver to a greater extent. The power management control that handles the sleep function of the device is a very critical function, which can save energy and prevent unnecessary power loss. Many new developments in the layout of these drives, such as speed adjustment, input keyboards and meters, are used to provide information about maintenance and communication with other equipment. These features in the drive not only beautify the product, but also increase its capacity and utilization.
Automobiles are being used in industry or households all over the world. Electricity is the most powerful full-featured tool for running any electric motor. Among all motors, AC induction motors are most commonly used and widely used due to their large number of applications. But there is a need to eliminate the problems associated with AC induction motors and operate it in a very efficient manner. In this regard, many devices are used, but the best of all devices is a variable frequency drive, which is used to control the speed and frequency of the motor, and by reducing the speed, the motor can run under various loads. There is a direct relationship between motor speed and motor operating frequency. Therefore, by changing the frequency of the AC voltage, the motor speed can be adjusted according to the desired value.
N = f * 120 / P(1)
N = motor speed (RPM)
F = motor electrical frequency
P = number of motor poles
These frequency converters are very important for HVAC systems, because the HVAC system consumes a very large amount of power before the motor reaches full speed, and the motor absorbs a very large amount of inrush current, resulting in this huge energy loss. By using a variable frequency drive, the starting current can be reduced, thereby saving energy to a large extent. Inverters have various applications in different equipment such as fans, pumps, tower cooling systems, microwave ovens, air conditioners and ship propulsion systems. It is said that from the energy consumption of AC motors, 10% of the motors will idling when running at full load, and 12%-15% will be lost. Therefore, users very much hope to reduce this kind of energy waste, which can only be achieved by using equipment such as variable frequency drives, because its biggest advantage is energy saving.
The variable frequency drive basically consists of three parts. These are as follows.
AC to DC converter
Inverter (DC to AC converter)
When a fixed AC voltage is fed into the AC-to-DC rectifier, the AC voltage is converted into a DC voltage. This is further directed to the DC bus, which includes capacitors and is used to store voltage and eliminate ripples in the DC voltage, thereby smoothing the waveform. The inverter is the most important part, because it performs DC to AC conversion through an approximate square wave waveform, and the pulses of the sinusoidal waveform are adjusted to control the voltage and frequency of the motor. A very important tool for variable frequency drive is PWM, which is a key technology to control the speed of a motor. There are many other techniques, but in this article, only the PWM method is discussed.
The variable frequency drive includes three main parts as described above, each of which has its key importance. The first part is the rectification part, then the DC bus, and finally we have an inverter part where the load is connected.
This section can include diodes, transistors or silicon controlled rectifiers. But diodes are usually used because of their lower cost. The AC voltage from the main line has a positive peak value and a negative peak value. When these voltages are fed into the bridge configuration of the diodes of the rectifier section, the negative peaks disappear and only the positive peaks remain. In this way, the frequency of the upcoming voltage is doubled. The rectifier part is also called an AC to DC converter. In addition, this pulsating DC current passes through a capacitor to eliminate ripples in the waveform. These ripples can cause distortion and hinder the smooth operation of the appliance, so it must be removed with a filter.
The DC bus is used to store the voltage from the AC to DC converter. This includes capacitors and some other items such as inductors or chokes to smooth the power from the previous part. Therefore, the ripple is further removed by storing voltage in the DC bus. Therefore, this DC bus can not only be used to eliminate ripples, but also help to improve power factor correction.
PWM based on microcontroller
Pulse width modulation is a basic technique that is very widely used to control the speed and frequency of a motor. This can be done by using a microcontroller. In this study, we selected a frequency range of 5 Hz to 50 Hz using PWM. The basic principle of PWM is to generate a sine wave in the microcontroller, which is imposed on the triangle wave. This results in a square wave, which is then fed to the inverter section. The width of the square wave can be controlled by changing the duty cycle of the pulse. Basically, the duty cycle describes the time for the pulse waveform to turn on and off, so by switching the waveform between two discrete levels, the square wave is approximated by a sine wave with the required duty cycle.
The above-mentioned technology of PWM based on software and hardware first uses a microcontroller to program and simulate on Proteus.
In addition, this is implemented on the controller hardware, and the results are observed and matched with the software results on the oscilloscope.
The inverter will then feed the square wave generated using PWM into the inverter section. This section consists of insulated gate bipolar junction transistors (IGBTs), which are power transistors. These IGBTs have very fast switching speeds and their voltage and current ratings are also high, so they are best used in frequency converters. In the inverter section, these IGBTs are connected in an H-bridge configuration. These IGBTs are fed by two PWMs, one is normal and the other is supplementary. This is because first we need to excite the No. 1 and No. 4 IGBTs, which will generate a waveform rising in a clock manner. Although the complement of PWM is used to generate the rise of the waveform in an anti-clock manner, a complete cycle of the waveform will be realized. But one thing to note is that it is necessary to introduce a small dead time.
The reason is simple, but the important thing is that the time to turn off the power device is much longer than the time to turn on the device. For this reason, this dead time is inserted into the switching action of the complementary channel. The power transistors turn on when a square wave is fed into them. Since the pulse width of the square wave varies from the microcontroller at very high switching speeds, when applied under load, the square wave pulse is approximated by a sine wave. The inductance of the load also helps shape this waveform into a sine wave. Therefore, the use of inverters and PWM achieves the desired goal.
Advantages This drive has many advantages.
Some of them are listed below.
Reduce inrush current
Improve power factor
Low fever and
A very common problem in motors is that when they are not running at full load, their power factor is very low, which can cause serious damage. Therefore, this power factor must be improved. For this purpose, capacitors are being used. But another advantage of using a VFD is that the capacitor used in the DC bus performs the same action. Therefore, power factor correction can be performed without adding additional equipment. These attributes of variable frequency drives entice the industry to choose it and enjoy additional savings in energy and losses.
VFD as an energy-saving device As mentioned earlier, the biggest advantage of VFD is energy-saving. This not only attracts consumers, but is also very helpful in countries where we need to reduce energy consumption. To this end, we describe a simple example of a pump and water flow to illustrate the energy-saving work. The equation mentioned below shows the relationship between power and pump speed.
Flow2 / Flow1 = Speed2 / Speed1 (2)
Head2 / Head1 = (Speed2) 2 / (Speed1) 2 (3)
Power2 / Power1 = (Speed2) 3 / (Speed1) 3 (4)
Here Tables 1 and 2 below show the different operating points. The above formula tells us that by using a frequency converter, about 85% of energy can be saved at different working points of the pump.
VFD VS other technologies use various methods to overcome the current inrush problem in induction motors. Use soft starter, autotransformer why-delta starter and VFD. The reason for choosing a VFD is that the VFD reduces the current surge to full load current. Soft starters and autotransformers are not so energy efficient. Both methods operate at a fixed frequency (50 Hz in Pakistan), which is different from a VFD that operates a motor at a variable frequency. The following table is based on experimental results.
The above table is based on experimental results. Direct start means to start the motor directly on the line voltage without any current reduction device. IFL refers to the full load current of the motor, which is unique for each motor.
From the above discussion and findings, we infer some useful results. This drive VFD is very beneficial to industry and household appliances, because it can improve the efficiency of electrical equipment and save a lot of energy, and we know that they exist between energy and current. Is directly related to
P = I2 * R (5)
P = power (watts)
I = load current (amperes)
R = load resistance (ohm)
So by reducing the current power will also be reduced.
By keeping in mind the current form of VFD, it is almost impossible to change anything in the future. But with the rapid development of technology and new research proposed by people, everything can be predicted. The emerging technology of FPGA, the advancement of DSP, genetic algorithm, fuzzy logic, innovative PWM technology and the revolution in the power electronics field can enable us to experience any magical equipment.
In addition, VFD can also be miniaturized using NANO technology. This kind of micro VFD is very useful in small devices, so it can also save energy on a smaller level.
By designing this device, we can now bring many improvements to this drive. Just like we can introduce a numeric keyboard, through which the required frequency value can be given as input. This is how the frequency of our own choice drives the load. In addition, we can use the GSM module to remotely monitor the speed and frequency of the drive. And can control the start and stop operation of the drive from a distance.
From the above discussion and results, we have seen that the variable frequency drive is the best solution to solve the inherent motor problem, and the drive can best solve the energy saving problem. Other technologies like soft starters are not as effective as variable frequency drives, because variable frequency drives have many advantages, for example, it can control the start and stop of the motor, and it also provides the versatility of motor action. Overload protection, power reduction when not needed and dynamic torque control are other key features of the inverter. We can suggest that this kind of equipment is very useful in a country like Pakistan, because It is very necessary for us to minimize energy consumption, so VFD should be used in HVAC systems for industrial and household appliances. The only disadvantage associated with this device is cost. This is very expensive and requires maintenance. However, the importance of variable frequency drives cannot be denied.