Basics of ac drives pdf

Process control: controlling the process output to match the need; synchronising different parts of the main process to secure smooth flow between subprocesses; easily changing the setup when the process requirements change.

In system stress: Reducing the start-up current, which allows the use of smaller fuses and supply connections and reduces peak loads on the electrical network; reducing the mechanical shock in start and stop situations.

Energy: Saving electrical energy compared to traditional methods of process control. For instance in pump and fan applications, energy savings are typically per cent. In heating, ventilation and air conditioning HVAC applications the main processes are related to heating, cooling, drying and circulating air. Supporting processes are mostly related to taking the extra heat out of the building or providing additional heat energy to the building.

The ac drive controls the speed of the pump and fan by changing the electrical energy supplied rather than damping the air- or water flow.

It is like reducing the speed of a car by pressing less on the accelerator instead of using the brake to slow down the speed. The payback time of an ac drive is typically one year or less. Motor Speed Control The speed is controlled by the ac drive converting the frequency of the network up to anything between Hz or even higher. The technology behind ac motor speed control consist of: Rectifier Unit : The ac drive is supplied by the electrical network via a rectifier.

The rectifier unit can be unidirectional or bidirectional. When unidirectional, the ac drive can accelerate and run the motor by taking energy from the network. If bidirectional, the ac drive can also take the mechanical rotation energy from the motor and process and feed it back to the electrical network.

DC Circuit: The circuit will store the electrical energy from the rectifier for the inverter to use. In most cases, the energy is stored in high-power capacitors. Inverter Uni: The ac motor drive inverter unit takes the electrical energy from the dc circuit and supplies it to the motor. 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.

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. 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.

The components of a modern electric drive system are illustrated in below figure. In the above block diagram of an electric drive system, electric motor, power processor power electronic converter , controller, sensors e. The electric motor is the core component of an electrical drive that converts electrical energy directed by power processor into mechanical energy that drives the load.

The motor can be DC motor or AC motor depends on the type of load. 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 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. 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.

In this article we are going to discuss about the AC drives. 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. 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 is shown below. The various sections of the variable frequency drive VFD include. 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. 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 correction , temperature control and PC connectivity for real time monitoring. 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. The above figure shows the torque and speed variation of an induction motor for voltage and frequency control.

In the figure, voltage and frequency are changed at a constant ratio up to the base speed. Thus the flux and thereby torque remain almost constant up to the base speed. This region is called as a constant torque region. Since the supply voltage can be changed up to the rated value only and hence the speed at rated voltage is the base speed. If the frequency increased, beyond the base speed, the magnetic flux in the motor decreases and thereby torque begins falling off.

This is called flux weakening or constant power region. Suppose the induction motor is connected to a V, 60Hz supply, then the ratio will be 7. As long as this ratio maintained in proportion, the motor will develop a rated torque and variable speed. There are different speed control techniques implemented for variable frequency drives.

The major classification of control techniques used in modern VFDs is given below. The motor is fed with variable voltage and frequency signals generated by the PWM control from an inverter.

The inverter can be controlled with a microcontroller, microprocessor, or any other digital controller depending upon the type of manufacturer. This control scheme is widely used because it requires a little knowledge of the motor to perform the speed control.

The scalar control can be implemented in a number of ways and some of the popular schemes include. In this method, the frequency of the switch is varied depending on the sped reference input and the average or RMS value of the voltage for that frequency is determined by number of pulses and width of the pulses.

If the width of the pulse is varied, the voltage across the motor is also varied. This voltage creates the sinusoidal current through motor which is much closer to true sine wave. Only little calculations are needed to achieve this method.

In this method, sinusoidal weighted values are stored in the microcontroller or microprocessor and are made to available at the output port at user defined intervals which are then applied to the inverter in order to produce a variable supply to the motor. In this method, the inverter of the VFD has six distinct switching states and they are switched in a specific order so as to produce the variable voltage and frequency to the motor.

The direction reversal of the motor is readily accomplished by changing the inverter output phase sequence by means of the firing angle. This method can easily be implemented as there is no intermediate calculations are required and also the magnitude of fundamental voltage is more than the DC bus. However the low order harmonics are high in this method which cannot be filtered by the motor inductance and hence it results more losses, motor jerky operation and high torque ripple.

In this technique, three phase voltage vectors of an induction motor are converted into a single rotating vector. The inverter of the VFD can be driven to eight unique states. The PWM voltage to the load is accomplished by properly selecting the switch states of the inverter and by calculating appropriate time period for each state. By using space vector transformation, three phase sine waves are generated for each state, which are then applied to the motor.

The main advantage of this technique is that the harmonic magnitude is less at the PWM switching frequency.