India saw the real boom of AC variable speed drives in the 1990s. The market today is increasing at an average pace of 20-30 per cent. Post-liberalisation, the growth has augmented enormously, much to the advantages of consumers. Consumer awareness has gone up drastically and the end result is that almost every industry - small and large - in the country is keen on having an AC variable speed drive system for varying the speed of an AC motor. AC variable speed drives are principally meant for the control of the speed of an AC motor. The traditional methods of controlling the speed of AC motors are electromechanical starters or sold state motor controllers. Each type is suitable for certain applications and many installations use multiple types of controllers. However, electromechanical drive controllers do have many limitations.
The electronic motor controllers like AC variable speed drives offers numerous features such as soft start, which are essential for protection and improving the process. Once the motor starts, it keeps running at the same speed until it is turned off. Variable speed drives are used when the process requires variable speeds. Adjustable speeds can optimise a process to save energy. Variable speed drives are common in heating, ventilation, and air conditioning applications. In these applications, variable speed operation makes it simple to slow down the fan when air demand is lower to reduce electricity use in a facility. Another application is pump control, where variable speed control can help protect equipment during high or low volume of operation.
DC drives have many disadvantages by way of limitation of features and advantages. Basically DC drives are silicon-controlled rectifier (SCR) based whereas AC variable speed drives are based on the concept of pulse width modulation (PWM). DC drives can deliver extremely precise and independent speed and torque control whereas AC drives have limitations in certain areas. PWM based drives use the high end, third generation, insulated gate bipolar transistor (IGBT) to control the switching required for pulse width modulation. PWM drives typically can reduce most harmonics to the motor by controlling the occurrence and duration of each pulse.
Vector control drives
The advanced vector controlled drives use field oriented control to make the AC drives. They provide accurate speed and torque control to AC motors. These are typically used for application such as winders and rollers application where precision torque control is critical. Vector control methods let users use efficient, low maintenance AC motors in applications traditionally dominated by DC motors. Vector controlled drives produce flux current and torque independently to control motor torque and speed. Indirect and direct feedback allows these drives to have varying levels of speed and torque accuracy. Faster current control loops combined with precise control simplify installations and improve performance by sampling motor parameter information.
IGBT: an overview
The advances in AC drive technology have gone hand in hand with improvements in the size and performance of power switching devices known as insulated gate bipolar transistor (IGBT). One can find that there are immense developments taken place in this area because until the 90s we had only the thyristor based AC drives. Consequent improvements led to transistor based to MOSFET and BJT based and finally IGBT based AC drives. IGBT is ideal for low level signal to power control, AC drives use IGBTs to provide fast, accurate operation, electronic signals to the motor, and quiet operation. Although some electrical loss had been associated with the use of IGBT switching devices, drives manufacturers have taken full advantages of this technology by upgrading drive performance. The AC drive was a demanding application for earlier IGBTs. However, third generation IGBTs reduce losses and efficiently handle higher voltages. These devices have faster switching speeds, and lower conduction drops. Losses of the new devices are 39 W at 15 kHz, which are 40 per cent lower than earlier types of IGBTs. In addition, the newer IGBTs reduce voltage overshoot by employing a soft recovery free wheeling diode in the design. Improvement in both AC drives and AC motors has led to higher performance power technology. Drives with the latest IGBTs offer increased reliability and better performance at a lower cost. For 460 V or higher voltages, use of AC motors with improved insulation are required when long cables length exist between the drive and the motor.
Drive communication and software systems
Todays most complicated application requires communication and software features in AC variable speed drives. Most drives communicate through software, but often it has been observed that such systems are less reliable. Often, it is difficult to integrate such systems with total customers requirements. There are many factors to be considered when looking at communication software systems related to AC variable speed drives. Some of them are :
- RS 485/232 support
- Communication via Fieldbus, Profibus or DeviceNet
- Compatibility with sophisticated communication software like Ecofast
- Provision for links to SCADA systems
- Easy accessibility and simplicity in installing.
Latest communication system
The Ecofast system for decentralised installation offers all the necessary components with a standard system of connectors, standard DESINA interfaces, a high enclosure value of IP 65/67, uniform connection technology and standardised fieldbus communication/diagnostics. The objective is to arrive at a uniform standard for all automation and drive tasks in machine and plant construction - and all without a central switch cabinet. The correct components (field distributors, drives, plug connectors and cables) are selected, the system architecture for the (auxiliary) power supply and communication were defined with the Ecofast ES software that has been specially developed for this purpose.
There are few advantages which needs to be highlighted for users :
- Small amount of project planning
- Straightforward component selection from the suggestion catalogues
- High project planning security
- Tailor made solutions
- Documentation on project planning
A major aspect of drive selection is a thorough evaluation of facility and application parameters. Getting the most out of the variable speed drives requires looking not only at the drive part, but also at the motor it will control, the operating environment and the way these factors will fit together. Following are tips to consider:
Is the ampere rating on the drive comparable to the ampere and kW rating of the motor?
Matching drive and motor kW rating is not always sufficient. It is always advisable to know, before recommending a drive, the starting current, peak current, and normal current of the motor. This will help to make the correct drive selection rather than matching the amperes of the motor and drive system.
Does the application demand a constant or variable torque loads?
Matching the drive to the load characteristics of an application is a primary consideration in selecting adjustable speed drives. Constant torque loads are very common. The drive can provide constant torque operation because motor output horsepower is directly proportional to speed and rated torque is available at any speed. Using power rated motors with adjustable speed drives ensures the correct match between the drive, the motor and the application. Constant torque loads are essentially friction loads. Constant torque is required to overcome friction. Typical constant torque applications include conveyors, extruders, and surface winders and other applications involving shock, overloads, and high inertia loads.
Variable torque is often applied to help save energy. Common variable torque application includes centrifugal pumps, fans, and blowers. By applying adjustable speed drives, the air or liquid flow can be control directly by controlling the speed of the motor. Since losses due to the flow of air or liquid decreases as the flow decreases, power losses, associated with restricted control methods, can be eliminated by direct control of the motor speed. Generally, these types of applications do not require extra power for momentary peak loads, therefore the overload capacity of variable torque drives is suitable for most applications. Mostly AC variable speed drives does not need an air-conditioned place because most of the drive manufacturers have taken precautions on environmental aspects while designing itself.