Electrical drives was first designed in Russia in the year 1838 by B. S. Lakobi, when he tested a DC electric motor supplied from a storage battery and propelled a boat. But industrial adaptation occurred after many years as around 1870. Today almost everywhere the application of electric drives is seen.
In the past induction and synchronous motor drives were mainly used in fixed speed applications. Variable speed applications were dominated by DC motor drives. Emergence of thyristor in 1957 lead to development of variable speed induction motor drive. In late sixties which were efficient and could match the performance of DC drives. Consequently, because of the advantages of squirrel-cage induction motor over DC motors. It was predicted that induction motor drives will replace DC drives in variable speed applications. However, following hurdles forbidden for the prediction to come true:
1. Although squirrel-cage induction motor was cheaper than DC motor, the converter and control circuit of an induction motor drive was very expensive compared to those for a DC drive. Therefore, total cost of an induction motor drive was significantly higher than that of a DC drive.
2. While the technology of DC drives was well established, that of AC was new.
3. AC drives were not as reliable as DC.
4. Development in liner and digital ICs, and VLSIs were helpful in improving the performance and reliability of AC drives. But then this development also led to similar improvements in DC drives.
Improvement in thyristor capabilities, availability of power transistors in early seventies and that of GTOs and IGBTs in late seventies and late eighties respectively; reduction in cost of thyristor, power transistors and GTOs; developments of VLSIs and microprocessors; and improvement in control techniques of converters have resulted into reduction in cost, simple controllers, and improvement in performance and reliability for AC drives. Although even now majority of variable speed applications employ DC drives, the AC drive are preferred over DC drives in a number of application with the results, AC drive application are growing.
Induction motor drives find application with the result, AC drive application and synchronous motor drives are employed in very high power and medium power drives. The permanent magnet synchronous motor and brushless DC motor drives are being considered for replacing for replacing DC servo motors for fractional HP range. As the trend exists, application of AC drives will continue to grow. However, DC drives will also continue to be used for quite some time.
An electric drive (often referred to as an electric controller) is a device used to control the output of a motor used, for example, to produce linear motion in an electric actuator. In our article on electric motors we referred to the feedback mechanism from a servo motor, shown schematically below.
The drive will accurately control the motor output and the motor response against a controlling input (nset in the schematic above).
Electric drives require a three-phase AC supply. The incoming fixed frequency AC supply is rectified to provide a DC signal; this is then smoothed and circuitry provides a decoupling of the input and output stages as shown below.
The output stage uses software controlled Insulated Gate Bi-junction Transistors (IGBT’s) to switch and provide a variable frequency three phase AC supply to the motor. To understand the output stage in more detail, we can refer to the following equivalent circuit.
In the circuit, the IGBT’s are represented by the switches 1 to 6; a centre tap of each column of switches is connected to an individual phase of the motor windings. In the diagram switches 5 and 4 are closed which passes current through the W and V phases. In the next mode of operation, switches 5 and 4 may open and 3 and 2 close; this will pass current through the V and U phases. The software controls the successive opening and closing of the switches and the duration of each step; this creates a pulse width modulation.
The varying pulse width effectively simulates an AC waveform as seen by the load of the motor windings; for example, at A in the diagram above, a pulse width of ‘0’ will result in zero voltage across the windings, where as at B, the maximum voltage will be applied.
The drive must be capable of supplying and controlling the motor which, in turn, will be matched to the requirements of an application.
The motor voltage, power rating and full-load current will all need to be matched; any over-load requirements will need to be considered, for example if higher torque is required for start-up.
The feedback mechanism and any input/output (I/O) requirements should be accommodated.
Communication protocols– for example CANOpen, Profibus, etc, and the operating temperature should be considered and, hence, the need for any ventilation or forced cooling.
Drives will need to control either DC or AC motors, the latter single- or three-phase. Different variants will be required, dependent on the parameters mentioned above – for example, voltage.
Drives can also be classified into single-, group- and multi-motor drives. Single are the most basic and are often used in domestic appliances; group lend themselves to use in more complex systems and multi are used in heavy, or multiple motor applications.
Cabling will be required for power and control signals to the motor, along with suitable supply for operation of the drive itself.
It is good practice to have input fuses for protection; if EMI is a potential problem, then filters can be built into the circuitry. A cooling fan, with suitable ventilation, will be required if high operating temperatures are encountered.
Depending on application requirements a dynamic brake is available.
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