"Power Electronics & Drives: Electric Drives
In many applications, electric motors supply power to a load, hence, require a variable voltage or variable frequency control. The same can be achieved through power electronic converters. Figure below shows block diagram of an electric drive system.
Electric drives are typically of two types: AC Drives & DC Drives. These two mainly differ by the motor type (DC or AC) which supply power. Figure below shows block diagram of a modern electric drive system using power electronic converter.
Fig. Block diagram of a modern electric drive system using power electronic converter
DC Drives consists of a DC Motor, Power Electronic Converter. i.e. Rectifier (or) chopper. Speed Sensing Mechanism-Tachometer, Feedback circuit and intelligent device (Micro controller).
The following dc motors are suitable for speed control applications.
- Series motor
- Separately excited dc motor
In the speed control application, load remains same i.e., output current is continuous and assumed to be constant.
Single phase half wave converter drive:
The figure shown below shows single phase half wave converter supplying power to a DC motor. It is assumed that armature draws a constant current, Ia.
Fig. Single phase half wave converter drive
Fig. Waveforms for Single phase half wave converter drive
Larger DC motors are fed through three-phase converters for their speed control. For speeds below base speed, three-phase controlled converter feeds power to armature circuit and for speeds above base speed, the same is connected to field circuit. The output ripple frequency of three-phase converter is higher than that of single-phase converter. Hence for reducing armature current ripple, inductance required in three-phase dc drive is lower than of single-phase dc drive. Also, the motor performance in three-phase dc drive is superior to that of single-phase dc drive.
The three-phase dc drives may be subdivided as,
- Three phase half wave converter drive
- Three phase semiconductor drive
- Three phase full converter drive
- Three phase dual converter drive
The operation of three phase DC drives is similar to single phase DC drives except that output voltage equation should be used accordingly.
Three Phase half wave Converter Drive:
When AC voltage of frequency control of input voltage of an AC motor is required. The same can be achieved through AC voltage regulators or cycloconverter.
- AC drives have many advantages over DC drives like lighter weight for the same rating and low maintenance.
- Whereas the disadvantages are, ac drives are more complex and generate harmonics in supply system.
- Also, AC drives can be classified as induction motor and synchronous motor drives.
- Induction motors are the one generally used in AC drives.
Various methods of Speed Control of Induction Motors:
- Pole changing method
- Cascade method of speed control
- Stator voltage control
- Stator frequency control
- Stator v/f control
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