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Although an increasing interest and use of Permanent Magnet Synchronous Motors (PMSM) can be observed during the last decade, the standard 3-phase Induction Motor (IM) is still the most widely used electrical motor. The simplest way to start an IM is to connect the motor directly to the 3-phase main supply. Star-Delta Start and Soft-Starter were introduced in the past to overcome the extremely high starting current of a Direct-on-Line Start. All mentioned methods have in common that the frequency of the drive is fixed. Frequency converters allow adjusting the speed and current. For this purpose different control strategies adequate for the desired dynamic responds have to be considered. The simplicity of scalar control (V/f control) ensures ruggedness but on the other hand it satisfies only moderate dynamic requirements. Higher dynamic response can be achieved by vector control (Field Oriented Control). To enable these drives, semiconductor suppliers have traditionally relied on discrete IGBT solutions. Intelligent Power Modules are now replacing these discrete solutions. These new solutions enable designers to develop cost-effective solutions.
Induction motors as a commodity
IMs are the world's most common electrical motor in the power range from 100W to several 100kW. They are characterized by a rugged and a simple construction, a long lifetime, relatively low cost and fair-to-high efficiency. General purpose IMs are constitute a majority of the market whose main characteristics are standardized and can be grouped in different efficiency classes. The standardization of mounting arrangement, mechanical size, cooling and protection against contact and penetration of solid objects and protection against water allows motors to be easily exchanged around the world by motors made by different manufacturers meeting same requirements.
Rating Plate of Induction Motor
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The figure above shows a typical rating plate of an IM. This foot-mounted 3-phase IM can be mounted either on floor, wall or ceiling (IM B3), is design for uninterrupted operation at constant load (S1), is protected against dust and water jets (IP 55) and is dimensioned for a maximum temperature rise of 75K (I.CL. F).
Beside the operation of an IM fed with a frequency converter, the IM has captured the market of servo applications where such servo drives are an alternative solution if highest dynamic performance is not mandatory. Servo motors based on IM offer a very high dynamic response. Its compact design and low inertia results in an excellent density of performance. For this purpose a servo converter is mandatory.
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