13 Electric Motors 1.2.3 AC and DC Motors The fi rst electric motor, a DC motor, was built around 1833. Speed control of this type of motor is simple, and met the requirements of many diff erent types of applications at the time. The DC motor is controlled by supplying a DC voltage whose magnitude infl uences the speed of the rotor. Voltage applied to stator and rotor windings results in magnetic fi elds which attract or repel each other, leading to rotor movement. Energy supplied to the rotor is transmitted via brushes, typically made of graphite, to a commutator. The commutator ensures that the next winding is energised to achieve a continuous rotation.
The brushes are subject to mechanical abrasion and require maintenance or periodic replacement. The importance of DC motors has decreased over time and they are rarely used in power ranges above a few hundred watts today. Compared to DC motors, AC motors are much simpler and more robust. However, AC motors typically have a fi xed speed and torque characteristic. Because of these fi xed characteristics, for many years AC motors could not be used for many diverse or special applications. They are nonetheless used in most applications to transform electrical energy into mechanical energy. The functional principle of AC motors is based on the eff ects of a rotating magnetic fi eld.
The rotating fi eld is generated either from a multi-phase fed AC source (typically three-phase) or from a single phase source assisted by capacitors or inductances to achieve phase shift. This book focuses on AC motors, particularly on asynchronous motors, as the requirements for operation with FC’s in adjustable speed drive applications for various motor types can be derived from this motor technology. DC motors will not be addressed further. 1.2.4 Electromagnetic Induction Most electric motors operate through the interaction of magnetic fi elds and current- carrying conductors to generate force.
This is the reverse process of producing electrical energy from mechanical energy, performed by generators such as an alternator or a dynamo on a bicycle. a) Generator principle, induction by motion When a force (F) acts on a conductor and moves it across a magnetic fi eld (B), a voltage is induced. If the conductor is part of a closed circuit, a current (I) fl ows, see Fig. 1.3 Principle for electromagnetic induction.