Direct Current (DC) Motors in Various Designs
Besides AC motors, gear motors, and frequency inverters, we also offer DC motors. For higher quantities, we can offer you an extremely attractive price/performance ratio. The following DC motors can be purchased from us: permanent field DC motors, shunt-wound DC motors, and series-wound DC motors.
Permanent Field DC Motor
For smaller motors (actuators, blowers, and cooler fans in motor vehicles), the stator magnetic field is often generated by permanent magnets. With the development of DC motors, these magnets have become increasingly powerful. For this reason, newly built DC motors perform just as well as those with electric excitation. However, the costs of permanent magnets are often higher for larger motors than for an excitation winding. DC motors with permanent magnets and electrical machines with external excitation both have very high starting currents. Their operating behaviour can be explained by mathematical fundamentals. Electrical machines with permanent magnets are advantageous in the sense that no energy is required to generate the magnetic field. This improves efficiency, especially at low total power. The clear disadvantage is that field weakening is impossible which results in a decreased available speed range.
Shunt Wound DC Motor
The exciter and armature windings of the shunt machine are connected in parallel. An alternating voltage operation is hardly possible as the excitation and armature current have a different phase position. The reason for this is the highly reactive component of the current through the excitation winding (high inductance). The speed of large shunt motors is almost load-independent. Shunt motors can run through disruptions of the excitation circuit because if the exciter field collapses, the speed and current consumption increase drastically with the same supply voltage. Shunt motors can operate as a generator (e.g., for braking) if an auxiliary voltage source or a residual magnetisation ensures that excitation occurs when the braking process is started. With increasing excitation or speed, the generated voltage increases – the voltage also counteracts the feeding current even when the motor is running and ensures a constant speed. It is therefore referred to as a counter electromotive force. The connections of the armature are referred to as A1 and A2, and those of the excitation winding are referred to as E1 and E2.
Series-Wound DC Motor
The field winding of a series-wound motor is connected in series with the armature winding. In contrast to the shunt motor, the field winding must therefore have low resistance. When supplied with AC voltage, both the magnetic field of the field winding and the armature current change direction after each half-wave, so that the motor can also be operated with AC voltage. For this, however, the iron core of the stator must be laminated to avoid eddy currents. The starters of combustion engines are also series-wound motors. The speed of these series-wound-motors strongly depends on the load (series-wound-behaviour). If the output torque decreases, the speed of the armature increases simultaneously due to the lower current and the associated field weakening. This can result in the motor destroying itself due to the centrifugal forces that occur. For this reason, series motors must be operated with a baseload (motor fan, gearbox, etc.). Compared to shunt or permanently excited motors, series motors have a significantly lower inrush current, especially in AC operation (“universal motor”, e.g., hoovers). They do however provide a high starting torque for a short time. Due to this, they are used for starters, trams, and electric locomotives, where they can be significantly overloaded in short operation.
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