DC motors in various designs
Aside from AC motors, geared motors and frequency converters, we have DC motors in our product range. We will guarantee you will get a very attractive price/performance ratio for higher quantities. The following DC motors are available in our store: permanent magnet DC motors, shunt-wound DC motors, and series wound DC motors.
Permanent magnet DC motor
On many small motors (servo drives, blowers and cooler fans), the stator field is generated by permanent magnets. These magnets have become more and more powerful with the development of DC motors, and today they allow construction of motors with powers that come up to the powers of motors with field windings. However, for many big motors, the costs of permanent magnets are higher than those of field coils. DC motors with permanent magnets, just as electrical machines with external excitation, have very high starting currents. Their operating behaviour is explained in the mathematical fundamentals. Electrical machines with permanent magnets provide the advantage that no energy is required to generate the magnetic field. This improves motor efficiency, in particular with small total power. The disadvantage is that it is not possible to weaken the magnetic field, and hence the possible speed range is smaller.
Shunt wound DC motor
In a shunt-wound machine, the field magnets are excited by a winding connected in parallel with the armature winding. The machine cannot normally be operated with alternating voltage because the current that flows through the field coils and the current that flows through the armature have phases shifted from each other since the field coil produces reactive power due to large inductive reactance. The rotation speed of a big shunt-wound motor remains substantially constant when the load is varied. When the field circuit of a shunt-wound motor is opened, the motor can drive the armature many times as fast as the load demands because, when the magnetic field of the field winding collapses, the current draw and the corresponding motor armature speed will increase dramatically at constant supply voltage. A shunt-wound motor can operate as a generator (e.g. for braking), if an auxiliary voltage source or residual magnetization provide for excitation of the field magnets when the braking process is started. The generated voltage will increase with increasing excitation of the field magnets – it is equal to the electromotive force that opposes the electromagnetic force of the power supply and hence keeps the rotation speed constant when the shunt-wound machine operates as a motor. This back-electromotive force induced in the field circuit is described as left-inductance. The connections of the armature are designated with A1 and A2; those of the field winding, with E1 and E2.
Series-wound DC motor
A series wound motor has its field winding connected in series with the armature winding. Here, in contrast to a shunt-wound machine, the field winding needs to have a small resistance. When supplied with alternating voltage, the direction of the magnetic field of the field winding as well as that of the armature current are changed after each semi-amplitude, and hence the motor can also be operated with alternating voltage. The iron core of the stator (i.e. the field magnet) must consist of a metal sheet package in order to avoid eddy currents. DC series-wound motors can, for example, be used as starters of engines. The rotation speed of a DC series-wound motor depends strongly on the load. When load torque decreases, a smaller current flows through the field coils and the armature, which weakens the magnetic field and in response increases the armature speed. Theoretically, under no load, the motor can drive the armature many times as fast as a load would rotate, and this speed might wreck the armature because of centrifugal force. Therefore, a DC serious-wound motor must be connected to a base load (motor fan, gearbox, etc.) when it is operating. In particular when operated with alternating voltage, serious-wound motors (“universal” motors) have much smaller starting currents than shunt-wound DC motors or permanent magnet DC motors, giving high torque at low speeds (high starting torque). For this reason, they are used in starters, tramways and electrical locomotives where they can be subjected to extreme overload in short-time duty.
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