Starting and performance of DC motor
In order to limit the starting current, often in the armature circuit on specially designed variable resistor, in starting process with the speed rising step by step in a timely manner the piecewise resistance short circuit, the starting current limit in a permissible value within. This starting method known as the series resistance starting, very simple and portable equipment, widely used in a variety of small and medium sized DC motor. However, due to the large energy consumption during the starting process, the motor and the large DC motor are not suitable for the frequent starting..But for some special needs, such as urban tram although frequent starting, in order to simplify the equipment, reduce the operating weight and convenient maintenance, usually by series resistance starting method.
Because of the motor's armature circuit resistance and inductance are small, and the rotating body with certain mechanical inertia, so when the motor is switched on after, starting began armature phase speed and the corresponding counter electromotive force is small, the starting current is large. Maximum up to 15 ~ 20 times the current rating. This will make the grid current, the unit has been disturbed by mechanical shock, sparking on commutator. Therefore the direct starting only applies to power less than 4 kW motor (starting current rated current is 6 ~ 8 times).
For large capacity DC motors, starting with a voltage drop usually. Namely the individual can pressure regulating DC power supply to power the motor armature, control power supply voltage can make motor start smoothly, and can realize the speed control. This method power equipment is more complex.
Is closely related to the performance of DC motor and its excitation, usually DC motor excitation mode has four: DC he excited motor, DC and excited motor, DC on excitation motor and DC compound excited motor. Grasp the characteristics of the 4 ways:
1. DC series excited motor, series current and partial pressure, the excitation winding is and the armature series, so the motor magnetic field with the change of the armature current have significant changes. In order to make the excitation winding in causing big losses and voltage drop, excitation winding resistance of the smaller the better, so DC on excitation motor usually thicker wire is wound into a, his turn number less.
2. DC shunt motor: parallel circuit, diversion, and excitation winding voltage across is the armature voltage across, but excitation winding with thin wire wound, it has a large number of turns, so it has larger resistance, making through his excitation current is small.
3 DC motor: the magnetic flux of the motor is generated by the excitation current of the two windings..
4 DC he shunt motor: the excitation winding and the armature is not connected, the excitation circuit is supplied by the other DC power supply.. Therefore, the excitation current is not affected by armature current or armature current..
