Dc motor in order to limit the starting current, often in series with the armature circuit designed variable resistance, in the process of starting with the speed of rising step by step in a timely manner to each sub section resistance, make the starting current limit within a certain value. The starting method, called string resistance starting, is very simple, portable equipment, widely used in various kinds of small and medium-sized dc motor. But, because of the large energy consumption in the process of starting often is not suitable for starting motor and medium and large dc motor. But for some special needs, such as city tram is often starting, in order to simplify equipment, reduce weight, and convenient operation and maintenance, usually with series resistance starting method. Due to the small motor armature loop resistor and inductor, and turn the body has a certain mechanical inertia, so when the motor after turning on the power supply, the start of the starter armature speed and corresponding counter electromotive force is small, large starting current. Up to 15 ~ 20 times the rated current. Makes the current grid disturbance, mechanical shock unit and commutator spark. Direct starting is only applicable to power switching is not greater than 4 kw motor (Starting current is rated current of 6 ~ 8 times)。 For large capacity of dc motor, usually USES the reduced voltage starting. Or by a separate adjustable voltage power supply, dc power supply to the motor armature supply voltage can smooth the motor starting, and can realize speed regulation. This method power equipment is more complex. Dc motor performance is closely related to its excitation mode, dc motor excitation method usually has four kinds: dc separately excited motor, dc shunt motor, dc series motor and dc compound motor. To master the respective features of four ways: 1. Dc series motor: current series, partial pressure, exciting winding and armature in series, so this kind of motor with the change of the armature current in the magnetic field change dramatically. In order to make the field winding does not cause big loss and voltage drop, exciting winding resistance as small as possible, so direct current series motor is usually use coarser wire coils, his less number of turns. 2. Dc shunt motor, circuit in parallel, shunt, both ends of the shunt winding voltage is the voltage on both ends of the armature, but exciting winding with fine wire coils, the number of turns a lot, so has larger resistance, made by his exciting current is small. 3. Compound motor: dc motor flux produced by the two winding excitation current. 4. Dc separately excited motor: exciting winding and armature contact without electricity, excitation circuit is made up of other dc power supply. So the field current is not affected by the armature terminal voltage or armature current.
