When the DC motor is started directly, the impulse current can reach 10-20 times of the rated current. However, since the armature resistance Ra is small, the starting current at the time of direct starting becomes large. Therefore, DC power supply requirements are high, and the motor speed is not controlled, which is likely to cause mechanical damage to the aircraft in flight. Therefore, the general requirement: DC motors with a power of less than 2 kilowatts can be started with a rheostat. A DC motor with a power greater than 2 kilowatts needs to use a multi-stage rheostat or voltage regulator to start gradually after the voltage is reduced.
Specifically, the reasons why the DC motor cannot be started directly are as follows:
Because the resistance and inductance of the armature circuit of the DC motor are very small, and the rotating body has a certain mechanical inertia, when the DC motor is switched on, the back-EMF electric power R corresponding to the armature speed at the beginning of the starting phase is small, and the starting current is large, up to 15-20 times the rated current. This current can cause grid failure, mechanical shock of the unit, and commutator sparks. Therefore, direct closed start is only suitable for motors with a power of less than 4 kilowatts (starting current is 6 to 8 times the rated current).
In order to limit the starting current, a specially designed variable resistor is often inserted into the armature circuit. During the starting process, as the speed increases, the segment resistors are short-circuited in stages to limit the starting current within a specific tolerance range. This kind of starting method is called series resistance starting. It is very simple and light in weight. It is widely used in various small and medium-sized DC motors. However, due to the high energy consumption during startup, it is not suitable for frequently-started motors or large and medium-sized DC motors. However, if there are special needs, such as starting urban trams, the series resistance starting method is usually used to simplify equipment, reduce weight, and make operation and maintenance easier.
For large DC motors, voltage drop start is usually adopted. In other words, the motor armature is driven by a separately adjustable voltage DC power supply, and the motor can be started smoothly by controlling the power supply voltage to achieve speed regulation.
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