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Briefly explain the working principle of diesel generator set starter motor

July 22, 2022
When the diesel generator set is started, the crankshaft must be driven by external force to rotate, so that it completes the process of air intake, compression, combustion, exhaust, etc., until the combustion to do work, the set can rise to work. The starter motor is a device to complete this work, which consists of series-excited DC motor, electromagnetic switch and transmission device, which can make the engine start quickly and reliably. Most of the modern diesel generator set starting methods use electric starting. Electric starting is simple, reliable, easy to operate, can work at any time, and easy to control from a distance. The electric starter uses the battery as the power source and the DC motor as the power source, and works through the transmission and control mechanism.
The so-called series-excited motor is a DC motor with the excitation coil in series with the armature coil. After the motor is energized, the armature rotates under the action of electromagnetic force, and the electrical energy is turned into mechanical energy and output.
When the starter is energized, the current is high, the torque is high, and the speed is low. When the armature is rotating, its coil is also cutting the magnetic lines of force, so it will also generate the induction electromotive force. The direction of the induced electromotive force can be determined by Ampere's right-hand rule, which is always opposite to the direction of the current supplied by the power supply. It always prevents the current from passing through, so it is called counter-electromotive force. The magnitude of the counter-electromotive force is proportional to the speed of the motor. The counter-electromotive force is small at the beginning of starting, and the higher the motor speed is, the larger the counter-electromotive force is, the smaller the current through the armature coil is, and the torque of the motor will drop with the increase of speed, which is common to all kinds of DC motors. As for series-excited motor, the counter-electromotive force not only affects the current in the armature coil, but also affects the current in the excitation coil. When the rotational speed increases, the current in the armature coil and excitation coil will decrease at the same time, so the torque further decreases with the increase of rotational speed, which is the working characteristic of series-excited DC motor.
When the diesel generator starts, on the one hand, it needs to overcome the large static inertia force, on the other hand, the friction parts are in the dry friction state, the friction resistance is large, and it needs a large starting torque. At this time, the motor is rotating from standstill, and the counter-electromotive force in the armature is increasing from zero, so in the initial moment of starting, the armature can pass a larger current, which ensures that the starter has a larger torque to drive the crankshaft to rotate and start the internal combustion engine.
When the crankshaft is driven to rotate, the above resistance decreases, and after the speed increases, the current flowing through the motor coil decreases accordingly due to the increasing counter-electromotive force, and the torque force of the motor decreases, and this characteristic is exactly what the unit needs. Because the unit has been rotated, at the same time, as the motor speed increases, the current taken from the battery decreases, which can reduce the consumption of battery power, and reduce the heat of the motor itself, so the diesel generator set with electric starting, are used series excitation DC motor.
DC motor starting system has the advantages of compact structure, convenient operation, and can be operated from a distance, etc. It is widely used in various vehicle engines and small and medium power diesel generator sets.