Starting And Performance of DC Motor

In order to limit the starting current, a specially designed variable resistor is often connected in series in the armature circuit. During the starting process, with the continuous increase of the speed, the segmented resistors are short-circuited in time step by step to limit the starting current to a certain allowable value. This starting method is called series resistance starting, which is very simple and portable. It is widely used in various small and medium-sized DC motors. However, due to the large energy consumption in the starting process, it is not suitable for the motors that are often started and the medium and large DC motors. However, for some special needs, for example, although the city tram is often started, in order to simplify the equipment, reduce the weight and facilitate the operation and maintenance, the series resistance starting method is usually adopted.

Since the resistance and inductance of the armature circuit of the motor are small, and the rotating body has certain mechanical inertia, when the motor is powered on, the armature speed and the corresponding back electromotive force at the beginning of starting are small, and the starting current is large. Up to 15~20 times the rated current. This current will cause the power grid to be disturbed, the unit to be mechanically impacted, and the commutator to spark. Therefore, direct closing starting is only applicable to motors with power not more than 4 kW (starting current is 6-8 times of rated current).

For DC motors with large capacity, voltage drop starting is usually adopted. That is, the motor armature is powered by a separate adjustable voltage DC power supply. Controlling the voltage of the power supply can not only make the motor start smoothly, but also realize speed regulation. This method is complex for power supply equipment.

The performance of DC motor is closely related to its excitation mode. Generally, there are four excitation modes of DC motor: DC separately excited motor, DC shunt excited motor, DC series excited motor and DC compound excited motor. Master the characteristics of the four methods:

1. DC series-excited motor: the current is in series, the voltage is divided, and the excitation winding is in series with the armature, so the magnetic field in this motor changes significantly with the change of the armature current. In order to avoid large loss and voltage drop in the excitation winding, the smaller the resistance of the excitation winding is, the better. Therefore, the DC series excitation motor is usually wound with thicker wires, and its turns are less.

2. DC shunt motor: the circuit is parallel and shunt. The voltage at both ends of the shunt winding is the voltage at both ends of the armature. However, the excitation winding is wound with thin wires and has many turns, so it has large resistance, making the excitation current passing through it small.

3. DC compound excitation motor: the magnetic flux of the motor is generated by the excitation current in the two windings.

4. DC separately excited motor: the excitation winding has no electrical connection with the armature, and the excitation circuit is supplied by another DC power supply. Therefore, the excitation current is not affected by the armature terminal voltage or armature current.

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