Why are DC motors more powerful than AC motors of the same power?
In order to limit the starting current, a specially designed variable resistor is often inserted into the armature circuit. In the starting process, as the speed continues to rise, each segment resistance is shorted in time, so that the starting current is limited to a certain allowable value. The working principle and starting method of this kind of DC motor is called series resistance starting. It is very simple and easy to use. It is widely used in various medium and small DC motors. However, due to the large energy consumption during starting, it is not suitable for motors that are frequently started and medium and large DC motors. But for some special needs, for example, city buses are often started. In order to simplify equipment, reduce weight and facilitate operation and maintenance, it is usually adopted serial resistance starting method.
Because the motor armature circuit resistance and inductance are small, and the rotating body has certain mechanical inertia, so when the motor is switched on, starting the beginning of the armature speed and anti electromotive force of the corresponding small starting current, large. Up to 15~20 times the rated current. This current will disturb the power grid, the unit will be subjected to mechanical shocks, and the commutator will spark. Therefore, the direct closing start is only applicable to motors with power not greater than 4 kW (starting current is 6~8 times the rated current).
This statement is inaccurate, and the output torque is the same under the same power and configuration.The reason for this intuitive illusion is that there is a difference between the two. AC motor as the power supply connection method or different structure, small starting torque, and starting current of DC motor often exceeds the rated current, starting torque exceeds the rated torque, thus starting load is more common. In order to start smoothly, the AC motor must not enlarge the output torque in order to overcome the insufficiency of the starting torque or change the connection method to increase the starting current. This gives a very intuitive appearance, the same power, AC motor can not drive, while the DC motor is easily competent.
