The difference between the performance characteristics of DC motors and AC motors

Speed ​​regulation performance
DC motor: DC motors have excellent speed regulation performance, which is one of their significant features. It can achieve smooth and stepless speed regulation under heavy load conditions. The speed regulation range is also relatively wide. The speed regulation of DC motors can usually be achieved by changing the armature voltage U (such as changing the armature voltage through pulse width modulation - PWM technology), changing the excitation current to adjust the magnetic field strength and thus change the motor speed) or changing the armature voltage and excitation current at the same time. For example, in some industrial equipment that requires precise speed control, such as papermaking machines and rolling mills, DC motors can accurately adjust the printing speed, steel rolling speed, etc. according to the production process requirements, and the speed regulation accuracy can reach a very high level. Moreover, the DC motor has good speed regulation stability, and the motor speed changes relatively little when the load changes.

AC motor: Traditional AC asynchronous motors are relatively weak in speed regulation performance. However, with the development of inverter technology, AC motors can also achieve better speed regulation functions with the help of inverters. However, compared with DC motors, there are still some limitations in the speed regulation of AC motors, such as insufficient torque output at certain low speeds, relatively low speed regulation accuracy, and long speed response time. Although vector frequency conversion technology can improve the speed regulation performance of AC motors to a certain extent and simulate DC power, it is still difficult to completely replace the speed regulation performance of DC motors in some special occasions with extremely high speed regulation requirements.

Starting performance
DC motor: The starting torque of DC motors is large and can bear heavier loads at startup. This is because the torque formula of DC motors is T=K TφI. If a suitable starting method (such as armature series resistance starting and other methods to limit the starting current and increase the starting torque) is used at the moment of starting, a large electromagnetic torque can be provided to start the motor. For example, in some electric vehicles, crane lifting mechanisms and other equipment that require a large starting torque, DC motors can meet the starting requirements.

AC motor: The starting torque that AC asynchronous motors can generate at startup is relatively small (especially squirrel cage asynchronous motors). If a high-power AC motor is started directly, the instantaneous current of the motor starting is large (generally 4-7 times the rated current), which may cause a large impact on the power grid, and additional starting equipment (such as a step-down starter or soft starter, etc.) may be required to reduce the starting current, which also increases the cost and complexity of the equipment. However, the wound asynchronous motor can increase the starting torque by inserting a resistor in series in the rotor winding, but it is still slightly insufficient in terms of the size and flexibility of the starting torque compared to the DC motor.

Operation efficiency
DC motor: The operating efficiency of DC motors varies by type. The operating efficiency of traditional brushed DC motors is not very high by modern standards due to factors such as contact resistance between the brush and the commutator and friction loss. However, brushless DC motors overcome many of the shortcomings of brushed motors, and the operating efficiency is significantly improved. The efficiency of some high-efficiency brushless DC motors can be comparable to that of AC motors. Under low load and low speed conditions, some DC motors can still maintain a certain efficiency, which is their advantage over AC motors (the efficiency of AC motors will drop significantly at low load and low speed).

AC motor: Modern AC motors, especially three-phase asynchronous motors, are widely used in industry, and their operating efficiency is high near the rated load. AC motors do not have the problems of brush contact loss of DC motors, and with the continuous improvement of design and manufacturing technology, they can work near the efficient operating point, and are a relatively economical and efficient power source. For example, in some large industrial fans, water pumps and other equipment, AC motors are used as power sources, which can achieve higher energy conversion efficiency while meeting the equipment operation requirements.

Maintenance Difficulty
DC motor: Since the structure of DC motors contains brushes and commutators, the maintenance of DC motors is more complicated. Brushes will wear out after long-term work and need to be regularly inspected and replaced (generally when the brushes are worn to 1/3-1/2 of the original length, they need to be replaced); commutators are also prone to wear and sparks due to friction of brushes, resulting in surface oxidation, and need to be cleaned and maintained regularly. In addition, the control circuit of DC motors is relatively complex. If DC brushed motors are used, there is also the problem of electromagnetic interference generated by brushes.

AC motor: AC motors have a relatively simple structure and do not have brushes and commutators, two components that require regular maintenance, so they are relatively easy to maintain. The structure of its stator winding and rotor (especially the squirrel cage rotor) is stable, and the probability of failure is relatively low. However, for higher-power AC motors, it is necessary to check the insulation of the windings and the lubrication of the bearings, but in general, the frequency and complexity of maintenance are much lower than those of DC motors.