when the power of induction motor is large, such as motors with more than tens of kilowatts, the efficiency can reach nearly 95%, but the efficiency of low-power motor is very low. for example, the efficiency of single-phase fan is even about 30 ~ 40%. the motor efficiency is not only related to the power (related to the proportion of its excitation current), but also related to the environment in which the motor operates. if the design of a motor is optimized, but it is always allowed to operate in a very unfavorable situation, such as less than 20% of the full load, the efficiency will be very low. in addition, the power factor of the motor itself is another key index affecting the efficiency.
induction motors have different performances according to different motor power levels, but if they are basically in the load range of about 40 ~ 80%, the utilization rate of the system for the motor itself is acceptable. therefore, when considering the control system, what kind of interval the system should operate in, including the power factor, is very important to the efficiency.
some advantages of the wide application of induction motor: 1. the cost is relatively low, and the power grid can run directly; 2. it's ok without a controller (of course, the cost is mainly in energy efficiency); 3. the torque coefficient is low, and a very stable magnetic field space can be generated under the condition of single-phase current. 4. there is no permanent magnet, no loss of magnetism, no fear of vibration or rust, so it is relatively reliable. in terms of control, it is very easy to enter the high-speed field weakening control area, and a variety of motors are available in a large power range. but there are also disadvantages: the efficiency is very poor under light load, and the size is much larger than that of permanent magnet motor under the same process. in terms of control, vector transformation and vector control are expected to be used when realizing the high efficiency of the motor. because the current at the rotor side is large and the temperature change is very large, the resistance at the rotor side changes very large, which makes it difficult for the parameter based control strategy of induction motor vector control, which is a great technical challenge.