In the historical process, the AC motor appeared earlier than the frequency conversion speed control device, and the frequency converter basically operated "supported" for the existing motor. However, there is a problem of mutual adaptation between the motor and the frequency converter. In today's increasingly popular frequency control, combined with the characteristics of the frequency converter to design and manufacture a special motor, you can get more reasonable results.
In the design of the traditional asynchronous motor, it is particularly necessary to consider that the starting torque is large enough to ensure the start of the motor load and shorten the starting process. According to the motor theory, increasing the rotor resistance at start-up can increase the starting torque of the motor, but in normal operation, the larger rotor resistance will increase the loss during normal operation. Therefore, the rotor can be designed as a double squirrel cage winding or deep groove structure: the magnetic field has been rotated at the synchronous speed when starting, the rotor winding is still at a low speed, the relative motion speed of the two is relatively large, the skin effect is significant, and the rotor current is “squeezed†to The outer surface layer of the outer squirrel cage or deep groove, so that the effective resistance of the rotor winding increases, the starting torque increases; while in normal operation, the rotor speed is close to the magnetic field speed, the relative motion speed of the two is relatively small, and the skin effect is almost When it disappears, the rotor current is evenly distributed throughout the rotor winding, the rotor resistance returns to normal, and the loss is reduced to a normal value. This is a good design idea, but it will inevitably increase the radial size of the rotor, and the stator size will inevitably increase. The effective material and structural materials of the whole motor will increase. But this has been considered to be very normal and impeccable.
However, under the variable frequency condition, the motor is started according to the gradual increase of the applied frequency, the magnetic field rotation speed is synchronized with the frequency, and the rotor can always be close to the magnetic field speed at that time, without obvious effect; on the other hand, the torque of the motor - The speed curve is shifted to the low speed side as the frequency changes. Under the variable frequency condition, the torque at low frequency approaches the maximum torque. That is to say, the starting torque of the motor under the variable frequency condition is not a problem, and it is not necessary to increase the rotor resistance to increase the starting torque like the conventional motor, and the rotor winding has no obvious skin effect when the frequency conversion is started. Therefore, the structure of the double squirrel cage or deep groove is unnecessary for the frequency conversion start. Under the same power, the special motor for variable frequency operation can design the rotor groove shape to be shallow and flat, the radial size is obviously reduced, the overall size and material consumption of the motor are reduced accordingly, and the motor efficiency is improved. This is a beneficial side of the use of frequency conversion conditions.
On the other hand, harmonic currents and corresponding harmonic magnetic fields appear in the motor under variable frequency conditions. They cause harmonic losses and increase the heat of the motor. Harmonic torque can also occur, causing additional oscillations in the motor during starting. This puts new requirements on the motor design: choose a reasonable number of stator and rotor slots, and take appropriate chute measures to reduce the effects of harmonic torque. In addition, the harmonics in the motor are divided into two types: one is the time harmonic magnetic field excited by the current harmonics; the other is the spatial harmonic magnetic field formed by the discontinuous distribution of the windings. If the appropriate scheme is selected through simulation, it is worthwhile to properly compensate the two harmonics. Therefore, strictly speaking, the traditionally designed motor needs to consider the capacity reduction operation in the frequency conversion operation; the frequency conversion special motor can greatly weaken the influence of harmonics when it is designed and manufactured. Therefore, weigh the effects of positive and negative, and design a special motor suitable for variable frequency working conditions, which can get the effect of positive and negative.