Due to the special nature of variable frequency power supplies, the winding insulation of variable frequency motors has stricter requirements than that of ordinary motors. Let's take a closer look at the details!
First, select electromagnetic wire with good corona resistance to meet the requirements of motor resistance to high-frequency pulses and partial discharge.
Generally, polyesterimide/polyamideimide composite layer enameled wire is used, which is corona resistant and anti-corona electromagnetic wire.
II. Construction process for winding and embedding wires.
The processing of variable frequency motors, including winding, winding, and binding, must be strictly managed. In particular, damage to the conductors must be prevented during winding and winding. During winding, slot insulation, phase insulation, and interlayer insulation must be properly placed. Phase insulation should use materials that are easily penetrated by insulating varnish, and the coil ends should be reinforced and fixed to ensure that the ends form a single unit.
Strengthening insulation at the bottom of the motor slot, between phases, between layers, and at the first and last turns of the coil can improve the motor's electrical withstand strength.
3. The main insulation must be gapless insulation.
Air gaps in the insulation structure of variable frequency motors are a major factor contributing to corona discharge. To ensure that the overall insulation structure of the motor is free of air gaps, according to the national standard GB/TZ1707-2008 "Insulation Specification for Three-Phase Asynchronous Motors for Variable Frequency Speed Control," the impregnating varnish used in these motors must be solvent-free varnish of at least Class F, with a volatile content of less than 10%, and must employ the VPI (Vacuum Insulation Process) technology. This process also improves the overall mechanical strength of the insulation structure.
Fourth, ensure proper matching between the frequency converter, cables, and motor, and limit the length of the cable between the motor and the power supply.
Due to impedance mismatch in the power lines, the overvoltage amplitude at the motor terminals increases with the length of the cable between the inverter and the motor, which can easily lead to partial discharge. Therefore, the length of the connecting cable should be shortened as much as possible, based on the specific characteristics of the inverter power supply and actual needs, to reduce the overvoltage amplitude and partial discharge at the motor terminals and extend the motor's lifespan.
Variable frequency motor power cables generally use professional cables, also called symmetrical conductor variable frequency cables, which are 3P+3N/E series, meaning that the original 3+1 is split into 3 strands.
The difference between variable frequency motors and ordinary motors
Variable frequency motors can achieve the required speed by changing the motor frequency according to the work needs. Of course, a powerful cooling fan is also added to ensure the cooling of the motor at low speeds.
Features of variable frequency motors
1. Electromagnetic Design
For ordinary asynchronous motors, the main performance parameters considered during design are overload capacity, starting performance, efficiency, and power factor. However, for variable frequency motors, since the critical slip is inversely proportional to the power supply frequency, they can start directly when the critical slip is close to 1. Therefore, overload capacity and starting performance are no longer as important considerations. The key issue to address is how to improve the motor's adaptability to non-sinusoidal power supplies. The general approach is as follows:
1) Minimize stator and rotor resistance as much as possible.
Reducing the stator resistance can lower the fundamental copper loss, thus compensating for the increased copper loss caused by higher harmonics.
2) To suppress high-order harmonics in the current, the inductance of the motor needs to be appropriately increased. However, a larger rotor slot leakage reactance results in a larger skin effect and increased copper losses from high-order harmonics. Therefore, the magnitude of the motor leakage reactance must take into account the rationality of impedance matching throughout the entire speed range.
3) The main magnetic circuit of a variable frequency motor is generally designed to be unsaturated. This is because higher harmonics will deepen the saturation of the magnetic circuit, and also because at low frequencies, the output voltage of the frequency converter needs to be appropriately increased to improve the output torque.
