A distributor said that the motors they sold had a batch occurrence of inter-turn faults. We sent someone for after-sales service and found that these motors were originally designed for mains frequency but were being used with variable frequency equipment.
The distributor's experience prompted us to discuss the requirements of electromagnetic wires for variable frequency motors.
Variable frequency motors require special electromagnetic wires
During the operation of a variable frequency motor, the impact of a large amount of harmonics and transient pulse voltage changes can cause damage to the electromagnetic wires.
Therefore, the insulation performance of the electromagnetic wires has a particularly important influence on the insulation structure reliability of variable frequency motors. Variable frequency motors should use anti-corona varnished wires.
Variable frequency enameled wire, also known as corona-resistant enameled wire or anti-corona enameled wire, is a special type of enameled wire that can withstand corona discharge and prolong the service life of variable frequency motors. It is mainly used in various variable frequency motors, speed-regulating motors, hoist motors, and elevator motors。
When the IGBT-PWM variable frequency speed control device is powered, the power supply cannot be a pure sine wave.
The residual modulation pulse is applied to the motor terminal, forming a high peak (high voltage peak) waveform with steep rise time.
In addition, factors such as variable output voltage frequency or extremely high frequency and unavoidable local discharge and local dielectric heating inside the motor cause serious damage to the insulation coating of enameled wire, making it easy for the variable frequency motor to experience inter-turn breakdown during use.
This greatly shortens the service life of enameled wire, so it is necessary to develop corona-resistant enameled wire.
Variable Frequency Control Terminology Explanation
IGBT (Insulated Gate Bipolar Transistor) is a composite fully controllable voltage-driven power semiconductor device composed of BJT (Bipolar Junction Transistor) and MOS (Metal-Oxide-Semiconductor).
It combines the advantages of high input impedance from MOSFET and low conduction voltage drop from GTR.
Pulse Width Modulation (PWM) is the abbreviation of "Pulse Width Modulation" in English. It is a very effective technique for controlling analog circuits using digital outputs from microprocessors. It is widely used in many fields ranging from measurement, communication to power control and conversion.
Practical Experience Sharing
For variable frequency motors with power below 300kW, electromagnetic wire manufacturers recommend using round enameled wire. When it comes to the quality of round enameled wire, special attention needs to be paid to the heat resistance, softening breakdown, continuity of enamel film, dielectric loss curve, pulse voltage resistance performance, and flexibility of the electromagnetic wire. Currently in China, special dedicated enameled wires are used for variable frequency motors. It is reported that their pulse impact voltage resistance performance has been improved by 200 times compared to Q (ZY/XX)-2.
According to the special nature of variable frequency motors, electromagnetic wire manufacturers pay special attention to the thickness and uniformity of the insulation film on enameled wires used for variable frequency motors.
Generally speaking, for variable frequency motors above 300kW, polyimide film-wrapped sintered wires are used.
There are also polyimide film-wrapped sintered wires specifically designed for high-frequency pulse resistance in foreign countries, and similar film-wrapped sintered wires have been successfully developed domestically as well.
Practice has proven that variable frequency motors manufactured using variable frequency electromagnetic wires perform well after leaving the factory.
Variable frequency electromagnetic wire comes into being
AC variable frequency motors use corona-resistant enameled wire, winding wire, and corona-resistant enameled wire enamel.
It has been a hot topic in the electromechanical industry and electromagnetic wire industry in recent years.
China's self-developed AC variable frequency motors have a history of many years since their development, but as the main insulation material for variable frequency motors - corona-resistant enameled wire, domestic production only started in 1998 and is now produced in large quantities.
Early developers of corona-resistant enameled wire and enamel in Europe and America mainly use powders of metal oxides such as titanium, chromium, silicon, and aluminum. These powders are filled into commonly used high-temperature resistant enamel for wire coating. After thorough mixing, it becomes corona-resistant enamel for wire coating.
The powders of these metal oxides are distributed between the molecules of the polymer.
When the solvent evaporates and dries, and the resin completes cross-linking curing, they become fixedly "embedded" in the middle of the polymer's molecules, acting as reinforcing agents for organic polymers. However, the disadvantages are quite obvious.
According to the data, this structure can only improve the resistance to partial discharge and withstand voltage capability, but cannot improve the corona inception voltage (DIV value).
Due to its three-layer insulation coating, it also brings great inconvenience to production management. At the same time, if this type of insulated wire is stretched during use, its corona resistance performance will be significantly reduced. If stretched by 5%, corona resistance performance will decrease by half; if stretched by 10%, lifespan will decrease by 90%.
With the improvement of product quality from electromagnetic wire manufacturers, the current performance of variable frequency electromagnetic wire insulation has been significantly enhanced.
It is particularly suitable for developing highly efficient and ultra-efficient motors. It not only meets the requirements for corona resistance but also effectively improves slot fill factor of motor windings through thinning of insulating film.
