Requirements Of Variable Frequency Motors For Magnet Wires

A motor manufacturer encountered mass interturn faults on its delivered motors. After after-sales technicians arrived on site for troubleshooting, they found that these motors, originally designed for mains frequency operation, had been matched with variable frequency drive equipment. Today we will elaborate on this issue.

When variable frequency motors are in operation, massive harmonics and transient pulse voltage surges will damage the magnet wire. Therefore, the insulation performance of magnet wires exerts a decisive impact on the reliability of the insulation system for motors, and corona-resistant enamelled magnet wires dedicated for variable frequency applications must be adopted.

Variable frequency enamelled wire, also known as corona-resistant enamelled wire, is a special type of magnet wire capable of resisting corona discharge and extending the service life of variable frequency motors. It is widely applied in various variable frequency motors, adjustable-speed motors, crane motors and elevator motors.

When powered by an IGBT-PWM variable frequency speed control unit, the power supply waveform is far from a pure sine wave. Residual modulated pulses are transmitted to the motor terminals, forming steep-fronted voltage spikes with extremely high peak values. Coupled with variable or ultra-high output frequencies, inevitable partial discharge and localized dielectric heating inside the motor, the insulating film of the enamelled wire suffers severe degradation. This easily leads to interturn breakdown of variable frequency motors in service, drastically shortening the service life of the magnet wire, which necessitates the development of corona-resistant enamelled wires.

IGBT (Insulated Gate Bipolar Transistor) is a composite fully-controlled voltage-driven power semiconductor device integrating BJT (Bipolar Junction Transistor) and MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor). It combines the merits of high input impedance of MOSFETs and low conduction voltage drop of GTRs.

PWM stands for Pulse Width Modulation. It is an efficient technology that controls analog circuits via digital outputs of microprocessors, widely used in measurement, communication, power control and power conversion fields.

For variable frequency motors with rated power below 300 kW, magnet wire manufacturers recommend round enamelled wires. For round enamelled wires, key quality indicators include thermal endurance, thermal shock breakdown, film continuity, dielectric loss curve, pulse voltage resistance and flexibility. Domestic manufacturers now exclusively produce special enamelled wires for motors. It is reported that their pulse surge withstand capacity is 200 times higher than standard Q(ZY/XX)-2 wires.

With continuous product upgrades by magnet wire manufacturers, the coating performance of modern variable frequency magnet wires has been greatly improved. Such wires are highly compatible with the development of high-efficiency, ultra-high-efficiency and premium ultra-high-efficiency motors. They not only satisfy anti-corona requirements, but also effectively boost the slot fill factor of motor windings by adopting thinner insulation film.