Applications Drive Technology
Drive Technology
Inverter duty: why variable speed drives changed the winding wire
Almost every new industrial motor is now fed by a variable frequency drive. That saves energy, but it also changed what the wire inside the motor has to survive. This is what inverter duty means for the enamel, and how to specify for it.
The problem with a fast edge
A drive does not deliver a sine wave. It delivers a string of rectangular pulses, each rising in a fraction of a microsecond. When such a pulse arrives down a long cable it reflects at the motor terminals, so the first coil sees up to twice the DC bus voltage. And because the rise time is so short, that voltage does not divide evenly across the winding; most of it lands on the first few turns.
Between two adjacent turns that share a thin layer of enamel, the result is partial discharge: tiny sparks in the air gaps that erode the enamel a molecule at a time. On a motor fed by a pure sine wave this never happens. On an inverter-fed motor with ordinary wire it can destroy the winding in months.
Three answers: thicker, tougher, filled
The first defence is covering thickness. Moving from Grade 1 to Grade 2, or Grade 2 to Grade 3, raises the voltage at which discharge starts. The second is the material: polyesterimide with a polyamide-imide overcoat resists the chemical attack of the discharge far better than plain polyester. The third, for the hardest duty, is a corona-resistant enamel that carries a dispersed inorganic filler; the filler does not stop the discharge but it stops the discharge from eroding the film.
For general-purpose inverter motors up to 415 V we recommend Grade 2 polyesterimide dual coat, Class 200. For long cable runs, 690 V drives, or servo motors with very fast switching, Grade 2 corona-resistant or Grade 3 dual coat.
Thermal cycling and speed range
A drive lets the motor run from near zero to well above base speed. At low speed the fan does little and the winding runs hot; at high-speed the rotor stress rises. The winding goes through far more thermal cycles than a fixed-speed motor, so the enamel needs flexibility as well as heat resistance. We test elongation and adherence on every batch, and use polyesterimide for its balance of flex and thermal endurance.
Servo, gear and pump motors
Servo motors for machine tools and robotics are the extreme case: compact, hot, fast-switched and expected to run for a decade. We supply them in fine and medium gauges with Class 200 and 220 corona-resistant enamel. Gear motors for conveyors, and pump and fan motors for process plant, cover the middle ground on standard Class 200 dual coat. Across all of them we hold SWG 8 to 43 in stock, so one specification can run across a whole motor family and the winding shop does not need to carry two wires.
Key takeaways
- Inverter pulses concentrate voltage on the first turns; ordinary Grade 1 polyester wire fails early.
- Baseline for inverter duty: Grade 2 polyesterimide dual coat, Class 200. Fast servo drives or 690 V: corona-resistant or Grade 3.
- Flexibility matters as much as heat rating; ask for elongation and adherence data with each batch.
