Tom Bishop, PE
EAS A Senior Technical Support Specialist 

With the advent of solid-state electronic variable frequency drives (VFDs) in the late 1980s, it was found that the windings of motors used on VFDs failed more frequently than when powered by a utility (sine wave) supply. By the turn of the century, motor manufacturers had gained a better understanding of how VFDs affected motor windings, and motor manufacturers and suppliers of winding materials had developed materials and methods to improve the reliability of motor windings supplied from VFDs. The general term for the windings is “inverter duty.” In this article, we will describe the materials and methods associated with inverter duty windings.

Magnet Wire
Before “spike-resistant” wire was developed in the late 1990s, a common practice for winding motors used with inverters was to use a wire with thicker polyester-based insulation. Some wires used triple or quad-film as insulation. These wires are very effective when subjected to sine wave voltage or intermittent transient voltages. The heavy build wires are effective against corona (Figure 1) because the distance between the actual conductors is greater with the added insulation. This larger gap between individual conductors forces any build-up of voltage between the conductors to be smaller. However, when stressed by the VFD waveform, the dielectric strength of the heavy build wires is not as effective. 

The newer magnet wires used for motors with inverters have a higher dielectric capability with significantly increased life (Figure 2). These wires can withstand voltage spikes better than the heavy film wire but with the same build as standard magnet wire. Figure 3 is an illustration of the impact on magnet wire life as the switching frequency of the drive increases. The life of the heavy film wire is greatly impacted, whereas the inverter grade system does not see a shorter life based on the switching frequency. 

Using a larger diameter wire will increase the voltage where corona begins to occur. That is why it can be important to use as few in hand of larger diameter wires when rewinding inverter duty motors. 

Conversely, smaller diameter wires have lower skin effect losses at higher frequencies, such as the carrier frequency of a drive. Skin effect causes the current in a round conductor to be near the surface, and the carrier frequency is the rate at which the DC voltage is “chopped” into segments to simulate sine wave power. If the carrier frequency is high, e.g., 12kHz or greater, use smaller wire diameters if possible; otherwise, consider using larger diameter wires. 

Slot Fill and Insulation System
Even the best insulation system will eventually begin to break down, especially with the use of a VFD. For added electrical and mechanical strength, a typical inverter duty design will maximize slot fill. Not only does this increase efficiency and allow the motor to run cooler, it will also help prevent coil movement that can begin to break down the insulation. It is a good practice to use ties on at least every 3rd or 4th coil on both connection end and opposite connection end coil extensions to further brace the winding. The most common failure of windings used with VFDs occurs in the first turns of a lead coil as illustrated in Figure 4, so some windings may have sleeving over the first turn of the lead coils for extra electrical protection. 

Phase insulation is designed to separate the coils of different phases. Most of the magnitude of the voltage spikes seen by the winding is concentrated on the lead end coils. The beginning and end turns of a random wound coil may touch each other. A voltage spike may occur between those two adjacent wires, as well as across the coils. Because voltage spikes can reach 2000 volts or more, additional slot insulation for the higher voltage should be used as well, provided that the wire area per turn does not need to be reduced to fit the winding in the slots. Maximize the insulation and use separators in the slots and end turns.

A motor operating on a VFD typically runs hotter than the same motor on sine wave power. If the winding temperature is 10°C higher, the thermal insulation life is cut in half. Class H (180°C) insulation has a higher temperature rating than Class B or F (130°C or 155°C) factory windings so that the life of the winding can be extended. When the motor is operating at a slower than rated or base speed, the decreased airflow will cause the motor winding to heat up more. For this reason, a Class H (180°C) insulation system is advantageous. 

Varnish and Impregnation
A double dip and bake process should be used. If available, a better alternative would be a dip and bake followed by vacuum pressure impregnation (VPI) and bake. Make certain to follow the varnish/resin manufacturer guidance for winding pre-heating temperature, and for bake temperature and time. Bear in mind that the bake time does not begin until the winding has been heated to the minimum recommended bake temperature for the varnish/resin. 

Caution: Most magnet wires have a lubricant coating on them that was used to facilitate manufacture of the wire. The winding preheating process serves two purposes. One is to evaporate the lubricant off the wire, which then allows the varnish/resin to bond to the wire. The other purpose is that the preheating helps relieve residual stresses in the wire insulation coating so that the coating does not crack (crazing). 

Inverter Winding Technique
When a motor that is used with a VFD is wound or rewound, special care must be taken when putting the coils in the slots so that the magnet wire insulation film does not become scratched or nicked. It is a good practice to use mylar feeder paper in the slots to aid in the insertion of coils to protect them from damage. 

Some manufacturers use a winding technique that makes the winding less “random” by aligning the wire in the slots with a more orderly spacing of the turns. The idea is to keep the beginning and the end of the coil as far away from each other as possible to reduce the magnitude of the voltage between adjacent conductors. Semi-automated winding machines used in service centers approach this level of orderly turn spacing.

Inverter duty rewind specification
The following is a guideline specification for an inverter duty winding system. 

General 

• Class H or higher rated insulation system 

Magnet wire 

• Inverter duty wire 

Wire area 

• Maintain or increase wire area per turn 
• Maintain or increase the number of conductors (reduce eddy current loss) per turn 

Insulation 

• Phase insulation between phases 
• Ground insulation 0.015” (0.38 mm) minimum 
• Over 80 volts per coil use phase insulation between the middle coils of each group 

Lacing and bracing 

• Lace at least every third or fourth coil 
• Wrap end turn OD with a minimum of 3 half lap layers of untreated Dacron glass tape [1 inch (25 mm)] from core to 1 inch (25 mm) from coil noses 

Impregnation 

• Preheat per varnish manufacturer instructions 
• Double dip and bake 
• Bake/cure for longest time recommended by varnish manufacturer 
• Note that cure time does not begin until winding has been heated to curing temperature

AVAILABLE IN SPANISH

Categories: Technical topics, AC motors, Stators/windings, Lacing/bracing, Random wound, Winding insulation, Dip & bake, Vacuum pressure impregnation (VPI), Wire

Tags: Variable frequency drives Magnet wire Inverter Duty

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