Product category:
Industrial Motors
News Release from: WEG Electric Motors (UK) | Subject: W21 motors
Edited by the Engineeringtalk Editorial
Team on 07 October 2002
Insulation quality is key to motor
reliability
The recent debate about the quality and reliability of electric motors has brought into sharp focus critical elements of the motor insulation system.
The recent debate about the quality and reliability of electric motors has brought into sharp focus critical elements of the motor insulation system As possibly the most important component in this system, impregnation is particularly in the spotlight
Especially so, as opinions vary as to which is the best method to use: VPI (vacuum pressure impregnation) or CRFI (continuous resin flow impregnation).
One global motor manufacturer, WEG, has solved this problem by employing both types, employing each process in motor applications where it is shown to offer distinct advantages over the other.
The process of impregnation consists of adding varnish or resin (varnish is resin plus solvent, resin is an organic polymer) to the motor stator in order to fill all gaps, thus increasing motor thermal conductivity and mechanical and dielectric strength.
Of the two filling agents, better results are achieved with resins, which can be polyester, epoxy or silicon.
Essentially, there are five major types of impregnation: dipping, flooding, dripping, VPI and CRFI.
Most people believe that VPI (vacuum pressure impregnation) is the best impregnation system.
This is true for form wound motors due to the high porosity of insulating tapes and fleeces.
However, random wound motors are far superior when impregnated by the CRFI (continuous resin flow impregnation) system.
CRFI (sometimes known as trickle) should not be confused with dripping.
Dripping impregnation is achieved by pouring varnish on top of the windings, which may or may not be preheated.
In the CRFI system, the whole stator is preheated to facilitate resin flow through the slots.
The stator is then tilted and rotated.
The combination of gravity with the centrifugal force created by the rotating movement ensures that the resin adheres to the windings and slot insulation, filling all gaps.
Moreover, the use of resin instead of varnish increases the percentage of retained solids at the end of impregnation from 40 to over 80%.
This results in an overall increase in mechanical and dielectric strength of the insulation, eliminating vibration between turns which prevents short-circuits and improves heat transfer.
CRFI impregnation brings high voltage integrity and mechanical endurance to the motor windings, features that are crucial for reliable operation in applications where motors are used with AC inverters.
The CRFI impregnated motor also presents a six times lower amplitude discharge than motors impregnated using standard dipping systems at an applied voltage of 2600V.
Finally, the voltage amplitude threshold from which partial discharges occur is 41% higher for a CRFI system, providing greater immunity against voltage spikes.
With its new W21 line of high-efficiency motors, WEG has addressed the insulation problems experienced with motors controlled by inverters.
These problems - essentially higher stress on the insulation - are caused by the fast rise times of the IBGTs used in modern inverters.
The W21 motors are impregnated with higher insulation class H varnish, providing a state-of-the-art insulation system that withstands the voltage spikes commonly encountered with inverter use.
In addition, bearing protection against EDM (electrical discharge machining), which can cause motor failure, is offered as an option for frame sizes 225 and above.
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